ANESTHESIOLOGY VMED 7412
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F5 W quot VETERINARY EH HEALTH SCIENCES AIRWAY MANAGEMENT ampVENHLATION Lyon Lee DVM PhD DACVA Intmdurtion Animals under anesthesia increase the risk of developing airway complications as anesthesia induces a number of abnormal physiologic conditions Most anesthetics obtund or abolish the swallowing re ex a protective mechanism to prevent accidental inhalation of foreign materials that may be present in the mouth Maintaining a secure airway during perianesthetic period comes as a primary requirement for safe anesthesia and is a fundamental responsibility of the anesthesia provider Failure to do so can result in brain damage and death In some situations maintaining patent airway can be performed by a simple maneuver such as extending the neck of a patient However placing a tracheal tube into the trachea without question ensures a more secure airway This lecture covers some basic anatomy and physiology of the respiratory system the necessary devices required to perform endotracheal intubation and techniques of airway management basic principles of the ventilator and ways how best to apply such knowledge into clinical practice Defhitlbns Airway is the passage way into or out of the lungs Proper airway ensures adequate delivery of oxygen 02 and elimination of carbon dioxide C02 to and out of the body Respiratian is the total process of oxygen supply and carbon dioxide elimination Ventilatian is the movement of gases in and out of the alveoli Anatomic and leblogk39 mmHeIatlans Airways 0 Upper airways o nares nasal cavity pharynx larynx Trachea 0 Lower airways o bronchi bronchioles alveoli Upper airway obstruction Anesthetics relax nasal alar and pharyngeal musculature and abolishes cough re ex increasing the risk of causing upper airway obstruction Brachycephalic dogs are predisposed to the obstruction because of higher incidence of elongated soft palate stenotic nares everted laryngeal ventricles and hypoplastic trachea Treatment Remove the source of obstruction physical objects regurgitated materials etc Extend the head and pull the tongue out 0 Perform endotracheal intubation or when intubation is not feasible perform tracheostomy but this is rarely needed 0 o Airway Management Ventilation 1 of 14 Veterinary Surgery I VMED 7412 Lower airway obstluction May develop due to accidental aspiration of regurgitants saliva or due to pulmonary edema microbes induced infection and presence of underlying small airway obstruction Increased delivery of tidal volume increased inspiratory pressure all have shown to improve ventilation in this patient but in general harder to treat than upper airway complications Depending on the magnitude of the complications e g asthma or COPD treatment with bronchodilators may be indicated Ventilation and gas exchange VE total minute ventilation VT tidal volume f respiratory rate VE VT X f Suppose the volume exhaled with each breath is 500 ml and there are 15 breathsmin then the total volume leaving the lung each minute is 500 X 15 7500 mlmin total minute ventilation VE To increase ventilation VT or f or both need to be increased Poiseuille 3 law and airway resistance Airway Management ampVentilation P r4 Q 8nL Q Gas ow P pressure difference r tube radius n viscosity of the gas L tube length Ohm s law states V IR or Pressure Flow X Resistance From above as defined by Poiseuille s equation and Ohm s law Resistance changes relative to several factors Resistance 4 r 0 Therefore any reduction in the radius of the airway is INVERSELY related to the l res1stance s1nce R 4 r o A 50 reduction in the radius results not in a doubling of the resistance but rather a 16fold increase 0 However doubling the length L through which the patient is breathing will simply double the resistance 2 of 14 Veterinary Surgery I VMED 7412 Changes af aw restsmnce In response m changes afradms Q ow R resxstance The photo above shows d ereht sues ofmasks Use the smallest mask that Wm m yourpauent rmxmmxze dead space hunmn n quotH recogmze limitations 9 They do NOT protect the auway from aspiration They do NOT provxde a patent auway a Lxmxted abxlxty to supportvenulauon hf apnea orhypovennlauon occurs Airway Manamam avnulmon 3 uf u vmrrmy surpry I vuzn 7412 Inchll Endntrachul tubs Murphy mug cummunly used Cuffed ururreuffed hula anhe up Murphy eye Murphy eye allu bypass ufthe alrwhen the pauem end rs acndmtally blanked Ma 11 e Cuffed ururreuffed hu hula anhe up chle e Tapered end pmmdehghtsalmu euffrru hula Murphy Eye Paxiem and Murphyeye lnl39btm l cu Mmyhmumm hVvnilmm A of IA Venomn Sunny I vuzomz Ideal tube size Select the largest possible diameter without a danger to damaging larynx or trachea Length should be from the tip ofthe nose to the point ofthe shoulder see photo below Too long tube can result in endobronchial intubation and unintentional one ung ventilation Excessive tube length extending out ofthe oral cavity results in increased dead space ventilation cut to the length ifnecessary 1 Point of shoulder Intubation techniques Direct visualization dogs cats small ruminants swine etc laryngoscopy useful 394 t t o Blind horses easier than in most species Am Minnine QVenIIlllbn 5 of IA Valanan Surgery I VMED 7412 Proper cuff inflation o Overin ation of the endotracheal tube cuff may cause 0 Pressure necrosis of the tracheal epithelium o Collapse of the endotracheal tube and airway occlusion more common with softer silicone type endotracheal tubes 0 To properly in ate the endotracheal tube cuff the minimal leak method should be employed 0 Intubate the patient 0 Administer a positive pressure breath reaching a peak airway pressure of about 20 cmHzO or watch for a moderate amount of chest rise if a pressure manometer is not available While administering the positive pressure breath listen for an airway leak or sniff for trace of anesthetic gas at the level of your patient s mouth If you do not hear any air leaking around the endotracheal tube do not in ate the cuff If you do hear an air leak around the endotracheal tube in ate the cuff with a small amount of air volume is relative to tube size Repeat the above process just until you no longer can hear a leak or smell anesthetic gas at a peak airway pressure of just under 20 cmHzO pressure 0 You may need to recheck cuff in ation 0 After 15 20 minutes of anesthesia as your patient s anesthetic depth deepens the laryngeal and tracheal muscles may relax and an airway leak may develop Special techniques for endotracheal intubation dif cult airways o o o o 0 Guide tube stylet technique 0 Retrograde intubation technique 0 Endotracheal intubation through nasal passage nasotracheal intubation o Endotracheal intubation through lateral pharyngostomy o Endotracheal intubation using a fiberoptic endoscope o Endotracheal intubation by tracheostomy 0 For full technical details of these techniques see pp52853l of Veterinary Anesthesia Thurmon et al 1996 Airway Management ampVentilation 6 of 14 Veterinary Surgery I VMED 7412 Larynmpy uThelannowpc quot L LL L 39 amino quot 39 i d lui N39BUsed1e laryngoscope to depress d1 base om tongue not d1 epiglot s Depressing d1 r inducing airway complications m l r A for most small animals llamas and big cats McIntosh curved and char are numerous modi cations based on hes two Laryngeal blade Laryngoscopehandle AllwnyNanagarmntRVantlla nn 7 of 14 Veter nary Surgery I WED 7412 Type Size Miller 0 McIntosh 1 Miller 2 McIntosh 3 McIntosh 4 Spedesged cpmblemsmdtmm m adequate cuffin ation ofche endotracheal tubes is essential Tn mminnnc 39 39 39 39 39 mi 1 n n ml 39 39 t L39 and hi n limc clinical signi cance I I I r I indie cat quot 39 39 Tomcal 39 39 39 39H L i n n 39 39 Ashort Almy Mmunrmnl Wonulauon a of 14 Veterinary Surpry I WED 7412 C ontml of respiration Respiratory function is controlled by central respiratory centers central and peripheral chemoreceptors pulmonary re exes and nonrespiratory neural input Blood gas tensions and hydrogen ion concentration are monitored by central and peripheral chemoreceptors that return signals to central neural controller to provide necessary feedback adjustments in ventilation 0 Central neural control mechanism regulate the activity of the primary and accessory respiratory muscles 0 Central chemoreceptors are located on the ventral surface of the medulla and bathed by cerebrospinal uid and plays a significant regulatory role in conscious animals by affecting total minute ventilation and alveolar ventilation 0 Peripheral chemoreceptors are located on the carotid and aortic bodies and sensitized to increase respiratory drive when Pa02 falls below 60 mmHg Diug s effect on contra of ventilation o Anesthetics alter the central and peripheral chemoreceptor response to C02 and 02 in a dosedependent manner 0 0pioids shift the C02 response curve to the right and cause additive depression of the respiratory center At the clinical dose respiration complication is very rare 0 Tranquillizers o Phenothiazine and benzodiazepines often reduce respiratory rate but do not appreciably alter arterial C02 0 Alpha 2 adrenergic receptor agonists produces laryngeal relaxation and an alteration of pulmonary mechanics compliance and resistance a PaC02 is not significantly altered but a fall of Pa02 as low as 3050 mmHg in the sheep has been reported although in most domestic species this severe reaction is not observed a There is an increase of lower respiratory airway resistance Changes of ventilationperfusion VQ relationship during anesthesia o Pa02 decreases more dramatically during general anesthesia in large animals than smaller animals The primary attributor is believed to be increased V Q mismatches o Hypoxic pulmonary vasoconstriction a protective mechanism redirects blood ow to better ventilated area in the lung Anesthetics cause marked reduction in this protective response resulting in further VQ mismatch Airway Management ampVentilation 9 of 14 Veterinary Surgery I VMED 7412 Ventilator Anesthesia ventilators provide controlled ventilation to patients under general anesthesia Simply anesthesia ventilator is a reservoir bag a bellow or concertina bag in a closed container bellows housing that can substitute for the reservoir bag of an anesthesia breathing system The anesthesia ventilator performs the same job as the anesthetist who periodically squeezes the breathing system s reservoir bag to ventilate the patient lessening laborious workload of the anesthetist and thereby minimizing the potential for distraction Classi cation is through power source drive mechanism and cycling mechanism 0 Power source electrical or compressed gas or both 0 Drive mechanism commonly compressed gas even when electrical source is used 0 Cycling mechanism is typically time cycled o Other mechanisms include pressure cycled volume cycled but timing mechanism plays a major role in ventilatory function even in these two For additional details about anesthesia ventilators see pages 535556 of Veterinary Anesthesia Thurmon et al 1996 A variety of ventilators are manufactured by different vendors and may come in a stand alone unit or built into the anesthetic machine 0 The decision to choose which type will depend on what suits better for the clinical need a practitioner has based on the animal species and economic consideration The cost for most standalone ventilator adequate for small animal use should fall into the range of 2000 to 5000 Airway Management ampVentilation 10 of 14 Veterinary Surgery I VNED 7412 Asequotding hulluws sea phom buluw Most new Venmators are othxs type Safe know that a leak exxsts Alrwly Mangumsquot avnuluan l1 of 14 vmnmry Surpry I wen 7412 Descending Bellows Metomatic see photos below older design but still very functional The ventilators that we routinely use in our hospital Metomatic controls Airway Mlnl mlnt EVomIlItbn Power onoff Expiratory ow rate used to adjust character ofventilation lnspirato ow rate used to adjust character ofventilation Expiratory time controls respiratory rate phat in 39 39 r 39 39 39 lungs l piiaio 39 in pirato lnspiratory trigger effort can be adjusted to make ventilator function in assist rnode 39 39 39 39 39 39 39 39 39 39 oluine rarely used cquaLUI Tidal volume knob on back ofbellows adjusts tidal volume 11 of 14 vumry 5mm I VMED 7412 am h WE m a ham haw A venulator IS nothmg more than a mechamcal rebreaLhm b 33th ths prevean delwenng a hypoxlc gas mlxture R g ag Thaefore the Venulator always anaches where the rebreathmg bag attaches w a breaLhmg mum see above photo on the nght Alrwly Mangumsquot avnuluan n of 14 vmnmry Surpry I wen 7412 Respiratory assist devices 0 Manual resuscitator ambubag see photo below inexpensive and portable Demand valves mostly used in horses see photo on the right Clinical considerations to ensure adequate ventilation in respiratory distressed patienB Establish and secure a good airwa Increase inspiratory Oz fraction of delivered gas ontrol or assist ventilation using desirable devices mechanical ventilator ambubag Antagonize When available and indicated an overdose of respiratory depressants With reversal agents eg opioid antagonist Airway Management Ventilation 14 of 14 Veterinary Surgery I VMED 7412 Hm EQVETERINARY HEALTH SCIENCES PHARMACOLOGYPREMEDICATION Lyon Lee DVM PhD DACVA Introduction Preanesthetic medication or premedication is used to help both the anesthetist and the animal Premedication implies administration of sedatives tranquilizers and analgesics with or without anticholinergics before anesthetic induction Premedication is aimed to o relieve anxiety thus apprehension fear and resistance to anesthesia counteract unwanted side effects of agents used in anesthesia reduce the dose of anesthetic provide extra analgesia Modern methods of obtaining balanced anesthesia have clouded the issue as to the aims and the exact nature of premedication because they have to some extent made dif cult the de nition of anesthesia For example If a low dose of medetomidine and ketamine combination is given IM to a cat then once the cat is sedated prior to anesthetic inductionmaintenance this mixture is a premedicant However if a high dose of medetomidine and ketamine combination is given IM and the cat becomes anesthetized is the mixture still a premedicant The de nition is unimportant as long as the part which each drug premedicant or anesthetic plays in the production of suitably balanced anesthethesia and in the reduction of unwanted side effects The sedative and anxioloytic drugs play the major role in premedication by improving the quality of anesthesia and recovery and counteracting unwanted side effects such as ketamineinduced muscle rigidi The type of sedative drug chosen for premedication depends on a variety of factors such as species health status procedures an age Anticholinergics such as atropine and glycopyrrolate may be used as premedicant to avoid excessive salivation bradycardia and hypotension There is no premedicant or combination of drug protocols that can safely and routinely administered to all patients Type of surgery duration of procedure anticipated complications postoperative needs age temperament and physical condition should all be carefully considered in the choice ofpremedicants Sound understanding of premedication pharmacology helps avoid dangerous dosing and choice and increase the safety of anesthetic management In clinical situation what is most commonly practiced in preanesthetic medication is to provide more than one class of drugs to achieve sedation and analgesia This technique is termed as neuraleptanalgesiu which is de ned as a state of quiescence altered awareness and analgesia produced by the administration of a combination of a neuroleptic agent and a narcotic opioid analgesic This lecture provides a brief pharmacological description of various drugs used as premedicants o o o Phenothiazines Some examples of the phenothiazines in veterinary use are acepromazine promazine chlorpromazine propiopromazine and tri upromazine of which acepromazine is the most commonly used in veterinary medicine Can be given orally subcutaneously intramuscularly or intravenously Some examples of butyrophenones in veterinary use are droperidol azaperone and lenperone Although the phenothazines and butyrophenones differ in chemical structure they share many similar pharmacological properties so these two will be discussed together These agents are classi ed as major tranquilizers neuroleptics They induce sedation by depressing brain stem and the neuronal signal transmission to the cerebral cortex and also by antagonizing dopamine a multipurpose neurotransmitter excitatory receptors through inhibiting dopamine release at the neuronal synapses Major clinical indications for their use are to provide anxiolysis reduce the concurrently administered anesthetic dose when used as part of preanesthetic medication or provide additional sedation synergistic with other prescribed sedatives or analgesics Caridovascular effects A well known side effect is a tendency to cause hypotension due to alphaladrenergic blockade Therefore its use is contraindicated in shocky hypotensive or anemic patients phenothiazines possess antiarrhythmic effects CNS effects Phenothiazine derivatives induce CNS depression by affecting the basal ganglia hypothalamus limbic system brain stem and reticular activating system They lack any generalized hypnotic effect and do not produce analgesia They block dopamine receptors and the action of 5 hydroxytryptamine They act centrally on the chemoreceptor trigger zone as well as the vomit center in the medulla to induce antiemesis Thermoregulation is depressed Other side effect includes lowering seizure threshold so avoid using it in a patient that is likely to induce seizure eg myelogram Respiratory effects Pharmacology amp Premedication At therapeutic dose there is negligible respiratory effects They may decrease rate but this is usually compensated with increase in tidal volume and the minute volume is maintained Large dose can depress ventilation When combined with opioids and hypnotics the phenothiazines have additive effect and respiratory depression may occur 2 of 10 Veterina Surge I VMED 7412 2 Other physiological effects Some skeletal muscle relaxation delayed gastric emptying time decreased packed cell volume and total plasma protein and increase in plasma volume as a result of peripheral vasodilation and subsequent shifts of extravascular water into the vascular space Decreased body temperature as a result of peripheral vasodilation reduced skeletal muscle tone and depressed thermoregulatory center Metabolism and excretion Biotransformation include oxidation conjugation with glucuronic acid in the liver Metabolites are excreted in urine for several days No speci c antagonists are available so depend on metabolism and excretion when overdosed Although the manufacturer s recommended dose for acepromazine found in the package insert ranges l 2 mgkg these doses are rarely prescribed in the clinical situation Most common clinical dose in the dogs and cats ranges between 001 to 01 mgkg and doses exceeding this range will simply increase the duration incidence of hypotension without much bene t in the degree of sedation Most of these agents have relatively long acting duration of drug effect lasting for several hours Elimination of the drugs is mainly through hepatic metabolism Some owners report unhappily that their dog is still sedate a day after a dose of acepromazine It is more likely easily noticed in larger breed dogs when they are sedate and groggy compared to small toy breeds under similar in uence which explains why the complaints usually come from large breed dog owners Butyrophenones Pharmacology amp Premedication Three bytyrophenone derivatives are used in veterinary medicine droperiodol Inapsine azaperone Stresnil and lenperone HCl ElanoneV Butyrophenones have very similar physiologic effects to the phenothiazines Droperidol 04 mgml is combined with fentanyl citrate 20 mgml to form Innovar Vet The combination has little to no effect on cardiac output but decreases arterial blood pressure systemic vascular resistance and heart rate This product is no longer marketed in the US 3 of 10 Veterina Surge I VMED 7412 3 Benzodiaz epines These provide minor tranquilization at clinically used dose Diazepam Valium midazolam Versed and zolazepam in Telazol are most commonly used agonist in this class in veterinary practice Although speci c antagonists such as umazenil Romazicon and sarmazenil Sarmasol are available its use is limited by the expense and lack of enough clinical information Benzodiazepine agonists bind at benzodiazepine receptor sites in the CNS and these receptors potentiate the effects of GABA gammaamino butyric acid an inhibitory neurotransmitter which lead to enhanced function of chloride ion channel gating The resulting enhanced opening of the chloride ion channel leads to hyperpolarization of cell membranes making them more resistant to neuronal excitation These mechanisms explain how the central nervous depression is achieved with use of the drugs Despite less degree of sedation compared to more potent sedative eg acepromzinae or alpha 2 agonists benzodiazepines are favored in animals that are in increased risk of cardiopulmonary failure The cardiopulmonary effect from these drugs is very minimal although accompanies occasionally mild hypotension and respiratory depression Mild to moderate sedation is achieved but clinically it is rarely given as the sole sedative For synergistic effect it is most commonly combined with opioid analgesics in dogs or dissociatives in cats which provides greater degree of sedation as well as analgesia Other common use of this class of drug is to treat seizure In some dogs paradoxical excitement or aggression can be observed when given alone believed to be through disinhibition of suppressed behavior The excitement is seen most notably in hyperactive and young animals but less frequently in depressed and geriatric animals Coadministering the drug with opioids or dissociatives should avoid incidence of such adverse behavioral alteration The sedative effect lasts shorter than phenothiazine class drugs ranging from 30 minutes to a few hours at most The drugs are eliminated due primarily to hepatic metabolism with renal and fecal excretion Diazepam Pharmacology amp Premedication Physicochemical characteristics 0 insoluble in water needs propylene glycol to increase solubility irritating to tissues erratic absorption from 1M injection often incompatible with other solutions 0 02 710 mgkg IV SQ in the dog and cat 0 IV should be injected slowly to prevent pain and venous thrombosis o Propylene gloycol is a cardiopulmonary depressant and rapid infusion may cause hypotension bradycardia and apnea 4 of 10 Veterina Surge I VMED 7412 4 CNS effects 0 Works primarily on parts of the limbic system the thalamus and the hypothalamus o The sedative effect varies considerably among individuals It may produce calming or taming effects in animals but paradoxical excitement is also a possibility o It is known to be amnesic in human 0 Anxiety is reduced without marked sedation o Benzodiazepine receptors appear to have wide spread distribution in the brain however these receptors appear to be lacking in the white matter 0 Anticonvulsant Muscle relaxing effect is believed to occur at the spinal cord level and at the reticular formation of the brain stem 0 A number of neurotransmitter systems including acetylcholine glycine serotonin gammaamino butyric acid GABA may be involved in the CNS effects produced by benzodiazepines 0 Could be used as appetite stimulant in cats and dogs Cardiopulmonary effects 0 At clinical dose minimal cardiopulmonary effects prevail 0 High IV dose may produce respiratory depression and decreased blood pressure Metabolism and excretion 0 Up to 96 of diazepam is protein bound 0 Diazepam is metabolized in the liver to Ndesmethyldiazepam 3 hydoxydiazepam and oxazepam all of which are pharmacologically active 0 Most metabolites go through renal excretion Midazolam Physicochemical characteristics 0 It is water soluble and has a pH of 35 At a pH above 40 it turns into lipid soluble form 0 It is 25 times more potent than diazepam but the unit cost is still more expensive 0 The typical clinical dose is 01 7 0 5mgkg in dogs and cats 0 Due to a good water solubility it is well absorbed from IM or SQ injections and physically compatible with many other solutions unlike diazepam it devoid of the irritant solvent propylene glycol Cardiopulmonary effects 0 It has similar cardiovascular effect to diazepam with minimal alterations in this 0 It may be more respiratory depressant than diazepam Metabolism and excretion o It is similar to diazepam in this respect 0 Clinical duration of midazolam is shorter than diazepam but onset of clinical effects are more predictable Zolazepam This is only used in combination with tiletamine in Telazol Class III Each vial contains powder of 250 mg of zolazepam and 250 mg of tiletamine and typically is reconstituted to make 5 ml solution therefore 100 mgml of zolazepam tiletamine mixture This mixture is essentially identical to Ketaminediazepam Ket Val mixture in many pharmacophysiologic aspects Their use is popular for exotic species Pharmacology amp Premedication 5 of 10 Veterina Surge I VMED 7412 5 Alpha2 adrenergic agonists Examples 0 xylaZine Rompun o medetomidine Dormitor o 39 39 quot D J D J o romifidine Sedivet o clonidine Catapres The alpha 2 adrenoceptor is a subclass of the alpha adrenergic receptors The prejunctional inhibitory receptors exist within the sympathetic nervous system Alpha2 receptors are found in the CNS gastrointestinal tract uterus kidney and platelets Mild analgesia is also achieved Good muscle relaxation is usually present Among the alpha2 agonists medetomidine and xylaZine are the most commonly used in small animals These drugs provide moderate to heavy sedation Animals appear very sedate but still respond to stimuli Beware aggressive dogs should still be muZZled when handled and horses may still kick in response to sudden touch Alpha2 antagonists such as atipamezole yohimbine and tolazoline are available to reverse the alpha2 agonistic side effects such as excessive CNS or CVS complications Cardiovascular effects include initial transient hypertension followed by prolonged hypotension biphasic changes bradycardia and second degree atrioventricular block and decreased cardiac output Respiratory effects include decreased respiratory rate with a variable effect on tidal volume but at clinically useful dose it is of minor concern Other effects of clinical importance are increasing blood glucose level decreasing intestinal motility 39 39 urine 1 39 quot 39 39 uterine 39 which may lead to premature delivery or abortion and inducing vomition The drug s mechanism of action is mainly through its agonist activity at presynaptic alpha2 adrenergic receptors that results in decrease in release of norepinephrine from adrenergic nerve terminals in CNS and periphery This causes sedation decreased sympathetic activity analgesia and hypotension Main clinical uses are to decrease anxiety provide chemical restraint with relatively dependable sedation addition of opioids recommended for more predictable outcome potentiate effects of other drugs and provide analgesia The duration of action is dose dependent and typically lasts 10 to 30 minutes of sedation and restraint for xylaZine and l to 3 hours for medetomidine The drugs are metabolized by the liver and undergo urinary and biliary excretion One good advantage of this class of drugs is its ability to enable pharmacological reversal with alpha2 adrenergic antagonists atipamizole 20 100 mcgkg yohimbine 00503 mgkg and tolazoline 0515 mgkg Xylazine 20 mgml 01 100 mgml Pharmacology amp Premedication A thiazine derivative that has sedative analgesic and muscle relaxant effect Onset of effect is within 35 minutes following IV administration and 5 7 15 minutes following IM administration Typical dose is 01 7 10 mgkg in dogs and cats 05 7 20 mgkg for horses but lower in ruminants in the range of 005 7 02 mgkg Analgesic effect is relatively short lived 1530 minutes but sedation outlasts this Initial increase in blood pressure is due to intense peripheral vasoconstriction but this is followed by prolonged hypotension biphasic BP due to decrease in sympathetic out ow and decrease of norepinephrine from sympathetic nerve terminal 6 of 10 Veterina Surge I VMED 7412 6 Single therapeutic dose does not induce much repiratory depression but at large doses and concurrent administration opioids inhalants and injectable anesthetics signi cant respiratory depression may occur Xylazine causes laryngeal relaxation and cough suppression Vomiting is frequently seen in the cat and the dog due to central alpha 2 activation The thermoregulatory center is depressed and hypothermia is observed Hyperglycemia and glucosuria are due to depressed insulin release in the pancreas Inhibition of ADH release results in diuresis Xylazine goes through extensive hepatic metabolism and metabolites are excreted mainly in the urine Xylazine has marked ecbolic effects and should not be used in the last third of pregnancy nor at conception for example in ovum transplants Combining with anticholinergics is not recommended due to excessive hypertension and tachydysrhythmia Medetomidine It is the most potent and specific of this group It has displaced the use of xylazine in the dog and the cat It causes marked ataxia in the horse even at low doses Its alpha2 to alphal ratio is 1620 in comparison to 160 of xylazine thus making it approximately 10 times more potent than xylazine Dose is arroximately 530 mcgkg IM SQ IV Detomidine It is a potent alpha 2 agonist primarily used in the horse In horses maximum sedation effect is achieved at 20 mcgkg IV equipotent of 1 mgkg of xylazine or 40 mcgkg IM It has less ecbolic effect than zylazine so is preferred over xylazine in late pregnancy Dose requirement is similar for horses and cattle Alpha 2 antagonists Example 0 atipamezole Antisedan o yohimbine Yobine o tolazoline Tolazine o idazoxan Alpha 2 agonistic side effects such as excessive sedation or bradycardia can be reversed using the antagonists Atipamezole has alpha 2 to alpha 1 seletivity ratio of 200 to 300 times higher than yohimbine or idazoxan Equal volume of atipamezole 5 mgml is administered to reverse medetomidine 1 mgml Reversal is also possible for other alpha 2 agonists using atipamezole but is more costly than using telazoline or yohimbin Atipamezole has no activity at betaadrenergic histaminergic serotonergic dopaminergic GABAergic opioid or benzodiazepine receptors Rapid IV administration is associated with hypotension and excitatory emergence Slow titrated IV dosing or IM SQ administration will minimize these Pharmacology amp Premedication 7 of 10 Veterina Surge I VMED 7412 7 Opioids Pharmacology amp Premedication Detailed pharmacologic information about opioids will be presented in the Pain lectures Examples of drugs in this class are morphine fentanyl oxymorphone meperidine butorphanol pentazocine buprenorphine nalbuphine and naloxone The drugs bind to opioid receptors in the CNS which usually have inhibitory effects on neurons Mechanism of action 0 When opioid agents are bound to their specific receptors the membrane G proteins are activated Activated G proteins open potassium channels in the neuronal membrane hyperpolarizing the neuron o The neuron becomes unresponsive to excitatory input and blocks the neurotransmission 0 Primary clinical effects are analgesia and sedation Opioids are classified into agonist agonistantagonists partial agonist and antagonist depending on the pharmacological effect Main clinical uses are to provide analgesia decrease anxiety provide sedation chemical restraint variable depends on species agent and dose and decrease doses of other agents for synergism The cardiopulmonary effects include bradyarrhythmias usually easy to correct with anticholinergics minimal effect on blood pressure and cardiac output The respiratory depression accompanies with opioid agonist administration and is dose dependent plateau effect with partial agonists Respiratory rate may decrease or increase and induce compensatory changes in tidal volume The opioids raise chemoreceptor threshold to PaCOz thereby further exacerbating the respiratory depression Other effects include vomition defecation initial effect urinary retentionconstipation with continued use excitatory effects at high doses in some species cat horse pig best used with neuroleptics to avoid the excitement reset of the thermoregulatory center resulting in panting in dogs Duration of action depends on the type of agents used as well as dose and route of administration widely ranging from 30 minutes fentanyl to several hours buprenorphine These drugs are metabolized by the liver extensive first pass metabolism hence not very effective orally administered and excreted in bile and urine The pharmacological reversal of agonists is achieved by antagonistic effects with a pure antagonist eg naloxone but due to short duration of action be aware relapse to the agonistic effect monitor closely particularly respiratory depression Partial agonists e g nalbuphine and mixed agonistantagonist butorphanol can reverse some of the effects e g sedation excitement respiratory depression Combination of opioid with tranquilizer is the most popular practice for sedating or chemically restraining animals neuroleptanalgesia These combinations provide heavy sedation and analgesia for minor surgical procedures or allow endotracheal intubation airway support or anesthetic induction It is noted that patients may be hyperresponsive to noise so quiet surrounding is desirable o 8 of 10 Veterina Surge I VMED 7412 8 Some examples of commonly used combinations are 0 morphine acepromazine o morphine diazepam o morphine medetomidine o butorphanol medetomidine o butorphanol diazepam o hydromorphone acepromazine o hydromorphone diazepam o hydromorphone medetomidine o oxymorphone acepromazine o oxymorphone diazepam o oxymorphone medetomidine o fentanyl droperidol 0 etc Dissociatives The examples of dissociative agents in veterinary use are ketamine and tiletamine These are generally classed as anesthetic agents rather than sedatives but at low doses and particularly used in combination with another sedatives these can produce very useful premedication particularly in intratable cats or very aggressive dogs The mechanism of action is through its NMDA nmethyldaspartate 7 glutamate antagonistic effect which inhibits neurotransmission and causes a variety of pharmacological effects It is described as having cataleptic analgesic and anesthetic action quotr 39 39 y muscle v t and hypertonicity are not uncommon The dissociative agents are highly abused and there have been sporadic reports of break ins of the veterinary practices Ketamine This is the most commonly used dissociative used in veterinary medicine At sedative doses 510 mgkg the effects of ketamine alone on the cardiovascular system is minimal Although ketamine can be used by itself in small animals as a sedative for procedures which do not require muscle relaxation for example radiography it is commonly combined with other sedatives for synergistic effect and to counteract muscle rigidity The useful adjunct sedatives include but are not limited to acepromazine xylazine medetomidine diazepam and midazolam Pharmacology amp Premedication 9 of 10 Veterina Surge I VMED 7412 9 Anticholinergics Its vagolytic effect induces J J quot quot 39 39 Jquot quot decreased gastrointestinal motility and tachycardia Its routine use as premedication is not recommended but still is a useful agent in procedures that may cause vagal stimulation e g ocular surgery Its use is contraindicated in patients with preexisting tachycardia Atropine sulfate 054 mgml for small animals 2 or 15 mgml for large animals Atropine is belladonna alkaloid that blocks acetylcholine at the postganglionic terminations of cholinergic bers in the autonomic nervous system Atropine does not inhibit Ach release When administered rapidly IV initial bradycardia may be observed due to vagal stimulation which is then followed by cholinergic blockade and tachycardia Atropine crosses bloodbrain barrier and placenta barrier Relaxation of iris sphincter muscle induces dilation of the pupils Therefore it is contraindicated in patient with glaucoma and synechia Works as antidote for overdose of cholinergic drugs such as neostigmine and edrophonium Works as antidote for organophosphate poisoning Typically used in the dose range of 002 7 004 mgkg IV IM SQ in dogs and cats but at a lower dose than 002 mgkg in large animals Duration is between 60 7 90 mins Rabbits and rats have atropinase in the liver so higher dose is needed in this species Glycopyrrolate RobinulV 02 mgml Large synthetic quartemary ammonium molecular size prevents easy crossing of the bloodbrain barrier and placental barrier Therefore it may be a better choice for CNS diseased patients or in pregnant animals than atropine The mechanism of action is similar to atropine and duration is longer lasting 90180 minutes Typical dose is in the range of 0005001 mgkg IV IM SQ Tachycardia is not as prevalent as seen with atropine and is less arrhythmogenic It is more expensive than atropine Clinical Notes Pharmacology amp Premedication A number of agents and techniques are available for premedicating animals but the choice would depend on the patient s physical state and the type of procedure It must be remembered numerous combinations exist depending on the circumstances and a good review of the major pharmacological features of the drugs as well as the important concerns for the fragile patient will help guide the practitioner in avoiding complications In animals under premedication that are still conscious anesthetic monitoring is not usually carried out to the standard applied in animals under general anesthesia see Anesthetic monitoring notes However vigilant monitoring of cardiopulmonary system is encouraged as often as possible in severely sedate or unconscious animals under the in uence of premedicants 100f 10 Veterina Sur e VMED 741 ANESTHESIA FOR PATIENTS WITH CARDIOVASCULAR IGFAGFG LyonL WM PhD DACVA Gncral communion Must enestnetses pruduce same degree ufcardmvascular depressmn less ebmty tn enmpensete m enesmetsennaueea depressmn 39 y nst h h t chm51 symptsms nfthexr disease tulerate anesthesxafauly well Cardl upulmunary Yhysmlugy lecture Effects st enestnetses an the cardlnvascular system are e 1m smnent st cal1 uuhz ens mhalants barbxturates alterauun at systemic vascular resxstance heart rate bland p e devexnpment nfmtracellular amdnsxs seenn Many dl erenttypes st cardluvascular disease as n dary tn respxratury depressmn ay be encnuntered cangemt een disease acqmred muse disease la e cardlumyupathy Ynmary guals st enestnetse managementquot these penentgmups are tn prevent hypuvulemxa n uverhydrau nn mxmmxze changes In Innkupy myncardlal cankacuhty Allmush 1 HI vmrinry sumWIv wen 7411 haunts Mm Clrdlavlscmln Inspirth n lmolnhs n Dinu Ways to support the cardiovascularly challenged patients Anesthesia 2 of 8 Patients with Cardiovascular Respiratory amp Gastrointestinal Disease Stabilize heart rate amp rhythm prior to anesthesia if possible Optimize cardiac function prior to anesthesia if possible Physical examination observe jugular distension pulsation palpate peripheral arterial pulse quality auscultate heart for assessing characters of pulsation Thorough cardiac evaluation prior to anesthesia ECG Doppler echocardiograph thoracic radiographs blood pressure measurement ultrasonography cardiac catheterization Laboratory evaluation PCV TP hemoglobin content arterial blood gases electrolytes Choose anesthetic agents that produce minimal cardiovascular changes and preferably have drugs of short duration of action or that are reversible Preanesthetics rely mostly on opioids benzodiazepines neuroleptanalgesic combination Anticholinergics are used judiciously Employ local anesthetic technique under sedation or even general anesthesia Induction propofol etomidate ketamine mask with inhalant Maintenance usually iso urane or sevo urane rapid recovery and less cardiovascular depression than halothane Monitor cardiovascular performance 0 ECG rate and rhythm o arterial blood pressure BP CO X SVR 0 central venous pressure preload Treat arrhythmias if they develop 0 significant VPC 7 lidocaine betablockers 0 significant bradycardia or bradyarrhythmias 7 glycopyrrolate atropine isoproterenol or temporary pace maker implant if medically nonresponsive Support inotropy with o adrenergic agonists dobutamine dopamine doepxamine ephedrine norepinephrine epinephrine o phophodiesterase inhibitor milrinone amrinone enoximone theophylline pentoxyfylline 0 calcium channel sensitizer levosimendan pimobendan digoxin 0 calcium glucagon O O Veterinary Surgery I VMED 7412 Case example Taylor Signalment 6 month old intact male Maltese of 2kg in bwt History presented for evaluation of inappetence ataxia weakness and exercise intolerance Significant physical exam ndings ataxia muscle weakness heart murmur Laboratory finding no abnormalities noted Thoracic radiographs enlarged heart shadow Echocardiographic ndings patent ductus arteriosus Presented for PDA surgical ligation Preanesthetic medication Anesthetic induction Maintenance of anesthesia Monitoring Postoperative care Goal amp Plan Maintain diastolic blood pressure avoid alpha blockers phenothiazine Action Neuroleptanalgeisa midazolam 02 mgkg IM oxymorphone 01 mgkg IM glycopyrrolate 001 mgkg IM Little change of blood pressure myocardial contractility Diazepam 01 mg IV Etomidate 3 mgkg IV to effect Avoid deep plane of anesthesia Little change ofblood pressure myocardial contractility support ventilation Sevo urane endtidal 20 7 24 uids 10 mlkghr dopamine l 5 mcgkgmin PRN controlled ventilation IPPV Oxygenation circulation ventilation temperature ECG pulse oximetry capnography invasive ABP CVP temp ABG Patent airway avoid hypothermia pain control Leave the ET tube as long as possible Forced warm air blanket pulse oximetry oxymorphone 005 mgkg IV Anesthes 3 of 8 Patients with Cardiovascular Respiratory amp Gastrointestinal Disease Veterinary Surgery I VMED 7412 Patients with Respiratory Diseases General considerations Many anesthetics produce some degree of respiratory depression The respiratory depression in combination with cardiovascular depression induced by most anesthetics decreases oxygen availability in the tissues Avoid heavy sedation that may induce excessive respiratory depression May have impairments of ventilation oxygenation or both Ventilatory impairment affects acidbase balance Oxygenation impairment affects oxygen delivery to tissues Respiratory disease may be divided into upper or lower airway disease 0 With upper airway disease the key is to bypass the upper airway obstruction as quickly as possible 0 With lower airway disease our ability to correctmanage the problem may be more limited Patient with poor compliance of the lung restrictive disease such as pulmonary edema brosis or effusion tend to adopt rapid shallow ventilatory pattern Patients with obstructive disease laryngeal paralysis collapsing trachea small airway disease tend to adopt a slower pattern with increased respiratory effort Inspiratory dyspnea is usually associated with extrathoracic and expiratory dyspnea with intrathoracic lesion in origin If pneumonic that can be treated with antibiotics and other supportive therapy delay the surgery as long as possible until the symptom gets fully resolved Preoperative evaluations Thorough physical exam and ancillary investigation Does the patient exhibit dyspnea at rest with exercisestress Is there stridor present Thorough auscultation of the lungs and trachea Radio graphs ultrasono graphy ECG Pulse oximetry Wright s respirometer and tight fitting face mask to assess respiratory volume tidal volume and minute ventilation Blood gas analysis Keys to anesthetic management Preoxygenate if possible Thoracocentesis if needed remove air uid blood etc Minimize stress Tranquilizationsedation with short acting or reversible drugs 0 Opioids resp Depression 0 Benzodiazepines o Phenothiazines 0 Avoid excessive doses so as to prevent resp depression Rapid induction with short acting anesthetic agents 0 Thiobarbiturates o Propofol o Etomidate o Ketamine Anesthes a 4 of 8 Veterinary Surgery I VMED 7412 Patients with Cardiovascular Respiratory amp Gastrointestinal Disease minimize oxygen de cit period by allowing rapid intubation and ventilation Control airway as quickly as possible begin positive pressure ventilation esp with lower airway disease Nitrous oxide may be better avoided o It diffuses into gaseous pocket and worsens symptoms such as pneumothorax o It reduces the inspiratory fraction of oxygen Monitoring ECG O 0 Pulse oximetry o P o Capnography O O 0 DJ Serial blood gas analysis Tidal volume and peak airway pressure thoracic compliance Temperature Recovery 0 Maintain ET tube in situ as long as possible 0 Postoperative pulse oximetry 0 Support ventilation as long as possible 0 Consider post anesthetic oxygen supplementation mask nasal catheter oxygen cage 0 Minimize stress judicious use of tranquilizerssedatives if needed 0 If acute respiratory obstruction occurs post extubation be prepared to reinduce anesthesia amp reintubate rapidly 0 Treat chest pain so as to facilitate better use of respiratory muscle Case example 1 Anesthesia 5 of 8 Patients with Cardiovascular Respiratory amp Gastrointestinal Disease Jake Signalment 1 year old intact male Labrador retriever History presented for evaluation anorexia listlessness of one week39s duration Significant physical exam findings tachypnea fever Laboratory finding elevated white blood cell count Thoracic radiographs pleural uid lung lobe collapse suspect lung lobe torsion Presented for anesthesia 327 for thoracic exploratory Preanesthetic management Anesthetic induction Maintenance of anesthesia Monitoring Postoperative care Veterinary Surgery I VMED 7412 Case example 2 quotMiss Genuinesquot Signalment 1 week old Quarter Horse lly History presented for choanal atresia Significant physical exam ndings normal neonatal foal except for nasal obstruction Laboratory finding normal Referring DVM had performed a tracheostomy shortly after birth Presented for anesthesia 46 for laser surgical correction of choanal atresia Preanesthetic management Anesthetic induction Maintenance of anesthesia Monitoring Postoperative care Anesthesia 6 of 8 Veterinary Surgery I VMED 7412 Patients with Cardiovascular Respiratory amp Gastrointestinal Disease Patients with Gastrointestinal Diseases General considerations Variety of disease processes Malabsorption Derangement of electrolytes acidbase status hypovolemia Preoperative stabilization of uid balance electrolyte balance important if possible Gastric dilitationlvolvulus GDV Surgical emergency Present with Respiratory compromise Cardiovascular compromise Cardiac dysrhythmias VPCs V tach tachycardia Hypotension Hypoxemia Acidbase disturbances If possible decompress stomach prior to anesthesia Large volumes of IV uids rapidly multiple large bore catheters at 4090 mlkg Acidbase evaluation helpful Monitor amp treat cardiac dysrhythmias as they present lidocaine usually first line of defense Anesthetic management 0 Preanesthetic opioids benzodiazepines 0 Induction rapid induction to gain control of airway quickly is preferable initiate positive pressure ventilation may be able to intubate w neuroleptanalgesic combination eg oxymorphone diazepam propofol preferred low dose thiopental may be used but cautiously potential for aggravating arrhythmias mask induction w iso uranesevo urane may be used but it is still slower 0 Maintenance iso uranesevo urane supplemental opioids eg oxymorphone hydromorphone fentanyl IV to reduce inhalant concentration IPPV usually needed Monitor cardiovascular system closely ECG Blood pressure ll 000000 0 O Anesthesia 7 of 8 Veterinary Surgery I VMED 7412 Patients with Cardiovascular Respiratory amp Gastrointestinal Disease Equine Colic One of our most common emergency surgical procedures Patients present in a variety of conditions from minimally to severely compromised Respiratory compromise Cardiovascular compromise Dehydration Hypotension Hypoxemia Electrolyte imbalances AcidBase disturbances Again stabilize if possible 0 Large volumes of uids IV rapidly multiple large bore catheters o Bicarbonate if acidotic 0 Pain management usually w alpha2 NSAID such as FluniXin meglumine Our current anesthetic protocol 0 Premedicate with xylazine butorphanol or xylazine Induce with diazepam ketamine or GGE ketamine Maintain with sevo urane Monitor invasive blood pressure ECG capnography serial blood gases and electrolytes Controlled ventilation IPPV Multiple IV lines for rapid uid administration Dobutamine or other positive inotropes to support BP and CO Calcium supplementation if hypocalcemic Colloids if TP lt 4 gdl Recovery often slow postoperative pain management should be considered 0000000 000 00000 Anesthesia 8 of 8 Veterinary Surgery I VMED 7412 Patients with Cardiovascular Respiratory amp Gastrointestinal Disease