What is a left posterior fascicular block?

Left Posterior Fascicular Block: Definition and Clinical Significance

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

Diagnostic Criteria

• LPFB is defined by the following ECG criteria:

  • Frontal plane axis between 90° and 180° in adults 1
  • rS pattern (small r, deep S) in leads I and aVL 1
  • qR pattern (small q, tall R) in leads III and aVF 1
  • QRS duration less than 120 ms 1

• In children, the diagnostic criteria should be applied cautiously as they normally have a more rightward axis until age 16, and LPFB should only be diagnosed when a distinct rightward change in axis is documented 1

Epidemiology and Etiology

• LPFB is an extremely rare intraventricular conduction disorder, especially as an isolated finding 2

• The posterior fascicle is less vulnerable to damage than other parts of the conduction system due to its:

  • Thicker structure compared to the anterior fascicle 2
  • Dual blood supply (from both the anterior descending and posterior descending coronary arteries) 3

• Common causes of LPFB include:

  • Ischemic heart disease, particularly inferior myocardial infarction 4
  • Severe coronary artery disease, especially three-vessel disease 4
  • Mechanical disruption of the posterior fascicle, such as in ventricular septal defect 3
  • Cardiomyopathies 1
  • Conduction tissue disease 1

Clinical Significance

• LPFB is often associated with underlying cardiac pathology:

  • When found in patients with inferior myocardial infarction, LPFB has been shown to be a reliable marker for severe coronary artery disease, with a high correlation to three-vessel disease requiring invasive investigation 4
  • LPFB can mask the ECG signs of inferior myocardial infarction, making diagnosis challenging 4
  • The development of LPFB during inferior wall myocardial infarction may serve as a warning sign for possible septal rupture 3

• LPFB can occur in different forms:

  • Isolated (rare) 2
  • Associated with other conduction abnormalities (more common) 2
  • Permanent or intermittent (transient) 5
  • Rate-dependent or rate-independent 5

Clinical Evaluation

• When LPFB is detected, further evaluation is recommended:

  • Transthoracic echocardiography is reasonable to exclude structural heart disease 1
  • In symptomatic patients where atrioventricular block is suspected, ambulatory electrocardiographic monitoring is useful 1

• Special consideration should be given to patients with alternating left anterior and left posterior fascicular block, as this pattern suggests infra-Hisian disease with risk of progression to complete atrioventricular block 6

Differential Diagnosis

• The diagnosis of LPFB should always consider clinical context, as similar ECG patterns can be seen in:
  • Right ventricular hypertrophy (e.g., in COPD/emphysema) 2
  • Extensive lateral myocardial infarction 2
  • Extremely vertical heart orientation 2

Management Implications

• LPFB itself rarely causes symptoms but may indicate underlying heart disease requiring treatment 1

• When LPFB is associated with specific arrhythmias like verapamil-sensitive idiopathic left ventricular tachycardia (fascicular VT):

  • Catheter ablation is recommended as first-line treatment in symptomatic patients 1
  • When catheter ablation is not available or desired, treatment with beta-blockers, verapamil, or sodium channel blockers is recommended 1

• In patients with alternating bundle branch blocks or fascicular blocks suggesting infra-Hisian disease, electrophysiological study and consideration for permanent pacemaker implantation may be warranted 6