Intermittent pattern of ventricular pre-excitation

ECG: In the initial part of the tracing, there is a sinus rhythm with a normal PR interval, a narrow QRS (presence of a septal q wave in V6) and aspecific repolarization disorders; following slowing of the heart rate, the appearance of ventricular pre-excitation with short PR, delta wave and repolarization disorders; the ST segment depression and the quasi-systematic inversion of the T wave in all leads relative to the initial tracing can be attributed to the modification of activation and to the appearance of pre-excitation;

 Comments: This young patient has an accessory pathway with intermittent anterograde conduction. Conduction in an accessory pathway operates on the all-or-nothing principle: the bundle is either in a refractory period or not, and the impulse is or is not transmitted to the ventricles. The anterograde refractory period of this pathway is long, a slow atrial activity is thus necessary for the pre-excitation to become visible. During higher rate, the electrocardiogram is normal (phase 3 block). This type of accessory pathway is considered benign as a long refractory period protects the patient from the risk of sudden death secondary to rapid atrial fibrillation conducted by the pathway. On the other hand, this particular pattern is associated with a short retrograde refractory period of the pathway and this patient presented with orthodromic reentry tachycardia (good retrograde conduction) with a disabling symptomatology. Given the frequency of the seizures (daily tachycardias), the patient underwent the ablation of an anteroseptal pathway.

Take-home message: Conduction in a bundle of Kent is achieved according to the all-or-nothing principle. The danger of this type of accessory pathway resides in the presence of a short refractory period. An intermittent pattern in sinus rhythm (onset during a bradycardia) is indicative of a long refractory period and of its benign nature.