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?