Intermittent failure of local conduction during ventricular tachycardia.

Forty-three patients with sustained ventricular tachycardia (VT) caused by prior myocardial infarction underwent intraoperative endocardial activation mapping during a total of 122 episodes of VT. Electrograms obtained during mapping were analyzed to determine the prevalence of local conduction failure during VT (defined as a portion of the local electrogram that did not repeat with every tachycardia cycle). Local conduction failure during VT was observed in 37 (86%) patients and 73 (65%) tachycardias. VT in which local conduction failure was observed were faster than VTs without local conduction failure (cycle length 315 vs 345 msec; p less than .05). Local conduction failure occurred most frequently at or near sites having the earliest recorded electrical activity during VT ("site of origin"). Twenty-three patients also had sinus rhythm endocardial mapping at the time of surgery. Areas with abnormal or fractionated electrograms in sinus rhythm were more likely to demonstrate local conduction failure in VT than areas with normal electrograms in sinus rhythm (16% vs 8%; p less than .01). Although the mechanism responsible for local conduction failure in VT is unclear, it is a common occurrence and is significant in that it can occasionally mimic "early" sites of endocardial activation, unless enough VT cycles are observed at a given site.