Narrow QRS complex tachycardia: What is the mechanism?

A 30 year old female with history of atrio-ventricular nodal reentrant tachycardia (AVNRT) who underwent slow pathway modification at age of 12 years presented with recurrent palpitations and presyncope for one year. She was found to have recurrent supraventricular tachycardia sensitive to adenosine and was referred for electrophysiology study and catheter ablation. Baseline electrocardiogram showed sinus rhythm, RBBB, with no evidence of preexcitation. Electrophysiology study showed an AH 73ms, HV 48 ms, and RR 876 ms. Atrial extra-stimulus pacing showed no evidence of dual AV node physiology. Ventricular extra-stimulus pacing showed midline decremental conduction, figure 1A, and no change in the retrograde atrial activation sequence. Supraventricular tachycardia was consistently induced with single atrial extra-stimuli without an AH jump. Ventricular extra-stimuli never induced tachycardia. At baseline, the tachycardia had a cycle length of 450 ms, which decreased to 320–330 ms on isoproterenol, and occasionally, showed cycle length variability, figure 1B. Tachycardia always terminated with ventricular activation and the response to adenosine showed no retrograde VA conduction. Ventricular overdrive pacing at a cycle length of 310 ms when tachycardia had a CL of 330 ms is shown in figure 2A, and the response to atrial pacing at a cycle length of 310 ms, when tachycardia cycle length was otherwise stable at 320 ms, is shown in figure 2B. Entrainment from the ventricle repeatedly terminated tachycardia. Ventricular overdrive pacing at a CL much faster than the tachycardia lead to VA dissociation. Single His bundle refractory ventricular extra-stimuli reproducibly showed the response in figure 3 and on one occasion, terminated tachycardia. What is the mechanism of tachycardia? Figure 1 Panel A shows the response to ventricular extra-stimuli at 400/230 ms prior to ventricular effective refractory period. Panel B shows occasional oscillation in cycle lengths noted during tachycardia. HRA = high right atrium, His p = His bundle proximal, ... Figure 2 Panel A shows the response to ventricular over drive pacing at cycle length of 310 ms during tachycardia (tachycardia CL was 330 ms on isoproterenol). The arrow points to the first captured ventricular beat that leads to a change in the atrial activation ... Figure 3 The response to a His bundle refractory ventricular extra-stimulus at a time of stable tachycardia cycle length of 450 ms is shown. HRA = high right atrium, His p = His bundle proximal, His m = His bundle mid, His d = His bundle distal, CS = coronary ...