Overdrive stimulation of functional reentrant circuits causing ventricular tachycardia in the infarcted canine heart. Resetting and entrainment.

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Abstract
BACKGROUND Clinical electrophysiology studies have used, for the most part, models of anatomic reentrant circuits to explain entrainment of ventricular tachycardia. Our studies use activation maps to directly determine mechanisms of entrainment of functional circuits that cause tachycardia. METHODS AND RESULTS Electrograms were recorded from 192 sites on reentrant circuits in the epicardial border zone of canine myocardial infarcts during sustained ventricular tachycardia. Overdrive stimulation from different sites and at different cycle lengths was investigated. The reentrant circuits were shown to be functional, yet stimulated impulses could enter and repetitively reset the circuits (entrainment), demonstrating the presence of an excitable gap. Entrainment could occur from different stimulation sites with the stimulated impulses from each site activating the circuit with a different pattern. Entrainment, however, did not occur when the stimulated wave fronts obliterated the lines of functional block in the circuit. Fusion on the ECG occurred during entrainment when the stimulated impulses activated the ventricles concurrently with a previous stimulated impulse leaving the reentrant circuit at a different site. The first postpacing QRS was captured but not fused because it was caused by the last stimulated impulse emerging from the circuit. The first postpacing cycle length on the ECG was either equal to or longer than the overdrive cycle length depending on whether there was a fusion QRS during overdrive. The first postpacing cycle length at sites in the reentrant circuit equaled the pacing cycle length. At an appropriately short overdrive cycle length, stimulated impulses blocked in the circuit to terminate reentry. CONCLUSIONS Functional reentrant circuits causing ventricular tachycardia can be reset and entrained. Activation maps directly show the mechanisms.