CHARACTERIZATION OF CHRONOTROPIC AND DYSRHYTHMOGENIC EFFECTS OF ATROPINE IN DOGS WITH BRADYCARDIA

Objective-To characterize the magnitude, character, and time course of chronotropic and dysrhythmogenic responses of dogs with vagally media ted bradycardia to atropine sulfate. Design-Latin square design. Anima ls-Seven clinically normal adult mixed-breed dogs. Procedure-Vagally m ediated bradycardia was induced with morphine and fentanyl citrate. At ropine (0.02 mg/kg of body weight) was administered IV, SC, or IM. Ele ctrocardiograms were recorded continuously for 5 minutes before and fo r 35 minutes after atropine administration or until a sustained parasy mpatholytic response was observed. Data were digitized, analyzed indep endently for changes in atrial and ventricular rate, and compared betw een different routes of administration. Results-All dogs developed sec ond-degree atrioventricular (AV) block after IV administration of atro pine, and 71% of dogs developed AV block after SC or IM administration . The AV block arose and resolved more rapidly with IV administration than with SC or IM administration. The AV block was principally attrib utable to an increase in the atrial rate prior to increases in the ven tricular rate. Atropine, regardless of route of administration, potent iated baseline ventricular bradycardia in 62% of the experiments (mean heart rate decrease of 16 beats/min; decreased to < 20 beats/min in 2 dogs for less than or equal to 10 seconds). Duration of the bradycard ic potentiation was longer with SC administration (9.1 minutes, SC, vs 1.4 minutes, IV, and 4.6 minutes, IM). Parasympatholytic rate was hig her for IV than SC or IM administration (128 beats/min vs 92 beats/min and 101 beats/min). Two dogs given atropine SC failed to resolve the AV block and attain sinus rhythm. Conclusions-Administration of 0.02 m g of atropine/kg by IV, IM, and SC routes for vagally mediated bradyca rdia in dogs consistently induces AV block and occasional brief potent iation of ventricular bradycardia. Clinical Relevance-Parasympathomime tic effects occur and resolve most rapidly and consistently, and the s table parasympatholytic effect is of greatest magnitude after IV admin istration. Thus, vagally mediated bradycardia in clinically normal dog s appears to be best abolished by IV administration of atropine.