ABSTRACTIn this study we sought to determine whether characteristics of ventricular-induced atrial preexcitation during reciprocating tachycardia could help differentiate atrioventricular (AV) nodal reentry from orthodromic AV reentry using an accessory pathway and to identify the site of accessory pathways in patients with Wolff-Parkinson-White syndrome. Fifty-five patients with orthodromic AV reciprocating tachycardia and 22 patients with AV nodal reentrant tachycardia were studied with standard electrophysiologic techniques. There were 24 left free wall, 23 posterior septal, seven anterior septal, and one right free wall accessory pathways. Progressively premature right ventricular complexes (V2) were introduced during reciprocating tachycardia (V1V1). The VIV, interval during tachycardia minus the longest VIV2 at which atrial preexcitation occurred defined a preexcitation index (PI). Atrial preexcitation occurred in 49 of 55 (89%) patients with AV reentry compared with only three of 22 (14%) patients with AV nodal reentry (p < .001). In the three patients with AV nodal reentry who demonstrated atrial preexcitation, the PI was distinct from that of the septal pathways and was in the upper range of values for left free wall pathways. The percentage of tachycardias demonstrating atrial preexcitation was not different between the free wall and septal pathways, but His bundle activation was visible at the time of atrial preexcitation in only six of 17 (35%) left free wall compared with 13 of 16 (81%) posterior septal and seven of seven (100%) anterior septal pathways (p < .05 free wall vs posterior or anterior septal). Mean PI was 88, 38, and 17 msec for left free wall, posterior septal, and anterior septal pathways, respectively; all values were significantly different from each other. PI differentiated septal from left free wall pathways in 37 of 48 (77%) patients; a PI of 75 msec or greater occurred only with left free wall pathways and a PI under 45 msec only with septal pathways. Thus characteristics of ventricular-induced atrial preexcitation during reciprocating tachycardia help differentiate between AV nodal reentry and AV reentry and between left free wall and septal accessory pathways. Circulation 74, No. 493–500, 1986.