Innocent Bystander or the Heel of Achilles.

Abstract

A 29-year-old male presented with palpitations without a documented tachycardia. ECG showed minimal preexcitation (Figure 1A), and echo confirmed a normal heart. At electrophysiological study in the drug-free state, the HV interval was 13 ms in sinus rhythm, and ventricular (V) activation in the right para-Hisian region preceded the surface delta wave (Figure 2A). Programmed stimulation from the septal right atrium (A) increased preexcitation at shorter S1–S2 intervals (below 600–360 ms), as evidenced by a subtle loss of physiological left-to-right ventricular (RV) septal activation (loss of r wave in V1 and q waves in I, aVL) and an increasingly negative HV interval. The S2–δ interval also prolonged with progressive preexcitation. Figures 1B and 2B show increased preexcitation elicited at an S1–S2 interval of 220 ms. Atrioventricular (AV) effective refractory period was shorter than 600–220 ms, and dual AV nodal physiology was absent. These features along with the existence of escape junctional preexcited beat (Figure 1C and 2C) supported the presence of a right-sided manifest nodoventricular (NV) pathway bypassing a portion of the AV node, and ruled against typical anterograde AV bypass tracts, Mahaim fibers from AV ring, and fasciculoventricular pathways. Figure 1. 12-lead ECGs during sinus rhythm ( A ), programmed extrastimulation from septal right atrium at S1–S1=600 ms and S1–S2=220 ms ( B ), and a junctional beat ( C ). Figure 2. Intracardiac electrograms during sinus rhythm ( A ), programmed extrastimulation from septal right atrium at S1–S1=600 ms and S1–S2=220 ms ( B ), and a junctional beat ( C ). C , His-potential (H) preceded the onset of ventricular and atrial activation, which was consistent with a junctional beat. Surface preexcitation and HV interval (13 ms) during the junctional beat were identical to that in sinus rhythm, suggesting the origin of the accessory pathway from a portion distal in the atrioventricular junction. Channels shown from top to bottom are surface ECG …