Orthodromic AVRT and functional left bundle branch block
This patient presented repeated episodes of orthodromic tachycardia due to a hidden left lateral accessory pathway. Different elements allow arriving at this diagnosis:
- the accessory pathway was not identifiable in the sinus rhythm electrocardiogram;
- this patient presented various episodes of reciprocating tachycardia with both types of conduction aberration (right bundle branch block or functional left bundle branch block). It is relatively common to observe, at the onset of a tachycardia, the appearance of a conduction aberration during a few complexes which disappear thereafter (adaptation to the refractory periods of the blocked branch or disappearance of the hidden retrograde conduction from branch to branch thus maintaining the bundle branch block). If the disappearance of this bundle branch block leads to an acceleration of the tachycardia, this signals the presence of a bundle of Kent on the side of the branch whose block is slowing conduction. In this patient, the right bundle branch block did not alter the rate of the tachycardia (identical RR cycle length) while the left bundle branch block prolonged the RR (cycle length) by approximately 30 ms. The prolongation of the tachycardia cycle length is secondary to the prolongation of the circuit length in its ventricular portion (slowed ventricular depolarization in relation to muscle conduction characteristic of a bundle branch block). The prolongation of the RR cycle length is often clear-cut when the accessory pathway is of lateral topography, and less definite when it is septal. It should be noted however that the prolongation of the ventriculo-atrial delay can be compensated by a reduction in nodal conduction time (AH) fostered by the occurrence of this delay. The end result is therefore a decrease in tachycardia rate, which is less sizeable than that resulting from the modification in ventriculo-atrial conduction time;
- the diagnosis of left lateral bundle of Kent is confirmed by the presence of a negative P' wave in leads I and aVL. Indeed, during the tachycardia, a negative P' wave in lead I reflects the excitation of the atrium stemming from a left lateral connection. The atrial deviation is negative given that the left lateral atrial wall is depolarized before the septum, the left atrium before the right atrium, with the atrial activation vector pointing downward and to the right.
The careful analysis of the surface electrocardiogram allows for probable identification of the accessory pathway location and hence may guide the ablation procedure.