Determination of the minimum alveolar concentration (MAC) and cardiopulmonary effects of sevoflurane in sheep.

Objective: To determine sevoflurane's minimum alveolar concentration (MAC _SEVO ) and its cardiopulmonary effects in sheep.
Study Design: Prospective experimental study.
Animals: A group of 10 female nonpregnant Sardinian milk sheep.
Methods: Anesthesia was induced in each sheep twice with sevoflurane in oxygen. After a 30 minute equilibration at end-tidal sevoflurane concentration (Fe'Sevo) of 2.8%, an electrical stimulus (5 Hz/1 ms/50 mA) was applied to the right thoracic limb for 1 minute or until gross purposeful movement occurred. The Fe'Sevo was then changed using a 0.2% up-and-down protocol, dependent on whether or not the response was positive, and then noxious stimulation was repeated. The MAC _SEVO was defined as the mean Fe'Sevo between that allowing purposeful movement and that not. The group of 10 sheep were re-anesthetized and MAC _SEVO was re-determined. Thereafter, Fe'Sevo was maintained for 15 minutes each at concentrations corresponding to 1.0, 1.3, 1.6, 1.9 and 0.75 MAC _SEVO multiples, and cardiopulmonary, blood gas, acid-base variables and plasma electrolytes were determined. Also, time to induction of anesthesia, extubation and recovery were recorded.
Results: The mean ± standard deviation of the MAC _SEVO was 2.74 ± 0.38%. Median (interquartile range) time to intubation was 3.13 (2.98-3.33) minutes, time to extubation was 6.85 ± 2.65 minutes and time to recovery was 13.4 ± 5.2 minutes. With increasing Fe'Sevo, arterial blood pressures progressively decreased as did minute ventilation, which in turn caused end-tidal carbon dioxide, arterial partial pressure of carbon dioxide and bicarbonate values to steadily increase without significantly affecting arterial partial pressure of oxygen.
Conclusions and Clinical Relevance: The reported MAC _SEVO agrees with published data in this and other species. Administration of sevoflurane in sheep caused marked hemodynamic and respiratory depression, but soon after turning off the vaporizer, sheep could be extubated and recovered rapidly and event-free.
(Copyright © 2018 Association of Veterinary Anaesthetists and American College of Veterinary Anesthesia and Analgesia. Published by Elsevier Ltd. All rights reserved.)