Tachycardia with varying atrio‐ventricular relationships: What is the mechanism?

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An otherwise healthy 25‐year‐old man presented with paroxysmal palpitations for 6 months. The electrocardiogram (ECG) during tachycardia demonstrated a QRS morphology with right bundle branch block (RBBB) pattern, left inferior axis at a rate of 265 bpm. The ECG during sinus rhythm was normal with narrow QRS complexes and normal PR intervals.
The baseline H‐V interval during sinus rhythm was 50 milliseconds (Figure 1, Panel A). Atrial extra‐stimulus did not demonstrate preexcitation or dual AV nodal (AVN) physiology. Ventricular pacing demonstrated decremental retrograde conduction with the earliest A at the His region. The tachycardia was induced by burst ventricular pacing. The QRS morphology during tachycardia matched the clinical tachycardia with complete RBBB (QRS duration 133 milliseconds). The total cycle length (TCL) of tachycardia was between 220 and 300 milliseconds at different periods.
In total, three types of V‐A patterns were observed: (1) long RP interval (V‐A interval longer than A‐V interval) with 1:1 V–A relationship (Figure 1, Panel B), (2) A‐V dissociation with V greater than A rate (Figure 1, Panel C), and (3) regular tachycardia with atrial fibrillation (AF, Figure 1, Panel D). The H‐V interval remained constant at 50 milliseconds during all above V–A relationships. We delivered ventricular stimuli during His bundle refractoriness (RS2) during these variable V–A relationships. With a 1:1 V–A relationship, RS2 reset the A‐A interval, H‐H interval, and the subsequent V‐V interval (all these intervals decreased from 300 to 286 milliseconds (Figure 2, Panel A). We also delivered RS2 repeatedly during tachycardia with AF which not only advanced the next His and reset tachycardia (Figure 2, Panel B), but also terminated tachycardia without conduction to His bundle (Figure 2, Panel C), while AF continued. Burst RV pacing (S1 = 280 milliseconds) was also performed during tachycardia at a TCL of 300 milliseconds with a 1:1 V–A relationship. This resulted in ventricular dissociation from the His bundle and atrium. Following ventricular overdrive pacing at the RV apex (S1 = 250 milliseconds) during tachycardia at a TCL of 300 milliseconds, the return cycle length was 561 milliseconds, 281 milliseconds longer than TCL (Figure 2, Panel E). The morphology of QRS complexes during overdrive pacing showed full ventricular pacing.
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