Characterization of subforms of AV nodal reentrant tachycardia.

BACKGROUND: Different subforms of AV nodal reentrant tachycardia (AVNRT) have been described ("Slow/Fast", "Slow/Slow" and "Fast/Slow"). Our aim is to improve definition of these subforms, based on systematic evaluation, in a large cohort of patients, of the site of earliest atrial activation, timing intervals, and evidence for the presence or absence of a lower common pathway (LCP). METHODS AND RESULTS: In 344 patients, AVNRT using a slow pathway (SP) for antegrade conduction and earliest atrial activation at the superior septum (i.e. retrograde fast pathway) was present in 81.4% (Slow/Fast). AVNRT using an SP for antegrade conduction and earliest atrial activation at the inferior septum or proximal coronary sinus (i.e. retrograde slow pathway; Slow/Slow) was present in 13.7%. AVNRT with a short A-H interval and retrograde SP conduction (Fast/Slow) was present in 4.9%. All timing intervals during tachycardia are dependent on autonomic tone. H-A intervals during tachycardia (H-A(t)) overlap in Slow/Slow and Slow/Fast AVNRT: Slow/Slow therefore may mimic Slow/Fast AVNRT. The H-A interval during pacing at the tachycardia cycle length (H-A(p)) better discriminates both subforms. The difference between H-A(p) and H-A(t) (Delta H-A) was significantly longer in Slow/Slow compared with Slow/Fast AVNRT (isoprenaline 0.5 microg/min: 27+/-18 ms vs. 1+/-9 ms; p<0.001). Delta H-A>15 ms had a specificity and sensitivity for Slow/Slow of 94% and 64%, respectively. A Delta H-A>15 ms, combined with other data, pointed to the presence of a long LCP in 36 of 43 evaluable Slow/Slow (84%) and all Fast/Slow, but in only 10% of Slow/Fast (p<0.001). Retrograde conduction during ventricular pacing at the tachycardia cycle length was present in only 6% of Fast/Slow. CONCLUSIONS: AVNRT subforms can be distinguished based on a systematic evaluation of atrial activation sequence, timing intervals and evidence for the presence of an LCP.