Intermittent pattern of ventricular pre-excitation
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.