Phenomenon of the Gap in Atrioventricular Conduction in the Human Heart

The phenomenon of the "gap in atrioventricular conduction" was studied in eight human subjects, using a catheter technique for recording electrical activity of the His bundle. Premature atrial stimuli were applied throughout the basic atrial cycle, either during sinus rhythm or atrial pacing. As the coupling interval between the basic (A 1 ) and premature (A 2 ) atrial depolarizations was decreased, a point was reached where A 2 was no longer conducted to the ventricles. The region of conduction block was localized distal to the His bundle. The interval between basic and premature His bundle depolarizations at which block occurred provided a value for the effective refractory period of the His-Purkinje system. If A 2 was then made to occur earlier in the basic cycle, a point was reached where conduction of the premature response to the ventricles resumed. When this occurred, conduction delay of A 2 in the atrioventricular node had increased sufficiently to allow for recovery of excitability of the His-Purkinje system (the interval between successive His bundle depolarizations was greater than the effective refractory period of the His-Purkinje system). The gap phenomenon could be abolished by decreasing the basic cycle length or by beta-receptor blockade, both of which prevented conduction block of A 2 in the His-Purkinje system.