Effects of diazepam, acepromazine, detomidine, and xylazine on thiamylal anesthesia in horses

Abstract

The cardiorespiratory effects of thiamylal (10 mg/kg of body weight, IV) and the effects of preanesthetic medication with diazepam, acepromazine, detomidine, or xylazine administered prior to a thiamylal dosage of 6 mg/kg, IV, were evaluated in 6 adult horses. The quality of recovery from thiamylal anesthesia also was evaluated. Intravenous administration of thiamylal at a dosage of 10 mg/kg increased heart rate, systemic arterial, pulmonary artery, and central venous blood pressures, as well as cardiac output and arterial partial pressure of CO$D2 (Pa(CO$D2)). The maximal rate of right ventricular pressure increase (RVdP/dt(max)), respiratory rate, and arterial partial pressure of O$D2 (Pa(O$D2)) decreased, whereas atrial pH and systemic vascular resistance remained unchanged. Preanesthetic medication with diazepam prior to IV administration of thiamylal (6 mg/kg) did not change the pattern of this response, but diazepam did increase heart rate, cardiac output, and respiratory rate during the recovery period. Administration of acepromazine (0.1 mg/kg, IV) prior to administration of thiamyl increased heart rate and decreased systemic arterial and central venous blood pressures and systemic vascular resistance. Detomidine (10 _gmg/kg, IV), administered prior to thiamylal, decreased heart rate, cardiac output, and respiratory rate, and increased right atrial blood pressure. Administration of xylazine (0.5 and 1.0 mg/kg, IV) prior to thiamylal induced effects qualitatively similar to detomidine. Thiamylal decreased RVdP/dt(max) and Pa(O$D2) in horses that received diazepam, acepromazine, detomidine, or xylazine. Horses receiving 10 mg of thiamylal/kg, IV, or 0.1 mg of diazepam/kg, IV, prior to thiamylal (6 mg/kg, IV) had the most difficulty in attaining a standing position. Horses receiving 10 mg of thiamylal/kg, IV, or acepromazine, detomidine, or xylazine (1 mg/kg, IV) prior to thiamylal (6 mg/kg, IV) had the longest anesthesia time. Results indicated that bolus injections of thiamylal used to induce short-term anesthesia in horses increase heart rate, systemic arterial, pulmonary artery, and central venous pressures, as well as Pa(CO$D2), but decrease RVdP/dt(max), respiratory rate, and Pa(O$D2). Further, the recovery process was improved by administration of acepromazine, detomidine, or xylazine, but not diazepam, as preanesthetic medication.