Quantitative analysis of concealed conduction into accessory atrioventricular pathways in Wolff-Parkinson-White syndrome.

Concealed conduction is demonstrated to occur in an accessory AV pathway (AP). To test the hypothesis that anterograde and retrograde concealed conduction in the AP would have different characteristics, 35 consecutive patients with single APs were studied. The anterograde or retrograde ERP of the AP could be determined in 23 of those patients. Anterograde concealed conduction in the AP was assessed in the first 13 patients with retrograde AP conduction (6 APs with retrograde conduction only and 5 with both directions) (group A). Retrograde concealed conduction in the AP was evaluated in the remaining 10 patients with anterograde AP conduction (6 APs with anterograde conduction only and 4 with both directions) (group B). The concealed conduction in the AP was quantified by determining the ERP of the AP using a "probe" extrastimulus (Sp) introduced in the opposite chamber. The ERP was determined both during conventional extrastimulus (S1S2 method; ERPc) and during that with an Sp (S1SpS2 method; ERPp). The Sp was delivered before or after the last S1 with various S1Sp intervals. The ERPp was determined at each S1Sp interval. Three distinct patterns in concealed conduction in the AP were noted. In the first pattern, the ERPp was always shorter than the ERPc, whereas the reverse relation was noted in the second pattern. The third pattern showed a combination of the two. In group A, only the first pattern was noted. In group B, the first, second, and third patterns were noted in 4, 2, and 4 patients, respectively. The first pattern was noted only in septal APs and the second and third were seen only in left free-wall APs. The second pattern was seen in patients with retrograde AP conduction, whereas the third one was mainly noted in patients without retrograde AP conduction. These observations indicate that anterograde and retrograde concealed conduction in the AP have different characteristics. Shortening of the ERPp might be due to the "peeling back" phenomenon, and its lengthening might be caused by the presence of the inhomogeneous refractory periods of the AP.