Simulated 2:1 left bundle-branch block. Normalization of the left bundle-branch block pattern by ventricular extrasystoles.

V ENTRICULAR extrasystoles can occur anywhere in the cardiac cycle. When they occur toward the end of diastole they may be synchronous with the next conducted impulse resulting in a fusion complex. We report a case of left bundle-branch block associated with end-diastolic ventricular extra-systoles that occur in bigeminal rhythm; this results in fusion complexes that apparently normalize the bundle-braneh block pattern. The resulting arrhythmia simulates 2 :1 left bundle-branch block.