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

How to Identify Left Bundle Branch Block on ECG

To diagnose left bundle branch block (LBBB) on ECG, look for QRS duration ≥120 ms with specific morphologic features: QS or rS pattern in leads V1-V2, broad monophasic or notched R waves in lateral leads (I, aVL, V5, V6) without q waves in these leads, and mid-QRS notching or slurring in at least 2 of these lateral leads. 1

Essential Diagnostic Criteria

QRS Duration Requirements

• QRS duration must be ≥140 ms in men or ≥130 ms in women for strict diagnosis of LBBB 2
• The broader definition accepts QRS duration >120 ms, though this may include cases of left ventricular hypertrophy with intraventricular conduction delay rather than true LBBB 1
• QRS duration >155 ms has relatively high specificity for identifying true complete LBBB 1

Morphologic Features in Precordial Leads

• Lead V1 and V2 must show QS or rS pattern (predominantly negative deflection with slurred S wave) 1, 3
• Leads V5 and V6 must show monophasic notched or plateau-topped R waves without preceding q waves 1, 4
• The absence of septal q waves in leads I, V5, and V6 is mandatory due to abnormal right-to-left septal activation 4
• Slurred predominant R waves in left precordial leads are characteristic 1

Morphologic Features in Limb Leads

• Mid-QRS notching or slurring must be present in ≥2 of the following leads: V1, V2, V5, V6, I, and aVL 2
• Dominant R waves should be present in leads I and aVL (present in 97.2% of true LBBB cases) 3
• Notched or slurred QRS complexes in at least one of leads I, aVL, V5, or V6 are present in 100% of true LBBB cases 3

Intrinsicoid Deflection

• R peak time (intrinsicoid deflection) in leads V5 and V6 must be >60 ms 4
• This reflects the delayed activation of the left ventricle 4

Common Pitfalls to Avoid

Distinguishing True LBBB from Mimics

• Do not diagnose LBBB if q waves are present in leads V5, V6, or lead I - this indicates preserved septal activation and suggests intraventricular conduction delay rather than true bundle branch block 4, 3
• Broader definitions that only require QRS >120 ms with slurred R waves may misclassify LVH with conduction delay as LBBB 1
• The distinction between "complete" LBBB and LVH with intraventricular conduction delay can be difficult, as complete LBBB may not be truly complete 1

Strict vs. Broad Criteria

• Strict criteria (requiring monophasic notched R waves in lateral leads) have higher specificity but lower sensitivity 1
• Using strict criteria improves identification of patients who will respond to cardiac resynchronization therapy (82% response rate vs. 21% without strict LBBB) 2
• When ECG and echocardiographic criteria for LBBB are concordant, 95% of patients respond to CRT, validating the importance of strict criteria 2

Clinical Significance

Association with Cardiac Disease

• LBBB is generally associated with poorer prognosis compared to normal conduction or right bundle branch block 4
• LBBB may be the first manifestation of diffuse myocardial disease 4
• Abnormal asynchronous ventricular contraction can lead to asymmetric hypertrophy and left ventricular dilatation 4

Impact on Other ECG Diagnoses

• The diagnosis of left ventricular hypertrophy should generally not be attempted when LBBB is present, except when specific high-specificity criteria are met (QRS >155 ms plus precordial voltage criteria SV1 or SV2 + RV6 >4.5 mV) 1, 5
• When these specific criteria are present, LVH can be diagnosed with 86% sensitivity and 100% specificity despite LBBB 5