Effects of the Prophylactic Administration of Atropine on the Ephedrine-Induced Cardiac Baroreceptor Reflex in Anesthetized Dogs

Objective: To confirm the reproducibility of the cardiac baroreceptor reflex (CBR) and bradycardia induced by ephedrine administration and to evaluate the safety of administering prophylactic atropine to prevent the CBR in anesthetized dogs. Design: Experimental, single-blind, crossover study. Setting: Veterinary university research facility. Animals: Six healthy Beagle dogs. Interventions: Dogs were anesthetized with propofol and maintained with isoflurane. After various biological monitors were attached, dogs received IV premedication with saline (saline–ephedrine [SE] group) or atropine (0.04 mg/kg; atropine–ephedrine [AE] group). Subsequently, ephedrine (0.1 mg/kg) was administered intravenously. Measurements and Main Results: Cardiovascular parameters (invasive arterial blood pressure, heart rate [HR], stroke volume index, and cardiac index) and immediate parasympathetic tone activity (PTAi) were recorded 15 min after premedication (baseline [BL]) and for 30 min after ephedrine administration. All data were recorded by the same blinded recorder. Consequently, the MAP in the SE and AE groups significantly increased after ephedrine administration. The HR in the SE group significantly decreased from BL; two dogs exhibited bradycardia (<50/min), and atrioventricular block was seen in one dog. In the AE group, the HR significantly increased from BL; no tachycardia was observed. The cardiac index in the SE group was maintained at BL, but the cardiac index in the AE group was significantly higher than that at BL during the experimental period. In both groups, PTAi rapidly decreased after ephedrine administration at 1 min. In the SE group, PTAi recovered to BL after 2 min. However, PTAi in the AE group remained significantly lower at 1–3 min than at BL and was significantly lower than that at 1–4 min in the SE group. Conclusions: We confirmed the CBR-induced bradycardia after ephedrine administration in anesthetized dogs, showing that atropine administration for prevention of ephedrine-induced bradycardia is safe with no adverse events.