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 H A-interval during tachycardia (HAt) from that during ventricular pacing at exactly the same cycle length (HAp) (delta HA = HAp-HAt). 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 HAt and HAp was possible in 44 of the 61 patients (72%). In these 44 patients, HAp was longer than HAt in 12 patients, indicating the presence of a LCP. All patients with delta HA > or = 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 = 1). On the other hand, 31 of the 32 patients without evidence of a substantial LCP (delta HA < or = 0) had typical Slow/Fast AVNRT. Moreover, although it appears logical for Slow/Fast AVNRT to have a shorter HAt than Slow/Slow AVNRT, an HAp of > or = 70 ms was a better discriminator between the two forms of AVNRT than any HAt value. Therefore, delta HA > or = 15 ms (sens. > or = 86%; spec. > or = 97%) or HAp > or = 70 ms (sens. = 100%; spec. > or = 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.