Initial shortening of the tachycardia cycle length after the induction of fast-slow atrioventricular nodal reentrant tachycardia may support slow pathway as an antegrade limb of the reentry circuit

Abstract

Introduction: It remains controversial whether fast pathway (FP) is actually incorporated as an antegrade limb of the reentry circuit in fast-slow (F-S) atrioventricular nodal reentrant tachycardia (AVNRT) with the earliest atrial activation at coronary sinus ostium, presenting long RP tachycardia, because antegrade conduction over slow pathway (SP) is occasionally observed during the tachycardia. Although double ventricular responses (DVR) in which single atrium impulse conducts to the ventricle simultaneously over FP and SP are frequently observed during slow-fast (S-F) AVNRT, these phenomena have not been recognized during F-S AVNRT. The purpose was to verify our hypothesis that SP is incorporated as an antegrade limb of the reentry circuit in F-S AVNRT, by the detection of DVR during F-S AVNRT. Methods: In 6 F-S AVNRT, 8 S-F AVNRT and 8 atrioventricular reentrant tachycardia (AVRT) reproducibly inducible with atrial extrastimulation (S1-S2) or overdrive pacing (S2-S2), we examined an initial cyclic variation after the induction of the tachycardia by measuring interhisian intervals of H2-H3, H3-H4 and H4-H5. Results: As shown in table, the tachycardia cycle length immediately after the induction was paradoxically shorter than the subsequent tachycardia cycle length in F-S AVNRT only. Temporal P-QRS variation during F-S AVNRT and a complication of S-F AVNRT with the same earliest atrial site in 5 F-S AVNRT (83%) suggested the presence of low common pathway (LCP) located below the AVN reentry circuit. Values are expressed in ms. *p<0.05 vs H4-H5 interval minus H3-H4 interval. Conclusions: Paradoxical shortening of H2-H3 in F-S AVNRT may be explained by DVR in which H2 and H3 is antegradely captured over FP and SP, respectively, followed by slow-slow AVNRT with a long LCP, presenting long RP tachycardia.