A study in intracardiac conduction. 'Normalization' of bundle-branch block by fusion with ventricular escape beats.

A ventricular fusion beat reflects the simultaneous activation of the ventricles by two impulses usually a sinus impulse and an ectopic ventricular impulse each activating part of the ventricles. The resulting QRS complex has a configuration that is in between that of the 'pure' uncomplicated sinus beat, and the 'pure' uncomplicated ectopic ventricular beat. If, however, the sinus beat is conducted with a bundle-branch block pattern, and it forms a fusion beat with a ventricular ectopic impulse originating in the same ventricle as the bundle-branch block, the bundle-branch block pattern is 'normalized'. This is because the=-ectopic ventricular impulse activates the region beyond the block, and the simultaneous activation of the ventricles by both impulses results in normal or near-normal activation (see Fig. 3A). This normalization of the bundlebranch block pattern has been produced experimentally by fusion with end-diastolic ventricular extrasystoles (Bisteni et al., I960). The phenomenon has also been observed clinically when left ventricular end-diastolic extrasystoles occurred in bigeminal rhythm during sinus rhythm with left bundle-branch block (Schamroth and Chesler, 1I962). Here, the resulting normalization of alternate QRS complexes simulated 2: I left bundle-branch block.