Clinical Overview

Left posterior fascicular block (LPFB), also called left posterior hemiblock, is a conduction abnormality in which the posterior fascicle of the left bundle branch fails to conduct normally. Below the bundle of His, the left bundle branch divides into an anterior (anterosuperior) fascicle, which normally activates the anterior and lateral wall of the left ventricle, and a posterior (posteroinferior) fascicle, which activates the posterior and inferior wall. In LPFB, conduction down the posterior fascicle is slowed or blocked, so the inferior and posterior wall is instead activated late, indirectly, by depolarization spreading from the intact anterior fascicle (LITFL, 2021; Merck Manual, 2024). Like its mirror-image counterpart, the finding is defined electrically rather than anatomically: it is diagnosed from a rightward, inferior shift in the frontal-plane QRS axis together with a specific QRS morphology, not from imaging.

Mechanistically, depolarization first travels down the intact anterior fascicle to the anterior and lateral left ventricular wall, producing an initial vector directed superiorly and leftward — a small r wave in leads I and aVL. The remaining, larger mass of the inferior and posterior wall is then activated late, without the normal competing input from the posterior fascicle, so the dominant, unopposed vector swings inferiorly and to the right. This produces the deep S waves in I and aVL and the tall R waves in II, III, and aVF that define the pattern, along with right axis deviation (LITFL, 2021; Merck Manual, 2024). Because conduction still reaches the inferior wall — just later, by an indirect route — the QRS widens only modestly (80-110 ms), which is what separates LPFB from a complete bundle branch block, where an entire ventricle depolarizes through slow, cell-to-cell myocardial conduction instead of the specialized conduction system.

LPFB is far less common than LAFB and is rarely seen on its own: the posterior fascicle is broader than the anterior fascicle, which makes it more resistant to injury (LITFL, 2021). Because isolated LPFB is unusual, its presence should prompt a look for a coexisting conduction defect — most often right bundle branch block, producing a bifascicular block pattern (LITFL, 2021). Reference guidance on hemiblocks in general notes that their associations with structural heart disease parallel those of complete left bundle branch block, and that the presence of bifascicular or trifascicular block after a myocardial infarction implies extensive cardiac damage (Merck Manual, 2024). [CLINICAL REVIEW NEEDED: a 2021 study (Calò et al., J Am Coll Cardiol) examined LPFB as a possible marker of increased sudden cardiac death risk in young people; the study population, effect size, and whether the association holds independent of bifascicular block could not be confirmed against a second source this run.]

LPFB itself produces no symptoms — it is an ECG pattern, not a hemodynamic event, since conduction still reaches the entire ventricle, just later than normal. Any symptoms a patient reports come from an accompanying cause, or from progression to a higher-grade block: dizziness, fatigue, or syncope can appear if conduction disease advances toward complete heart block, particularly when LPFB coexists with right bundle branch block as a bifascicular block (Merck Manual, 2024).

Recognized causes and risk factors parallel those described for left bundle branch block and hemiblocks generally: coronary artery disease and myocardial infarction, hypertensive heart disease, and cardiomyopathy (Merck Manual, 2024; WikiDoc, 2013).

Interpretation Guide

Key Features:

• Rate: not a defining feature — LPFB is a QRS-morphology finding superimposed on whatever rate accompanies the underlying rhythm
• Rhythm: not a defining feature — assessed against an underlying supraventricular rhythm; the fascicular block itself does not describe the rhythm
• P waves: within normal limits unless a coexisting atrial finding is present
• PR interval: within normal limits unless a separate, coexisting AV conduction abnormality is present
• QRS complex: the defining feature — normal or only slightly prolonged duration (80-110 ms, under 120 ms), with an rS pattern (small r, deep S) in leads I and aVL and a qR pattern (small q, tall R) in leads II, III, and aVF; an R-wave peak time (intrinsicoid deflection) in aVF of 45 ms or more supports the diagnosis (LITFL, 2021)
• ST segment: not a defining feature of isolated LPFB; no associated strain pattern — ST depression here instead suggests right ventricular hypertrophy
• T waves: not a defining feature; the absence of a strain-pattern T-wave inversion in the inferior leads helps distinguish LPFB from right ventricular hypertrophy, which can also produce right axis deviation
• QT interval: not a defining feature; within normal limits unless the underlying cause independently affects repolarization
• Other findings: frontal-plane QRS axis right axis deviation, most commonly cited as more positive than +90° (LITFL, 2021), though some reference sources set the threshold at +120° (Merck Manual, 2024) [CLINICAL REVIEW NEEDED: the exact axis cutoff differs between the two sources read this run]; before diagnosing LPFB, other causes of right axis deviation must be excluded, including right ventricular hypertrophy, acute pulmonary embolism, tricyclic antidepressant overdose, and a lateral-wall STEMI (LITFL, 2021)

Key Leads

• Lead aVF — a qR complex with an R-wave peak time of 45 ms or more is the most specific finding for LPFB
• Leads II and III — qR pattern with a dominant R wave, confirming the axis points inferiorly and rightward
• Leads I and aVL — rS pattern with a deep S wave, confirming the axis has shifted away from the left and superior direction
• No single lead is optional here — the diagnosis rests on the specific combination of findings across aVF, II, III, and the lateral leads together, not any one lead in isolation, and only after excluding other causes of right axis deviation

Differential Diagnosis

• Left Anterior Fascicular Block (LAnFB) — the mirror-image fascicular block. Distinguishing clue: LAFB produces left axis deviation (leftward and superior) with a qR pattern in I/aVL and an rS pattern in II/III/aVF — the opposite of LPFB’s pattern — and is considerably more common, since the anterior fascicle is more vulnerable to injury than the posterior fascicle.
• Axis Right Shift (ARS) — the general rightward-axis finding that a right axis deviation cause, including LPFB, must be identified within. Distinguishing clue: the specific rS-in-I/aVL and qR-in-inferior-leads morphology, together with a prolonged aVF R-wave peak time, identifies LPFB specifically; a rightward axis without this conduction pattern (from right ventricular hypertrophy or a normal variant in a tall, thin patient, for example) is ARS without LPFB.
• Complete Right Bundle Branch Block (CRBBB) — LPFB is rarely seen in isolation and most often coexists with RBBB as a bifascicular block. Distinguishing clue: RBBB alone widens the QRS to 120 ms or more with an rsR’ pattern in V1 and a wide, slurred S wave in I and V6; when RBBB and LPFB coexist, both the RBBB morphology in the precordial leads and the LPFB axis/limb-lead pattern are present together.
• Right Ventricle Hypertrophy (RVH) — a structural cause of right axis deviation that must be excluded before attributing an axis shift to LPFB. Distinguishing clue: RVH typically shows a dominant R wave in V1, a strain pattern with ST depression and T-wave inversion in the right precordial leads, and often other voltage criteria for right-sided enlargement, none of which are features of isolated LPFB.

Treatment Brief

LPFB is a descriptive ECG finding, not a rhythm or condition to treat directly — the response depends on whether it is new, what accompanies it, and the patient’s overall cardiac status.

• Compare against a prior ECG whenever one is available; a new fascicular block deserves more attention than a longstanding, stable one.
• Before treating a rightward axis shift as LPFB, confirm the rS-in-I/aVL and qR-in-inferior-leads pattern and rule out other causes of right axis deviation such as right ventricular hypertrophy, acute pulmonary embolism, or a lateral-wall STEMI.
• Because isolated LPFB is uncommon, its presence should prompt a check for a coexisting right bundle branch block; notify the provider if bifascicular block is identified, especially in a patient with syncope or near-syncope.
• Bifascicular block generally requires no direct treatment unless second- or third-degree AV block develops; a true trifascicular block pattern requires immediate, then permanent, pacing (Merck Manual, 2024).
• If bundle branch block with unexplained syncope is present, an electrophysiology evaluation may be considered per current pacing guidelines to assess the need for permanent pacing (ESC Guidelines on Cardiac Pacing, 2021).
• Confirm lead placement and repeat the tracing if the finding is new or unexpected.