Effect of concealed anterograde impulse penetration on retrograde atrioventricular nodal conduction in man

The manner in which concealed anterograde impulse penetration may affect retrograde atrioventricular nodal conduction was studied systematically in 12 patients with intact ventriculoatrial (VA) conduction. After the last beat of a basic atrial drive (A1), an extrastimulus (A2) was introduced 20 msec inside the effective refractory period of the atrioventricular node. A ventricular extrastimulus (Vp) was then introduced at a progressively shorter A1Vp coupling interval both in the presence (method I) and absence (method II) of A2. In two patients, Vp was never conducted retrogradely to the atria with method I despite the presence of VA conduction during method II. In the remaining 10 patients, the VpAp interval was longer with method I vs method II; moreover, retrograde block of Vp ultimately occurred at a mean A2Vp coupling interval of 359 +/- 153 msec (range 190 to 540 msec) during method I despite the persistence of VA conduction during method II at comparable A1Vp coupling intervals. Before onset of retrograde block in method I, the VpAp curve took one of the following three forms: (1) crescendo, a progressively increasing VpAp interval; (2) flat, a constant VpAp interval, (four cases); or (3) discontinuous, a marked jump in the VpAp interval before the onset of retrograde block (two cases). Our findings may serve to elucidate some poorly understood electrophysiologic phenomena.