The differential effects of ouabain on sinus, A-V nodal, His bundle, and idioventricular rhythms

The influence of ouabain on various spontaneous and experimentally-induced cardiac rhythms was determined. Twenty-three mongrel dogs, anesthetized with sodium pentobarbital (30 mg. per kilogram) were digitalized by the administration of ouabain, 10 μg per kilogram every 10 minutes, until toxicity developed. A Lead II electrocardiogram, His bundle electrogram, and aortic blood pressure were continuously monitored. During sinus rhythm (average HR: 152 ± 25 beats/min.) the rate of this pacemaker steadily declined in response to ouabain, then abruptly increased prior to the onset of toxicity (ventricular tachycardia). During A-V nodal rhythms, induced by crushing or cooling the S-A node in 11 dogs, the average heart rate was 94 ± 22 beats/min. Progressive digitalization consistently decreased the A-V nodal rate (average: -44 per cent) until toxicity developed. His bundle (average HR: 39 ± 9.0 beats/min.) or idioventricular rhythm (average HR: 42 ± 5.2 beats/min.) with complete heart block was induced by the local injection of 0.3 to 0.5 c.c. of formaldehyde into the A-V nodal or His bundle region, respectively. Progressive digitalization produced no significant change in the rate of the His bundle pacemaker, whereas ouabain consistently decreased the idioventricular rate (average: -50 per cent). We conclude that ouabain exerts distinctly different chronotropic effects on various cardiac areas exhibiting automaticity. These differential effects on A-V nodal and His bundle rhythms suggest the existence of at least two areas of latent pacemaker activity which are anatomically contiguous but functionally distinct.