Entrainment mapping in patients with sustained atrioventricular nodal reentrant tachycardia: insights into the sites of conduction slowing in the slow atrioventricular nodal pathway.

The inferoposterior region of the triangle of Koch is hypothesized to be the location of the atrial insertion of the slow atrioventricular (AV) nodal pathway. However, the actual site of conduction slowing in the slow AV nodal pathway is unknown. Entrainment mapping during AV nodal reentry can localize the reentrant pathway as follows: the AH interval measured from the mapping catheter = A'H (where A' is the exit site of the reentrant circuit) minus A'A (the conduction time from A' to the site of mapping); the SH interval during entrainment = SA' (the conduction time from stimulus into the reentry circuit) plus A'H. Thus, in all cases, the SH interval should be greater than or equal to the AH interval, and the deltaAH-SH should increase as distance and conduction time (SA' and A'A) from the reentry circuit increases. Fourteen patients with typical AV nodal reentry (cycle length 346 +/- 62 ms) and 1 with fast-slow (cycle length 430 ms) underwent activation and entrainment mapping from 8 to 12 sites in the triangle of Koch and coronary sinus. Pacing was performed at 2 to 3 mA above threshold, at a cycle length 10 ms shorter than tachycardia. A mapping site was defined as being in close proximity to the circuit if the deltaAH-SH was within 120% of the shortest 20th percentile deltaAH-SH value from all measured sites. In the 14 typical cases, 45 of 83 sites (54%) in the anatomic slow pathway region fulfilled criteria for close proximity to the reentry circuit compared with 13 of 50 sites (26%) outside of this region (p = 0.005). For these patients, the shortest SH interval measured from any entrainment site was 294 +/- 58 ms (89 +/- 10% of tachycardia cycle length, range 70% to 119%), indicating that the site of slow conduction in the slow pathway during AV nodal reentrant tachycardia was distal to all mapped sites. Thus, during typical AV nodal reentry, the "slow" pathway does not conduct slowly, and its insertion is located at or within the inferoposterior or midseptal regions in most cases.