The effect of variable retrograde penetration on dual AV nodal pathways: observations before and after slow pathway ablation LDD.

Retrograde VA conduction is usually over the fast pathway and rarely over the slow pathway in patients with dual AV nodal pathways. It is unknown whether this apparent unidirectional conduction of the slow pathway is due to the lack of its retrograde conducting ability or the result of concealment. The effect of variable retrograde AV nodal penetration on antegrade AV nodal conduction was determined in patients with typical AV nodal reentrant tachycardia before and after the slow pathway ablation. Variable retrograde penetration was produced by delivering a ventricular extrastimulus simultaneously with (VE-0), 50 ms after (VE-50), or 100 ms after (VE-100) the last basic atrial stimulus, while atrial extrastimuli were used to determine changes of anterograde AV nodal effective refractory period (ERP) and A-H interval. The AV nodal functions measured without the ventricular extrastimuli served as the baseline. Although the mean slow pathway ERP was not significantly different among the different stimulation protocols, a significant shortening of the slow pathway conduction time (A-H from 348 +/- 60 to 324 +/- 119 ms, P < 0.05) was observed with upper level retrograde penetration of the AV node (VE-0). This facilitating effect became a prolonging effect when the retrograde penetration level moved to the lower level (VE-100, A-H from 324 +/- 119 to 366 +/- 122 ms, P < 0.05). The fast pathway ERP shortened with an upper level penetration (VE-0) but tended to prolong with a lower level retrograde-penetration (VE-100) both before and after the slow pathway ablation (preablation, from 348 +/- 143 of the baseline to 302 +/- 114 to 360 +/- 143 ms, P < 0.05; postablation, from 314 +/- 101 of the baseline to 274 +/- 118 to 361 +/- 143 ms, P < 0.05). The mean A2-H2 interval of the slow pathway was significantly shorter than the baseline (350 +/- 44 ms) with VE-0 (249 +/- 48 ms, P < 0.05) and VE-50 stimulation (285 +/- 82 ms, P < 0.05) but not with VE-100 stimulation (330 +/- 83 ms, P = NS). Before slow pathway ablation, the A2-H2 interval of the fast pathway at equal coupling intervals was shorter than the baseline (165 +/- 53 ms) with VE-0 (144 +/- 47 ms, P < 0.01) and VE-50 stimulation (152 +/- 43 ms, P < 0.05) but tended to be longer with VE-100 stimulation (175 +/- 47 ms, P = NS). After slow pathway ablation, the mean A2-H2 interval at the same coupling interval was shorter than the baseline (173 +/- 39 ms) with VE-0 (139 +/- 35 ms, P < 0.05), VE-50 (153 +/- 32 ms, P = 0.05) but tended to be longer with VE-100 stimulation (178 +/- 49 ms, P = NS). We conclude that: (1) concealed retrograde conduction can be demonstrated in both the slow and the fast AV nodal pathways; and (2) concealed retrograde conduction may either shorten or prolong anterograde refractoriness and conduction time, depending on the level of retrograde penetration.