Emergency administration of ephedrine for general anesthesia-induced hypotension requires preemptive atropine in dogs but not cats

Objective: This study aimed to investigate the effects of IV ephedrine on heart rate (HR) and mean arterial pressure (MAP) in anesthetized hypotensive dogs and cats and whether the presence or absence of preemptive atropine could influence these effects. Methods: This single-center retrospective study collected the data of dogs and cats that received an initial single IV bolus of ephedrine during anesthesia. The cases were analyzed based on the species, presence or absence of preanesthetic atropine administration, and dose of ephedrine. Mean arterial pressure and HR before and after ephedrine administration; response rate to ephedrine (RRE), defined as the probability of MAP > 70 mm Hg after IV ephedrine administration, with/without atropine; and factors contributing to RRE were analyzed. Results: A total of 1,679 dogs and cats were analyzed. In dogs, IV ephedrine decreased the HR, but preanesthetic atropine prevented this decrease and increased the RRE. In cats, IV ephedrine did not decrease the HR and the RRE did not change significantly, regardless of preanesthetic atropine administration. Multivariate logistic regression analysis revealed that the significant factors contributing to RRE were MAP before ephedrine administration and atropine premedication in dogs and MAP before ephedrine administration in cats. Conclusions: In anesthetized dogs, preemptive atropine enhanced the blood pressure-increasing effect of ephedrine by preventing decreases in HR. In anesthetized cats, IV ephedrine with/without atropine increased MAP without decrease in HR. Clinical Relevance: When IV ephedrine is used for treating hypotension during anesthesia, preemptive atropine is recommended in dogs but has less clinical significance in cats.