B-po04-160 the atrium is likely a part of the circuit in typical av node reentrant tachycardia

The circuit for typical AVNRT is not well defined. To determine whether typical AVNRT circuit includes atrial tissue between AV node breakout near the His bundle and the coronary sinus (CS) floor. If so, atrial fusion should be present during resetting from the CS Os, which is a downstream site near the atrionodal connection. In patients with typical AVNRT, atrial overdrive pacing (AOP) 10-30ms faster than the tachycardia cycle length (TCL) was performed from the CS floor and His bundle sites, looking for evidence of resetting (advancing or delaying the His EGM or termination of SVT) and atrial fusion relative to the first beat of resetting (Figure A). Twenty consecutive patients were included. Mean age was 59±16yrs and mean TCL was 363±57ms. Number needed to reset the tachycardia after local capture was similar while pacing from CS Os and His bundle sites (2.5±1.1 vs 2.9±0.9 respectively; p=0.35). During entrainment from the CS Os, atrial fusion (antidromic/orthodromic wavefront collision between the CS Os and His A sites) was present for 4.2±1.6 beats and persisted after resetting for 1.7±1.5 beats (Figure B). During entrainment from the His A, atrial fusion was present for 1.3±0.6 beats and not associated with resetting. Stable constant fusion was demonstrated in 2 cases during entrainment from the CS os (His A driven by orthodromic wavefront (Figure C). Resetting of typical AVNRT circuit occurs with atrial fusion from a downstream site (CS Os) but not from an upstream site (His A) confirming the existence of separate exit and entrance points to the AV nodal portion of the circuit and that atrial tissue is likely a critical part of the reentry circuit.