What is a left posterior fascicular block?

Left Posterior Fascicular Block: Definition and Clinical Significance

Left posterior fascicular block (LPFB) is a conduction disorder defined by specific ECG criteria including a frontal plane axis between 90° and 180° in adults, rS pattern in leads I and aVL, qR pattern in leads III and aVF, and QRS duration less than 120 ms. 1

Diagnostic Criteria

• LPFB is diagnosed when the following ECG findings are present:

  • Right axis deviation with frontal plane axis between 90° and 180° in adults 1
  • rS pattern (small r wave, deep S wave) in leads I and aVL 1
  • qR pattern (small q wave, tall R wave) in leads III and aVF 1
  • QRS duration remains normal (<120 ms) 1

• In children, diagnosis requires caution as they normally have a more rightward axis until age 16; LPFB should only be diagnosed when a distinct rightward change in axis is documented 1

Epidemiology and Etiology

• LPFB is an extremely rare finding both in isolated form and in specific patient populations 2
• Common causes include:
  • Cardiomyopathies 1
  • Conduction tissue disease 1
  • Inferior myocardial infarction (IMI) - LPFB has been found to be a reliable marker for IMI and associated severe coronary artery disease 3
  • Mechanical disruption of the posterior fascicle, such as in ventricular septal defects 4

Clinical Significance

• LPFB itself rarely causes symptoms but often indicates underlying heart disease requiring treatment 1
• When LPFB is detected, transthoracic echocardiography is recommended to exclude structural heart disease 1
• In patients with IMI, the presence of LPFB often correlates with severe three-vessel coronary artery disease, warranting invasive investigation 3
• LPFB can mask the ECG signs of inferior myocardial infarction, making diagnosis challenging 3
• Alternating left anterior and posterior fascicular block may indicate risk for progression to complete atrioventricular block 5

Management Considerations

• For symptomatic patients where atrioventricular block is suspected, ambulatory electrocardiographic monitoring is useful 1
• In patients with LPFB and fascicular ventricular tachycardia:
  • Catheter ablation is recommended as first-line treatment for symptomatic patients 1
  • Beta-blockers, verapamil, or sodium channel blockers are alternatives when catheter ablation is not available or desired 1
• Patients with alternating fascicular blocks may require electrophysiological study and possibly pacemaker implantation due to the risk of progression to complete heart block 5

Differential Diagnosis

• LPFB must be distinguished from:
  • Right ventricular hypertrophy (RVH) due to COPD/emphysema 2
  • Extensive lateral myocardial infarction 2
  • Extremely vertical heart position 2

Special Considerations

• LPFB can occur as an intermittent phenomenon, either rate-dependent or rate-independent 6
• When LPFB is associated with right bundle branch block and acute inferior MI, PR interval prolongation is very frequent 2
• The development of LPFB during inferior wall myocardial infarction may serve as an alert for possible underlying septal rupture 4