What is the significance of a left posterior fascicular block on an electrocardiogram and how should it be evaluated and managed?

Left Posterior Fascicular Block on ECG

Direct Answer

Left posterior fascicular block (LPFB) is a rare but clinically significant finding that strongly suggests underlying inferior myocardial infarction and severe coronary artery disease, requiring comprehensive cardiac evaluation including coronary angiography. 1, 2

Diagnostic Criteria

To diagnose LPFB, all four of the following ECG criteria must be present simultaneously:

• Frontal plane QRS axis between 90° and 180° (right axis deviation) 1
• rS pattern in leads I and aVL (small r wave followed by deep S wave) 1
• qR pattern in leads III and aVF (small q wave followed by tall R wave in inferior leads) 1
• QRS duration less than 120 ms (preserved QRS duration distinguishes this from bundle branch block) 1

Critical Diagnostic Pitfalls

LPFB cannot be definitively diagnosed in the presence of:

• Right ventricular hypertrophy (COPD/emphysema) 3
• Extensive lateral myocardial infarction 3
• Extremely vertical heart position 3

These conditions can mimic the ECG pattern of LPFB, making clinical correlation essential 3.

Clinical Significance and Associations

Strong Association with Coronary Disease

Isolated LPFB is an extremely rare finding (0.24% prevalence) but serves as a reliable marker for serious cardiac pathology 3, 2:

• 89% of patients with LPFB have three-vessel coronary artery disease 2
• LPFB is reliably connected with inferior myocardial infarction, which it often masks on ECG 2
• The diagnosis of inferior MI is frequently missed initially because LPFB alters the typical ECG pattern 2

Bifascicular and Trifascicular Block

• LPFB is usually associated with right bundle branch block (bifascicular block), not isolated 1
• Bifascicular blocks reflect extensive involvement of the specialized conduction system and carry increased risk of clinically significant AV block 1
• When first-degree AV block accompanies bifascicular block (trifascicular involvement), mortality risk is higher 1
• PR interval prolongation is very frequent when LPFB is associated with RBBB and acute inferior MI 3

Evaluation and Management Algorithm

Immediate Evaluation

1. Confirm the diagnosis by verifying all four mandatory ECG criteria are met and excluding mimicking conditions (RVH, lateral MI, vertical heart) 1, 3

2. Assess for acute coronary syndrome:

   • Look specifically for inferior MI, which LPFB commonly masks 2
   • Check for associated RBBB (bifascicular block) 1
   • Evaluate PR interval for first-degree AV block (trifascicular involvement) 1

3. Obtain cardiac biomarkers (troponin) given the strong association with myocardial infarction 2

Comprehensive Cardiac Work-Up

Given the high prevalence of severe coronary disease, invasive investigation is warranted 2:

• Coronary angiography should be strongly considered, as 89% have three-vessel disease 2

• Transthoracic echocardiography to assess:

  • Left ventricular function and wall motion abnormalities 4
  • Ventricular septal integrity (LPFB can indicate septal rupture post-MI) 5
  • Right ventricular size to exclude RVH as a mimic 3

• 24-hour ECG monitoring to assess for:

  • Intermittent higher-grade AV block 6
  • Progression of conduction disease 4

Ongoing Monitoring

• Serial ECGs to monitor for progression to higher-grade AV block, particularly in bifascicular or trifascicular block 4, 1
• Follow ACC/AHA/HRS bradycardia guidelines for pacing indications if symptoms develop or conduction disease progresses 4
• Permanent pacing may be indicated for persistent symptomatic second- or third-degree AV block 6

Special Considerations

Pediatric Patients

• In children up to 16 years of age, apply the rightward axis criterion only when a distinct rightward change in axis is documented, since children normally have more rightward axes than adults 1
• Congenital heart disease with left axis deviation present from infancy does not meet criteria for acquired fascicular block 4

Mechanical Disruption

• Development of isolated LPFB complicating myocardial infarction may alert to possible ventricular septal rupture, requiring urgent echocardiographic evaluation 5