Atrial tachycardia with Wenckebach atrioventricular conduction mechanism: What is the origin of the beat following the pause?

Abstract

Case presentation A 65-year-old woman was suffering from recurrent paroxysmal atrial fibrillation and was being treated with propafenone 150 mg tid. Continuous ECG recording strips of lead V1 are shown in Figure 1. The QRS complexes are associated into groups separated from each other by pauses. Although several P waves are not clearly discernible because they are superimposed on the T waves, analysis of the recordings reveals regular atrial activity with P-P intervals of 520 ms (Figure 2). Often the P waves are partially buried within a coincidental ventricular complex, causing subtle QRS alterations that result in relatively wide “pseudo-r” waves. This occurs with the fourth beat in the top strip of Figure 1, the 13th beat of the middle strip, and the third beat of the bottom strip. The P-R intervals undergo progressive prolongation until a ventricular pause occurs. Such a sequence indicates a Wenckebach atrioventricular (AV) nodal conduction mechanism. During the tachycardia, the morphology of the QRS complexes is typical for left bundle branch block (LBBB). However, the ventricular beats that follow a pause at times are relatively narrow but on other occasions show an LBBB configuration. These late QRS complexes are clearly dissociated from, and independent of, the near-synchronous P wave (Figures 1 and 2). What is the mechanism underlying the beats ending the pauses?