To the Editor—Supernormal conduction or masked bundle branch block?

Abstract

Carbone et al present an interesting case report in which both right bundle branch block (RBBB) and left bundle branch block (LBBB) aberrancy are seen during different initiations of an atrial tachycardia. They postulate that this is caused by phase 3–dependent block in the left bundle branch (LBB; long-short coupling interval) resulting in RBBB aberration with a prolonged PR interval, followed by intermittent supernormal conduction over the right bundle branch (RBB) resulting in LBBB aberration and a nearnormal PR interval. The existence of true supernormal conduction in the intact human heart has not been shown conclusively, however, and before invoking this phenomenon, alternative physiological mechanisms can be considered. In Figure 3, impulse 1 conducts with an LBBB morphology and a PR interval of 220 ms. Carbone et al propose that conduction is entirely blocked in the LBB at this time, when it may simply be significantly delayed by the premature atrial impulse. Impulse 2 (Figure 3) occurs slightly earlier and may block completely in the RBB and unmask the slowly conducting LBB, leading to a longer PR interval (340 ms) and an RBBB morphology. The principle of supernormal conduction does not need to be invoked because there is delayed conduction in the LBB with both impulse 1 and 2; it is simply masked by greater delay in the RBB after