Diagnostic Criteria for Left Bundle Branch Block (LBBB)
Complete LBBB requires three essential components: QRS duration ≥120 ms in adults, broad notched or slurred R waves in lateral leads (I, aVL, V5, V6), and absent Q waves in these same lateral leads. 1
Complete LBBB Criteria
QRS Duration Requirements
Morphologic Features (All Must Be Present)
- Broad notched or slurred R waves in leads I, aVL, V5, and V6 (occasional RS pattern may appear in V5-V6 due to displaced transition) 1
- Absent Q waves in leads I, V5, and V6 (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 V1-V3 when small initial r waves are discernible 1
ST-T Wave Changes
- ST and T waves are typically opposite in direction to the QRS complex (appropriate discordance) 1
- Positive T waves in leads with upright QRS may be normal (positive concordance is acceptable) 1
- Depressed ST segments and/or negative T waves in leads with negative QRS are abnormal (negative concordance indicates additional pathology) 1
Axis Considerations
- LBBB may shift the mean QRS axis rightward, leftward, or superiorly, sometimes in a rate-dependent manner 1
Incomplete LBBB Criteria
Incomplete LBBB demonstrates similar morphologic features but with shorter QRS duration:
- Adults: QRS duration 110-119 ms 1
- Children 8-16 years: QRS duration 90-100 ms 1
- Children <8 years: QRS duration 80-90 ms 1
- Presence of left ventricular hypertrophy pattern 1
- R peak time >60 ms in leads V4, V5, and V6 1
- Absent Q waves in leads I, V5, and V6 1
Critical Pitfalls to Avoid
Do Not Confuse with Nonspecific IVCD
- Nonspecific intraventricular conduction delay is defined as QRS >110 ms without meeting the morphology criteria for RBBB or LBBB 1
- QRS prolongation alone is insufficient for LBBB diagnosis—all morphologic criteria must be met 1
Rate-Dependent Considerations
- LBBB appearance may be rate-dependent in some cases, so evaluate at different heart rates when possible 1
Masking of Underlying Pathology
- LBBB can mask underlying myocardial pathology due to altered ventricular activation sequence, making diagnosis of ischemia or infarction challenging 1
Special Circumstance: Diagnosing LVH in Presence of LBBB
The diagnosis of left ventricular hypertrophy in the presence of LBBB is extremely challenging and should generally not be attempted. 2 However, when specific criteria are met, LVH diagnosis is reasonable:
- Left atrial P-wave abnormality present 2, 1
- QRS duration >155 ms 2, 1
- Precordial lead voltage criteria met 2
These criteria have relatively high specificity but low sensitivity, so LVH diagnosis should only be made when all three features are present 2
Controversy Regarding Stricter Criteria
While some research has proposed stricter criteria (QRS ≥140 ms in men, ≥130 ms in women, with mandatory mid-QRS notching) for cardiac resynchronization therapy patient selection 3, 4, the established AHA/ACCF/HRS guideline criteria with QRS ≥120 ms remain the standard for general LBBB diagnosis 1. The stricter criteria may improve specificity in research settings but have not demonstrated superior clinical outcomes in all studies 5.