CARDIAC ELECTROPHYSIOLOGIC MEASUREMENTS IN DOGS BEFORE AND AFTER INTRAVENOUS ADMINISTRATION OF ATROPINE AND PROPRANOLOL

Objectives-To assess baseline cardiac electrophysiologic (EP) measurem ents in dogs undergoing a clinically used anesthetic protocol, and to study the effects of IV administered atropine and propranolol on these EP variables. Animals-15 adult dogs with cardiac function within refe rence ranges, as assessed by physical examination, electrocardiography , and echocardiography. Procedure-13 cardiac EP variables were measure d in isofluorane-anesthetized dogs before and after IV administration of atropine and propranolol. Multipolar electrode catheters were posit ioned against the endocardium of the dorsal portion of the right atriu m, His bundle region, and right ventricular apex. Incremental pacing a nd pacing-extrastimulus techniques were used to obtain EP measurements of the sinoatrial node, atrioventricular node, and atrial and ventric ular myocardia in the control state and after IV administration of 0.0 4 mg of atropine and 0.2 mg of propranolol/kg of body weight. Results- Only the atrial effective refractory period changed significantly afte r muscarinic and beta-adrenergic receptor antagonism. Marked individua l variation in response to these agents, however, was apparent. Two do gs had substantial decreases in sinoatrial and/or atrioventricular nod al measurements, and 7 dogs had notable increases in atrioventricular nodal measurements. Conclusions-Cardiac EP measurements vary widely am ong clinically normal, isofluorane-anesthetized dogs. Individual dogs can have variable degrees of autonomic tone, which can be minimized by pharmacologic receptor antagonism. Clinical Relevance-Although effect s of receptor antagonism at clinically applicable dosages were not sig nificant for 12 of 13 measurements, withdrawal of vagal tone can induc e marked EP changes and may be important during a clinical study.