[Alternating bundle branch block in a supraventricular tachycardia: Which is the mechanism ?]

Functional bundle branch block during a supraventricular tachycardia can be observed with shorter cycle lengths and represent a physiologic response by the specialized intraventricular conduction system to accelerated AV nodal conduction. The present case corresponds to a young patient with exercise induced orthodromic A-V reentrant tachycardia and alternating bundle branch block. This unusual response is explained by the finding obtained during the electrophysiology study. An accelerated AV nodal conduction made the depolarizing wave front reach the bundle branches during their refractory period. Once block in one bundle was stablished, block persisted due to the linking phenomenon that is repetitive retrograde concealed conduction from the contralateral bundle. After catheter ablation of a concealed left-sided accessory A-V pathway, rapid atrial pacing at the same cycle length of the tachycardia reproduced the same aberrancies observed during tachycardia. This response proved that functional bundle branch block is due to the short cycle length and not the presence of an accessory A-V pathway.