Actions of isoflurane and halothane in pregnant mares

Eighteen healthy, pregnant mares scheduled for laparotomy and uterine manipulation were randomly allotted to 2 equal groups. After IV administration of xylazine hydrochloride and thiamylal sodium, general anesthesia was maintained with halothane (HALO) or isoflurane (ISO) in oxygen. Results of cardiovascular measurements were similar with both inhalant anesthetics; mean arterial blood pressure was 79 and 82 mm of Hg with HALO and ISO, respectively. Respiratory rate decreased most with ISO (mean frequency was 4 and 9 breaths/min with ISO and HALO, respectively). Partial pressure of arterial CO2 was increased similarly with HALO and ISO. Partial pressure of arterial O2 varied greatly among mares and decreased with duration of use of both anesthetics. Recovery time from anesthesia was significantly (P < 0.05) shorter after use of ISO vs HALO. Minor superficial injuries were associated with recovery from both anesthetics (in 5 mares with ISO and in 1 mare with HALO). Physical signs of postanesthetic myopathy or vital-organ dysfunction were not associated with either agent.