Fascicular premature beats with narrow QRS in the presence of bundle-branch block.

The electrocardiogram of a sixty-two-year-old male revealed right bundle-branch block, left anterior superior fascicular block, and right precordial Q waves consistent with anteroseptal myocardial infarction. Premature beats with narrow QRS (100 ms) occurred intermittently; coupling intervals varied between 560-600 ms. His bundle electrogram confirmed the fascicular origin of the premature beats. During atrial prematuring studies, it was shown that an appropriately timed premature beat could prolong infra-His conduction time, but QRS normalization was not achieved. The possible electrophysiologic mechanisms that could result in a normal QRS complex with fascicular premature beats in this patient are discussed; a new mechanism is suggested.