Electrophysiologic demonstration of concealed conduction in anomalous atrioventricular bypass tracts

To demonstrate the occurrence of concealed conduction in anomalous atrioventricular (AV) bypass tracts, 11 patients were selected for study. Two had a right-sided and nine had a left-sided bypass tract. Electrode catheters were placed in the right atrium, coronary sinus, AV junction and right ventricle. After every eighth atrial or ventricular driving beat (A1or V1) at a constant cycle length, two successive atrial or ventricular premature beats (A2and A3or V2and V3) were delivered. The A1A2or V1V2interval was fixed at 30 ms greater than the effective refractory period of the atrium or right ventricle, but less than the effective refractory period of the bypass tract in the anterograde or retrograde direction. This allows A2or V2to capture the atrium or ventricle, but not conduct in the bypass tract. The A3or V3was delivered from late diastole with a progressively shorter A2A3or V2V3interval until atrial or ventricular refractoriness was encountered.In the anterograde direction, the presence of A2prevented A3conduction in the bypass tract despite A1A3intervals being longer than the anterograde effective refractory period of the bypass tract in 8 of the 11 patients. In the retrograde direction, the presence of V2prevented V3 conduction in the bypass tract despite V1V3, intervals being longer than the retrograde effective refractory period of the bypass tract in 3 of the 11 patients. Thus, using the technique of programmed electrical stimulation, concealed conduction in anomalous AV bypass tracts can be demonstrated in both anterograde and retrograde directions