Case:
A 35 year-old-lady underwent EP study for recurrent narrow QRS tachycardia (Fig 1A) terminating with adenosine. Baseline ECG showed no pre-excitation (Fig 1B). EP study was performed with decapolar catheter in coronary sinus (CS), quadripolar catheters placed in HRA (high right atrium) and His; and one roving catheter in right ventricle (RV). Baseline intervals: AH interval=76 ms, HV interval 34 ms. During antegrade study AH jump, AV node duality and intermittent rate related aberrancy (RRA) with unusual axis ( RBBB with right axis deviation, HV 34 ms) was noted. Retrograde study -VA conduction was concentric and decremental (VAERP=270 ms), no VA jump. No sustained tachycardia could be induced by standard protocols at baseline. On isoprenaline, a short VA tachycardia (SVT1) with near simultaneous A and V activation [HV=34 ms, tachycardia cycle length (TCL)= 305 ms, Fig 1C) was induced with atrial premature depolarisation (APD) and ventricular premature depolarisation (VPD) with similar RRA (Fig 1C and 1D). Maneuvers confirmed SVT1 to be slow-fast atrioventricular nodal reentrant tachycardia (AVNRT) (VAV response, SA-VA=160 ms, cPPI-TCL=151 ms). During programmed decremental APDs from CS to differentiate it from junctional tachycardia (JT), faster SVT2 was induced with a longer VA (Fig 2A and 2B). SVT2 had TCL of 260 ms after initial wobble, septal VA = 140 ms. What could be the mechanism of SVT2 ?