What do you see in a Left Bundle Branch Block (LBBB)?

From the Guidelines

In a Left Bundle Branch Block (LBBB), you see characteristic ECG changes including a widened QRS complex greater than 120 milliseconds, absence of septal Q waves in leads I and V6, broad monophasic R waves in leads I, aVL, V5, and V6, and a broad, slurred S wave in leads V1 and V2. These findings occur because electrical impulses cannot travel through the left bundle branch, forcing them to spread through the myocardium in an abnormal sequence, as described in the 2009 American Heart Association/American College of Cardiology Foundation/Heart Rhythm Society recommendations for the standardization and interpretation of the electrocardiogram 1. The right ventricle depolarizes first, followed by delayed activation of the left ventricle, which creates the distinctive ECG pattern. LBBB is significant because it can mask signs of myocardial infarction, complicate interpretation of stress tests, and may indicate underlying heart disease such as coronary artery disease, hypertension, cardiomyopathy, or valvular heart disease. Some key points to consider in LBBB include:

• QRS duration greater than or equal to 120 ms in adults, as defined by the American Heart Association/American College of Cardiology Foundation/Heart Rhythm Society 1
• Broad notched or slurred R wave in leads I, aVL, V5, and V6 and an occasional RS pattern in V5 and V6 attributed to displaced transition of QRS complex
• Absent q waves in leads I, V5, and V6, but in the lead aVL, a narrow q wave may be present in the absence of myocardial pathology
• R peak time greater than 60 ms in leads V5 and V6 but normal in leads V1, V2, and V3 when small initial r waves can be discerned in the above leads
• ST and T waves usually opposite in direction to QRS When encountering LBBB, it's essential to compare with previous ECGs to determine if it's new or chronic, as new LBBB in the setting of chest pain may represent acute myocardial infarction requiring immediate intervention, as suggested by the 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay 1. Additionally, the presence of LBBB on ECG markedly increases the likelihood that left ventricular systolic dysfunction will be diagnosed by echocardiogram, and cardiac MRI may be considered in selected patients with LBBB and normal left ventricular function by echocardiography where sarcoidosis, connective tissue disease, myocarditis, or other dilated cardiomyopathies are suspected on clinical grounds 1. It is also important to note that LBBB can be associated with a higher risk of cardiac events, such as heart failure and sudden cardiac death, and that cardiac resynchronization therapy may be beneficial in patients with LBBB and heart failure with reduced ejection fraction, as reported in the 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay 1.

From the Research

Left Bundle Branch Block (LBBB) Characteristics

• LBBB may be due to conduction system degeneration or a reflection of myocardial pathology 2
• It may also develop following aortic valve disease or cardiac procedures 2
• LBBB can occur frequently in heart failure and may trigger a vicious cycle of progressive left ventricular dysfunction 3

Diagnosis and Treatment

• Correctly diagnosing LBBB is fundamental, as it is pivotal to guide cardiac resynchronisation therapy 3
• The European Society of Cardiology (ESC) recently updated the LBBB diagnostic criteria, which can help improve diagnostic accuracy 3
• Cardiac resynchronization therapy (CRT) can be an effective treatment for patients with heart failure and LBBB, especially when lead placement is done via the coronary sinus 2
• Physiological pacing is also being explored as an alternative option for CRT 2

Relationship between ECG and Echocardiography

• The relationship between myocardial electrical activation by electrocardiogram (ECG) and mechanical contraction by echocardiography in LBBB has been demonstrated 4
• New strict criteria for LBBB based on ECG and echocardiography have been established, which can help identify CRT candidates 4
• Echocardiographic coronary flow assessment can be affected by LBBB, but it can still be a useful tool for evaluating coronary artery disease (CAD) and stratifying risk 5

Recent Insights and Characterization

• LBBB is not just a simple electrocardiogram alteration, but a complex entity that affects patient diagnosis, treatment, and prognosis 6
• Recent insights into the pathophysiology of human LBBB have improved our understanding of this condition, and recruiting the left bundle branch with conduction system pacing depends on the complex interaction between anatomy, site of pathophysiology, and delivery tools 6