BackgroundThe differential diagnosis of supraventricular tachycardia with concentric atrial activation usually requires the inducibility of sustained tachycardia and needs a complex and time-consuming electrophysiological evaluation. To develop a simple test to establish if ventriculoatrial conduction uses a posteroseptal accessory pathway or the normal conduction system, we compared the ventriculoatrial intervals during right ventricular pacing from apical and posterobasal sites.Methods and ResultsContinuous pacing was performed from an apical and a posterobasal right ventricular site in 34 patients with retrograde conduction over the normal conduction system (group A) and in 22 patients with conduction over a posteroseptal accessory pathway (group B). During apical pacing, ventriculoatrial intervals in group A (176±40 milliseconds) were not significantly different than those in group B (197±47 milliseconds,P= NS). During posterobasal pacing, group B patients had significantly shorter ventriculoatrial intervals than group A patients (158±46 versus 197±39 milliseconds,P< .01). The difference between the ventriculoatrial interval obtained during apical pacing and that obtained during posterobasal pacing (ventriculoatrial index) discriminated between the two groups without overlapping: It was positive in all group B patients (39±19; range, +10 to +70 milliseconds) and negative in all except two group A patients (−21±13; range, −50 to +5 milliseconds;P< .001).ConclusionsThis ventriculoatrial index can identify accurately and in the absence of tachycardia whether concentric retrograde conduction is proceeding over a posteroseptal accessory pathway or over the normal conduction system.