Localization of an area of maximum refractoriness or “gate” in the ventricular specialized conduction system in man

Abstract

The area of maximum refractoriness (AMR) or “gate” in the ventricular specialized conduction system (VSCS) has not been previously demonstrated in the human A-V conduction system. Animal studies have shown an AMR proximal to the Purkinje-muscle junction. Seven patients were studied using His bundle electrograms. Single atrial premature depolarizations (APD) were introduced progressively earlier during the basic atrial cycle, until a point was reached where the APD no longer conducted to the ventricles. The region of block in these seven selected patients was distal to the bundle of His. Single ventricular depolarizations were then likewise introduced prematurely throughout mid-late diastole. In all seven patients, the ventricle could be depolarized earlier in the basic cycle (30 to 220 msec.) than the point at which block below the His occurred, localizing the AMR between the bundle of His and the ventricular muscle. A similar AMR was found during retrograde conduction. A mechanism analogous to the A-V nodal “gap” phenomenon exists in the VSCS. Thus, late premature atrial impulses may fail to conduct distal to the bundle of His while earlier beats encounter delay in an area of the VSCS proximal to the AMR, allowing propagation to the ventricle. This represents another mechanism of so-called supernormal conduction. The present study emphasizes simultaneous evaluation of the refractory state of specialized cardiac conducting tissue and muscle. Such an approach may be useful in future studies of drugs which depress subjunctional tissues.