Prospective evaluation of the length of the lower common pathway in the differential diagnosis of various forms of AV nodal reentrant tachycardia

The conduction time over the lower common pathway (LCP) in AVNRT can be assessed by subtracting the HA-interval during tachycardia (HA(t)) from that during ventricular pacing at exactly the same cycle length (HA(p)) (Delta HA = HA(p)-HA(t)). It has been suggested that H-A measurements may help in the differentiation, of Slow/Fast from Slow/Slow AVNRT. This study evaluated prospectively in 61 consecutive patients with AVNRT (43 +/- 15 y; 46 women, all with antegrade conduction during AVNRT over the slow pathway) how often a reliable measurement of the length of the LCP could be made, and in how far the results were concordant with mapping criteria for the differentiation of Slow/Fast from Slow:;Slow AVNRT. A new para-Hisian pacing technique (using only the His bundle catheter) was applied in all patients. Comparison of HA(t) and HA(p) was possible in 44 of the 61 patients (72%). In these 44 patients HA(p), was longer than HA(t) in 12 patients, indicating the presence of a LCP. All patients with Delta HA greater than or equal to 15 ms had earliest retrograde atrial activation in the posterior septum (Slow/Slow AVNRT; n = 6) or simultaneously in the anterior and posterior septum (n = I). On the other hand, 31 of the 32 patients without evidence of a substantial LCP (Delta HA less than or equal to 0) had typical Slow/Fast AVNRT. Moreover, although it appears logical for Slow/Fast AVNRT to have a shorter HA(t) than Slow/Slow AVNRT, an HA(p) of greater than or equal to 70 ms was a better discriminator between the two forms of AVNRT than any HA(t) value. Therefore, Delta HA greater than or equal to 15 ms !(sens. greater than or equal to 86%; spec. greater than or equal to 97%) or HA(p) greater than or equal to 70 ms(sens. = 100%; spec. 89%) were,, highly indicative for the Slow/Slow variant of AVNRT. Using a para-Hisian pacing technique, H-A measurements can be performed in 72% of AVNRT patients?. They can be used as an important tool in the differentiation of Slow/Fast and Slow/Slow AVNRT.