An isolated left anterior fascicular block is caused by LVH, coronary heart disease, and fibrosis of the infra-Hisian conduction system (also known as Lenègre disease). It is seen in patients older than 40 and is common in patients over 80. The left anterior fascicular block potentially masks an inferior wall or anterior wall infarction.

An isolated left posterior fascicular block is caused by an inferior wall MI 90% of the time. Among all patients with an inferior wall MI, only 6% present with left posterior fascicular block, but it is an important finding that that masks the classical ECG characteristics of the infarct. 

The QRS duration of the left anterior and left posterior fascicular blocks is usually longer (about 0.10–0.11 seconds) without LVH.

The Infra-Hisian Conduction System