Changes in the retrograde conduction time (ventriculoatrial [VA]) interval during functional bundle branch block (BBB) have been used to separate septal from free wall accessory pathways (APs), but different values of the VA interval prolongation (ΔVA) have been described in different reports. A total of 95 patients with single nondecremental APs who developed BBB during atrioventricular reentrant tachycardia were studied. Free wall APs were found in 60 patients, and 35 had septal APs. For patients with free wall APs, complete and incomplete BBB ipsilateral to the atrial insertion site of APs were observed in 39 of 60 patients (65%) and 31 of 60 patients (52%), respectively. For patients who had both complete (QRS ≥120 ms) and incomplete (QRS <120 ms) BBB during atrioventricular reentrant tachycardia, ΔVA for patients with complete BBB was significantly greater than in those with incomplete BBB, 59 ± 19 ms versus 30 ± 11 ms, p <0.001. For patients with septal APs and complete and incomplete BBB during tachycardia, the mean ΔVA for those with complete BBB was 31 ± 20 ms and was significantly longer than in patients with incomplete BBB (14 ± 6 ms), p <0.001. There was no significant difference in ΔVA between those with free wall APs and incomplete BBB compared with those with septal APs and complete BBB. The criteria of QRS ≥120 ms associated with ΔVA ≥40 ms served to best separate free wall from septal APs with a sensitivity of 88% and a specificity of 89%. Left anterior fascicular block was associated with marked lengthening of ΔVA for those with left free wall APs, whereas a left posterior fascicular block pattern resulted in a marked increase in the ΔVA for patients with posteroseptal APs. In the absence of fascicular block patterns, a ΔVA ≥40 ms provides strong evidence of a free wall AP, with a sensitivity of 95% and a specificity of 100%. The left posterior fascicle appears to provide predominant innervation of the posterior septum. Copyright (C) 2000 Excerpta Medica Inc.