Triggered ventricular rhythms in 1-day-old myocardial infarction in the dog.

Triggered activity developed in depolarized Purkinje fibers (maximum diastolic potential -59.3 +/- 9.9 mV) during superfusion with normal Tyrode's solution in 84% of subendocardial preparations from 1-day-old canine infarct. Triggered activity occurred when a delayed afterdepolarization attained threshold potential and spontaneously terminated after a subthreshold delayed afterdepolarization. Triggered activity was initiated either by stimulated beats or by the background slow Purkinje automaticity. When multiple stimulated beats were required, the amplitude of the afterdepolarization increased and the coupling interval decreased when the stimulation frequency and/or the number of stimulated beats increased. Varying degrees of entrance and exit block around sites of triggered activity were common, and some triggered activity manifested as a parasystolic rhythm. In preparations showing subthreshold delayed afterdepolarizations, both epinephrine (2.5 x 10(-6) M) and increasing Ca++o, (from 2.7 to 8.1 mM) increased the amplitude of afterdepolarizations and resulted in triggered activity. On the other hand, verapamil (2.2 x 10(-6) M) resulted in exit block around sites of triggered activity and/or completely suppressed afterdepolarizations and triggered activity. Isochronal mapping of endocardial activation during the initiation and perpetuation of triggered activity elucidated a focal site of origin from Purkinje fibers overlying the infarct and showed no evidence of a circus movement of excitation. The in vitro triggered activity may explain the spontaneous multiform ventricular rhythms seen in the intact heart.