State of the Art Lecture
Dr. Jan Ilkiw
Diplomat, ACVIM (Anethesiology)
Professor, Department of Surgical and Radiological Sciences, University of California, Davis.
Improved Intraoperative Anesthetic Management in Cats
Published studies on morbidity and mortality associated with anesthesia in cats as well as the pharmacology of inhalant anesthetic agents will form the basis for discussing some of the challenges that cats present during the maintenance of anesthesia. Recent literature from studies in cats as well as other species, will be used to discuss alternative techniques and strategies that may improve not only the anesthetic management but also the morbidity and mortality associated with anesthesia in cats.
COMPLICATIONS ASSOCIATED WITH ANESTHESIA IN CATS
Five studies examining anesthetic management in veterinary practice have been published in the last decade. Veterinary anesthesiologists associated with university veterinary schools reported all studies. Four were a result of questionnaires sent to veterinary practices, one in the United Kingdom,(1) two in the United States,(2,3) and the other in Canada.(4) Three of these concerned small animal practice only,(1,3,4) while the other included all species.(2) The fifth concerned complications and mortality associated with anesthesia in cats and dogs undertaken in a university teaching hospital in the United States.(1)
The first study from the United Kingdom included 40,917 anesthetics in which 20,103 were undertaken in cats.(1) For analysis, dogs and cats were divided into two groups based on American Society of Anesthesiologists (ASA) classification of anesthetic risk. Cats categorized as ASA 1-2 had a mortality of 1 in 552 (0.18%), whereas cats categorized as ASA 3-5 had a mortality of one in 30 (3.33%). Overall, 29.6% of deaths occurred during induction, 38.9% during surgery and 31.5% postoperatively. In healthy cats, death was equally divided between the intraoperative and postoperative periods, whereas in sick cats, more deaths occurred during induction and surgery than postoperatively.
The first study undertaken in the United States (Vermont) was the result of 41 returned questionnaires comprising of an excess of 32,000 cats anesthetized per year by the respondents.(3) Complication rate was 54%, although there was no break down between dogs and cats. In this study, the mortality rate in cats was reported as 0.06%.
The study from Canada reported on 8,087 dogs and 8,702 cats, for a total of 16,878 animals.(4) For cats, the overall incidence of complications was 1.3% and cardiac arrest 0.10%. Cats in category 3, 4 or 5 were 5.28 and 21.60 times more likely than expected to suffer a complication or cardiac arrest, respectively. The most commonly reported complication was related to respiration, with respiratory depression or apnea noted intraoperatively in 0.3% of cats. Problems associated with intubation and laryngeal spasm or edema was reported in 0.15% of cats. Cardiac arrhythmias were reported almost as frequently as problems related to respiration.
The study undertaken in the university teaching hospital was set up prospectively.(5) A certain level of monitoring of physiologic variables was required and patients tended to be watched more closely, albeit in some cases by inexperienced students. In this study, 2,556 dogs and 683 cats, for a total of 3,239 cases, were anesthetized. The most common complication was hypotension (systolic arterial blood pressure < 80 mmHg or mean arterial blood pressure < 60 mmHg) with an incidence of 8.5% in cats. Cardiac arrhythmias occurred in 2% of cats, while hypercapnia was documented in < 1% of cats. Cardiac arrest occurred in 0.4% of cats and was more common in healthy cats undergoing ovariohysterectomy.
The last and most recent study included all species and reported techniques and concerns, rather than complications and morbidity. While castration and ovariohysterectomy were the most commonly performed procedures requiring anesthesia in cats (together 54%), dentistry accounted for 22% of cats requiring anesthesia. Intubation was carried out in cats by 76% of practitioners and all small animal practitioners used inhalants for maintenance.
These studies indicate the significant risks and mortality associated with anesthesia in cats, especially if they are ASA category 3-5. The use of inhalants for maintenance of anesthesia in veterinary practice is frequent and the most commonly reported complications relate to intubation and respiratory arrest. While halothane was the mainstay of inhalant anesthesia in these studies, isoflurane is now probably the most commonly administered inhalant. In veterinary practice, the use of monitoring equipment that is likely to warn of impending complications is rare. In those patients that were monitored appropriately, hypotension was the most frequent complication.
MAINTENANCE OF ANESTHESIA WITH INHALANT ANESTHETIC AGENTS
Cardiopulmonary depression induced by inhalants
Inhalants, while providing progressive depression of the central nervous system, also induce cardiovascular and respiratory depression in dose-dependent fashion. In cats, the depressant effects of the inhalants, halothane, isoflurane, sevoflurane and desflurane, on the cardiovascular and respiratory systems have been documented and these effects appear to be greater than in other species.(6-11) Using similar methodologies, three separate studies have documented the cardiopulmonary effects of halothane, isoflurane and desflurane during spontaneous and controlled ventilation in cats. All studies document a progressive decrease in mean arterial blood pressure and cardiac output as anesthetic dose increases. In fact, for surgical anesthesia, mean arterial blood pressure is often close to a level that may compromise blood flow to vital organs. Likewise, significant dose-dependent respiratory depression is reported and circulatory variables are further reduced when ventilation is controlled.
In our practice, hypotension is a more frequent complication of anesthesia in cats than dogs and is especially a problem intraoperatively in old or sick cats. Cardiovascular depression is likely responsible for the significant intraoperative and postoperative mortality in cats, especially those in ASA category 3-5. Hypotension was the most common complication reported in cats in the one study in which blood pressure was measured. Monitoring of blood pressure in practice would alert practitioners to the depressant effects of inhalant anesthetics on the cardiovascular system.
Attenuation of reflex responses
Anesthesia is a state in which, as a result of drug-induced unconsciousness, the patient neither perceives nor recalls noxious stimuli. However, noxious stimulation of surgery induces a variety of reflex responses that may be independently modified to the benefit of the patient. The body responds to noxious stimuli by somatic and autonomic responses. The concentrations required to eliminate a somatic motor response are higher than those necessary to induce unconsciousness and to eliminate perception of pain. Higher concentrations are required to suppress the respiratory response than the somatic motor response and autonomic responses, in the form of cardiovascular, sudomotor and metabolic responses, are the most difficult to suppress. With inhalants, increasing concentrations will usually block the respiratory response but may not block cardiovascular responses. Inhalant agents, in general, are unable to suppress any of the responses to stress, whereas balanced anesthetic techniques using systemic administration of potent opioids such as fentanyl or epidural administration of local anesthetics or opioids can partially or completely suppress hemodynamic and metabolic responses.
Modern anesthesia defines that the state of anesthesia as a spectrum of effects made up of separate pharmacologic actions. These pharmacologic actions include analgesia, anxiolysis, amnesia, unconsciousness, and suppression of somatic motor, cardiovascular, and hormonal responses to the stimulation of surgery. It is unrealistic to expect one drug to possess all these effects and modern anesthesia combines a number of drugs, each with specific effects, to safely create the state of anesthesia.
ALTERNATIVE ANESTHETIC TECHNIQUES
The term balanced anesthesia refers to the use of a mixture of drugs, such that the advantages of small amounts of drugs are utilized without having to contend with the disadvantages of large doses of any one drug. While balanced anesthetic techniques are the usual maintenance technique in people, their use in cats is rare. As well as providing specific pharmacologic actions, these techniques improve hemodynamics and provide improved intraoperative management.
Nitrous oxide/inhalant anesthesia
Since its introduction into human anesthesia, nitrous oxide has been a component of more general anesthetic techniques than any other single inhalant. Its widespread use resulted from many desirable properties including low blood gas solubility, limited cardiovascular and respiratory depression, and minimal toxicity. Its use in dogs and cats is controversial; some report minimal advantages in supplementing more potent anesthetic agents, while others agree that its analgesic properties enable excessive dosage of the more potent agents to be avoided and concurrent cardiopulmonary depression to be minimized.
In cats, 50 and 75% nitrous oxide have been reported to decrease halothane minimum alveolar concentration (MAC) by 19 and 31% respectively.(12) Recent studies with isoflurane, have not consistently documented a decrease in MAC.(13) Comparison of the cardiovascular effects of nitrous oxide in addition to 1.25 isoflurane MAC demonstrated an increase in mean arterial pressure, central venous pressure, mean pulmonary arterial pressure, packed cell volume, PaCO2, systemic vascular resistance index, and pulmonary vascular resistance index with 70% N2O, compared to 0 and, for most variables, 30%.(14) Cardiac index and stroke index did not change. In cats, unlike in dogs, addition of 70% N2O appears to act through vasoconstrictive rather than inotropic effects.
Opioid infusion/inhalant anesthesia
In human anesthesia, administration of high doses of opioids, either as primary or sole anesthetics, has become popular because they produce or promote stable hemodynamics both in the presence and absence of noxious stimuli. In fact, opioids are said to be superior to most, if not all other drugs in anesthesia, in achieving this goal. Thus, not only do they blunt and/or eliminate significant hemodynamic responses to noxious stimuli but also they induce minimal cardiac depression. Similar beneficial effects have been documented in cats, where administration during inhalant anesthesia has been shown to decrease the requirement for inhalant anesthesia and block autonomic responses to noxious stimuli resulting in better hemodynamic stability.(15,16)
The beneficial effect of opioids on MAC reduction, or lack of, was studied initially in cats by screening a number of drugs at both high and low doses. The reason for this approach was that opioid administration especially in high doses to cats might induce mania, which could increase MAC through central transmitter release. Low and high dosages of morphine, butorphanol, buprenorphine and U-50488H (kappa agonist) were found to induce significant MAC reductions of 14 and 27%, 19 and 22%, 13 and 15%, and 8 and 11%, respectively. A ceiling effect, within the two doses tested, was identified for buprenorphine and U-50488H, but not for morphine or butorphanol. Further studies identified maximal MAC reduction for alfentanil in cats as 35%.(15)
Significant beneficial cardiovascular effects were reported in cats when isoflurane alone was compared with an equipotent alfentanil/isoflurane MAC multiple. In that study, alfentanil was found to attenuate most of the hemodynamic and metabolic responses to a noxious stimulus.(16)
Epidural/spinal opioid/inhalant anesthesia
Opioids may be administered by injection into the epidural or subarachnoid (spinal) space to provide regional analgesia. Local anesthetic agents, such as lidocaine and bupivacaine, block sympathetic and pain fibers together with loss of sensation and motor function. Opioids, such as morphine, oxymorphone and fentanyl, selectively block pain conduction without interference with motor function. Epidural administration of morphine has been found to significantly reduce the inhalant anesthetic requirement in cats with a dose of 0.1 mg/kg morphine resulting in 31% isoflurane MAC reduction.(17) Although not reported, beneficial hemodynamic effects would be expected in cats at equipotent doses, since no change in mean arterial blood pressure was observed in non-anesthetized cats after epidural or spinal administration of morphine.
Transdermal opioid/inhalant anesthesia
A transdermal therapeutic system was developed for continuous delivery of the potent opioid fentanyl to provide ongoing analgesia in human patients with chronic pain. In veterinary practice, this method of administration has been adapted to provide postoperative analgesia.
In order to be effective during the postoperative period, patches must be placed prior to surgery so that plasma fentanyl levels are within the analgesic range immediately postoperatively. Thus, plasma fentanyl levels within the analgesic range will also be present during surgery and add a balanced component to standard inhalant techniques.
The isoflurane MAC reduction following application of 25 and 50 µg/hr patches to cats has been reported.(18) Patches were placed 24 hours prior to anesthesia and plasma fentanyl levels were measured from 0 to 6 days. Both 25 and 50 µg/hr patches reduced isoflurane MAC (17 and 18%, respectively) although the reduction was not significantly different between the two doses. Comparative hemodynamic and respiratory studies between inhalant alone and an equipotent transdermally administered fentanyl inhalant MAC multiple have not been reported.
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5. Gaynor JS, Dumlop CI, Wagner AE, et al. Complications and mortality associated with anesthesia in dogs and cats. JAAHA 1999;35:13.
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7. Ingwersen W, Allen G, Dyson DH et al. Cardiopulmonary effects of halothane/oxygen combination in healthy cats. Can J Vet Res 1988;52:386.
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10. Mc Murphy RM and Hodgson DS. Cardiopulmonary effects of desflurane in cats. Am J Vet Res 1996;57:367.
11. Hikasa Y, Kawanabe H, Takase K, et al. Comparisons of sevoflurane, isoflurane, and halothane anesthesia in spontaneously breathing cats. Vet Surg 1996;25:234.
12. Steffey EP, Howland D. Isoflurane potency in the dog and cat. Am J Vet Res 1977;38:1833.
13. Imai A, Ilkiw JE, Pypendop B. Nitrous oxide does not consistently reduce isoflurane requirement in cats. Submitted to Am Coll Vet Anesth Annual General Meeting.
14. Pypendop B, Ilkiw JE, Imai A, et al. Hemodynamic effects of nitrous oxide in isoflurane anesthetized cats. Submitted to Am Coll Vet Anesth Annual General Meeting.
15. Ilkiw JE, PJ Pascoe and LD Fisher. Effect of alfentanil on the minimum alveolar concentration of isoflurane in cats. Am J Vet Res 1997; 58:1274.
16. Pascoe PJ, JE Ilkiw and LD Fisher. Cardiovascular effects of equipotent isoflurane and alfentanil/isoflurane minimum alveolar concentration multiple in cats. Am J Vet Res 1997;58:1267.
17. Golder FJ, Pascoe PJ, Bailey CS, Ilkiw JE, Tripp LD. The effect of epidural morphine on the minimum alveolar concentration of isoflurane in cats. J Vet Anaes 1998;25:52.
18. Yackey M, Ilkiw JE, Pascoe PJ, et al. The effect of transdermally administered fentanyl on the minimum alveolar concentration of isoflurane in cats. Proc 6th Int Cong Vet Anaesth 1997:104.