Effects of preemptive atropine administration on incidence of medetomidine-induced bradycardia in dogs

Objective-To determine the cardiorespiratory effects of preemptive atropine administration in dogs sedated with medetomidine. Design-Randomized crossover trial. Animals-12 healthy adult dogs. Procedures-Dogs underwent 6 treatments. Each treatment consisted of adminis tration of atropine (0.04 mg/kg [0.018 mg/lb] of body weight, IM) or saline solution (0.9% NaCl, 1 ml, IM) and administration of medetomidine (10, 20, or 40 mug/kg [4.5, 9.1, or 18.2 mug/lb], IM) 10 minutes later. Treatments were administered in random order, with a minimum of 1 week between treatme nts. Cardiorespiratory effects before and after atropine and medetomidine a dministration were assessed. Duration of lateral recumbency and quality of sedation and recovery were assessed. Results-Bradycardia (heart rate < 60 beats/min) was seen in all dogs when s aline solution was administered followed by medetomidine, and the dose of m edetomidine was not associated with severity or frequency of bradycardia or second-degree heart block. However, a medetomidine dose-dependent increase in mean and diastolic blood pressures was observed, regardless of whether dogs received saline solution or atropine. Preemptive atropine administrati on effectively prevented bradycardia and second-degree heart block but indu ced pulsus alternans and hypertension. The protective effects of atropine a gainst bradycardia lasted 50 minutes. Blood gas values were within referenc e limits during ail treatments and were not significantly different from ba seline values. Higher doses of medetomidine resulted in a longer duration o f lateral recumbency. Conclusions and Clinical Relevance-Preemptive administration of atropine in dogs sedated with medetomidine effectively prevents bradycardia for 50 min utes but induces hypertension and pulsus alternans.