Triggered Activity Induced By Pacing During Digitalis Intoxication: Observations During Programmed Electrical Stimulation in the Conscious Dog With Chronic Complete Atrioventricular Block

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Abstract
In isolated Purkinje fibers, digitalis intoxication induces triggered activity, which is based upon delayed afterdepolarizations. The characteristics of delayed afterdepolarizations have been studied systematically by programmed electrical stimulation. The present investigations were done to study the role of triggered activity during digitalis intoxication in the intact heart. For this purpose, a pacing protocol, similar to that used in experiments of isolated Purkinje fibers, was used. The experiments were done on conscious dogs with chronic complete atrioventricular block. Ventricular tachycardia was induced with digoxin IV 0.1 mg/kg/1‐1½ hr. The effect of programmed electrical stimulation on the first post‐pacing interval was determined during sustained ventricular tachycardia and, following its spontaneous termination during an episode when ectopic activity could only be induced by pacing. During sustained ventricular tachycardia there was a direct linear relation between the interstimulus interval of regular pacing and the first post‐pacing interval. During the episode when ectopic activity could only be induced by pacing, shortening of the post‐pacing interval resulted in biphasic behavior of the first post‐pacing interval. Pacing with interstimulus intervals of more than 400 ms induced a first post‐pacing interval equal to the interstimulus interval, whereas shorter interstimulus intervals induced a first post‐pacing interval twice the interstimulus interval. When during regular pacing only the last pacing interval was changed, a similar biphasic response resulted. When toxicity had almost subsided, ectopic activity could only be induced following short pacing intervals (200– 320 ms). Again, a direct linear relation was found between the pacing interval and the first post‐pacing interval. Our findings strongly suggest that at different levels of digitalis intoxication triggered activity is the underlying mechanism for the first post‐pacing QRS complex.