Concealed conduction in accessory atrioventricular pathways: an important determinant of the expression of arrhythmias in patients with Wolff-Parkinson-White syndrome.

Concealed conduction into accessory atrioventricular pathways has been postulated to explain variability of R-R intervals during atrial fibrillation in patients with Wolff-Parkinson-White syndrome. We examined the occurrence of concealed conduction into atrioventricular pathways using extrastimulus techniques in 26 consecutive patients undergoing electrophysiologic studies for the Wolff-Parkinson-White syndrome. Anterograde pathway concealment was demonstrated (10 patients) by introducing a second atrial extrastimulus (A3) after block in the accessory pathway occurred following the first extrastimulus (A2). The apparent effective refractory period (ERP) of the atrioventricular pathway with A3 (after A2 blocked in the pathway), or ERPB, was always greater than the ERP of the atrioventricular pathway (505 +/- 100 vs 323 +/- 105 msec, mean +/- SD; p less than .001), a finding explained by concealment into the pathway by the blocked A2. A measure of the apparent prolongation of refractoriness due to anterograde concealment (delta ERPB), defined as the difference between ERP and ERPB at a given cycle length, was derived. The average R-R interval in atrial fibrillation correlated better with delta ERPB (r = .8, p less than .01) than with the ERP (r = .6, p = NS), supporting the influence of anterograde atrioventricular pathway concealment in modulating the ventricular response during atrial fibrillation. By similar techniques, concealed retrograde conduction in the atrioventricular pathway could be demonstrated in 16 of 26 patients. In two of these patients "bystander" atrioventricular pathway conduction during orthodromic reciprocating tachycardia that did not involve the atrioventricular pathway did not occur, even though the ERP of the pathway should have permitted it, a finding readily explained by repetitive retrograde concealment into the atrioventricular pathway during tachycardia. Concealed conduction can be demonstrated in most patients with Wolff-Parkinson-White syndrome and is an important factor in the clinical expression of their arrhythmias.