What are the diagnostic criteria for Left Bundle Branch Block (LBBB)?

Diagnostic Criteria for Left Bundle Branch Block (LBBB)

The diagnostic criteria for complete Left Bundle Branch Block (LBBB) include QRS duration ≥120 ms in adults, broad notched or slurred R waves in leads I, aVL, V5, and V6, absent q waves in leads I, V5, and V6, and R peak time >60 ms in leads V5 and V6. 1

Complete LBBB Criteria

The American Heart Association/American College of Cardiology Foundation/Heart Rhythm Society guidelines define complete LBBB with the following criteria:

• QRS duration ≥120 ms in adults, >100 ms in children 4-16 years of age, and >90 ms in children <4 years of age 1
• Broad notched or slurred R wave in leads I, aVL, V5, and V6 with occasional RS pattern in V5 and V6 due to displaced transition of QRS complex 1
• Absent q waves in leads I, V5, and V6 (though a narrow q wave may be present in aVL without indicating pathology) 1
• R peak time >60 ms in leads V5 and V6 but normal in leads V1, V2, and V3 when small initial r waves can be discerned 1
• ST and T waves usually opposite in direction to QRS 1
• Positive T wave in leads with upright QRS may be normal (positive concordance) 1
• Depressed ST segment and/or negative T wave in leads with negative QRS (negative concordance) are abnormal 1
• LBBB may change the mean QRS axis in the frontal plane to the right, left, or superior, sometimes in a rate-dependent manner 1

Incomplete LBBB Criteria

Incomplete LBBB is characterized by:

• QRS duration between 110 and 119 ms in adults, between 90 and 100 ms in children 8-16 years of age, and between 80 and 90 ms in children <8 years of age 1
• Presence of left ventricular hypertrophy pattern 1
• R peak time >60 ms in leads V4, V5, and V6 1
• Absence of q waves in leads I, V5, and V6 1

Newer "Strict" LBBB Criteria

Some research suggests stricter criteria for LBBB diagnosis, particularly for patients being considered for cardiac resynchronization therapy (CRT):

• Longer QRS duration (≥140 ms for men, ≥130 ms for women) 2, 3
• Mid-QRS notching or slurring in ≥2 contiguous leads 4, 3

However, studies have shown that these stricter criteria do not necessarily improve response to CRT compared to the conventional AHA criteria 4. The conventional criteria with a minimum QRS duration of 120 ms remain the standard recommendation 2.

LBBB in Special Circumstances

LBBB and Left Ventricular Hypertrophy

• The diagnosis of left ventricular hypertrophy (LVH) in the presence of LBBB is challenging 1
• In LBBB with suspected LVH, parameters based on the amplitude of S waves in V2 or V3 and R waves in leads aVL, V5, and V6 have the highest clinical value 5
• Left atrial P-wave abnormality and QRS duration >155 ms have relatively high specificity for LVH in the presence of LBBB 1

Clinical Pitfalls in LBBB Diagnosis

• Do not confuse LBBB with nonspecific intraventricular conduction delay, which is defined as QRS duration >110 ms in adults without meeting morphology criteria for RBBB or LBBB 1
• Be aware that LBBB can mask underlying myocardial pathology due to altered ventricular activation sequence 1
• Remember that the appearance of LBBB may be rate-dependent in some cases 1
• When evaluating for LBBB, ensure all criteria are met, not just QRS duration, as QRS prolongation alone is insufficient for diagnosis 1