EFFECTS OF DIAZEPAM, ACEPROMAZINE, DETOMIDINE, AND XYLAZINE ON THIAMYLAL ANESTHESIA IN HORSES

The cardiorespiratory effects of thiamylal (10 mg/kg of body weight, I V) and the effects of preanesthetic medication with diazepam, aceproma zine, detomidine, or xylazine administered prior to a thiamylal dosage of 6 mg/kg, IV, were evaluated in 6 adult horses. The quality of reco very from thiamylal anesthesia also was evaluated. Intravenous adminis tration of thiamylal at a dosage of 10 mg/kg increased heart rate, sys temic arterial, pulmonary artery, and central venous blood pressures, as well as cardiac output and arterial partial pressure of CO2 (Pa(CO2 )). The maximal rate of right ventricular pressure increase (RVdP/dt(m ax)), respiratory rate, and arterial partial pressure Of O2 (Pa(O2)) d ecreased, whereas arterial pH and systemic vascular resistance remaine d unchanged. Preanesthetic medication with diazepam prior to IV admini stration of thiamylal (6 mg/kg) did not change the pattern of this res ponse, but diazepam did increase heart rate, cardiac output, and respi ratory rate during the recovery period. Administration of acepromazine (0.1 mg/kg, IV) prior to administration of thiamylal increased heart rate and decreased systemic arterial and central venous blood pressure s and systemic vascular resistance. Detomidine (10 mug/kg, IV), admini stered prior to thiamylal, decreased heart rate, cardiac output, and r espiratory rate, and increased right atrial blood pressure. Administra tion Of xylazine (0.5 and 1.0 mg/kg, IV) prior to thiamylal induced ef fects qualitatively similar to detomidine. Thiamylal decreased RVdP/dt (max) and Pa(O2) in horses that received diazepam, acepromazine, detom idine, or xylazine. Horses receiving 10 mg of thiamylal/kg, IV, or 0.1 mg of diazepam/kg, IV, prior to thiamylal (6 mg/kg, IV) had the most difficulty in attaining a standing position. Horses receiving 10 mg of thiamylal/kg, IV, or acepromazine, detomidine, or xylazine (1 mg/kg, IV) prior to thiamylal (6 mg/kg, IV) had the longest anesthesia time. Results indicated that bolus injections of thiamylal used to induce sh ort-term anesthesia in horses increase heart rate, systemic arterial, pulmonary artery, and central venous pressures, as well as Pa(CO2), bu t decrease RVdP/dt(max), respiratory rate, and Pa(O2). Further, the re covery process was improved by administration of acepromazine, detomid ine, or xylazine, but not diazepam, as preanesthetic medication.