Local slow potential preceding the surface QRS complex detected at the subvalvular mitral annulus in patients with a left-sided concealed accessory pathway. Incidence, electrophysiological characteristics and the possible mechanism, with demonstration of anterograde concealed conduction through the pathway.

AIM: We sought to evaluate the incidence and electrophysiological features of the local slow potential preceding the surface QRS complex (pre-QRS potential) which was detected more frequently at successful sites of catheter ablation of left parietal concealed accessory pathways, than at unsuccessful sites.

METHODS AND RESULTS: Thirty eight consecutive patients with a single left sided concealed accessory pathway underwent radiofrequency catheter ablation exclusively from the subvalvular mitral annulus. The local bipolar electrograms during sinus rhythm from the target sites were carefully analysed and the incidence of pre-QRS potentials was compared between successful and unsuccessful ablation sites. All ablation sessions attained a successful outcome with a total of 84 radiofrequency current applications (38 at successful sites, 46 at unsuccessful sites). The incidence of pre-QRS potentials (preceding by 10 ms or more) was 12/38 at successful sites (32%) and 1/46 at unsuccessful sites (2%) (P<0.001). The QV interval, defined as the interval between the upstroke of the QRS complex and the ventricular electrogram, including the pre-QRS potential, was -5.6+/-9.1 ms at successful sites, while it was 1.2+/-6.1 ms at unsuccessful sites (P<0.001). The pre-QRS potential disappeared during atrioventricular reciprocating tachycardia and right ventricular pacing, and was eliminated by successful ablation.

CONCLUSIONS: Detection of the pre-QRS potential was clinically relevant and could be distinguished from artifact. This potential may be caused by anterograde concealed conduction through the accessory pathways.