Comparison of resetting and entrainment of uniform sustained ventricular tachycardia. Further insights into the characteristics of the excitable gap.

BACKGROUND Resetting and entrainment have both been used to characterize the electrophysiological properties of the reentrant circuit in ventricular tachycardia. Several entrainment studies have suggested that the circuit has decremental properties, because the return cycle increases at faster pacing rates. Resetting, however, demonstrates a fully excitable gap in the majority of tachycardias. METHODS AND RESULTS The response to resetting and overdrive pacing was analyzed in 18 ventricular tachycardias. Resetting demonstrated some duration of a fully excitable gap in 14 of 18 tachycardias. Overdrive pacing was performed at several cycle lengths with an incremental number of stimuli (1-15 beats) such that the first beat that interacted with the tachycardia (the nth beat) could be identified. The return cycles measured during resetting and the nth beat of pacing were identical (r = 0.99). At relatively long paced cycle lengths, paced beats after the nth beat resulted in a constant return cycle in most tachycardias with a fully excitable gap. At rapid paced cycle lengths, an increase in the return cycle from the nth to the nth + 1 beat was associated with progressive prolongation in the return cycle with each incremental paced beat until a longer equilibrium return cycle was reached or the tachycardia terminated in response to pacing. CONCLUSIONS We propose that the responses to resetting and overdrive pacing with or without entrainment appear to provide conflicting information about the characteristics of the circuit because they in fact measure entirely different electrophysiological parameters. The nth beat of pacing foreshortens the excitable gap to the extent that it arrives prematurely. Subsequent paced beats interact with an altered tachycardia circuit that has had less time to recover excitability. Resetting is the interaction of a single paced beat with the tachycardia and, as such, provides a more accurate assessment of the characteristics of the unaltered tachycardia circuit.