Management of Left Posterior Fascicular Block
In isolated left posterior fascicular block (LPFB) without symptoms or AV block, no specific treatment or permanent pacing is required—observation alone is appropriate. 1
Initial Diagnostic Confirmation and Risk Assessment
Verify all ECG diagnostic criteria are met: 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 <120 ms. 1
Obtain a 12-lead ECG to document the conduction abnormality and screen for structural heart disease or systemic illness. 1
Perform transthoracic echocardiography to exclude structural heart disease, cardiomyopathy, or other cardiac pathology when LPFB is detected. 2
Assess for symptoms including bradycardia, syncope, presyncope, heart failure, or palpitations that might indicate progression to higher-degree AV block or associated arrhythmias. 1, 2
Management Based on Clinical Presentation
Asymptomatic Isolated LPFB
No intervention is required for isolated LPFB in the absence of symptoms or AV block—permanent pacing is not indicated. 1, 2
Monitor for progression to bifascicular block (LPFB + RBBB) or development of AV block, particularly in patients with known structural heart disease. 1
Post-Myocardial Infarction Context
Permanent pacing is NOT recommended for transient AV block in the presence of isolated LPFB or for acquired LPFB in the absence of AV block. 1
Permanent pacing IS indicated for persistent second-degree AV block in the His-Purkinje system with bilateral bundle-branch block or third-degree AV block after STEMI. 1
Symptomatic Fascicular Ventricular Tachycardia Associated with LPFB
This represents a distinct clinical entity requiring specific management:
Acute Management of Hemodynamically Stable VT
Administer intravenous verapamil as first-line acute treatment for verapamil-sensitive idiopathic left ventricular tachycardia (fascicular VT) presenting with RBBB morphology and left axis deviation. 2, 3
Beta-blockers are an alternative if verapamil is contraindicated or unavailable. 3
Perform immediate synchronized cardioversion if the patient is hemodynamically unstable with hypotension, altered mental status, or signs of shock. 3
Definitive Treatment Strategy
Catheter ablation is recommended as first-line therapy for symptomatic patients with fascicular VT, particularly in young patients who would otherwise require lifelong drug therapy, with acute success rates exceeding 90% and recurrence rates of 0-20%. 2, 3
Target the distal insertion of the anterograde limb of the Purkinje system along the inferior LV septum near the left posterior fascicle junction during ablation. 2
Development of left posterior fascicular block on surface ECG can serve as an effective endpoint for successful ablation. 4, 5
Long-Term Medical Management When Ablation Not Performed
Beta-blockers, verapamil, or sodium channel blockers (class IC agents) are recommended when catheter ablation is not available, not desired, or has failed. 2, 3
Chronic oral verapamil can be useful for preventing recurrences, though it may not be effective long-term in all patients. 2
Avoid class IC agents in patients with any history of myocardial infarction or structural heart disease, as they are contraindicated in this population. 3
Critical Pitfalls to Avoid
Do not diagnose LPFB in the presence of right ventricular hypertrophy (COPD/emphysema), extensive lateral MI, or extremely vertical heart position, as these conditions can mimic LPFB criteria. 6
In children up to age 16, apply LPFB criteria cautiously due to their normally more rightward axis—only diagnose when a distinct rightward change in axis is documented. 1
Do not use IV amiodarone as first-line therapy for fascicular VT when verapamil or beta-blockers are available, as these are more specific and effective. 3
Recognize that isolated LPFB is extremely rare—its presence should prompt thorough evaluation for underlying structural heart disease or other conduction abnormalities. 6