Background
Right ventricular pacing (RVP) during supraventricular tachycardia (SVT) 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 re-entrant 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 63 patients with SVT 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 (SA) interval from the end of the TZ and with each QRS complex thereafter until pacing was terminated or ventriculoatrial block occurred. A fixed SA interval was defined as variation less than or equal to 10 ms during RVP. Atrial pre-excitation, postexcitation, or SVT termination with abrupt ventriculoatrial block was observed within the TZ in 19 of 22 patients with ORT. A fixed SA interval was established within the TZ in 22 of 22 patients with ORT. At least one of these two responses was observed in all patients with ORT. Only four patients out of 36 with atrioventricular nodal re-entrant tachycardia showed atrial pre-excitation within TZ and only four patients showed fixed SA within TZ. None of the atrial tachycardia patients had atrial timing perturbed or a fixed SA interval established within the TZ.
Conclusion
During RVP within 40 ms of the tachycardia cycle length, ORT is the likely mechanism when atrial timing is perturbed or a fixed SA interval is established within the TZ.