1 Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California, Davis, USA 2 Present address: B32-322-26-6 B Heiwa-Machi Kanazawa, Ishikawa 921-8105, Japan
*Corresponding author. E-mail: lsbarter@ucdavis.edu This study was presented in abstract form at the American College of Veterinary Anesthesiologists Annual Meeting in Orlando, Florida, October 2002.
Accepted for publication: August 19, 2003
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Abstract |
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Methods. Six female mongrel cats, age 2 yr (range 1.82.3) and mean weight 3.5 (SD 0.3) kg, were studied on three separate occasions over a 12-month period to determine the MAC of isoflurane, sevoflurane and desflurane. Induction of anaesthesia in a chamber was followed by orotracheal intubation and maintenance of anaesthesia with the inhaled agent in oxygen delivered via a non-rebreathing circuit. MAC was determined in triplicate using standard tail-clamp technique.
Results. Mean MAC values for isoflurane, sevoflurane and desflurane were 1.90 (SD 0.18), 3.41 (0.65) and 10.27 (1.06)%, respectively. Body temperature, systolic pressure and SpO2 recorded at the time of MAC determinations for isoflurane, sevoflurane and desflurane were 38.3 (0.3), 38.6 (0.1) and 38.3 (0.3)°C; 71.2 (8.3), 74.6 (15.9) and 88.0 (12.0) mmHg; 99.2 (1.1), 99.1 (1.3) and 99.4 (0.8)%, respectively. Both the anaesthetic agent and the individual cat had significant effects on MAC. Correlation coefficients for comparisons between desflurane and isoflurane, desflurane and sevoflurane, and sevoflurane and isoflurane were 0.90, 0.89 and 0.97, respectively.
Conclusions. These findings show that an individual has a consistent degree of sensitivity to a variety of inhaled anaesthetics, suggesting a genetic basis for sensitivity to inhaled anaesthetic effects.
Br J Anaesth 2004; 92: 2757
Keywords: anaesthetics volatile, desflurane; anaesthetics volatile, isoflurane; anaesthetics volatile, sevoflurane; cat; lung, minimum alveolar concentration
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Introduction |
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Methods and results |
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Mean MAC values for isoflurane, sevoflurane and desflurane were 1.90 (SD 0.18), 3.41 (0.65) and 10.27 (1.06)%. Oesophageal temperatures, systolic pressures and SpO2 values recorded at the time of MAC determination for isoflurane, sevoflurane and desflurane were 38.3 (0.3), 38.6 (0.1) and 38.3 (0.3)°C; 71.2 (8.3), 74.6 (15.9) and 88.0 (12.0) mmHg; 99.2 (1.1), 99.1 (1.3) and 99.4 (0.8)%, respectively. The anaesthetic agent and the individual cat had significant effects on MAC values (P=0.0001 and P=0.0185, respectively). Rank order of an individuals MAC values was similar for the three anaesthetic agents (Fig. 1). The correlation coefficients for isoflurane and sevoflurane, isoflurane and desflurane, and sevoflurane and desflurane were 0.97, 0.90 and 0.89, respectively. No correlation was found between age of cat and MAC for any agent.
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Comments |
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Each cat was anaesthetized with one agent at a time. However, for each agent, the order in which individual cats were anaesthetized was random. Lack of randomization of the inhaled agents may have affected MAC values for each agent for reasons such as variations in age or day length.5 Such factors should not, however, have consistently influenced the order of susceptibility of the cats to different agents.
Body temperature, oxygenation and systemic pressure have been shown to affect inhaled anaesthetic requirements.1 Cats in this study were normothermic at all times and not considered hypoxaemic based on SpO2 values of at least 95% at all times. Systolic pressure of cats in these studies averaged 77.9 mmHg over all MAC determination points. This may be considered to represent mild hypotension. No experimental studies have investigated the effects of hypotension on inhaled anaesthetic requirements in the cat. In dogs, reducing mean arterial pressure to 4050 mmHg reduced halothane MAC by 20%.6
In the cat, the MAC of isoflurane has been reported to be 1.282.06%; that for sevoflurane 2.58% and desflurane 9.79%.710 Our reported MAC values for isoflurane, sevoflurane and desflurane are 1.90%, 3.41% and 10.27%, respectively. Variation in MAC may result from differences in laboratory technique but the findings of our study suggest that individual variation should also be considered as a contributory factor. This is consistent with current literature suggesting a genetic basis for variations in anaesthetic requirements.24 The genetic background of the cats in this study is not known, other than they were not direct litter mates. The consistent degree of sensitivity to isoflurane, sevoflurane and desflurane displayed by these cats lends support to the hypothesis that these three agents have a similar mechanism of action.
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Acknowledgements |
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References |
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