New criteria during right ventricular pacing to determine the mechanism of supraventricular tachycardia.

BACKGROUND Right ventricular pacing (RVP) during supraventricular tachycardia produces progressive QRS fusion before the QRS morphology becomes stable. This transition zone (TZ) may provide useful information for differentiating orthodromic reciprocating tachycardia (ORT) from atrioventricular nodal reentrant tachycardia and atrial tachycardia independent of entrainment success. METHODS AND RESULTS We studied the effect of properly timed RVP on atrial timing during the TZ in 92 patients with supraventricular tachycardia who had RVP within 40 ms of the tachycardia cycle length. The TZ during RVP includes progressively fused QRS complexes and the first paced complex with a stable QRS morphology based on analysis of the 12-lead ECG. We also measured the stimulus-atrial interval from the end of the TZ and with each QRS complex thereafter until pacing was terminated or ventriculo-atrial block occurred. A fixed stimulus-atrial interval was defined as variation <10 ms during RVP. Atrial preexcitation, postexcitation, or supraventricular tachycardia termination with abrupt ventriculo-atrial block was observed within the TZ in 32 of 34 patients with ORT. A fixed stimulus-atrial interval was established within the TZ in 33 of 34 patients with ORT. At least 1 of these 2 responses was observed in all patients with ORT. None of the patients with atrioventricular nodal reentrant tachycardia or atrial tachycardia had atrial timing perturbed or a fixed stimulus-atrial interval established within the TZ. CONCLUSIONS During RVP within 40 ms of the tachycardia cycle length, ORT is the likely mechanism when atrial timing is perturbed or a fixed stimulus-atrial interval is established within the TZ.