left posterior fascicle

**Anatomy/physiology** - The His bundle splits into the right bundle branch and the left bundle branch (LBB). The LBB then divides into the left anterior fascicle (LAF), left posterior fascicle (LPF), and in many people a variable left septal fascicle. - The LPF is shorter and thicker than the LAF. It fans out over the posteroinferior LV wall and the posteromedial papillary muscle, and it connects to the Purkinje network. - Blood supply: dual, from the AV nodal artery (a branch of the PDA, off the RCA in right-dominant hearts) and septal perforators of the LAD. This is why it is more resistant to ischemic injury than the LAF, which is supplied mainly by LAD septal perforators.

**Left posterior fascicular block (LPFB)** - Depolarization is delayed in the inferoposterior LV. The initial forces are directed superiorly and leftward, and the main vector then swings inferiorly and rightward. - ECG findings: - Right axis deviation (+90° to +180°) - qR pattern in II, III, and aVF (small q, tall R) - rS pattern in I and aVL - Normal or only slightly prolonged QRS (<120 ms) - LPFB is a diagnosis of exclusion. First rule out other causes of right axis deviation: right ventricular hypertrophy, cor pulmonale or pulmonary hypertension, lateral MI, and normal variants (tall, thin patients). Also exclude dextrocardia and lead misplacement.

**Causes** - Rarely isolated. It is usually seen with right bundle branch block (RBBB) as part of bifascicular block. - Ischemic heart disease or MI, including anterior MI, which carries a worse prognosis. - Degenerative conduction disease (Lev disease, Lenègre disease), cardiomyopathies, myocarditis, Chagas disease, aortic valve disease or calcification, hyperkalemia, and cardiac surgery or TAVR.

**Clinical significance** - Bifascicular block (RBBB + LPFB) has a higher risk of progressing to complete heart block than RBBB + LAFB, because the LPF is more robust and its involvement implies more extensive disease. - Trifascicular block is bifascicular block plus first-degree AV block (this is a classic board concept). Strictly, it implies conduction disease in all three fascicles. - Asymptomatic patients are usually just observed. Syncope with bifascicular block raises concern for intermittent high-grade AV block, so evaluate with an EP study or ambulatory monitoring and consider a pacemaker. - Do not confuse this with left anterior fascicular block (LAFB), which gives left axis deviation (−45° to −90°), qR in I and aVL, and rS in II, III, and aVF. LAFB is the more common of the two.

**High-yield pearls** - LPFB gives right axis deviation, and LAFB gives left axis deviation. - LPFB is rare because the LPF is thick, short, and dual-supplied. - RBBB + LPFB = bifascicular block. Add a prolonged PR interval and it becomes "trifascicular" block. - Always exclude RVH and other causes of right axis deviation before diagnosing LPFB.