New observations on the effects of atropine on the sinoatrial and atrioventricular nodes in man.

Previous observations of slowing of the heart rate after administration of atropine in doses smaller than 0.4 mg and recent reports of development of rhythm disorders in patients with acute myocardial infarction given atropine prompted us to evaluate systematically the effects of various doses of atropine (0.1 to 0.8 mg) on the response of the sinoatrial (S-A) and atrioventricular (A-V) nodes in healthy volunteers. The response of the S-A node to atropine was characteristically bimodal, slowing at smaller doses and accelerating at larger doses. In contrast, the A-V node showed acceleration of conduction in response to all doses of atropine used. A hypothesis based on current understanding of the electrophysiologic parameters governing impulse formation and impulse conduction is advanced to explain the apparent paradox in the S-A and A-V nodal responses to small doses of atropine. The results suggest the need for caution and continuous rhythm monitoring when giving atropine to patients with acute myocardial infarction.