Transient left septal fascicular block in a patient with stable effort angina and critical proximal obstruction of left anterior descending coronary artery.

Several publications from our group and others have shown that a main cause of middle fiber block or left septal fascicular block of the left bundle of His is proximal obstruction of the left anterior descending coronary artery, before its first septal perforator branch, since this fascicle is the only one of the three fascicles irrigated exclusively by this artery. We present a case in which the phenomenon occurred transiently, causing prominent anterior QRS forces, confirmed by coronary angiography.