Funding Statement: None

Case PresentationA 13-year-old boy presented with recurrent episodes of palpitations and a normal baseline electrocardiogram (ECG). He underwent a cardiac electrophysiology study with the intent of radiofrequency ablation. Four diagnostic electrophysiology catheters were placed – a decapolar catheter in the coronary sinus (CS) and three other quadripolar catheters placed at AV junction in the ‘His bundle’ region, right ventricular (RV) apex, and right atrium (RA). Baseline intervals were normal. A narrow QRS tachycardia (Tachycardia 1) was induced during catheter placement with a cycle length of 300ms and AH longer than HA interval (Figure 1a). The earliest atrial activation was in the proximal CS electrodes. Ventricular pacing at a cycle length of 280ms showed ventricular capture without reset of the atrium or change in its activation sequence. However, on cessation of ventricular pacing, a different narrow QRS tachycardia (Tachycardia 2) was seen at a cycle length of 290ms with AH interval shorter than HA interval, and VA interval of 180ms (Figure 1b). Differential atrial overdrive pacing from three different sites showed a maximum Δ V-A interval of the first return beats post-cessation of pacing to be less than 10ms for tachycardia 1. This manoeuvre could not be utilised for tachycardia 2 as the tachycardia got terminated. On performing “simultaneous atrial and ventricular pacing” (RA+RV pacing) at a cycle length of 270ms during tachycardia 2 – the first event after cessation of pacing was always an atrial electrogram (Figure 2a). The same manoeuvre when performed for tachycardia 1 showed an atrial EGM as the first event with change to tachycardia 2 (Figure 2b) every time, excepting for one occasion where the initial event was a His bundle electrogram (HBE) with tachycardia 1 which continued (Figure 2c). How do we electrophysiologically reconcile the Janus or duality of responses with - RA+RV pacing during tachycardia 1 and what is the likely mechanism for the two tachycardias?