Characteristics of triggered-activity and delayed afterdepolarization in responses to the electrical stimulation

Delayed afterdepolarization (DAD) and triggered-activity (TA) were induced in dog and guinea pig papillary muscles in low K+, high Ca2+ solutions. A single or train stimuli with higher rate than basic rhythm produced TA or accelerated triggered automaticity. A bigeminal rhythm was also elicited by train stimulations. Amplitude of DAD showed dual dependence on cycle length of basic stimuli and became larger at the shorter (less than 350 msec) and at the intermediate (between 500-800 msec) cycle length. Incidence of TA also increased at these zones of cycle length. The coupling interval of TA had a direct relation to basic cycle length or coupling interval of premature stimuli, at certain ranges of intervals, but outside of these zones, inverse or no relation to the latter were found. The transient inward current in single ventricular myocytes revealed similar type of cycle length dependent changes to those of DAD and TA. Termination of TA by premature stimuli could not be obtained consistently at fixed zone of coupling intervals. These characteristics are somewhat different from those of reentry and can be used as differential signs between TA and reentry.