Left Posterior Fascicular Block: Diagnostic Criteria and Clinical Significance

Diagnostic Criteria

• The American College of Cardiology defines left posterior fascicular block (LPFB) by the following ECG criteria: 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
• In children, LPFB diagnostic criteria should be applied cautiously due to their normally 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

• Cardiomyopathies are a common cause of LPFB 2
• Conduction tissue disease is also a common cause of LPFB 3

Clinical Evaluation

• The American College of Cardiology recommends transthoracic echocardiography to exclude structural heart disease when LPFB is detected 2
• In symptomatic patients where atrioventricular block is suspected, ambulatory electrocardiographic monitoring is useful 2

Management Implications

• LPFB itself rarely causes symptoms but may indicate underlying heart disease requiring treatment, according to the American College of Cardiology 2
• The European Heart Society recommends catheter ablation as first-line treatment for symptomatic patients with verapamil-sensitive idiopathic left ventricular tachycardia (fascicular VT) associated with LPFB 4
• The European Heart Society also recommends treatment with beta-blockers, verapamil, or sodium channel blockers when catheter ablation is not available or desired for patients with LPFB and fascicular VT 4

Management of Left Posterior Fascicular Block (LPFB) with Symptomatic Fascicular Ventricular Tachycardia

Initial Treatment Options

• Catheter ablation is recommended as first-line therapy in experienced centers, particularly for young patients who would otherwise require lifelong drug therapy, with acute success rates exceeding 90% and recurrence rates of 0-20%, according to the European Heart Journal and the American College of Cardiology 5, 6
• The target for catheter ablation is typically the distal insertion of the anterograde limb of the Purkinje system along the inferior LV septum near the left posterior fascicle junction, as recommended by the American College of Cardiology 6

Alternative Medical Management

• Beta-blockers, verapamil, or sodium channel blockers (class IC agents) are recommended when catheter ablation is not available, not desired, or has failed, with intravenous verapamil recommended for acute VT termination in hemodynamically stable patients, according to the European Heart Journal and the American College of Cardiology 5, 6
• Chronic oral verapamil can be useful for preventing recurrences, though it may not be effective long-term in all patients, as noted by the European Heart Journal and the American College of Cardiology 5, 6

Ambulatory Monitoring Recommendations for Left Posterior Fascicular Block

Symptomatic Patients

• The American College of Cardiology recommends ambulatory electrocardiographic monitoring to document suspected atrioventricular block in patients who experience intermittent bradycardia symptoms such as lightheadedness, presyncope, or syncope. 7

Asymptomatic Patients

• For patients with isolated left posterior fascicular block who are asymptomatic, the American College of Cardiology suggests that ambulatory monitoring may be considered, although it is not mandatory. 7

Follow‑up Assessment

• The American College of Cardiology advises that new onset symptoms (e.g., syncope, presyncope, palpitations) should prompt reassessment with ambulatory monitoring or an electrophysiology study. 7