Linking in accessory pathways. Functional loss of antegrade preexcitation. BACKGROUND. Concealed retrograde activation has been proposed as a mechanism for antegrade conduction block in the bundle branches and atrioventricular accessory pathways. We studied this hypothesis (linking) in 10 patients with the Wolff-Parkinson-White syndrome in whom antegrade preexcitation could be persistently blocked by overdrive atrial pacing. METHODS AND RESULTS. An atrial pacing protocol, with a decremental ramp followed by an incremental ramp, defined a range of atrial paced cycle lengths (linking window) associated with both persistent conduction and block in the accessory pathway. Within the limits of the linking window, the ability of an atrial impulse to conduct over the accessory pathway was dependent on the preceding state (i.e., conduction or block). The observed linking window ranged from 70 to 290 msec (mean, 185 +/- 68 msec) and closely approximated the measured delay in retrograde activation of the accessory pathway during persistent antegrade block. The mean antegrade effective refractory period of the accessory pathways was long (486 +/- 156 msec), and in each case, it exceeded the antegrade refractory period of the normal atrioventricular pathway. Critically timed premature ventricular extrastimuli, delivered while linking was maintained in the accessory pathway, were able to interrupt the linking and restore antegrade accessory pathway conduction. CONCLUSIONS. These observations suggest that accessory pathway linking is associated with bidirectional block in the accessory pathway. The ability to initiate linking (and the stability of the phenomenon) depends on a critical relation between antegrade accessory pathway refractoriness and the magnitude of retrograde accessory pathway activation delay. 