The effect of intermittent bundle branch block on the coupling interval of ventricular premature depolarizations.

The mechanism of origin of ventricular premature depolarizations (VPDs) is explained by automaticity or reentry. We studied three patients with intermittent left bundle branch block (LBBB) and unifocal VPDs of right bundle branch block (RBBB) morphology (assumed left ventricular origin). We examined the effect of different types of intraventricular conduction on the coupling intervals of the VPDs. Specifically, we proposed that the coupling intervals of VPDs would be longer during LBBB conduction of sinus beats (ipsilateral to the ventricle of origin of VPDs) compared with the coupling intervals during normal intraventricular conduction. We found that the coupling intervals during LBBB were significantly longer than those during normal conduction in all three cases (patient 1, 596 +/- 7 vs 484 +/- 5 msec; patient 2, 639 +/- 9 vs 534 +/- 11 msec; patient 3, 444 +/- 4 vs 382 +/- 9 msec) (p less than 0.005). We also examined the length of the preceding RR intervals and counted sinus beats intervening between successive VPDs (S values). One case demonstrated S values suggestive of concealed bigeminy; the other cases had S values suggesting concealed bigeminy and its variants. We have demonstrated that some VPDs are dependent on an initiating sinus beat. This dependence on a preceding beat is consistent with both a reentrant mechanism or triggered automaticity.