Orthodromic AVRT due to a hidden accessory pathway

ECG 5A: This electrocardiogram performed between tachycardia episodes is within normal limits;

ECG 5B: Recording of the onset of a tachycardia; an atrial extrasystole is conducted through the normal pathways with a prolonged PR interval; beginning of a tachycardia with narrow QRS and identical to the sinus complexes; atrial activity difficult to identify but seemingly present with 1:1 conduction in the ascending portion of the T wave; the positioning of atrial activity points to an orthodromic tachycardia due to accessory pathway;

Comments: On the previous tracing, we detailed the electrocardiographic characteristics of an orthodromic atrio-ventricula reentrant tachycardia due to accessory pathway. It is evident that the presence of a pre-excitation in sinus rhythm (short PR, delta wave) is suggestive of a tachycardia due to accessory pathway. In contrast, as in this patient, a normal intercritical tracing does not allow eliminating reentry due to accessory pathway through a hidden bundle of Kent. This type of accessory pathway only conducts in a retrograde manner (from the ventricles to the atrium), which renders identification impossible on a sinus activity (no anterograde conduction). The electrophysiological study allowed inducing the same tachycardia and to reveal a hidden left posterior accessory pathway, i.e. conducting only in retrograde manner. These patients are at risk of reciprocating tachycardia but cannot present rapid atrial fibrillation conducted by the accessory pathway (no anterograde conduction), which limits the severity and the risk of sudden death. In view of the symptoms presented by the patient, an ablation of the accessory pathway was nonetheless proposed.

In a hidden bundle of Kent bundle, there is some degree of anterograde conduction due to the accessory pathway which is not sufficient to depolarize the ventricular cells and yield a pre-excitation pattern although sufficient to prevent a systematic retrograde conduction and the occurrence of permanent tachycardias.

During a tachycardia with normal sinus electrocardiogram, certain elements may suggest the presence of a hidden accessory pathway: the diagnosis is made if a conduction-depressant homolateral bundle branch block is detected; similarly, a negative P’ wave in lead I is highly suggestive of reentry through the left lateral accessory pathway. Electrophysiological study allows confirming the diagnosis.

Take-home message: An accessory pathway is said to be hidden when there is no anterograde conduction; the occurrence of orthodromic tachycardia is possible if the conduction through the retrograde accessory pathway is preserved; there is however no risk of sudden death due to rapidly conducted atrial fibrillation.