left anterior fascicle

**Anatomy and conduction** - Conduction pathway: SA node → AV node → bundle of His → right and left bundle branches. The left bundle splits into the left anterior fascicle, left posterior fascicle, and (variably) a left septal fascicle. - The LAF is long and thin. It supplies the anterior and superior part of the left ventricular free wall, including the anterolateral papillary muscle. - Blood supply: mainly septal perforators of the LAD. The AV-nodal artery (RCA or LCx) and the posterior descending artery also contribute in some patients. The LAF is therefore susceptible to ischemia in anterior MI, and it is the most commonly blocked fascicle. - The left posterior fascicle is thicker and has a dual blood supply (LAD septal perforators plus PDA). It is more resistant to injury, so isolated left posterior fascicular block is rare.

**ECG findings in LAFB** - Left axis deviation, usually −45° to −90°. - qR pattern in I and aVL (small q). - rS pattern in II, III, and aVF (small r, deep S). S in III is larger than S in II. - QRS is normal or only mildly prolonged (<120 ms), because the posterior fascicle and the septal fibers still activate the ventricle rapidly. - Delayed intrinsicoid deflection in aVL. - Exclude other causes of left axis deviation, such as inferior MI, LVH, WPW, and ventricular pacing.

**Clinical significance** - Causes: fibrosis or degenerative disease of the conduction system (Lev and Lenègre disease), anterior MI, hypertension/LVH, cardiomyopathies, aortic valve disease or calcification, myocarditis, Chagas disease, hyperkalemia, and congenital heart disease (e.g., ostium primum ASD, tricuspid atresia). - Isolated LAFB is usually asymptomatic and generally needs no treatment. - Bifascicular block is RBBB plus LAFB (the most common form) or RBBB plus LPFB. Trifascicular block is bifascicular block plus first-degree AV block. Both are at risk of progressing to complete heart block, though the risk is low in asymptomatic patients. - A pacemaker is indicated if there is symptomatic high-grade or intermittent complete AV block, but not for asymptomatic bifascicular block alone. - Syncope with bifascicular block warrants evaluation (EP study) for intermittent high-grade AV block.

**High-yield board pearls** - LAFB = left axis deviation with qR in aVL and rS in the inferior leads. - LPFB = right axis deviation with rS in I and aVL and qR in the inferior leads. It requires exclusion of RVH and lateral MI. - The LAF is more commonly affected than the LPF because it is thinner and has a single blood supply. - LAFB can mask or mimic inferior MI. It produces small q waves in I and aVL and may cause poor R-wave progression. - Remember that RBBB plus LAFB is the classic bifascicular block.