What is the treatment for left posterior fascicular block?

Treatment of Left Posterior Fascicular Block

Catheter ablation in experienced centers is recommended as first-line treatment for left posterior fascicular block (LPFB) associated with ventricular tachycardia, as it is more effective than long-term drug therapy with verapamil. 1

Diagnosis and Clinical Significance

• LPFB is diagnosed by ECG criteria including: 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, 2
• LPFB may occur in isolation or in association with other conduction abnormalities, such as right bundle branch block 3, 4
• LPFB itself rarely causes symptoms but may indicate underlying heart disease requiring treatment 2

Treatment Algorithm for LPFB

For LPFB Associated with Ventricular Tachycardia (Fascicular VT)

1. First-line treatment: Catheter ablation

   • Recommended for symptomatic patients with left posterior fascicular VT 1
   • Target site is typically the midseptum of left ventricle where earliest Purkinje potentials are recorded 5
   • The development of LPFB on ECG can serve as an endpoint for successful ablation 5, 6
   • Success rates after ablation range from 0-20% with recurrence rates of approximately 13% 1, 6

2. Pharmacological therapy (when ablation is not available or desired)

   • Verapamil is effective for acute termination of fascicular VT associated with LPFB 1, 5
   • Beta-blockers can be used for long-term management 1
   • Sodium channel blockers (class IC agents) may be considered in some cases 1

For Isolated LPFB Without Ventricular Tachycardia

• Isolated LPFB without symptoms generally requires no specific treatment 2
• Management should focus on identifying and treating any underlying structural heart disease 2
• Transthoracic echocardiography is recommended to exclude structural heart disease when LPFB is detected 2

For LPFB with Alternating Fascicular Block

• Patients with alternating left anterior and posterior fascicular block should be evaluated for risk of progression to complete atrioventricular block 7, 3
• Electrophysiological study may be warranted to assess for infra-Hisian disease 7
• Pacemaker implantation may be necessary in cases with evidence of infra-Hisian conduction disturbance 7

Clinical Pearls and Pitfalls

• LPFB is extremely rare in isolation and is often associated with underlying heart disease, particularly cardiomyopathies and coronary artery disease 2, 3, 4
• LPFB can be misdiagnosed in cases of right ventricular hypertrophy, extensive lateral myocardial infarction, or extremely vertical heart position 4
• In children, LPFB diagnostic criteria should be applied cautiously due to their normally more rightward axis until age 16 1, 2
• When LPFB is associated with right bundle branch block and acute inferior myocardial infarction, PR interval prolongation is common and may indicate higher risk of complete heart block 4
• Intermittent LPFBs are never complete blocks and may represent transient or second-degree block 4