The influence of butorphanol dose on characteristics of xylazine-butorphanol-propofol anesthesia in horses at altitude

Objective: To characterize behavioral and physiological responses to short-term, unsupplemented intravenous (IV) anesthesia in healthy horses at high altitude (2240 m), and to test the hypothesis that the dose of butorphanol modifies the response of the horse to propofol anesthesia following xylazine pre-medication. Study design: Randomized prospective butorphanol dose cross-over experimental design. Animals: Eight healthy horses, 13 ± 6 (mean ± SD) years of age, and weighing 523 ± 26 kg. Methods: Each horse was anesthetized three times with at least 3 weeks between each anesthesia. After collecting pre-drug data, xylazine (0.5 mg kg^-1) was given IV. Five minutes later butorphanol was given IV according to a randomized order of three doses: 0.025, 0.05 and 0.075 mg kg^-1. Five minutes later, anesthesia was induced with propofol, 2 mg kg^-1 IV. Data on heart rate (HR) and respiratory rate (f_r), mean arterial blood pressure, P_aO₂, P_aCO₂ and pH_a were collected before, during and for 60 minutes following anesthesia, and quality of induction and recovery was scored. Results: The pre-drug values for the three butorphanol groups did not differ. The combined pre-drug values from the 24 studies were HR, 33 ± 7 beats minute^-1; f_r, 11 ± 3 breaths minute^-1; P_aO₂, 67 ± 7 mmHg; P_aCO₂, 36 ± 4 mmHg; and pH_a, 7.42 ± 0.04. Five minutes after anesthetic induction P_aO ₂ decreased and P_aCO₂ increased 14.5 ± 7.7 and 5.1 ± 4.9 mmHg, respectively, but returned to pre-drug levels within 15 minutes of anesthetic recovery. There were no significant butorphanol dose-related differences in physiological results, anesthetic induction and recovery quality scores or recovery time. Conclusions and clinical relevance: Dose of butorphanol did not markedly influence study results. Notably, low P_aO₂ values related to geographic location of study and general anesthesia indicates a narrow margin of error for hypoxemia-related complications in anesthetized horses breathing unsupplemented air at high altitude.