Double atrial responses to a single ventricular impulse in long RP' tachycardia.

Double atrial responses (DARs) to a single ventricular impulse have been described in patients with long RP' tachycardia. To define the determinants for the occurrence of DARs, 8 cases with long RP' tachycardia were examined. The mechanism of long RP' tachycardia was the orthodromic atrioventricular reciprocating tachycardia (AVRT) involving a slow conducting concealed accessory pathway in 4 cases and uncommon (fast-slow) type of atrioventricular nodal reentrant tachycardia (AVNRT) in the other 4 cases. Programmed and rapid ventricular pacing was performed during sinus rhythm and also rapid ventricular pacing during tachycardia (i.e., entrainment). The retrograde effective refractory period (ERP) and the retrograde maximal 1:1 conduction rate of the fast and slow conducting pathways were examined. In 1 of the 4 cases with AVRT, DARs were observed during programmed and rapid ventricular pacing, performed during sinus rhythm and also during entrainment. In 1 of the 4 cases with AVNRT, DARs were observed only during entrainment. The determinants of DARs in cases with long RP' tachycardia were: (1) presence of two different retrogradely conducting pathways; (2) short ERP of the retrograde fast and slow conducting pathways and a short minimal pacing cycle length at which 1:1 ventriculoatrial conduction occurs via these pathways; (3) crucial conduction delay in the slow conducting pathway; and (4) preexisting antegrade unidirectional block in the slow conducting pathway or the antegrade block in the slow conducting pathway produced by collision with a previous retrograde impulse during entrainment.