A quantitative evaluation of refractoriness within a reentrant circuit during ventricular tachycardia. Relation to termination.

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Abstract
Programmed ventricular stimuli introduced during sustained monomorphic ventricular tachycardia frequently reset the tachycardia, resulting in a less than fully compensatory pause. A resetting response curve is generated when the set of return cycles is evaluated as the function of the coupling intervals of the extrastimuli delivered during the ventricular tachycardia. If the stimulated wave front encounters tissue within the tachycardia circuit that is not fully recovered, interval-dependent conduction changes should occur producing an increasing resetting response pattern. We quantified the magnitude of this interval-dependent conduction slowing in 17 morphologically distinct ventricular tachycardias. The slope of the increasing limb of the resetting response curve was determined by linear regression analysis and ranged from -0.30 to -1.14 (mean +/- SD, 0.70 +/- 0.25). Seven of the 17 ventricular tachycardias (41%) terminated during introduction of ventricular extrastimuli. The slope of the resetting response pattern in those ventricular tachycardias that terminated were significantly steeper than in those that did not terminate (-0.85 +/- 0.15 versus -0.61 +/- 0.21, respectively, p = 0.025). Six of the seven ventricular tachycardias terminated with programmed ventricular stimuli had a slope steeper than -0.75, whereas only one of 10 ventricular tachycardias that did not terminate exceeded this value. In conclusion, the slope of the increasing portion of the resetting response curve correlates with ability to terminate uniform sustained ventricular tachycardia by timed extrastimuli. This slope is the quantification of the magnitude of interval-dependent conduction slowing. Additionally, tissue within the reentrant circuit displaying greater degrees of interval-dependent conduction slowing may also have relatively longer effective refractory periods.