What are the electrocardiographic (ECG) criteria for diagnosing a left bundle branch block (LBBB)?

From the Guidelines

Left bundle branch block (LBBB) is diagnosed on ECG using specific criteria, including a QRS duration of ≥120 ms in adults, broad notched or slurred R wave in leads I, aVL, V5, and V6, and absent q waves in leads I, V5, and V6, as outlined in the 2009 American Heart Association recommendations 1. The diagnostic criteria for LBBB can be broken down into several key points:

• QRS duration: ≥120 ms in adults, >100 ms in children 4 to 16 years of age, and >90 ms in children less than 4 years of age 1
• R wave morphology: broad notched or slurred R wave in leads I, aVL, V5, and V6, with an occasional RS pattern in V5 and V6 1
• Q wave absence: absent q waves in leads I, V5, and V6, but a narrow q wave may be present in lead aVL in the absence of myocardial pathology 1
• ST and T wave direction: usually opposite in direction to QRS, with positive T wave in leads with upright QRS and depressed ST segment and/or negative T wave in leads with negative QRS 1 These criteria, as outlined in the 2009 American Heart Association recommendations 1, are essential for accurately diagnosing LBBB on an ECG. The presence of LBBB can have significant implications for patient management and outcomes, particularly in the context of cardiovascular disease, and therefore accurate diagnosis using these criteria is crucial. In clinical practice, it is essential to carefully evaluate the ECG for these specific criteria to ensure accurate diagnosis and appropriate management of patients with LBBB.

From the Research

Electrocardiographic Criteria for Left Bundle Branch Block

The electrocardiographic (ECG) criteria for diagnosing a left bundle branch block (LBBB) include:

• A QRS complex that is notched or slurred 2
• The initial portion of the QRS complex (the "septal Q waves") is absent 2
• A QRS duration of at least 120 milliseconds in duration 2
• Some studies suggest a minimum QRS duration of 130 to 140 milliseconds 2, 3

Additional Considerations

• The QRS prolongation should not be gradual 2
• Stricter criteria, including longer QRS duration and slurring/notching of the QRS, may better identify responders to cardiac resynchronization therapy 3
• ST/T concordance and discordance, as well as vectorcardiography, may further improve the diagnosis of LBBB 3

Specific ECG Criteria

• An R wave in V1 or V2 of greater than 30-ms duration may indicate ventricular tachycardia in wide complex LBBB morphology tachycardias 4
• Any Q wave in V6 may also indicate ventricular tachycardia in wide complex LBBB morphology tachycardias 4
• A duration of greater than 60 ms from the onset of the QRS to the nadir of the S wave in V1 or V2 may indicate ventricular tachycardia in wide complex LBBB morphology tachycardias 4
• Notching on the downstroke of the S wave in V1 or V2 may indicate ventricular tachycardia in wide complex LBBB morphology tachycardias 4

Diagnosis of Left Ventricular Hypertrophy in LBBB

• ECG parameters based on the amplitude of S wave in V2 or V3 and R wave in the leads aVL, V5, V6 have the highest clinical value in the prediction of left ventricular hypertrophy (LVH) in patients with LBBB 5
• New cut points for LVH parameters have been proposed, including S2+SV3 〉 6 mV, SV2,V3+RV5,V6 〉 4 mV, RaVL+SV3 〉 3.5 mV 5