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

Abstract

Summary 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 CO2 (PaCO2). The maximal rate of right ventricular pressure increase (RVdP/ dtmax), respiratory rate, and arterial partial pressure of Ο2 (PaO2) decreased, whereas arterial 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 thiamylal increased heart rate and decreased systemic arterial and central venous blood pressures and systemic vascular resistance. Detomidine (10 μg/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/dtmax and PaO2 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 PacO2, but decrease RVdP/dtmax, respiratory rate, and PaO2. Further, the recovery process was improved by administration of acepromazine, detomidine, or xylazine, but not diazepam, as preanesthetic medication.