Reentrant ventricular arrhythmias in the late myocardial infarction period. II. Burst pacing versus multiple premature stimulation in the induction of reentry.

Isochronal maps of ventricular activation were analyzed in dogs 1 to 5 days after infarction utilizing a 64 channel multiplexer. Only dogs in which circus movement reentry could not be induced by a single premature stimulus were analyzed. Reentrant rhythms could be successfully induced equally by multiple (double or triple) premature stimuli and by burst pacing. Successive premature stimuli as well as successive beats during burst pacing resulted in progressively longer arcs of functional conduction block or slower circulating wave fronts, or both, that succeeded in reexciting myocardial zones on the proximal side of the arc of block to initiate reentry. However, for manifest reentry to be induced by burst pacing, the paced run had to be terminated after the beat that resulted in a critical degree of conduction delay. Otherwise, reentrant activation could be confined (concealed) by the subsequent paced wave front, which could also arrive earlier to the reentrant circuit zone of slow conduction resulting in block and interruption of reentry. Termination of a paced run after this beat would not result in reentry. If the paced run was extended past this beat, a new sequence of ventricular activation patterns characterized by progressively longer arcs of block or slower conduction, or both, developed again. The number of beats in a paced run that could initiate reentry varied with the cycle length of pacing, as well as in different experiments, and was difficult to standardize. It is therefore concluded that random burst pacing as a technique for induction of reentrant rhythms should probably be abandoned in favor of multiple premature stimulation.