The Supernormal Phase of Atrioventricular Conduction

Abstract

Various manifestations of a supernormal phase of conduction in the atrioventricular (AV) junction were observed in the ecgs of 18 patients. Based on a detailed analysis of the 1st 6 cases and a reevaluation of 48 cases of the literature, the following conclusions were reached with regard to fundamental mechanisms Supernormal conduction is found only in association with depressed AV conduction. It is an unusual, but not extremely rare, manifestation of AV block that becomes apparent during an abnormally prolonged absolute or relative refractory period. In this respect it differs from supernormal excitability, which seems to operate in normal cardiac tissue after termination of the normal refractory phase. There are 3 types of manifestation of supernormal AV conduction: in type A, it involves 2 or more consecutive impulses propagated in the same (forward) direction; in type B, occasional supernormal forward conduction is induced by an impulse propagated in the opposite (retrograde) direction; in type C, supernormal forward conduction is induced by one retrograde impulse and then maintained for some time by repetitive (supernormal) forward conduction. Each of these 3 types of supernormal AV conduction is associated with some specific features: Type A causes "paradoxical" P-R shortening in the course of a Wenckebach period; this may precede a "dropped beat," or may prevent its occurrence and replace it by continued alternation of the P-R interval. Type B is characterized by the occurrence of sporadic "early" captures during incomplete AV dissociation; to explain this mechanism a region of unidirectional block in the AV junction must be postulated which, after traversion by a retrograde impulse, becomes a region of supernormal conduction for an appropriately timed atrial impulse. The characteristic manifestation of type C is sudden interruption of a constant AV conduction, when some irregularity occurs in the sequence of the atrial impulses; protracted ventricular asystole may follow until undisturbed AV conduction is resumed subsequent to the escape of a subsidiary pacemaker. The 3 types of supernormal AV conduction are closely linked to concealed forward and retrograde conduction. In cases of types B and C, retrograde impulse propagation through the region of unidirectional block may or may not remain concealed, but only when it is concealed does the presence of a supernormal phase of forward conduction become evident. Concealed forward conduction of atrial impulses stops, as a rule, at the site of unidirectional block, but it may come to a halt below it, after it has passed the block during its supernormal phase. Thus supernormal conduction may develop after complete or after concealed conduction of the preceding impulse and, in turn, may give rise to either complete or concealed conduction of the subsequent impulse. The application of the concept of supernormal conduction in conjunction with that of concealed conduction and of unidirectional block permits a satisfactory interpretation of some otherwise inexplicable features of AV block encountered in clinical electrocardiography.