Functional block in the intraventricular conduction system.

WHEN a normal impulse of supraventricular origin arrives at the His bundle, the entire specialized conduction system has had ample time to recover from the preceding response; the activation sequence of the ventricles and the resulting QRS complex are <'normal." When a lesion permanently damages one of the major divisions of the common bundle, the ventricular activation sequence is permanently abnormal. When the electrocardiographic pattern of bundle-branch block is intermittent or occasional, and is dependent upon the preceding cycle length, the cause is clearly not a destructive lesion of the conducting system; the block is "functional." Perhaps the most common situation in which functional block occurs is in atrial fibrillation; the pattern of irregular transmission across the atrioventricular (A-V) node provides the occasional opportunity for early activation of the His bundle after a relatively long preceding cycle. This combination of events often results in aberrant ventricular conduction,' for reasons which we shall demonstrate. Like any biologic system, cardiac tissue is nonhomogeneous. It is heterogeneous in the