ANESTHESIOLOGY VMED 7412
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Ml 7 VETERINARY HEALTH SCIENCES PHARMACOLOGY INHALANT ANESTHETICS Lyon Lee DVM PhD DACVA lntmdurtlon Maintenance of general anesthesia is primarily carried out using inhalation anesthetics although intravenous anesthetics may be used for short procedures Inhalation anesthetics provide quicker changes of anesthetic depth than injectable anesthetics and reversal of central nervous depression is more readily achieved explaining for its popularity in prolonged anesthesia less risk of overdosing less accumulation and quicker recovery see table 1 Table I Comarison 0 inhalant and in 39ectable anesthetics Injectable Technique Inhalant Technique Expensive Equipment Patent Airway and high 02 Better control of anesthetic depth Ease of elimination ventilation Pollution Cheap needles syringes Not necessarily Once given suffer the consequences Onlythrough metabolism amp Excretion No Commonly administered inhalant anesthetics include volatile liquids such as iso urane halothane sevo urane and des urane and inorganic gas nitrous oxide N20 Except N20 these volatile anesthetics are chemically halogenated hydrocarbons and all are closely related Physical characteristics of volatile anesthetics govern their clinical effects and praticality associated With their use Table 2 Physical characteristics of some volatile anesthetic agents The volatile anesthetics are administered as vapors a er their evaporization in devices known as vaporizers Ideal properties of an inhalant anesthetic o Nonexplosive o Non ammable Pharmacology Inhalant Anesthetics 1 of 13 Veterinary Sumgry I VMED 7412 o Nontoxic 0 Safe with C02 absorbent o Potent 0 Pleasant to inhale 0 Minimal metabolism 0 Low blood gas solubility 0 Good analgesia 0 Good shelf life 0 Minimal organ depression 0 Inexpensive The mechanism by which inhaled anesthetics produce the CNS depression is not clearly understood and a single theory to explain it is unlikely Most evidence is consistent however with inhibition of synaptic transmission through multineuronal polysynaptic pathways particularly in the reticular activating system see CNS amp Anesthesia lecture Practical aspects of the use of inhalational volatile anesthetic agents Anesthetic potency The minimum alveolar concentration MAC Pharmacology lnhalant Anesthetics The term potency refers to the quantity of an inhalational anesthetic that must be administered to cause a desired effect such as general anesthesia and the standard index of inhalation anesthetics is the minimum alveolar concentration MAC which was proposed by Merkel and Eger in 1963 MAC is defined as the alveolar concentration of anesthetic that prevents muscular movement in half the test subjects in response to a painful stimulus It is usually expressed as a but this assumes a normal sea level atmospheric pressure As blood anesthetic gas levels are difficult to measure end tidal levels of inhalant anesthetics are usually accepted as approximating to alveolar and therefore to blood gas tensions The anesthetic potency of an inhaled agent is inversely related to MAC MAC is also inversely related to the oil gas partition coefficient PC The partition coefficient is defined as the ratio of the amount of substance e g inhalant present in one phase oil blood etc compared with another gas the two phases being of equal volume and in equilibrium 0 A bloodgas PC of 05 means that the concentration of inhalant in the blood is half that present in the alveolar gas when the partial pressure of the anesthetic is identical at both sites PC in an inhalation anesthetic is most commonly used to refer to its solubility in a given solvent eg oil blood etc o A very potent anesthetic eg methoxy urane has a low MAC value and a high oil gas PC whereas a low potency agent e g N20 has a high MAC and low oil gas PC In other words an anesthetic with a high oil solubility ie high oil gas PC is effective at a low alveolar concentration and has a high potency The anesthetic dose is commonly defined in terms of multiples of MAC ie 15 times MAC or 15 MAC Surgical depth is usually achieved at 12 to 15 times of MAC values In a single species the variation in MAC values is generally small Even between species the variation is not usually large 0 o 2 of 13 Veterinam Sumem Il VMED 7412 0 One exception is N20 where MAC in man is 104 whereas in most animals close to 200 making the agent far less effective in domestic animals In man MAC is greatly in uenced by age in inverse relationship MAC is also reduced by any sedative analgesic or parental anesthetic agents which has been used 0 Factors to decrease MAC 0 hypotension o anemia PCVlt13 o hypothermia o metabolic acidosis o hypoxia 0 premedicants 0 pregnancy 0 aging o hypothyroidism 0 concurrent use of analgesics 0 Factors to increase MAC 0 increased body temperature 0 hyperthyroidism o hypematremia 0 concurrent use of central nervous stimulant e g doxapram 0 Factors known not to affect MAC 0 type of stimulation to test 0 duration of anesthesia 0 sex 0 PaCOz between 1595 mmHg 0 hypertension 0 potassium Methoxyflurane 023 023 028 Halothane 087 082 088 Isoflurane 128 163 131 Sevoflurane 21236 258 236 Desflurane 72 98 76 Nitrous oxide 188 255 205 Pharmacology lnhalant Anesthetics 3 of 13 Veterinam Sumem Il VMED 7412 Uptake and elimination nf volatile agenu the properuse and subsequently 1mproved safety for the patrert Alveolar pamd prexxwe curve 11 D Pbr ma al a m 11H anesthetre pamal pressure m the bran PA Pa thme 1 Ken 5 alveolar pomm prexxwe cum afmhalant dmmg uptake Alvealar pzmzl Ta pressure 47 Initial rise Time mmam 1m 1 and tau hto the lungs Followmg thrs the lmee stage amves where the rate othe agent start to slow down brarrt hver etc whrrh recewes geater than 70 quota of eardrae output has hrgh ougas Pc trssue and blood see gure 2 and 3 For aiamp e low blood orussue solubrhty VatHan Surj l VNED 7412 Pharmacoan Inhalam Annitheths 4 at 13 Figure 2 The rise of alveolar partial pressure toward the inspired partial pressure Pl in di erent volatile anesthetics N20 des urane sevo urane PA P1 Iso urane en urane Haluthane Methoxyflurane Time Anesthetic elimination or recovery from inhalation anesthesia results from the elimination of anesthetic from the brain This process is simply the reversal washout of the anesthetic uptake so the prominent factors affecting the recovery are e same as those for anesthetic induction veolar concentration of inhalant re ects the amount in the brain Factors governing the alveolar concentration of inhalants FDPPST Table 4 Solubility of inhalant in blood and tissues Cardiac output Tissue capacity and blood ow to the tissues Inspired concentration 0 The higher the vaporizer setting the higher the inspired concentration or partial pressure P1 of the inhalant by the patient Which results in the higher alveolar concentration Volatility boiling point 0 o Pharmacology Inhalant Anesthetics In uc on can be speeded by increasing the concentration of anesthetic gas inspired However the concentration that can be obtained is governed by the volatility of the agent NB With very volatile agents it may not be safe to use maximal possible concentrations for other reasons 5 of 13 Veterinaljy Sumely l VMED 7412 Alveolar ventilation o The inspired gases reach the alveoli where gas exchange takes place 0 Initially there should be a rapid rise in alveolar concentration but this is contrabalanced by the solubility of the inhalant anesthetics o The better a patient breathes in the better the gas exchanges and the faster the patient gets anesthetized 0 Increased alveolar ventilation like P1 promotes input of anesthetic to offset uptake The net result is a more rapid rate of increase in PA toward the P1 ie the faster the alveolar concentration PA will achieve the inspired concentration P1 and thus induction of anesthesia Solubility o Uptake of inhalant by blood washin is determined by 0 bloodgas solubility S cardiac output blood ow CO the pressure gradient of the partial pressure of anesthetics between the alveolus and venous blood returning to the lung PAPv expressed in millimeters of mercury mmHg where PbZr equals the barometric pressure in mmHg 0 o Uptake S X CO x H Pbar 0 Note that if any of these three factors equals zero there is no further uptake of anesthetic by the blood 0 Blood gas solubility o The lower the solubility of the anesthetic agents the faster the equilibration of gas in the blood and the faster the speed of induction and recovery Figure 3 below shows for uptake how the magnitude of change for B flow solubility agent is less than that for Ahigh solubility agent 0 Pharmacology lnhalant Anesthetics 6 of 13 Veterinam Sumem Il VMED 7412 Figure 3 in uence of alveolar ventilation and cardiac output on alveolar partial pressure and PAPI in inhalants with di erent blood39gas solubility Alveolar Ventilation Cardiac output 8 Lmin 2 Lmin B low solubility anesthetic 2 Lmin 8 Lmin Cpl 2 8 Lmiu 2 Lquotmin A high solubility anesthetic 2 Lmin 8 Lmin Time I Bloodtissue fat solubility o The less soluble the anesthetic gas in fat the lower the uptake in fat tissue 0 In short anesthetics this factor is not important but for long anesthetics it affects recovery o If the bloodfat solubility of the anesthetic gas is high then in a fat animal recovery from a long anesthetic will be slow even with agents with a low bloodgas solubility Cardiac output The higher the CO the higher the uptake This result in slower rate of rise in PAPl lt mainly affects drugs with a high solubility see figure 3 A positive feedback dangerl o The deeper the patient gets the further decrease of CO while PA approaches Pl more quickly as there is less uptake of the anesthetic by the blood which further deepens the patient 0 ln sick animals with reduced cardiac output anesthetics have more profound cardiopulmonary effects because of increased Pbr vicious cycle in a way Tissue capacity and the blood flow With the higher blood flow to the tissue and its capacity the greater the uptake of the anesthetic by the tissue When the tissue gets saturated with the anesthetic the alveolar concentration would remain constant and PA would approach P1 through equilibration Pharmacology Inhalant Anesthetics 7 of 13 Veterinam Sugem x VMED 7412 Agents in cunent we Isoflurane F0rane Is0 0 Generics This agent was the first which really appeared to be as good if not better than halothane For years the signi cant difference in pricing 10 times has been the primary deterrent for veterinarians to use it over halothane but more recently due to the generic product competition the pricing of iso urane has come down substantially making it more affordable for veterinarians as well as for physicians MAC is about 13 so surgical anesthesia is produced with end tidal concentrations of about 1517 in oxygen 2 7 24 at vaporizer This will be further reduced with premedicants and induction agents It is very insoluble in blood and a lot less soluble than halothane in fat Thus induction and recovery is faster Signs of anesthesia are identical to those seen with halothane Analgesia is poor and muscle relaxation moderate ie as halothane Dose dependent depression of respiratory and cardiovascular system is seen These changes are similar to those occurring with equipotent doses of halothane but the myocardial depression with iso urane is less whilst respiratory depression is greater to the extent that in horses iso urane should only be used with IPPV Liver metabolism is very low decreasing the risk of hepatitis Nevertheless there has been at least one report of a human patient previously sensitized by exposure from halothane suffering hepatitis after an iso urane anesthetic If administered by rebreathing circuits especially those with in circle vaporizers the quantities used are reduced therefore cost can be reduced Sevo urane Ultane Pharmacology lnhalant Anesthetics A halogenated ether which underwent trials some years ago then was dropped as it is metabolized is unstable in the presence of soda lime compound A formation which has been shown to be nephrotoxic in rats However it was licensed in Japan for human usage over a decade and has now been used safely on over 8 millions of people As a result the potential toxicity has been reinvestigated and is now clinically used in the USA and Europe It has low blood gas solubility even lower than iso urane so induction and recovery is quick In a horse this quick recovery may cause emergence excitement so a low dose of sedative usually xylazine is given to slow the recovery and avoid the adverse reaction at the time of recovery Unlike iso urane it does not cause mucous membrane irritation so it is considered that it will totally displace the last remaining reasons for using halothane eg neonatal mask induction by pediatricians MAC in horses has been shown to be 236 Cardio respiratory effects were similar to those induced by other agents Similar results have been obtained in the goat although MAC is slightly higher Has taken on well recently both in human and veterinary market due to market push by the pharmaceutical company Just as iso urane when the patent expires soon the price is expected to drop substantially due to generic product competition 8 of 13 Veterinam Sumem Il VMED 7412 Agents no longer in regular current use Nitrous oxide Presented in blue cylinders filled with liquid N20 Boiling point low so vaporizes in cylinder and obtained from cylinder as a gas MAC above 100 so cannot be used on its own Much more wide spread use in human medicine than in veterinary medicine Ensure at least 30 02 is given Advantages 0 Very insoluble in blood and in fat so rapid uptake and elimination 0 May also speed up induction with other agents by the second gas effect 0 Excellent analgesia and its use reduces the concentration of halothane required 0 Little cardiovascular or respiratory depression and very safe as long as sufficient oxygen 30 is given Disadvantages 0 Limited potency o Tends to diffuse into all gasfilled cavities such as gut so some authorities DO NOT use in horses or ruminants 0 Danger of hypoxia 0 Diffusion hypoxia during recovery A 510 minutes of 100 oxygen supplementation should prevent the diffusion hypoxia 0 Increase environment pollution 0 Drug abuse by personnel Practical use 0 Do not use unless the anesthetic machine has an oxygen deficit alarm o In fit animals can be used in a nonrebreathing circuit at 50 N20 with 50 02 In rebreathing circuits accumulation of N20 and of Nitrogen increases the danger of hypoxia This danger can be avoided by emptying rebreathing bag several times in early stages to remove nitrogen however not recommended to deliver N20 using such system and use of nonrebreathing system is recommended if N20 is used 100 02 should be given at the end of the procedure approximately 510 minutes to prevent diffusion hypoxia Higher concentrations of 02 must be used in pulmonary disease It should not be used in ruminants or horse unless blood gas oxygen analysis is available Probably not very practical for private veterinary practice setting as disadvantages outshadow the real useful advantages 0 o o o o o Halothane Fluothane generics Pharmacology lnhalant Anesthetics Once the most widely used volatile anesthetic agent in veterinary practice but taken over more recently by iso urane and to lesser extent by sevo urane Moderately insoluble in blood and adequate volatility therefore fast induction Fairly fat soluble so recovery from long anesthetic in fat animals may be prolonged Halothane has been very widely used in human and veterinary anesthesia for around 50 years However it is no longer marketed in the US Some equine practices will still carry it in their inventory and preferably use it over iso urane due to advocated superior recovery quality It is a very effective but potent and dangerous agent as are most halogenated inhalation anesthetics The following properties must be considered 9 of 13 Veterinam Sumem Il VMED 7412 A potent respiratory depressant However depressed respiration often consists of shallow rapid panting respiration which being inefficient causes C02 retention and subsequent muscle tension and twitching This makes judging the depth of anesthesia by older conventional re ex signs difficult A marked hypotensive agent which is dose dependent and can be used to assess the depth of anesthesia It has a particularly marked myocardial depressant action as well as causing peripheral vasodilation It is not a good analgesic and is not good at suppressing re ex movements Analgesics e g N20 may have to be given to avoid the need for deep anesthesia Muscle relaxation is moderate Shivering is frequently seen in the recovery period reason unknown calcium imbalance Halothane is significantly metabolized in the liver 20 Some people who have received a halothane anesthetic become sensitized to the metabolites and respond to a second challenge with a severe sometimes fatal hepatitis It must be emphasized that this reaction is very rare and halothane is NOT directly hepatotoxic However for legal reasons in human anesthesia another agent would be chosen for a second anesthetic main danger period is 36 weeks after first exposure It has not been conclusively proven to occur in animals The significance to the veterinarian is that of preventing pollution and exposure of staff to the agent Sensitizes the heart to epinephrine induced arrhythmias This is rarely a practical consideration unless the animal is very frightened at induction or in some animals where stress induced epinephrine release is expected such as with conditions of GDV and severe trauma 0 MAC in animals for halothane is about 09 this gives for stable anesthesia in practical situations an endtidal value of about 1 12 when oxygen alone is used as the carrier gas and about 09 1 when nitrous oxide oxygen mixtures are used This will be further reduced by premedicants and induction agents 0 NB Remember the normal distribution curve and the fact that the MAC value only relates to 50 of the subjects Thus after acepromazine premedication and induction of anesthesia using 715mgkg thiopental sodium an end tidal concentration as delivered via a nonrebreathing circuit of about 06 10 12 at vaporizer halothane should be adequate for most surgical procedures Methoxy urane Penthrane Metofan o Soluble agent with a very low saturated vapor pressure so that induction and recovery are very slow Analgesia is excellent and some post operative analgesia remains Nephrotoxic if used for very prolonged periods and for this reason now no longer used in man This meant the ow of the product into the veterinary market also was in uenced and this product is no longer available in the US 0 Good for use in small animals but its low saturated vapor pressure prevents its use in horses 0 Very obvious smell gives some people very severe headaches so good scavenging essential 0 High liver metabolism 50 so best to avoid in liver diseased patient Pharmacology Inhalant Anesthetics 10 of 13 Veterinam Sumem l VMED 7412 En urane This is much less soluble than halothane so that induction and recovery are faster MAC is about 2 and clinically following intravenous induction endtidal concentrations of about 23 in oxygen appear to produce satisfactory anesthesia However in dogs the anesthetic dose is close to the convulsant dose making it very difficult to obtain smooth anesthesia Signs of anesthesia differ from usual the dog s eye becoming central at much lighter levels of anesthesia than for other agents In the horse the rapid recovery is associated with emergence excitement In man it is primarily used with neuromuscular blocking agents Liver metabolism is low but does occur Cyclopropane A very good agent in that it is very insoluble so that induction and recovery are rapid but explosive So now rarely used if still available Chloroform The earliest agent used dating back mid l9Lh century Fairly fast and quiet induction and recovery Good analgesia and fair muscle relaxation Dose related respiratory and cardiovascular depression However sensitizes heart to epinephrineinduced arrhythmias far worse than halothane in this respect Liver toxic Not used now except in the horse usually by those who feel comfortable with this agent through their previous exposure and experiences where Standing chloroform can still be useful Premed with acepromazine to reduce the chance of epinephrine induced fibrillation Trichlorethylene Ether Pharmacology Inhalant Anesthetics Excellent analgesic but causes rapid jerking respiration which makes stabilization of anesthesia difficult unless neuromuscular blocking agents are employed Must not be used with soda lime as breaks down to toxic compounds if still available but was still used in human cardiac work in the 1980 s This is still a useful agent It gives excellent analgesia and muscle relaxation has a direct effect on the neuromuscular junction It is respiratory depressant but cardiovascular function is well maintained at clinically suitable levels of anesthesia Induction and recovery are fairly slow Ether has a pungent smell and is irritant to mucous membranes It causes copious salivation and bronchial secretion so that atropine or similar drugs must be used for premedication The use of ether is mainly limited by the fact that it is in ammable It is inexpensive so still used in developing countries 11 of 13 Veterinam Sumem l VMED 7412 New agent limited veterinary experience to date Des urane Suprane o This is one of the latest of the volatile agents It is very insoluble and has a very rapid inductionrecovery note also fat insoluble Its boiling point is close to room temperature so a complicated heated vaporizer is required Experience in animals has shown it to be an excellent anesthetic agent with most cardiovascular effects similar to those of iso urane However in contrast to most other halogenated volatile anesthetics des urane causes a tachycardia In the horse the rapid changes in depth which can be achieved result in particularly controlable anesthesia and recovery is exceptionally rapid and if coupled with some postoperative sedation is of outstanding quality It would seem that des urane might be particularly useful in this species Currently its high cost limits its widespread use in veterinary practice Xenon Since 1994 research on the evaluation of the possible use of xenon as an anesthetic in inhalation narcosis has been conducted throughout the world Xenon Xe an inert gas is present in the atmosphere in extremely low concentration Xenon produces inhalation anesthesia similar to Nitrous Oxide however there is not enough xenon in the earth atmosphere to be used for this purpose Xenon has minimal hemodynamic effects and the lowest blood gas partition coefficient among the known anesthetic agents With a minimum alveolar concentration of 71 it is more potent than nitrous oxide 105 MAC It is an inhaled agent with analgesic and anesthetic effects Not metabolized in the body and is eliminated via the lungs Low ow and closed breathing systems are recommended Experiments conducted over the last several years in Germany Russia Sweden and Japan have proven that Xenon which possesses more pronounced narcotic properties than nitrous oxide due to its higher inertness is in fact safer for patients Due to its lower solubility in blood xenon is quickly eliminated from the body upon stopping the gas supply patient recovery is immediate So far no hazardous side effects from the use of xenon have been reported However despite all of the advantages of Xenon over the anesthetics in current use the use of this inert gas is limited by its high price the average retail price in Europe is about U 10 per liter and the small volume of world production By the beginning of the 1990s several medical centers both in Russia and abroad almost simultaneously approached a solution of this problem by developing equipment in which Xenon is trapped during and after surgery and recycled for repeated use Such a method would decrease the price of Xenon anesthesia to a level equal to or only marginally higher than that of the currently available anesthetics Many advantages such as analgesia hemodynamic stability quick induction and recovery make this gas a very promising inhaled anesthetic agent in the future However its routine use both in human and veterinary anesthesia is unlikely for considerable time to come Pharmacology Inhalant Anesthetics 12 of 13 Veterinam Sumem l VMED 7412 Clinical Notes 0 The use of inhalational anesthetics better enables prolonged surgery and diagnostic procedures in a safe and efficient manner 0 Inhalational anesthetics are used in combination with injectable premedicants induction agents and ancillary analgesics best know as balanced anesthetic technique 0 Cardiopulmonary depression is invariably anticipated with modern potent inhalant anesthetics o The degree of cardiopulmonary depression depends on the dose and patient health status and careful administration will ensure safe anesthesia Pharmacology Inhalant Anesthetics 13 of 13 Veterinam Sumem l VMED 7412 Hm EQVETERINARY HEALTH SCIENCES ANESTHETIC EQUIPMENT Breathing Circuits amp Scavenging System Lyon Lee DVM PhD DACVA lntmdurtion Anesthetic Equipm ent Terminology concerning anesthetic breathing circuits is very varied with no universal agreement multiple and inconsistent de nitions in North American and British literatures Traditionally the systems of terminology consist of closed and open themes with variations using Rebreathing as distinguishing factor but are of little value now 0 It is very dif cult to exactly determine the degree of Rebreathing by the use of such terms as semiclosed semiclosed with absorption open etc Most workers agree that semiclosed refers to partial rebreathing techniques It will give the same label regardless of the degree of rebreathing39 a system might have nearly complete rebreathing of the expired gases while another may have almost no rebreathing Clearly this nomenclature or system may allow erroneous interpretation regarding actual inspired concentration or tension of any inhalational esthetic o 0 Currently the terminology describes the breathing systems as Nonrebreathing or Rebreathing see Figures 1amp 2 For majority of procedures regardless of variations of teaching practice and geographical location provision of two simple pieces of information can be quite adequate for description of a breathing circuit First the actual equipment used needs to be described Bain etc 0 Second the fresh gas ow should be stated 0 Under certain and special circumstances more detailed information may be provided such as apparatus mechanical dead space volume type of valves type and location of the vaporizer in or out of the breathing circuit etc Properties of the ideal breathing system 0 Simple and safe to use 0 Delivers the intended inspired gas mixture 0 Permits spontaneous manual and controlled ventilation in all size groups 0 Ef cient requiring low fresh gas ow rate 0 Protects the patient from barotraumas o Sturdy compact and lightweight in design 0 Permits easy removal of waste exhaled gases 0 Easy to maintain with minimal running cost Patient size and anesthetic breathing circuits 0 Two factors must be considered in proportion to the animal s size v Apparatus mechanical dead space v Apparatus mechanical resistance 0 Resistance is always high with turbulent ow so narrow ori ces sharp bends which produce this should be avoided in the apparatus 1 of 11 Veterinam Surgeml VMED 7412 Breathing Circuits amp Scavenging System e For the Laminar ow ofa gas in a tube the Hagen Poiseille law see equation below states that the gas ow is proportional to changes in the tube length diameter to the power of 4 Viscosity resistance and pressure drop thus narrow tubes cause the greatest resistance but resistance is also increased by long tube lengths and increased Viscosity ofthe gas Len 1 1 X Viscoxi Resistance R Modern quot 39 nnteniznd39 39 39 1 L does not come under their acute or chronic in uence The removal L 39 gases L 39 of w upul Jl Occupational Health and Safety Agency OSHA guideline and installation ofa scavenger with use ofvolatile anesthetics ensures compliance Cmem t i is agents which are allowed to be present in the operating rooms gure 1 Functioning Systems of In Anesthetic Machine gure 2 Nonrabru iinI Circuits MEEIESyslem quotJud lrolll he Exhllld all tnvels quotpm In pl esevvlor bin down quotI n lor tubing to quotI z Fr oul the popon Iquot valve to me scavenging systemt connullon urvier Dal m m cums the a mm from the common mm and mm own tn innrtumn to in patient eonmt an Tha39Ayro39s Tpia mg m the Fruit an mm in commi gu aim at In W a nonmne mumquot ontoquot me synm ninem h l0 lhl parlon cannlcxlou oul in popo Exhalnd ns lravcls mam ni pann connection valve to the suvennlnn system down i c Iratan luvquot m m unmet baa Anesthetic Equipment 2 at 11 Veterinaig Surge l VMED 7412 Breathing Circuits amp Scavenging System C unent terminology of the Anesthetic breathing circuits Nonrebreathing circuits With these the patient breathes in from the reservoir and out to atmosphere The gases are not reused However in practical sense this terminology is incorrect because some rebreathing of exhaled gases occurs in most of these systems especially with lower recommended ow rates Satisfactory elimination of C02 is dependent on adequate gas ow and on minimal dead space in the circuit Examples Bain Ayre s T piece Magill Lack Rebreathing circuits Here the same gases are reused and C02 is removed by passage of the gas through soda lime Examples Circle To and Fro UniversalF A circle rebreathing circuit is composed of 0 Carbon dioxide absorbing canister o Ypiece o Inhalation and exhalation breathing tubes 0 Inhalation and exhalation unidirectional one way valves 0 Fresh gas inlet 0 Pressure manometer o popoff valve 0 A reservoir bag 01d Terminology of the Anesthetic breathing circuits There are exceptions but basically all systems using non compressed gases ie room air classify as open and all those using an oxygen cylinder classify as closed Table 1 Traditional terminolo o the anesthetic breathin circuits Reservoir Types 1 Open Draw over systems with non No No Bag and bottle rebreathing valves Partial Bag and bottle with 239 seml39Open39 NO COZ build up occlusive packing Bains modified Jackon Yes No Rees Ayre s T piece Lack Magill 3 Semiclosed without absorption 3 Semiclosed with Carbon diox1de absorbers Yes Partial with leak circle and to absorption and fro 4 Closed Yes Complete carbon diox1de absorbers With no leaks Anesthetic Equipment 3 of 11 Veterina Surge I VMED 7412 Breathing Circuits amp Scavenging System Opensemiopen circuits eg Chloroform Mask Advantages 0 Useful where complicated apparatus is not available The patient airway remains open to room air and no tubing valves or reservoir bag are used Disadvantages o Difficult to obtain stable anesthesia due to unknown dilution by air 0 Dilution depends on tidal volume and the resultant ow of gas through mask and around the sides of the mask 0 No means of IPPV 0 Room air may not supply adequate 02 if respiratory depression exists Agents vented to air therefore fire and toxicity risks Semiclosed circuits best considered as nonrebreathing or rebreathing Nonrebreathing semiclosed without absorption 0 Bain Ayre s Tpiece Jackson Rees for animals below 6 kg 0 Mapleson classifred these into A B C D E and F but currently only A D E and F are commonly used during anesthesia see table 2 and figure 3 Rebreathing semiclosed with absorption 0 Circle for animals weighing above 6 kg Mapleson nonrebreathing circuit systems Advantages 0 Simple inexpensive and rugged 0 Less resistance to breathing 0 Easy to disassemble and can be disinfected or sterilized in a variety of ways 0 Light weight and not bulky Less likely to cause excessive drag on the mask or tracheal tube facial distortion or accidental extubation Reduces the time for inhalant induction for face mask induction than using a rebreathing circuit Once induced and intubated patients larger than 6 kg can be converted to a rebreathing circuit for anesthesia maintenance 0 Anesthesia particularly easy to keep stable as animal breathes exactly what is delivered from the machine The fresh gas inlet in a nonrebreathing circuit is adjacent to the endotracheal tube connection and therefore changes in vaporizer setting affect the inspired gas concentration immediately Disadvantages o More cold dry gases are delivered resulting in less well preserved moisture and heat within the breathing circuits therefore danger of hypothermia o More vapor use due to higher fresh gas ow requirement so more expensive 0 More pollution to atmosphere Flow rates required 0 Magill and Lack l 15X minute volume 150 mlkgmin o Bain and T piece 7 25 X minute volume 250 mlkgmin 0 With the Magill circuit IPPV leads to C02 retention 0 IPPV can be carried out with the other circuits 0 Ideally a capnography is used so ow rates are adjusted as just enough to prevent the C02 rebreathing avoiding waste and expense 0 o Anesthetic Equipment 4 of 11 Veterina Surge I VMED 7412 Breathing Circuits amp Scavenging System Near the bag Present Present Magill Lack Near to the patient Present Present Obsolete anesthesia Near to the patient Near the Present absent sun used for patient erner ency resuscitation Near to the patient from the Present Present Bain Ayre s Near to the patient from the absent Present Tpiece Figure 3 Schematic diagram oflhe Mapleson classi cation of circuits rvvvva 39 m A P Anesthetic Equipment 5 of 11 Veterinam Surgem x VMED 7412 Breathing Circuits amp Scavenging System Rebreathing circuits These circuits will all contain sodalime for C02 absorption If run on minimal gas ows with no leaks they are closed circuits If run with an over ow leak of gas they are semi closed with absorption Advantages o Economical o Gases warm and wet through rebreathing disadvantage in hot weather Disadvantages 0 May have high resistance due to soda lime inhalation exhalation valves and popoff valve 0 Difficult to predict inspired anesthetic concentration because of volume buffer by breathing tubes reservoir bag and varying degree of rebreathing 0 Less control of anesthetic depth than nonrebreathing circuit Flow rates required 0 Low flow 10 20 mlkgmin of oxygen ow rate 0 Medium ow 20 40 mlkgmin of oxygen flow rate suitable for most clinical circumstances 0 High ow greater than 40 mlkgmin of oxygen ow rate 0 Using high ow will compensate the gas leakage better with better control of anesthetic depth than low ow but will be less economical with more loss of heat and moisture of the patient as well as polluting the environment more Rebreathing circuits may either have the vaporizer on the anesthetic machine therefore out of the circuit VOC or have the vaporizer within the rebreathing circuit VIC Rebreathing circuits and the vaporizer location Vaporizer out of Circuit VOC rebreathing circuits There are two methods of running VOC rebreathing circuits 0 Continuous and intermittent ow Continuous ow Low ow anesthesia o This is ideal if one can replace exactly the mount of oxygen and anesthetic which is used by the animal 0 This is quite practical in the horse where one will need a ow of about 23 litersminute o In small animals the flow required is so low that the vaporizers become inefficient and it is often difficult to keep the animal asleep 0 A compromise is to work with a ow of about 1 literminute and to leave the expiratory valves open to allow overspill This converts the circuit to semiclosed with absorption Intermittent ow 0 Here an intermittent high ow rate is used to fill the bag with oxygen and anesthetic mixture and left off until either the bag is empty or the depth of anesthesia requires changing o This system is very economical on gas but has the disadvantage that the level of anesthetic administered is constantly being changed as dilution in the reservoir bag occurs 0 The level of anesthesia thus oscillates and at times may be unnecessarily deep There are two systems of soda lime absorption that are used for types of VOC rebreathing circuits 0 the circle system and the to and fro system in which the canister may be horizontal or vertical Anesthetic Equipment 6 of 11 Veterina Surge I VMED 7412 Breathing Circuits amp Scavenging System To and fro 0 Advantages Comparatively cheap mobile 0 Disadvantages Absorption less efficient than in circle Increasing deadspace as soda lime used up Layering particularly when horizontal Hot dusty soda lime close to patient Weight of canister may kink endotracheal tube Circle 0 Advantages More efficient removal of carbon dioxide and use of soda lime o Disadvantages Expensive to buy now some cheap disposable circles available Can have high resistance due to length of tubing but OK if tubes are wide Valves must be efficient or rebreathing occurs Circle rebreathing systems with Vaporizer in Circuit VIC Anesthetic Equipment Ohio No 8 machine Goldman machine Stevens machine and Kommesaroff machine VIC systems are always circle In these vaporization of the volatile agent depends on the flow through the vaporizer which is pushed through by the animal s own respiration Thus with every breath more agent is vaporized Advantages 0 Very economical Oz requirement is that used by animal 510 mlskgmin Minimal volatile agent wasted and minimal pollution 0 With all rebreathing circuits retains heat and water Disadvantages 0 Cannot use N20 in this circuit 0 If oxygen ow is too high difficult to get adequate concentrations of some volatile agents 0 Cannot use safely for IPPV unless removing vaporizer 0 Low efficiency and nonprecision These circuits are claimed by the manufacturers to be very safe because if animal becomes more deeply anesthetized respiration is less so uptake of volatile is reduced However this only happens at a depth of anesthesia at which with iso urane and sevo urane there is severe hypotension and as with any anesthetic system it is perfectly possible to accidentally kill patients when using these machines The machines were originally designed to be used with ether or methoxy urane both of which are very much safer in this system As these machines work on minimal intake of 02 into the circuit nitrogen will accumulate reducing the concentration of 02 Thus before use the machine it is primed with 100 02 after about 5 minutes of anesthesia the bag is emptied and reprimed with 100 02 Nitrous oxide must not be used If these machines are used properly with good monitoring and if the animal is too deep the vaporizer setting is reduced they can be excellent and economical for small animal anesthesia However if used as sometimes advertised ie giving maximal anesthetic agent and counting on respiratory depression to limit uptake then with iso urane and sevo urane anesthesia such depths of anesthesia will be excessive and accompanied by severe hypotension with subsequent morbidity 7 of 11 Veterina Surge I VMED 7412 Breathing Circuits amp Scavenging System Components of the circle system CO2 absorbent Sodalime baralyme These are used to absorb the C02 in rebreathing circuits Sodalime consists of 90 calcium hydroxide 5 sodium hydroxide plus 5 silicate and water to prevent powdering Indicators are added to show when it is fully used but do not trust them they can get leached out by water vapor and most change color a little too late and also reverts to its original color when not in use The absorption of CO2 by these is exothermic ie the soda lime gets hot you can use this action to test your soda lime if in doubt blow on some and see if its gets hot The reaction of C02 with sodalime 002 H20 gt H2CO3 HzCO3 2 Na2C03 H2CO3 gt K2003 Na2003 or K2C03 CaOH2 gt 2NaOH or 2KOH Ca003 Soft and crushable granules are converted to hard and noncrushable granules calcium hydroxide changes to calcium carbonate limestone which indicates exhausted sodalime Increased inspired fractional concentrations of C02 detected by the capnography indicates exhausted sodalime Y pi ece Constructed of plastic and unites the endotracheal tube connector and the inspiratory and expiratory breathing tubes Contribute to the mechanical dead space but not signi cantly greater than that in a non rebreathing system 15 mm ID female port to accpept the ET tube connector Breathing tubes Large bore nonrigid breathing and usually corrugated conducting tubes typically made of rubber or plastic Corrugations increase exibility and resistance to kinking Clear plastic tubes are more lightweight absorb less halogenated agents have a lower compliance than rubber tubes and allow visualization of the interior of the tube Act as a reservoir in certain systems Provide a exible low resistance lightweight connection from one part of the system to another Have some distensibility but not enough to prevent excessive pressures from developing One way unidirectional valves They direct gas ow away from the patient on expiration and toward the patient on inspiration Prevents the rebreathing of exhaled gases before they pass through the absorbent canister Gases enter a unidirectional valve from below raise the disc and pass under the dome to the reservoir bag the absorbent canister or the inspiratory breathing tube Valve incompetence contributes to accumulation of C02 within the breathing circuits Anesthetic Equipment 8 of 11 Veterina Surge I VMED 7412 Breathing Circuits amp Scavenging System Fresh gas inlet The location at which gases from the common gas outlet of the anesthesia machine or from the outlet of the vaporizer enter the circle system Placed on the absorbent canister near the inspiratory one way valve or on the inspiratory one way valve Entry of fresh gases on the inspiratory side of the circle minimizes dilution of the gases with exhaled gases with a VOC prevents absorbent dust inhalation reduces loss of fresh gases through the popoff valve 0 o 0 Adjustable pressure limiting valve Pop o quot valve A valve which allows exhaled waste gases and fresh gas ows to leave the breathing system when the pressure within the breathing system exceeds the valve s opening pressure Also called as Popoff valve Exhaust valve Scavenger valve Relief valve Expiratory valve Overspill valve etc It is a one way adjustable springloaded valve The spring adjusts the pressure required to open the valve The patient may be exposed to excessive positive pressure if the valve is closed for prolonged period always pay great attention to the valve closure Some designs have a safety mechanism allowing the relief valve open when a pressure within the breathing circuit reaches about 60 cmHZO Pressure manometer A pressure gauge that is attached within the breathing circuits It is calibrated in cmHZO but may have a scale of mmHg or KPa l KPa 75 mmHg Typically a pressure buildup over 20 cmHzO for small animals and 30 for large animals is considered unsafe Rebreathing Reservoir bag Anesthetic Equipment The rebreathing reservoir bag is an important component of most breathing systems It is made of antistatic rubber or plastic Volumes of 05 l 2 3 and 5 liters are commonly used for small animals and 15 20 and 30 liters are used for large animals The typical size for a 20 kg dog is 2 liter bag 500 ml for small dogs and cats and 30 L for adult horses and cattle It accommodates fresh gas ow during expiration acting as a reservoir available for use of the following inspiration It acts as a monitor of the patient s ventilatory pattern but is inaccurate for assessing the tidal volume It can be used to assist or control the ventilation Because of its compliance the rebreathing bag can accommodate rises in pressure in the breathing system better than other parts When grossly overin ated the reservoir bag can limit the pressure in the breathing system to about 40 cmHzO This is due to the law of Laplace dictating that the pressure P will fall as the bag s radius r increases tension Pressure radius A small bag may not be large enough to provide a sufficient reservoir for a large tidal volume Too large a bag makes it difficult to act as a respiratory monitor 9 of 11 Veterina Surge I VMED 7412 Breathing Circuits amp Scavenging System ScaW ging For halogenated hydrocarbon anesthetic agents iso urane halothane sevofurane and des urane 2 ppm is the allowed concentration and 25 ppm for nitrous oxide When the halogenated hydrocarbon anesthetic agent is used with nitrous oxide the maximum permissible concentration is reduced to 05 ppm All anesthetic facilities including recovery rooms must be tested for levels of escaped gases Testing is done on an occasional basis at the moment most vets intend to test once a year the anesthetist wearing a badge for a prescribed time which is then sent away for analysis There are many scavenging devices suitable for veterinary purposes but care must be taken to ensure that their use does not have an adverse effect on the patient The following reference 0 v a J quot on control of waste quot quot gases in the workplace JAVMA 2091 pp7577 describes more in detail the precautions and measures necessary to minimize the waste gas exposure Methods of scavenging Passive scavenging Tubes from the expiratory valve of the patient circuit lead to outside Advantage 0 Cheap to install Disadvantage o Ineffective 0 High expiratory resistance 0 Can obstruct expiration 0 Not acceptable now Passive with adsorption The tube from the expiratory valve now goes to a canister of activated carbon Advantage 0 Effective adsorption of halogenated hydrocarbon anesthetics 0 Simple and portable fills the gap when moving the machine with the patient connected Disadvantage 0 High resistance 0 Needs frequent changing weighed to detect when capacity full 0 Will not adsorb N20 Active scavenging A central vacuum draws the gases away from the expiratory valve The suction ow must be at least 30 Lmin Advantages 0 Very effective now really the only acceptable method 0 Minimal resistance to breathing although need a method of also drawing in room air or else it would suck so hard that animal could not breath in Disadvantage o Fairly expensive to install a reasonable system is around 3000 Active scavenging with adsorption The best method of all Anesthetic Equipment 10 of 11 Veterina Surge I VMED 7412 Breathing Circuits amp Scavenging System Further References Veterinary Anesthesia Hall Clarke and Trim WB Saunders 2001 Veterinary Anesthesia Thurmon Tranquilli and Benson Williams amp Wilkins 1996 Handbook of Veterinary Anesthesia Muir Skarda Hubbel Mosby 2000 Understanding Anesthesia Equipments Dorsch and Dorsch Williams amp Wilkins 1999 HeP Nf Anesthetic Equipment 11 of 11 Veterina Surge I VMED 7412 Breathing Circuits amp Scavenging System PERIOPERATIVE PAIN MANAGEMENT Lyon Lee DVM PhD DACVA Introduction Pen39operative Pain management Pain is de ned as an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage Merskey 1979 Pain is a universal experience for every living being and we all know what it feels like but it is still very difficult to clearly de ne or quantify Pain management in animals at perioperative and postoperative periods presents a significant challenge not only to veterinarians but also to the owners of the animals Humans can express and describe the sensations they experience and these descriptors are well accepted However in animals the difficulty of obtaining feedback through communication complicates the management of pain as one can only attempt to help alleviate the pain when it is expressed It is therefore very important that one is familiar with the signs associated with pain and interpretation of these behavioral changes for better clinical pain management in animals The ability to feel pain has clear survival advantages 0 limits movement of the damaged part so that it promotes healing o the individuals learn to avert similar noxious stimuli in the future However ongoing pain has no bene t and offers disadvantages 0 Animals feeling pain postoperatively get distressed and the stress response interferes with normal resumption of activity 0 Delay in wound healing has been demonstrated in animals in more painful condition Decreased food intake impairs nutrition and speed of recovery from the operation 0 Impaired respiration especially in animals undergoing thoracic surgery contributes to development of hypoxia pulmonary atelectasis pneumothorax and retention of mucus and sputum etc 0 May selfmutilate 0 Causes central sensitization which is more resistant to conventional analgesic therapy 0 The reasons for relative reluctance in treating pain have been 0 The clinician is not absolutely sure whether the animals are in pain 0 Belief of overrated concerns regarding the pain and subsequent overprescription of analgesics is dangerous due to its adverse side effects 0 Many effective analgesics are controlled substances and require a tight inventory log 0 Belief of analgesics should be withheld so that protective function of pain is maintained 0 Insuf cient knowledge about the analgesics especially regarding opioids Sound knowledge in physiology of pain and pharmacology of pain control substances analgesics will maximize the therapeutic outcome in animals A variety of drugs are available to treat pain in animals It is not uncommon to combine a specific analgesic drug with an anxiolytic This combination works synergistic in controlling pain maximizing the analgesic effect of a drug A clinician may experience signs of pain clearly dissipate with administration of tranquillizer such as l 39 alone at l 39 period quot pain is a complex phenomenon which can be controlled by even through behavior modification 1 A good pain management in suffering animals is not only obligation but professional privilege 1 of 13 Veterinary Surgery I VMED 7412 Physiology of pain The receptors that mediate the noxious signal which ultimately converts to the pain sensation are called nociceptors The nervous system plays a key role in transducing transmitting modulating and projecting a signal that is generated when the nociceptors are sensitized Noxious stimuli activate nociceptors that are at the termination of free nerve endings of Adelta and C afferent bers The pain sensory fibers nociceptor have cell bodies in dorsal root ganglia and project to the dorsal horn of the spinal cord where they synapse in Rexed laminae I II V VII and X Following synapses at this level the information is transmitted further via several spinal tracts most notably spinothalamic tract to the supraspinal level The synapses at the level of the thalamus then transmit the information diffusely to the cortex thalamocortical projection Pain perception takes place when the cortex processes the information as noxious Different neurotransmitters are involved in these pathways including GABA glutamate endogenous opioids substance P norepinephrine and many other neuropeptides and the interaction can be modulated at each step in the pathway roviding basis for the multimodal or balanced analgesic approach Pain can be typed into acute or chronic based on the duration or somatic visceral or neuropathic based on the origininvolvement of the damaged tissues For full details of the mechanisms and de nitions review physiologypharmacology texts Assessing pain Pen39operative Pain management The obvious lack of communication with animals dictates the therapists to depend on animal s behavior to assess the degree of pain e g does the cat walk around is it puriing or is it hunched up in the back of its cage is the dog reluctant to move etc The ability to relate such signs with pain management will ultimately affects the success of pain control The experienced veterinary technician is probably better able to judge levels of postoperative pain than is the veterinarian Animals are generally less expressive of the pain than humans as it offers survival advantages to potential predators Altered behavior to human contact may indicate as first line signal that the animal is in discomfort Acute postoperative pain accompanies signs of guarding and aversion of the site particularly when it is manipulated Although vocalization may indicate discomfort lack of vocalization should not be interpreted as lack of pain Moderate dis comfort often leads to attempts to bite or scratch the site Animals in visceral pain tend to exhibit restless and agitation while those that are in somatic pain would be reluctant to move Cats and dogs may exert common signs when in pain but they also differ markedly in their behavioral response to sufferings Cats in general tend to demonstrate less signs of pain compared to dogs to a same degree of surgical procedures that may in ict similar tissue damages e g ovariohysterectomy However this does not necessarily indicate the cat feels less pain than the dog but rather indicates a fundamental difference in how each species responds to the pain which presents a need to study animal behavior in a species specific way The following table 1 describes some examples of differences in behavior in dogs cats and horses in response to pain Assessment of pain and of the efficacy of analgesia is difficult Studies of analgesics are based on experimental analgesiometry The results of which bear little relation to the apparent efficacy in the practice situation 2 of 13 Veterinary Surgery I VMED 7412 0 Pain assessment methods used by veterinary researchers include the followings o Verbal rating scales VRS 0 0 0 0 Simple descriptive scales SDS Numeric rating scales NRS Visual analogue scales VAS VRS and SDS comrare pain as none mild moderate or severe and although simple to use it lacks sensitivity 0 Using two or more of different pain assessment scales may provide better sensitivity of the pain measurement although it may require more time and effort Table 1 The behavioral characteristics in dogs cats anal horses which are related to pain response Modified from Carrol GL Small animal pain management Lakewood American animal hosital association ress 1998 Behavioral response Horses Vocalization Groan whine whimper growl scream cry Groan growl hiss scream cry Grunt moan quiet Facial expression Fixed stare glazed appearance ears pulled back Furrowed brow squinted eyes ears pulled back Dull eyes teeth grinding ears pulled back Body posture Hunched laterally recumbent prayer position Generally sternal laterally recumbent Standing with head down standing on 3 legs recumbent Self awareness guarding and self mutilating Protective of wounds licking chewing and rubbing wounds and surgical sites limping loss of weight bearing Protective of wounds licking chewing and rubbing wounds and surgical sites limping loss of weight bearing Protective of wounds licking chewing and rubbing wounds and surgical sites limping loss of weight bearing Activity Restless or restricted movement trembling shivering Restricted movement stereotypes meaningless encircling movement Restless or restricted movement trembling shivering Attitude Socialization and comfortattention seeking Increased aggressiveness or timidity fearfulness Comfortseeking or hiding aggressiveness Uncooperative aggressive hiding Appetite Decreased Decreased Decreased Urinary and bowel habits Increased urination failure of house training urinary retention Failure to use litter box Increased urination urinary retention fecal retention Grooming Loss of sheen in hair coat particularly in chronically painful dogs Failure to groom particularly in chronically painful cats Disinterested Response to palpation Protecting biting vocalizing withdrawing orienting escape Perioperative Pain management 3of13 Protecting biting scratching vocalizing withdrawing orienting escape Aggressive kicking biting fighting escape Veterinary Surgery I VMED 7412 Pain relief within the periopaative period eoyered o Preoperauyely 1f tn pam or preemptryely o operauye for surgery o Postoperatryely short and1ong term Inman 5 stress a more raprd reeoyery and a shorter penod of me tn hosprta1 anaestheuzed m r F r andrepeat doses be gryen at adequate rnterya1s d ntt n r V p 4 NB Tth eomes from quesuonable I as H r n w of a1esser p rst for neg1 Dtseomfort may a1so eause ddstxess In the postroperauve perrod TLC TenderLovmg Care Wth a quret warm eomfortable enytronme an redu gesr be a source of senous ddscomfort METHODS OF PmVIDING ANALGESIA thme 1 Stte afmtaufat muons mtgeetce m a dag Opioids alphaZ a artists muuutate nuctceptwe m urmattun Prunessmg m he spmat card A pram ocal A nesthetics stun transmtsstun ut mturmattun NSAID39S prevent pruductmn m memamrs ut mtlammattun ee ana1 e requtrements eonsrderably andor Loea1 analgesres Genera1 anestheue N39B uneonserousness does notmean that an responseto parnrsremoye some g nera1 anesthettes are poor ana1gesres Alpha 2 adrenoeeptor agonrsts A antagonrsts Adjuncnve methods uneonyenuona1 ana1gesres Choree of surtable one depends on type of pan ease of use how gryen speed of effect duratron of effect andthe srde effects aeeeptabl For example anestheue agents are m proyrde useful postrop analgesra e oxygen andIPPV butnotpostroperauvely Farragutmy pun quottangentrt 4 or u Veurlniry Surgery I men 7412 Local analgesic techniques These can provide useful even complete analgesia at all three stages of the perioperative period Where practicable local nerve blocks provide superb analgesia The longacting local anesthetic bupivicaine gives 68 hours analgesia Intercostal nerve blocks are particularly useful after thoracotomy Epidural analgesia may be used after abdominal pelvic or pelvic limb surgery although this will also cause muscle paralysis of the hind region The only contraindication to the use of local analgesia postoperatively is where it is important that the operated site must not be used e g a fracture Local analgesia is the only form of analgesia which is so complete as to allow this to happen NB Epidural analgesia may also be achieved with opioids andor alpha 2 agonists The advantage of this route of administration is that analgesia can be achieved with limited respiratory depression or sedation Another advantage is that loss of motor function is much less seen with these drugs However the drugs are slowly absorbed from the epidural site so delayed respiratory depression can occur and the animal should be kept under observation General anesthesia Main use is intraoperatively Not all general anesthetics are good analgesics with some e g iso urane very deep levels are needed for adequate analgesia In these cases other analgesics eg opioids alpha 2 agonists are useful intraoperatively as well as providing good post operative analgesia as long as antagonists have not been used NB The use of opioid antagonists to reverse opioids used intraoperatively will prevent often the use of further opioids for postoperative analgesia Opioids Perioperative Pain management Opium the plant has been used for its analgesic and euphoric effects over thousands of years for men All opioids referring all endogenous and exogenous substances that possess morphinelike properties are chemically related and subject to regulatory controls governed by the Controlled Substance Act 1970 In man and dog these are the mainstay of perioperative analgesia and can be used in all three phases preintrapostoperative periods The potential excitement in cat and horse limits the use of the very potent drugs The choice of opioid suitable for each phase is based on the following points 0 Premedication O This needs a long acting opioid to have effect before during and after surgery O Hypnosis and sedation is an acceptable side effect O Respiratory depression must be minimal O Ideally there would be no vomiting 0 Intraoperative O This requires a very short acting and very potent drug O Onset of action must be rapid O Respiratory depression is acceptable as long as oxygen and the ability to give arti cial ventilation are available for use if necessary O In the dog the use of the fentanyl group of compounds is suitable but oxygen and IPPV is essential O In the horse all studies have shown that intraoperative opioids do not decrease the dose of general anesthetic and in some instances may increase it presumably through an excitement effect 5 of 13 Veterinary Surgery I VMED 7412 o Postoperative Drugs used must cause minimal respiratory depression Although in general a long action is required e g buprenorphine care must be taken not to overdose whilst any anesthetic effects which may be synergistic remain For this reason a short acting analgesic such as fentanyl may be used initially followed later by a longer acting drug Also note that when sedatives and opioids are combined the effects are synergistic so postoperative sedation may be profound 99 O O Pharmacology of opioid analgesics Perioperative Pain management See pharmacology texts for details of modes of action receptor theories and of main properties Brie y the properties of the opioids of clinical importance are a balance between depression and stimulation The result of the balance depends on dose receptor type and species Depression gives analgesia wanted hypnosis possibly useful respiratory depression and bradycardia unwanted and depression of the cough re ex care needed during recovery Stimulation causes the generalized excitement seen in horses and cats often together with tachycardia euphoria tends to abuse dysphoria unwanted and vomiting and defecation unwanted All opioids exert similar mode of action but their activity varies depending on degree of affinity of the specific subtypes Mechanism of action 0 Mainly through its agonist activity at opioid receptors that results in modulation of inbibitoryexcitatory effects in the CNS 0 This causes analgesia and sedation to a lesser extent Clinically important subtypes of the opioid receptors are mu1 mu2 kappa delta subtypes 0 ul supraspinal analgesia euphoria sedation o uZ respiratory depression bradycardia increased gastroinstestinal transmit time hypothermia physical dependence 0 K spinal analgesia sedation meiosis o 5 spinal analgesia respiratory depression Ideal opioid o The purpose of looking for a new opioid is O To improve on morphine by reducing respiratory depression so far without success if respiratory depression is limited so is analgesia O To reduce the potential for abuse The partial agonists cause dysphoria therefore are less pleasant so are less likely to abuse O To alter the length of action longer or shorter andor the speed of onset of action There is now a good choice availa le O To increase potency The more specific a drug the less the side effects other than those related to stimulation of that receptor O To reduce other side effects vomiting excitement euphoria dysphoria vagal stimulation Cardiovascular effects of opioids o In general they do not cause myocardial depression and for this reason the potent agonists are routinely used as part of the induction technique for anesthesia of people with heart disease 0 However they can cause very severe 39 39 with anti 39 quot the dog some may cause hypotension and in the horse excitement is accompanied by tachycardia and hypertension I u r i i in 6 of 13 Veterinary Surgery I VMED 7412 Perioperative Pain management Classification of opioids Opioids are generally classified as pure agonists antagonists partial agonists and mixed agonistsantagonists Pure agonists Drugs that produce a maximal biologic response through binding opioid receptors 0 In general the pure mu agonist gives the best analgesia which with the most potent drugs is dose related in effect 0 0 Unfortunately it has been impossible to separate this analgesia from respiratory depression all claims to date have proved false in clinical practice or from the liability for addiction and abuse 0 e g morphine fentanyl meperidine oxymorphone hydromorphone meperidine Antagonists 0 Drugs that competitively reverse antagonize the effects of agonists by preventing their access to a receptor 0 They have low or no intrinsic activity at the receptors themselves 0 e g naloxone nalmefene naltrexone Partial agonists 0 Drugs that produce a submaximal response at a particular receptor type even at high doses 0 The slope of doseresponse curve is less steep that that of pure full agonist o Doseresponse curve also exhibit ceiling effect ie submaximal o Concomitant administration of partial pure agonist can reduce antagonize the pure agonist s clinical effect 0 Partial agonists give much more less analgesia the only really effective one is buprenorphine but also have less respiratory depression and are often less subject to the schedule regulations 0 Partial agonists often have a bell shaped doseresponse curve once maximal analgesia is achieved further doses only reverse the effect Thus if these partial agonists are ineffective do not give a further dose 0 Partial agonists also have a tendency to cause dysphoria with unpleasant hallucinatory effects so less potential for abuse Mixed agonistsantagonists 0 These have divergent activities on different receptors acting simultaneously as an agonist at one receptor e g kappa delta while acting as an antagonist at another eg mu 0 e g butorphanol pentazocine It is important to know into what category an opioid analgesia falls as it in uences the ease of antagonism o For example naloxone will antagonize morphine very well but will be less ef cient in its antagonism of the partial agonist pentazocine o If pentazocine is given to a dog which has had morphine then it partially antagonizes it reducing the total analgesia even though both drugs and their own exert analgesic properties 0 Ideally one should not mix the use of agonist and partial antagonist drugs when trying to obtain analgesia in an animal However if a partial agonist fails to be effective a pure agonist at higher than normal doses may be effective 0 The term sequential analgesia has been used to describe the use of the partial agonists buprenorphine or nalbuphine to reverse a pure agonist whilst allowing some analgesia to remain 7 of 13 Veterinary Surgery I VMED 7412 Opioid agonists Morphine This is the gold standard against which the others are compared the prototype opioid agonist Primarily mu agonist with less of kappa and delta agonism Provides excellent analgesia with some sedation most species Use with care in the horse maximum 0 lmg kg preferably with sedation Vomiting and defecation is a common side effect It can be used at all perioperative stages Inexpensive Time of onset after IV injection is 10 minutes IIVI takes longer Rapid IV injection causes more predictable histamine release give it slow in this route Doses of 02505 mgkg provide 4 h ours analgesia in the dog Urinary retention and constipation with prolonged us age Oxymorphone It is p agonist and approximately 10 times more potent than morphine Vomiting is rare and produces some mild sedation Unlike morphine IV injection is not associated with histamine release Duration of action is similar or marginally shorter than morphine It cause the dog to pant but is of little clinical signi cance More expensive than morphine and current supply is very limited due to contamination of the manufacturing plant Hydromorphone It is p agonist and approximately 57 times more potent than morphine the dose range is 005 7 02 mgkg Vomiting is not as common as morphine but more commonly seen than with oxymorphone It provides slightly better sedation than morphineoxymorphone It cause the dog to pant but is of little clinical signi cance Unlike morphine IV injection is not associated with histamine release Duration of action is slightly shorter than morphine More expensive than morphine but cheaper than oxymorphone so it is being used as a good substitute for oxymorphone Meperidine Pethidine It is p and kappa agonist It is l10Lh as potent as morphine and doses of 15 mgkg are required in the dog It can be used at doses of 13 mgkg in the cat and 1 mgkg in the horse It provides less sedation than morphine IV administration occasionally causes severe anaphylactic reaction It has myocardial depressant effect which is unusual as opioid Molecular structure is similar to atropine and it tends to increase heart rate Analgesia is short 1 N 2 hours Its advantage is that it causes smooth muscle relaxation therefore can be used in spasmodic colic in the horse and in biliary and renal colic in all species and dose not cause vomiting Methadone Pen39operative Pain management This is equipotent to morphine but gives more prolonged analgesia It causes less vomiting than morphine IV administration does not cause histamine release so maybe given this route for faster onset of drug action In horses use at 01 mgkg IM IV can get them excited if not with sedatives dogs and cats up to 05 mgkg but beware of respiratory depression 8 of 13 Veterinary Surgery I VMED 7412 Fen tanyl It is u agonist Very potent short acting quick onset analgesic Popular as intraoperative and postoperative CRI analgesic Transdermal patch offers advantage of prolonged duration of effect The patches come in several doses each recommended accordingly for different patient sizes They need to be applied at least 12 hours before the expected start of the operation to be effective in dogs much faster onset in horses However once the plasma level is established the analgesia lasts for a few days It does not cause histamine release Analgesia is dose related but is accompanied by respiratory depression and at high doses up to 20 mcgkg its use may be only with 02 and IPPV In dogs an infusion could start with a 35 mcgkg as bolus followed by an infusion of 3 to 6 mcgkghr In cats half of these doses can be used Cardiovascular effects are minimal but bradycardia may be seen It tends to cause excitement manifested by walking in the horse Other congeners e g sufentanyl remifentanyl alfentanyl etc are too expensive for current veterinary use Carfentanyl another analogue is very very potent and has been primarily used for darting wild life Premixed formulations typically with butyrophenones e g droperidol or uanisone may be available Opioid antagonists Naloxone Perioperative Pain management It is pure antagonist It attaches to opioid receptors but has no effect The most common use is for reversing opioidinduced sedation respiratory depression excitement It is recommended agent for treating opioid overdose in man Doses are l to 25 mcgk best given titrated to effect Naltrexone and nalmefene are longer acting pure antagonists but have been used less than naloxone NBl The duration of an antagonist may not be the same as that of the agonist so the animal may return to sleep or respiratory depressant Close observation must be continued until the negative agonist effect fully wanes NBZ If an opioid antagonist is used it will reverse analgesia and postoperative analgesia cannot be easily supplied with further opioids The term sequential analgesia refers to the use of a partial agonist such as buprenorphine or nalbuphine to reverse a pure agonist such as morphine or fentanyl the idea then being that some analgesia will remain There are differing views as to whether this works in practice 9 of 13 Veterinary Surgery I VMED 7412 Opioid partial agonists It is Principles of using antagonists are similar to those relating to sedative antagonists important that the length of action of the agonists and antagonists are matched Where opioid antagonists are used analgesia is also reversed Buprenorphine It is u partial agonist Although weak there is K antagonism so it can be also categorized as mixed agonist antagonist It has been widely used in dogs and cats 520 mcgkg It is a potent analgesic with the advantage of a prolonged duration of action 68 hours It has a very slow onset of effect up to 45 minutes for full action even after IV injection It delays recovery from anesthesia Its disadvantages include the prolonged induction time and a bellshaped doseresponse curve in which increasing doses antagonizes the analgesia already present so if following its use analgesia is inadequate further doses must not be given it may be followed by any mu agonist such as morphine or hydromorphone Its high af nity to mu receptos means that the reversal would be hard as it will have very slow dissociation as well Ifnaloxone is ineffective to reverse respiratory depression doxapram can be used as temporary reversal In man when used in ambulatory patients as opposed to postop it causes severe hallucination Class III drug used to be V Mixed agonistsantagonists Buto rphanol K partial agonist u antagonist It was initially used as a cough suppressant in animals It is 5 to 10 times more potent than morphine but analgesic efficacy is much less NB do not confuse potency with efficacy and is not very effective in managing severe pain exception is in bird as this seems provide a good analgesia It provides mild sedation Cardiopulmonary effects are minimal With increasing dose there is a ceiling effect on respiratory depression ie further increase do not increase respiratory depression In dogs 01 to 05 mgkg in cats 0104 mgkg and in horses 002005 mgkg can be given IV IIVI or SQ In the horse 01 mgkg IV causes marked walking behavior In dogs IM is painful so beware of being bitten It has a duration of action lasting approximately 2 hours Pentazocine Nalb K partial agonist u antagonist has some 5 activity at higher doses Frequently causes dysphoria and is now only occasionally used It lasts approximately 1 to 2 hours Class IV drug uphine K partial agonist u antagonist Relatively new and has proved to give effective analgesia and has been particularly widely used in laboratory animals It lasts 12 hours Not classified Diprenorphine Revivon Perioperative Pain management Used to reverse etorphine 10 of 13 Veterinary Surgery I VNED 7412 Use of opioids with sedatives When sedative and opioid drugs are used in combination their effects are synergistic ie the total effects are greater than the sum of the effects of the two drugs used individually Use of such combinations is not new there are veterinary reports as far back as 1932 However the first widespread use involved combining opioids with the neuroleptic agents ie the phenothiazines and the butyrophenones Thus the terms neuroleptanalgesia and neuroleptanesthesia were coined With neuroleptanalgesia the combinations were used to produce sedation with mild analgesia the use of much more potent and higher doses of opioids give increased analgesia so that surgery can be performed Since this time the concept has been extended further and any sedative type drug ie benzodiazepines and alpha 2 adrenoceptor agonists has been used in combination with the opioids thus the term neuroleptanalgesia has been replaced by that of sedativeopioid combinations The initial idea of using the combinations was not only that there would be synergism of sedative effect but that the sedative would counteract the unwanted side effects of the opioids in particular excitement and muscle rigidity but would not increase the opioid induced respiratory depression Unfortunately respiratory depression is often increased and in human anesthesia sedativeopioid combinations are only used together with the administration of oxygen often with IPPV intermittent positive pressure ventilation Unfortunately this simple safety precaution is not always carried out by vets Caution regarding the use of potent opioids The potent opioids are absorbed across mucous membrane so great care must be taken to avoid dangerous spillages Carfentanyl and Etorphine are the most dangerous but buprenorphine and fentanyl and its analogues are also effective by this route Always must be handled by the veterinary professionals and have emergency CPR kits and personnel who can perform the CPR available when handling Non steroidal antiin ammatory drugs NSAIDs Perioperative Pain management Until relatively recently it has been stated that NSAIDs were not effective in acute pain and therefore although useful later were inadequate for immediate postoperative analgesia However the modem potent NSAIDs such as carprofen and unixine give useful postoperative analgesia It is probable that they do not provide any surgical analgesia carprofen does not reduce the MAC of halothane in dogs by an appreciable amount and they do not reduce the sensitivity to acute pain in analgesiometric tests in sheep When compared with opioids for postoperative analgesia the NSAle have the advantage of not decreasing consciousness or causing respiratory depression Their antiin ammatory action speeds healing and decreases pain They are toxic and tend to be cumulative There are major species differences in elimination times and toxicity and data and doses cannot be transposed between species In general it is not advisable to administer these drugs intravenously during anesthesia as the delay in elimination through anesthetic induced cardiovascular depression again increases toxicity For all NSAIDS never exceed the data sheet dose in quantity or in frequency of administration 11 of 13 Veterinary Surgery I VNED 7412 Flunixine meglumate Banamin o It has been used in the horse for some years 0 There appear to be no problems with single dose IV use during anesthesia 0 However cumulation through multiple dosing can cause colitis particularly in sick horses 0 Overdose causes death It has been shown to produce worth while analgesia during and after surgery when given as premedication 0 Possible renal toxicity has been reported when it has been given to halothane anaesthetised dogs Carprofen Rimadyl o It is very widely used analgesic in the dogs Appears very effective at doses of 4mgkg IV or SQ given at premedication or induction It is very long acting and has a much greater safety margin than has unixine The range available is enormous For details of doses halflives and toxicity in dog and cat review the cited references below Many NSAIDs may be given by the oral route making them useful for the later stages of postoperative analgesia Other useful perioperative NSAID 39 include firocoxib etodolac and tepoxalin and much of the toxicity is claimed to have been lessened due to their selective COX particularly COXZ or LOX inhibitory action For full details of the dosing and duration etc review the cited references below 000 I A r I 1 Alpha 2 adrenoceptor agonists It provides some effective analgesia particularly effective in visceral pain in comparison to somatic pain When used during anesthesia it reduce the dose of other anesthetics required However as analgesia is accompanied by sedation in addition to all the other side effects their use for postoperative analgesia is not common Alpha 2 agonists and opioid combination shows marked synergism and it is possible to obtain all degrees of sedation and in some cases anesthesia with such combinations No preprepared combinations exist They can provide effective analgesia with minimal side effects when used by the epidural route If antagonists have been used analgesia is reversed alongside sedation NMDA antagonists Ketamine falls into this classification Tends to provide better somatic analgesia than visceral analgesia CRI infusion is now commonly used in some severely painful animals particularly in the ICU setting but usually is combined with other analgesics and sedatives to enhance the analgesic ef cacy and reduce dissociative side effects balanced analgesia Amantadine has been used in man with varying success and seems to have a place in controlling a neuropathic pain Other adjunctive analgesic therapy Pen39operative Pain management Acupuncture Transcutaneous electrical stimulation TENS Physical therapy Antidepressants Anticonvulsants etc For further information on these modalities review the cited references below 12 of 13 Veterinary Surgery I VNED 7412 Conclusion What determines which agents and methods for pain control should be used would depend on the type of procedures severity of pain and economic consideration for each individual circumstance Our understanding of pain in its manifestation animals is still although improving limited It is imperative to ensure continuous efforts in establishing information on safe and appropriate use of analgesics in animals However it must be remembered that no pharmacological intervention can supersede tender loving care which animals deserve most during their suffering I l and n u in Further suggested references Pen39operative Pain management Mathews KA ed Management of Pain Veterinary Clinics of North America Small Animal Practice WB Saunders July 2000 Flecknell P and WatermanPearson A Eds Pain Management in Animals WB Saunders 2000 Gaynor JS Muir WW Handbook of Veterinary Pain Management Mosby 2003 Carroll GL Small Animal Pain Management American Animal Hospital Association Press Lakewood 1998 Hall L Clarke K and Trim C Veterinary Anesthesia WB Saunders 2001 Thurmon J Benson J and Tranquilli W Veterinary Anesthesia Williams and Wilkinson 1996 Seymour C and Gleed R eds BSAVA Manual of Small Animal Anesthesia and Analgesia 1999 13 of 13 Veterinary Surgery I VNED 7412 FIN EQVETERINARY HEALTH SCIENCES Local Anesthesia amp Analgesia Lyon Lee DVM PhD DACVA Introduction Several features of local anesthesia render it particularly useful in veterinary practice Many surgical procedures can be carried out satisfactorily under local anesthesia eg C section in cows Whether or not sedation is necessary as an adjunct Will depend on the species temperament and health of the animal and on the magnitude of the procedure In adult cattle many operations are performed on standing animals and since sedation may induce the animal to lie down it is better avoided Enabling operation in standing animals also eliminates the dangers associated With forcible casting and restraint and prolonged recumbency In other animals sedation is o en employed to facilitate cooperation from animals by reducing fear and anxiety The sedation also reduces the likelihood of sudden movement in anim s Preemptive local anesthesia in animals undergoing general anesthesia Will reduce the amount of general anesthetic minimizing the cardiopulmonary depression that may accompany and also leading to quicker recovery It provides a useful pain relief even beyond the full recovery from general anesthesia In some situations With extremely depressed animals When they Will tolerate performing a surgical procedure under local anesthesia may be safer as well as more economical The techniques are not dif cult to learn and do not involve the use of expensive or complicated equipment Structure activity relationships 0 Local anesthetics LAs consist of a lipophilic and a hydrophilic portion separated by a connecting hydrocarbon chain see gure An ester CO or an amide NHC bond links the hydrocarbon chain to the lipophilic aromatic ring The hydrophilic group is usually a tertiary amine such as diethylamine Whereas the lipophilic portion is usually an aromatic ring such as paraaminobenzoic acid The nature of this bond is the basis for classifying drugs that produce conduction blockade of nerve impulses as ester local anesthetics or amide local anesthetics Some examples are39 0 Esters procaine cocaine chloroprocaine and tetracaine o Amides lidocaine bupivacaine Amides drugs have i in generieprei c before wine Exception ispiperaeaine an ester dmg Local Anesthesia amp Analgesia 1 of 18 Veterinary Surgery I VMED 7412 Figure I Chemical structure of local anesthetics Aromatic intermediary tertiary Ring bond amine i H c i 3 3 0 CH i 2 Lidocaine NH C 39CHZ N i i H C i i CH2 i 3 f I H30 l i o Hac Prilocaine H nyit39iio cn CH NCH2 2 L I I 2 2 CH 2 H36 Mechanism of action LAs block nerve conduction by inhibiting in ux of sodium ions through ionselective sodium channels in nerve membrane leading to impairment of the generation of action potential The sodium channel itself is a speci c receptor for local anesthetic molecules Figure 2 Mechanism ofaetion afloeal anesthetics Extrac ellular 2H RW lt gt Lipid bilayer Na channel HB lt gt BH Intracellular BH ionized form cation B unionized form free base HZO soluble lipid soluble Local Anesthesia amp Analgesia 2 M15 Veterinary Surgery I VMED 7412 Factors affecting onset intensity and duration of neural blockade Local Anesthesia amp Analgesia Local anesthetics in solution exist in a chemical equilibrium between the basic uncharged form B and the charged cationic form BH At a certain hydrogen concentration speci c for each drug the concentration of local anesthetic base is equal to the concentration of charged cation This hydrogen concentration is called the pKa This relationship is expressed as B BH Lower pKa means greater fraction of the molecules exist in the unionized form in the body so more easily cross nerve membranes leading to faster onset The pKa of currently used local anesthetic compounds lies between 77 and 85 The commercially available solutions are always acid so that they contain more ionized molecules Acidosis in the environment into which the local anesthetic is injected as is present in an infected pus tissue further increases the ionized fraction of drugs This is consistent with slower onset and poor quality of local anesthesia when a local anesthetic is injected into an acidic infected area Local anesthetics with a higher degree of protein binding have a prolonged duration of action Increased dose increases the duration of the block The halflife of esters is only a few minutes due to their rapid hydrolysis in the plasma and liver whereas the halflife of amides is a few hours Patients with reduced cholinesterase activity new born pregnant may have an increased potential for toxicity from ester local anesthetics Among the resulting metabolites from ester local anesthetics the paraaminobenzoic acid is believed to be an antigen responsible for subsequent allergic reactions Amides are mainly metabolized by the liver Patient with severe hepatic disease may be more susceptible to adverse reactions from amide local anesthetics Thin nerve fibers are more easily blocked than thick ones However myelinated fibers are more readily blocked than unmyelinated ones because of the need to produce blockade only at the node of Ranvier In general autonomic fibers B and C fibers small unmyelinated C fibers and small myelinated fibers B and A5 fibers will be more readily blocked than thick myelinated fibers AOL and AB fibers Thus a differential block can be achieved where the smaller pain and autonomic fibers are blocked while large touch and motor fibers are spared This difference is due to the fact that nerve fibers containing myelin are relatively impervious to local anesthetic solutions compared to those which contain little or no myelin The lipid solubility and pKa of the local anesthetic are the primary determinants of the degree of differential blockade pH pKa log 3 of 15 Veterinary Surgery I VNED 7412 Systemic and toxic effects of local anesthetics Accidental intravenous injection of local anesthetics is the most common cause of adverse reaction associated with local anesthetic administration In severe cases it can cause cardiac arrest When the plasma concentration of LAs is excessive sufficient cardiac sodium channels become blocked so that conduction and automaticity become adversely depressed For example excessive plasma concentration of Lidocaine may slow conduction of cardiac impulses through the heart manifesting as increased PR interval and widened QRS complex on the ECG Effects of LAs on calcium and potassium ion channels and local anesthetic induced inhibition of cyclic adenosine monophosphate cAMP production may also contribute to cardiac toxicity Bupivacaine is more cardiotoxic than Lidocaine General overdose depends on blood levels therefore is in uenced by total dose and speed of uptake from the tissues As a very rough guide the toxic dose of Lidocaine would be 8 mgkg much lower in the cat 2mgkg and 4 mgkg of Bupivacaine NB in very small animals such as domestic cats small dogs goat kids birds and small mammals this amount can be easily exceeded using solutions of standard concentration so dilute it carefully and use with caution Signs of overdose are initial sedation followed with increasing dosage by twitching convulsions coma and death Reports implicate prilocaine benzocaine lidocaine and procaine as causative agents to produce methemoglobinemia in some animals Clinically important points to recognize are Local Anesthesia amp Analgesia Spreading properties Good spreading properties mean that specific nerve blocks need less accuracy Speed of onset of action Duration of action and mechanisms limiting this which include speed of removal from tissues and metabolism and removal from the body Effect on local blood vessels Vasoconstriction cocaine only or vasodilation epinephrine is often added to cause vasoconstriction thus delay removal and lengthen action Local irritation and swelling particularly important in horses Toxicity 4 of 15 Veterinary Surgery I VNED 7412 Local anesthetics in common veterinary use Lidocaine o This is the most widely used generalpurpose local anesthetic in veterinary use It possesses reasonably rapid onset of action with good spreading properties being a good all round useful local anesthetic o It may cause some local irritation and swelling which is particularly a problem in the horse It is available in a variety of concentrations or injection with and without epinephrine and in the form of solutions creams jellies sprays etc 0 Duration of action is variable depending on uptake but will be around 1 hour without epinephrine and 2 hours with epinephrine Bupivacaine o This drug has a prolonged duration of action up to eight hours when combined with epinephrine o It is therefore used whenever long action is required postop analgesia prolonged surgery etc Mepfvacaine o This is the most widely used drug in the horse as it causes very little swelling and edema in the area of injection possibly as it lacks vasodilatory action 0 Onset of action is faster and reliability of block greater than with prilocaine Prilocaine o It has slower onset of action and spreads less well compared to lidocaine o The unique ability of prilocaine to cause dosedependent methemoglobinemia limits its clinical usefulness o The main use is in the horse as it causes less swelling but great accuracy is needed when doing specific nerve blocks Other local anesthetics o Proparacaine is used to anesthetize the cornea of the eye When dropped on the cornea it has a rapid onset of action within 1 minute and lasts for about 1530 mins It is nonirritant and does not affect the size of the pupil Amethocaine is well absorbed by surfaces and is used on mucous membranes Procaine an older drug with slow onset of action and poor spreading powers has been superseded by the more modern drugs 0 Cocaine the original local anesthetic is the only one to cause vasoconstriction It is now not used as a local anesthetic because of its potential for abuse Schedule II o Newer long acting local anesthetics with less cardiotoxicity eg ropivacaine or lovobupivacaine are now available for man but are currently very expensive for veterinary use Local Anesthesia amp Analgesia 5 of 18 Veterinary Surgery I VNED 7412 Common methods of producing local anesthesia Surface topical anesthesia Intrasynovial anesthesia In ltration anesthesia Spinal anesthesia Intravenous regional local anesthesia Regional anesthesia Surface topical anesthesia o This refers to the use of local anesthetics in solution sprays as well as in various creams and ointments on mucous membranes drops into the eye sprays or brush in laryngeal area infuse into the nostrils urethra or rectum Intrasynovial anesthesia o In joints bursa and tendon sheaths 0 Useful for both diagnosis of lameness and for general pain relief 0 The local anesthetic chosen must cause minimal irritation and great care in sterility is necessary as infection in these sites occurs easily In ltration anesthesia By this method the nerve endings are affected at the actual site of operation 0 Most minor surgery can be done this way excluding surgery on teats in cattle or small animal digits 0 Problems occur through infection never inject local analgesic through infected tissues irritation distortion of the wound swelling and some delay in postoperative healing o A variation of in ltration anesthesia designed to minimize these effects is eld anesthesia Here walls of anesthesia are made by in ltrating the tissues around rather than at the surgical site 0 Advantages include absence of distortion of the anatomical features in the line of incision muscle relaxation and no interference to healing An example of a field anesthesia technique which is widely used in cattle is the Inverted L or 7 block for anesthesia of the abdominal fossa 0 Ring blocks whereby the tissue all around a distal organ is in ltrated with local anesthetic is another form of eld anesthesia examples of where this is used is on the teats of cattle as vasoconstriction could lead to ischemic necrosis and sloughing of tissue or around the limb of cattle Spinal anesthesia Spinal anesthesia is the injection of local anesthetic around the spinal cord 0 When local anesthetics such as lidocaine or bupivacaine are used all the segmental nerves sensory and motor which pass through the anesthetic are paralyzed although when opioids are used only sensory block occurs 0 Spinal anesthesia is divided into two types epidural and true spinal o Epidural or extradural anesthesia refers to depositing of local anesthetics into the extradural space The needle enters the spinal canal but does not penetrate the meninges The anesthetic is therefore limited to the canal outside the dura mater 0 True spinal anesthesia refers to the subarachnoid access usually known as spinal anesthesia in which the needle penetrates the dura mater and the analgesic is injected into the cerebrospinal uid CSF Local Anesthesia amp Analgesia 6 of 18 Veterinary Surgery I VNED 7412 Effects of subarachnoid and epidural anesthesia The requirements from these techniques is paralysis of sensory nerves to the area in which surgery is going to be performed Muscle relaxation can be an added bonus or a disadvantage Muscle relaxation of the limbs causes recumbency and of the thoracic region limits respiratory movement If local analgesic reaches the cervical region and affects the phrenic nerves then respiration ceases Thus most spinal and epidural anesthesia is injected in the caudal regions of the animal although there are several variations in terminology used generally where injection of drug is in the coccygeal region and the dose of drug is such that the hind limbs are not affected it is termed caudal anesthesia where a higher dose of drug is given still at the coccygeal area so the hind limbs may be just affected the term epidural anesthesia is used and where the block extends to the abdominal region either because of the volume used or because the injection is carried out at the lumbosacral space the term used is anterior epidural Autonomic effects of epidural anesthesia Many of the spinal nerves also carry fibers of the autonomic nervous system which will also be blocked Most important to the are the J quot quot fibers r quot 39 for tone Thus spinal and epidural anesthesia always causes hypotension and if the block is sufficiently anterior to block the splanchnic out ow this hypotension can be severe even life threatening IV uid line is essential prior to performing an epidural block to treat a potentially dangerous hypotension d u The area blocked by epidural anesthesia will depend on The site of injection Common sites used in veterinary medicine depending on the species are the sacrococcygeal or intercoxygeal space and the lumbosacral space However it is possible to carry out segmental blocks using other points of injection widely used in man development in veterinary use only just under way Quantity volume of and specific local anesthetic injected Well examined in man in veterinary use work dates from 1940 s no recent work with newer anesthetics Size of the spinal canal This varies not only between species of the same weight but between breeds with age and with condition of the animal e g fatthin etc Position of animal effects of gravity on spread Removal of the anesthetic from the canal Again this depends on multiple factors including age in uences size of holes in the dura around the nerves condition blood ow etc The use of vasoconstrictors epinephrine will delay removal Thus epidural or spinal anesthesia is not a very precise technique and it is difficult to estimate the extent of block which will occur or its duration Dangers of spinal and epidural block Local Anesthesia amp Analgesia Infection Careful sterile precautions good clipping and scrubbing Irritation causing spinal damage most likely with subarachnoid Hindlimb motor paralysis problem in large animals acceptable in small Hypotension most likely with an anterior block Where this is being done uid therapy or inotropes should be available to maintain blood pressure Respiratory paralysis only if massive overdose of local analgesic used 7 of 15 Veterinary Surgery I VNED 7412 Epidural anesthesia in the Dog and Cat Local Anesthesia amp Analgesia The epidural injection site in dogs and cats is located at the lumbosacral junction between the seventh lumbar L 7 and the first sacral vertebra S 1 In dogs the spinal cord terminates at around 6 7Lh lumbar vertebrae Anterior epidural anesthesia may therefore be safely and easily induced at the lumbosacral junction In cats and small ruminants cattle however the spinal cord terminates at the 3rd sacral vertebra in ruminants it is in 81 and both the spinal cord and subarachnoid space are very close to the site of epidural injection The spinal cord is supported protected and stabilized by vertebral column ligaments and meninges The epidural space is located immediately below the ligamentum avum separating the dura mater from the vertebral periosteum the lining of the spinal canal To locate the site identify the iliac prominences on either side and take an imaginary line between them crossing the dorsal spinous process of the last lumbar segment The site for the needle insertion is immediately caudal to this in the midline Restrain laterally or in stemal recumbency personal preference The hair over the lumbosacral junction should be clipped and aseptically prepared Sterile surgical gloves should be worn during the procedure A 2022 gauge 1525 inch spinal needle may be used depending on the size of the animal a longer 35 inch spinal needle is used for obese and large animals To reach the epidural space the needle must be passed in a ventral direction through the skin subcutaneous fat supraspinous ligament interspinous ligament and ligamentum avum yellow ligament Ifthe needle continues ventrally past the epidural space it will encounter the dura mater arachnoid membrane pia mater and nally the spinal cord The dura mater arachnoid membrane and pia mater constitute the three meningeal membrane layers that protect the spinal cord The dura mater meaning tough mother is the outermost meningeal layer The arachnoid membrane thin and avascular lines the inner surface of the dura mater The innermost membrane layer the pia mater closely approximates the spinal cord This layer is a highly vascularized membrane through which all blood vessels pass to enter and leave the CNS Cerebral spinal uid CSF is located in the subarachnoid space which separates the arachnoid membrane and the pia mater To perform epidural anesthesia the injection must be made only into the epidural space between the ligamentum avum and dura mater Epidural injection should be differentiated from spinal or intrathecal injection in which the anesthetic is injected into the subarachnoid space between the arachnoid membrane and the pia mater this is also where the myelogram is performed in the disc problem dogs Anesthetic agents injected into the subarachnoid space spinal injection produce true spinal anesthesia because of the lack of protection provided by the dura mater and arachnoid meninges Consequently the volume of the anesthetic solution must be reduced by one third Hypotension can be a major complication This procedure should not be carried out unless there is an intravenous line in place so uids or antihypotensives can be given promptly The epidural space is identi ed by advancing the needle from an area of high resistance ligamentum avum to an area of low resistance epidural space This is usually accomplished using the hanging drop or the lack of resistance technique during injection For cranial laparotomies or hindlimb orthopedic procedures a dose of 01 mgkg of morphine withwithout 05 bupivacaine of approximately 1 ml per 10 Kg is required a of 15 Veterinary Surgery I VNED 7412 Epidural anesthetic techniques The hanging drop technique 0 This involves removing the stylet of the spinal needle lling the hub of the needle with saline or anesthetic solution and allowing one drop to hang from the hub 0 As the needle is advanced through the ligamentous structures the drop does not move 0 However upon penetration of the ligamentum avum the negative pressure in the epidural space will draw the drop of solution into the needle indicating proper placement in the epidural space 0 A pop felt through the needle is usually encountered when the spinal needle is passed through the ligamenturn avum o The chance for a successful hanging drop technique is greater in large dogs than in smaller dogs and cats 0 If the hanging drop technique fails the lack of resistance technique can be used The lack of resistance technique 0 This indicates proper placement of the injection needle in the epidural space based on the amount of resistance to the injection of air or saline 0 Once in the epidural space the injection of air saline or anesthetic solution will encounter minimal resistance 0 A separate syringe of normal saline 3 ml or air preferred by others should be prepared for the lack of resistance technique 0 When minimal resistance to the saline injection is encountered the saline syringe is replaced with a syringecontaining anesthetic and the injection is completed 0 To rule out the possibility of administering drugs into the venous sinus presence of the blood or subarachnoid space presence of CSF it is important to aspirate or allow a few seconds to check bleeding before epidural injection Figure 4 Diagram showing various anatomic landmarks and positions of needles to perform epidural block technique a 7 Hanging de Lumbosacral Supiaspinous Hg ski suhamchnnirr space Tquot x x lmratllecal Spite amp r plquot Emma equina Spinal com Sylllpalhelln gangllan Local Anesthesia amp Analgesia 9 M18 Veterinary Surgery I VMED 7412 Epidural anesthesia in bovine In the ox the spinal cord ends in the region of the last lumbar vertebra but the meningeal sac goes to the 3rd4th sacral segments For caudal and epidural anesthesia the injection site used is between coccygeal Cl and C2 located by raising tail in pump handle fashion the rst obvious articulation behind the sacrum being C1 C2 For a 500 kg bovine 5 10 ml 2 lidocaine will give caudal anesthesia without causing hind limb ataxia or paralysis Onset of paralysis of the tail should occur in 12 mins The block will last 12 hours Larger doses will produce increasingly anterior effects By the time 100150 mls 2 lidocaine is injected the block will be suf ciently anterior to allow surgery of the hindlimbs mammary tissue anks and abdominal wall However the bovine will be recumbent Injection of local anesthetics canbe carried out at the lumbosacral junction in order to produce an anterior block with less anesthetic However there is a danger of accidental subarachnoid injection Segmental epidural anesthesia where the anesthetic is injected into the epidural space at the region required can be used for analgesia of any segment with less overall side effects It is more dif th to perform39 penetration of the meninges is likely but in skilled hands it is a very useful technique Figure 5 Location ofSacralCoccygealIntercoccygealjunctionfor the epidural injection in cattle Note the tail can be lifted up and down to yield an indentation at thejunction The shaded areas show the blockage ofthe caudalepiduralfollowing lidocaine iry39ection It induces both analgesia and motor blockade From Thurmon et al 1996 A Epidural anesthesia in the sheep and goat In both sheep and goats anterior epidural anesthesia induced by injection at the lumbosacral junction is easily performed and provides excellent analgesia and muscle relaxation for abdominal sur e Recumbency may occur but is not a problem in these small animals As in cattle there is a ri75k of subarachnoid injection I Figure 6 Diagrams to show various epidural blocking sites lumbosacral space sacrococcygeal intercoccygeal What determines the site depends on the area of operation and the technical competence From Thurmon et al 1996 Local Anesthesia amp Analgesia 10 of 18 Veterinary Surgery I VMED 7412 Epidural anesthesia in the horse Hind limb ataxia is a serious problem so only caudal epidural techniques are used These are useful for various obstetrical manipulations and surgery on the rectum vagina and tail The technique is less reliable than in cattle Site of injection is usually sacrococcygeal junction but can be between Cl and C2 For 500 kg horse a mixture of 50 m of xylazine and 6 ml of 2 mepivacaine may be repeated for another dose may prove very effective Intravenous regiona39 1 local anesthesia Bier39s bladr In this technique a limb vein is catheterized see gure 7 The limb is then exsanguinated Esmarchs bandage and a tourniquet placed around the limb at a pressure adequate to prevent arterial circulation gt 150 mm g Local anesthetic preferably without epinephrine is then injected into the vein A er a period of 15 minutes the area distal to the tourniquet is anesthetized until the tourniquet is removed Potential problems are Dif culty in finding the vein once the limb is exsanguinated this is why it s best to have a catheter in place rst Cardiac arrhythmias or even arrest This is due to an inadequate tourniquet in man there have been more problems when bupivicaine was used than were found with lidocaine Failure to take effect Common reasons are inadequate tourniquet inadequate time and lack of exsanguination it does work without exsanguination but not so well Collapse when tourniquet is removed This is because of anoxic waste products reentering circulation It is preferable if the animal is recumbent at this time o Damage as a result ofthe tourniquet being left on too long This is rare It can be left on for 115 hours on the limb of cattle and dogs Intravenous regional anesthesia is commonly used in cattle for amputation of a digit The diagrams below show some examples of veins available for injecting local anesthetics The analgesic technique is particularly effective in the hind limb For details of methods see Thurmon et al 1996 pp 506508 It is also used to a lesser extent for dogs to enable amputation of the digit combined with systemic sedation o o 0 Figure 7 Easily recognized veins ofthe distalparts ofthe limbs in cattle I Medial view ofthe rightfore limb A radial vein B medialpalmar digital vein 2 Lateral view of the right hindlimb C lateral branch of lateral saphenous vein lateral plantar vein E lateral plantar digital veinFr0m Hall et al 2000 Local Anesthesia amp Analgesia 11 18 Veterinary Surgery I VMED 7412 Regional anesthesia This term is used where speci c nerves to the area concerned are blocked Examples include speci c nerve blocks to the limbs paravertebral blocks comual block for dehoming and many others A list of the blocks widely used in veterinary medicine with suitable references is given as below Paravertebral anesthesia Paravertebral anesthesia refers to the perineural injection of local anesthesia about the spinal nerves as they emerge from the vertebral canal through the intervertebral foraminae The technique may theoretically be carried out in any species and at any level of the spinal cord but in practice its main use is to provide anesthesia of the lumbar region in ruminants Its advantage is that it provides analgesia and muscle relaxation of the whole area covered by the segmental nerves blocked Several different methods of achieving paravertebral anesthesia have been described All methods approaching from the dorsal surface are equally effective The method described whereby the needle is inserted ventral to the transverse processes of the spine has the disadvantage that the dorsal branches of the segmental nerves are not blocked thus some skin sensitivity remains Paravertebral anesthesia is easy to carry out and almost always effective except in the very large beef breeds where it may be very dif cult to locate the necessary landmarks A description of one method generally found to be effective for the cow is as follows Proximal paravertebral block Farquharson Hall or Cambridge Technique Indicated for standing laparotomy surgery such as Csection rumenotomy cecotomy correction of gastrointestinal displacement intestinal obstruction and volvulus The dorsal aspect of the transverse processes of the last thoracic Tl3 and rst and second lumbar Ll and L2 vertebrae is the site for needle placement The dorsal and ventral never roots of the last thoracic T 13 and 1st and 2nd lumbar spinal nerves emerge from the intervertebral foramina are desensitized 1020 ml of 2 lidocaine is injected to each site onset occurs usually within 10 minutes of injection Analgesia of the skin scoliosis toward the desensitized side due to paralysis of the paralysis of the paravertebral muscles increased skin temperature due to vasodilation paralysis of cutaneous vasomotor nerves indicates effective block Duration of analgesia lasts approximately 90 minutes Distal paravertebral block Magda Cakala or Cornell technique Local Anesthesia amp Analgesia Indicated for same as proximal paravertebral block above The dorsal and ventral rami of the spinal nerves T13 L1 and L2 are desensitized at the distal ends of Ll L 2 and L4 A 75cm 18gauge needle is inserted ventral to the tips of the respective transverse processes in cows where approximately 1020 ml of a 2 lidocaine solution are injected in a fanshaped in ltration pattern The needle is completely withdrawn and reinserted dorsal to the transverse process where the cutaneous branch of the dorsal rami is injected with about 5 ml of the analgesic The procedure is repeated for the second and fourth lumbar transverse processes 1020 ml 2 lidocaine is used per site and onset and duration similar to proximal technique 12 of 18 Veterinary Surgery I VMED 7412 Figure 8 The proximal and distal paravertebral block at the T I3 LI andL2 in cattle This technique is one ofthe most commonly used regional analgesia in cattle for standing surgery C section and laparotomy From Thurmon et al 1996 Figure 9 Which side left or right ofthe cow is blocked with paravertebral techni e Left side the muscles are relaxed on the blocked side and the muscle tone remains at the unblocked side right side whichpulled the cow toward this side and with adequate weight bearing From Hall et al 2000 Local Anesthesia amp Analgesia 13 18 Veterinary Surgery I VMED 7412 Table 1 Advantages and disadvantages of four common local anesthetic techniques in cattle Techniques Advantages g Proximal 1 Small dose of analgesic 1 Technical difficulty Paravertebral 2 Wide and uniform area of 2 Arching up of the spine due to Block analgesia and muscle relaxation paralysis of the back muscles 3 Minimal intraabdominal 3 Risk of penetrating vital pressure structures such as the aorta 4 Increased intestinal tone and and thoracic longitudinal vein motility on the left side and the caudal 5 Absence of local analgesic from vena cava on the right side the operative wound margins Distal l The use of more routine size 1 Larger doses of anesthetic are Paravertebral needles no risk of penetrating a needed Block major blood vessel 2 Variation in efficiency exists 2 Lack of scoliosis minimal particularly if the nerves vary weakness in the pelvic limb and in their anatomical pathway ataxia Infiltration 1 Easiest and most commonly used 1 Edema andhematoma of the Anesthes1a mult1ple 1njectlons along the incision site may interfere with healing 2 Incomplete analgesia and muscle relaxation of the deeper layers of the abdominal wall 3 Toxicity after injecting significant amounts of analgesic solution 4 Increased cost due to large doses and longer time required for injection Inverted L or 1 Deposition of the analgesic away 1 Incomplete analgesia and 7 Block from the incision site thus muscle relaxation of the minimizing edema hematoma deeper layers of the abdominal and possible interference with wall healing 2 Toxicity after injecting significant amounts of analgesic solution 3 Increased cost due to large doses and longer time required for injection Local Anesthesia amp Analgesia 14of18 Veterinary Surgery I VMED 7412 Laalneme blades ofthe bead Cornual nerve block Cattle Indicated for dehornjng and treating horn injury 0 Ophthalmic division of the h cranial nerve Injected on the upper third of the temporal ridge about 25 cm below the base of the horn 0 The nerve is relatively super cial about 071 cm deep 35 ml of 2 lidocajne is injected Onset of analgesia occurs 1015 minutes and duration of analgesia is approximately one hour 0 In adult cattle with welldeveloped horns a ring block around the base of the horn may be necessary Figure 10 Diagrams ofneedle placement sitesfor cornual nerve block in a cow From Hall et al 2000 Cornual nerve block Goats Again for dehoming not as easy as in the cow as there are two branches of the nerve Iacrymal and infratrochlear branches 0 Do not use in kids as tend to use a total overdose of local anesthetics in these very small animals Figure 11 Diagrams ofneedle placement sitesfor cornual nerve block in a goat From Thurman et al 1996 Local Anesthesia amp Analgesia 15 18 Veterinary Surgery I VMED 7412 Auricuuplpebr buck Cums and harm Aunmlupalpebml name suppues mulur benu me urbxcmans ueuu muscle us branches m me way mjemun sue huur Thsblunk uues nutpmduceanalgesa urme eye urthehds In cumuncuunwnh mum analgesxaZ ehducane1ns useful furthe remuwl uffurexgqbudles rum me Emma andcunjuncuwl E Alsu useuuuuess frequemly m utha39 wanes Ths blank has nu sensury effeds bulpamlyses me muscles urme eyeuu Ins usedlu keep me eye upen fur example as an adjund m uphmalme surgery Thu 5 uue urme must cummunly useu techniques m blank me mulur runeuuu urme eyeuan gure 12 Auvtmlopalptzbm blockn a cow amid homz meHall 8131 2999 Muu etal 1999 Revomhar nam Muck cmla deely useu m prawns m enudeale me eye May alsu be used mmany utha39 wanes gure 15 Rzrrobulbm needlaplactzmtzmrhvough the marital mmhu A to the ovbtm apex 5 new numou ere I996 1 of II Vatdan sunny I WEB 7412 Lou Anmhdl Aunt1 Peterson s eye block Cattle Requires more skill and speci c anatomic knowledge than retrobulbar nerve block but involves less risk in damaging surrounding anatomic structure around the eye globe and less volume requirement reducing potential for systemic toxicity and expense The point of injection is the notch formed by the supraorbital process cranially the zygomatic arch ventrally and the coronoid process of the mandible caudally An one inch 14 gauge needle is inserted through a desensitized skin as far anterior and ventral as possible in the notch Insert a 45 inch 18 gauge straight or slightly curved needle at the point of injection mentioned above in a horizontal and slightly posterior direction until it hits the coronoid process of the mandible Gently manipulate the needle anteriorly until its point passes medially around the coronoid process then advanced to the pterygopalatine fossa rostral to the solid bony plate that is in close proximity of the orbitorotundum foramen Following aspiration 7 7 15 ml of local anesthetics are injected Oculomotor trochlear abducens and three branches of the trigeminal nerve ophthalmic maxillary and mandible are desensitized in 10 7 15 minutes following injection Facial nerves lnfraorbital Mandibular Mental and Supraorbital DogHorse Local Anesthesia amp Analgesia These nerve blocks may be used to allow surgery on the face Thurmon et a1 1996 pages 430432 449450 17 of 18 Veterinary Surgery I VMED 7412 We n M limbquot Hols In the horse yery speclflc nerve blocks are used both for dragnoses oflameness and to allow surgery othe lower llmb Intrartlcular lnjectlons are also used Thurmon et al 1 996 pages 4527460 ltd The anatomy oftheneryes to the llmbs ofcatzle rsyery complrcated Thurmon et al pages 5077512 I Althou W l mm common to carry outthe srmpler Inclml plexus bloc any area below drstal part ofhumerus The technrque should be performed ln a wellrsedahed or anesthetrzed ammal Thls block can be used ln dogs cats small rumrnants calves and foals anesthesla and relaxatlon ofthe llmb and analgesla w the forellmb lnclude the mdlal ulnar medlan musculocutaneous and axlllary nerves The bmchlal plerus nery es derryed from Co c1 ces and T71 spmal nerves roots t eofdmgsused em mslalastsfor r e b hours Flgwe 14 Bracth plays and nerve blocklng techrllque 7 57cm 2022 gauge splnal needle ls lnsa ted medlal to the shoulder Joint and drrected parallel to the vertebral column toward the costochondral Junctlon In larger SlZe ammal lfno blood ls asprrated mto the synn as e needle ls wl w approxrmately 1015 ml of 2 lldocalne or 0 5 buplyacalne at 3 mgkg wth dllutron up to 2030 ml ls slowly anected Local anesthetlcs are lnjecled to brachlal plerus whlchwoulddl semto areatoe ect crselocatronrstheef usually less lmportant thanth spreadlngpow the gand J cte Sources of reference Thurmon Tranqullll and Benson Veterrnary Anesthesla wllllams and wllklns 1996 Hall Clarke and Tnm Veterrnary Anesthesla WB Saunders 2000 Mulr Hubbel and skarda A handbook ofAnesLhesla Mosby l999 Lnu Amsmlsll a Amman 1a of 15 vmrlmry Sum I wen 7412
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