Chronotropic effects of ouabain and heart rate on canine atrium in vivo.

Abstract

Positive chronotropic effects from the combination of ouabain administration and rapid stimulation were sought in the atrium and ventricle of ten anesthetized vagotomized dogs with intact atrioventricular conduction during the infusion of ouabain (1 µg/kg min -1 ) until the onset of ventricular tachycardia. Sinus rhythm was interrupted every minute by 15 seconds of atrial pacing at either 150 or 200 beats/min. Positive chronotropic effects were recognized immediately after pacing as (1) early ventricular escape beats during vagal stimulation, (2) early atrial escape beats during vagal stimulation, or (3) blunting of the normal postpacing atrial depression. Positive chronotropic effects were rate dependent in both chambers, but they were seen less often in the atrium than in the ventricle. Early ventricular escape beats occurred in all ten dogs at a pacing rate of 200 beats/min when 69 ± 12% of the dose of ouabain required to induce ventricular tachycardia had been administered. They occurred in six of ten dogs at 150 beats/min after infusion of 81 ± 6% of the dose of ouabain which induces tachycardia. Early atrial escape beats did not occur when the hearts were paced at 150 beats/min. However, they appeared in three of ten dogs paced at 200 beats/min after infusion of 93 ± 5% of the dose of ouabain which induces tachycardia. Blunting of the normal postpacing atrial depression occurred in six of ten dogs at both 150 and 200 beats/ min at 100 ± 4% and 89 ± 5%, respectively, of the dose of ouabain required to induce ventricular tachycardia. Six additional dogs with heart block were paced at an atrial rate of 250 beats/min to facilitate positive chronotropic effects in the atrium with simultaneous ventricular pacing at 60 beats/min to delay the onset of positive chronotropic effects in the ventricle. This procedure allowed more consistent and relatively earlier detection of atrial positive chronotropic effects. In these experiments, early atrial escape beats occurred in five dogs after infusion of 80 ± 3% of the dose of ouabain required to induce ventricular tachycardia, and blunting of the normal postpacing atrial depression occurred in all six dogs after 78 ± 3% of the tachycardia-inducing dose of ouabain had been infused. It was concluded that toxic doses of ouabain induce positive chronotropic effects in the atrium of intact dogs. The effects occur later than do similar effects in the ventricle. Moreover, the positive chronotropic effects are heart rate dependent in both chambers and can be studied by atrial-ventricular; differential rate experiments.