Orthodromic AVRT and functional left bundle branch block

ECG 6A: Narrow QRS tachycardia with probable 1:1 relationship between atria and ventricles; fixed RR intervals of 300 ms (200 bpm); atrial activity observable at the peak of the T wave; it appears negative in the inferior leads and in lead I; pattern compatible with an orthodromic tachycardia due to left lateral accessory pathway;

ECG 6B

ECG 6B: Tracing recorded a few minutes after the first tracing; 1:1 tachycardia between atria and ventricles with broad QRS (left bundle branch block pattern); RR cycle lenght slightly slower than on the previous tracing (330 ms);

ECG 6C

ECG 6C: Tracing recorded a few minutes after the first tracing; 1:1 tachycardia between the atria and ventricles with broad QRS (right bundle branch block pattern); RR cycle lenght identical to the first tracing (300 ms);

ECG 6D

ECG 6D: Recording of onset of a tachycardia due to an atrial extrasystole; left bundle branch block pattern;

ECG 6E

ECG 6E: Recording of a tachycardia pause; left bundle branch block pattern followed by acceleration of the rhythm occurring during the narrowing of the QRS, then termination;

Comments: This patient presented repeated episodes of orthodromic tachycardia due to a hidden left lateral accessory pathway. Different elements allow arriving at this diagnosis: 1) the accessory pathway was not identifiable in the sinus rhythm electrocardiogram; 2) 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; 3) 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.

Take-home message: In a 1:1 tachycardia, the occurrence of a bundle branch block which slows the tachycardia cycle is suggestive of a tachycardia due to an accessory pathway on the side of the branch whose block is conduction-depressant (conduction-slowing left bundle branch block suggestive of a left accessory pathway).