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

Abstract

ObjectiveTo determine sevoflurane’s minimum alveolar concentration (MACSEVO) and its cardiopulmonary effects in sheep. Study designProspective experimental study. AnimalsA group of 10 female nonpregnant Sardinian milk sheep. MethodsAnesthesia was induced in each sheep twice with sevoflurane in oxygen. After a 30 minute equilibration at end-tidal sevoflurane concentration (FeʹSevo) 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 FeʹSevo 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 MACSEVO was defined as the mean FeʹSevo between that allowing purposeful movement and that not. The group of 10 sheep were re-anesthetized and MACSEVO was re-determined. Thereafter, FeʹSevo was maintained for 15 minutes each at concentrations corresponding to 1.0, 1.3, 1.6, 1.9 and 0.75 MACSEVO multiples, and cardiopulmonary, blood gas, acid-base variables and plasma electrolytes were determined. Also, time to induction of anesthesia, extubation and recovery were recorded. ResultsThe mean ± standard deviation of the MACSEVO 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 FeʹSevo, 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 relevanceThe reported MACSEVO 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.