Effect of low-dose atropine administration on dobutamine dose requirement in horses anesthetized with detomidine and halothane

Objective - To determine whether a low dose of atropine is associated with decreased requirement for cardiovascular supportive treatment in horses given detomidine prior to maintenance of general anesthesia with halothane. Animals - 3 groups of 10 healthy horses. Procedure - Detomidine (20 μg/kg of body weight, IM) was administered to all 30 horses. Then, 10 horses received atropine (0.006 mg/kg, IV) 1 hour after detomidine administration, 10 horses received atropine (0.012 mg/kg, IM) at the time of detomidine administration, and 10 horses served as a control group. Heart rate was measured prior to detomidine administration and at fixed intervals throughout anesthesia. The dobutamine infusion rate necessary to maintain mean arterial blood pressure between 70 and 80 mm of Hg was recorded. Systemic blood pressures, end-tidal halothane, end-tidal CO₂, and arterial blood gas tensions were measured at fixed intervals. Results - Mean heart rate was higher among horses receiving atropine IV or IM, compared with that in control horses. Horses that received atropine IV had higher systemic arterial blood pressure and required a lower dobutamine infusion rate than did horses of the other groups. Conclusion - Detomidine-treated, halothane-anesthetized horses given atropine IV required less dobutamine, compared with horses receiving or not receiving atropine IM. Complications, such as colic and dysrhythmias, from use of higher doses of atropine, were not observed at this lower dose of atropine. Clinical Relevance - IV administration of a low dose of atropine prior to induction of general anesthesia may result in improved blood pressure in horses that have received detomidine before anesthesia with halothane.