A 45-year-old man with palpitations and absence of preexcitation in baseline electrocardiogram (ECG) underwent a Holter recording, revealing several episodes of regular non-sustained broad QRS complex tachycardia that terminated following the occurrence of a single narrow QRS (Panel A).
During the electrophysiological study, short episodes of orthodromic tachycardia (OT) with left bundle branch block (LBBB) were induced and consistently terminated following the occurrence of a narrow QRS (Panel B). The resolution of left BBB reduced LV activation time by 70 ms (as measured on CS recording) and consequently, shortened the ventriculo-atrial interval, favouring anterograde conduction block at the AV node. A left free wall accessory pathway was ablated.
Other mechanisms such as ectopic atrial tachycardia and AVNRT are not influenced by intraventricular conduction delays. A ventricular tachycardia may be terminated by a capture beat, which is expected to be anticipated. However, the tachycardia cycle length (measured on RV recording and surface ECG) did not change before termination.
A change in RR or VA intervals following the resolution of ipsilateral functional BBB is specific of OT. To our knowledge, this is the first report that shows a new ECG finding, which strongly suggests an accessory pathway as the mechanism of SVT.
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