Effects of inhalation anesthetic agents on response of horses to three hours of hypoxemia

Abstract

Objective - To study the effects of inhalation anesthetic agents on the response of horses to 3 hours of hypoxemia. Design - Controlled crossover study. Animals - Five healthy adult horses. Procedure - Horses were anesthetized twice: once with halothane, and once with isoflurane in O2. Anesthetized horses were positioned in left lateral recumbency. Constant conditions for the study began at 2 hours of anesthesia. A constant agent dose of 1.2 minimum alveolar concentration, Pa(O2) of 50 ± 5 mm of Hg, and Pa(CO2) of 45 ± 5 mm of Hg were maintained for 3 hours. Circulatory measurements were made at 0.5, 1, 2, and 3 hours of hypoxemia (anesthesia hours 2.5, 3, 4, and 5). Blood was collected from horses for biochemical analyses before anesthesia, within a few minutes after standing, and at 1, 2, 4, and 7 days after anesthesia. Results - Cardiac index was greater (P = 0.018) during isoflurane than halothane anesthesia. Cardiac index remained constant during the 3 hours of hypoxemia during halothane anesthesia, whereas it decreased from the baseline during isoflurane anesthesia. Marginally nonsignificant P values for an agent difference were detected for arterial O2 content (P = 0.051), and oxygen delivery (P = 0.057). Serum activities of aspartate transaminase (P = 0.050) and sorbitol dehydrogenase (P = 0.017) were higher in halothane-anesthetized horses than in isoflurane-anesthetized horses. Circulatory function was better in hypoxemic horses anesthetized with isoflurane than with halothane. Isoflurane resulted in less muscular injury in hypoxemia horses than did halothane anesthesia. Halothane anesthesia and hypoxemia were associated with hepatic insult. Conclusion - Isoflurane is better than halothane for hypoxemic horses.