From the Guidelines
Right bundle branch block (RBBB) with left anterior fascicular block (LAPB) on an ECG represents bifascicular block, which requires clinical correlation but often needs no specific treatment if asymptomatic, however, pacemaker implantation is indicated for documented high-degree AV block, alternating bundle branch block, or syncope with HV interval prolongation (>70ms) on electrophysiology study, as recommended by the 2018 ACC/AHA/HRS guideline 1.
Management and Implications
The management of RBBB and LAPB focuses on identifying underlying causes such as coronary artery disease, cardiomyopathy, or valvular disease. For asymptomatic patients without structural heart disease, regular cardiac follow-up is recommended without medication.
- If symptoms like syncope occur, evaluation with echocardiogram, stress testing, and possibly Holter monitoring is warranted.
- Patients with syncope or presyncope may need electrophysiology studies to assess for complete heart block risk.
- The clinical significance lies in the disruption of normal ventricular activation sequence, with electrical impulses first traveling down the intact left posterior fascicle before spreading to other ventricular regions.
ECG Findings
This conduction pattern creates characteristic ECG findings:
- RBBB shows an RSR' pattern in V1-V3 with wide QRS (>120ms) and slurred S waves in leads I and V6,
- while LAPB manifests as left axis deviation (-45° to -90°), small Q waves in leads I and aVL, and small R waves in leads II, III, and aVF.
Risk of Progression
The combination of RBBB and LAPB suggests more extensive conduction system disease and carries higher risk of progression to complete heart block than either finding alone, as noted in the 2013 ESC guidelines on cardiac pacing and cardiac resynchronization therapy 1.
Recommendations
According to the 2018 ACC/AHA/HRS guideline, patients with syncope, BBB, and HV >70ms should receive permanent pacing (Class I) 1, and alternating bundle branch block is also an indication for permanent pacing (Class I) 1.
- Additionally, the 2019 ACC/AHA/HRS guideline recommends that patients with LVEF 36-50%, LBBB, QRS >150 ms, and Class II or greater HF symptoms may benefit from cardiac resynchronization therapy (Class IIb) 1.
- The European Society of Cardiology also recommends that patients with alternating bundle branch block should receive a pacemaker, even in the absence of a history of syncope, due to the high risk of progression to AV block 1.
From the Research
Implications of RBBB and LAPB on ECG
- RBBB and LAPB can manifest as aberrant conduction in apparently healthy individuals, with evidence of left bundle branch involvement in the limb leads 2
- The combination of RBBB and LAPB may be related to the simultaneous occurrence of block of conduction through these components of the Purkinje network 2
- RBBB can be associated with a poor prognosis, especially when accompanied by severe underlying heart disease, such as inferior myocardial infarction or severe inferior fibrosis in cardiomyopathies 3
Management of RBBB and LAPB
- Permanent pacing is often considered for patients with syncope and bifascicular block, including those with RBBB and LAPB 4
- Pacemaker implantation has been shown to reduce adverse events compared to implantable loop recorder monitoring in patients with syncope and bifascicular block, irrespective of the type of bundle branch block 4
- In patients with high-risk pulmonary embolism, the presence of RBBB with a QR pattern in V1 on ECG can be a predictor of sudden cardiac arrest and may require advanced treatments, including systemic thrombolysis or invasive embolectomy 5
ECG Characteristics
- RBBB is characterized by a widened QRS complex and changes in the directional vectors of the R and S waves on the 12-lead ECG 6
- LAPB can cause a small q wave in lead I, with the initial forces oriented rightwards and inferiorly 3
- The presence of a QR pattern in V1 on ECG can be an independent predictor for high-risk pulmonary embolism patients who may require advanced treatments 5