ECG Findings in Left Bundle Branch Block
Left bundle branch block on ECG is defined by a QRS duration ≥120 ms with broad notched or slurred R waves in lateral leads (I, aVL, V5, V6), absent Q waves in these same lateral leads, and an R peak time >60 ms in V5-V6. 1
Core Diagnostic Criteria
The essential ECG features that must be present for LBBB diagnosis include:
- QRS duration ≥120 ms – This is the minimum threshold, though durations ≥130-140 ms are more specific for true complete block 1, 2
- Broad, notched or slurred R waves in leads I, aVL, V5, and V6 – The notching/slurring is a critical morphologic feature 1, 2
- Absent Q waves in leads I, V5, and V6 – The loss of normal septal Q waves is pathognomonic 1, 2
- R peak time >60 ms in leads V5-V6 – This reflects delayed left ventricular activation 1
Lead-Specific Morphology Patterns
Lateral Leads (I, aVL, V5, V6)
- Dominant R waves with notching or slurring present in at least one of these leads 3, 4
- No q or Q waves should be present in V5 or V6 4
- The R wave morphology shows characteristic broad, slurred upstrokes 1
Right Precordial Leads (V1, V2)
- QS or rS pattern (predominantly negative QRS complex) 1, 4
- If an R wave is present in V1, it must be ≤20 ms in duration 3
- Never shows the typical qR pattern seen in other conditions 4
Inferior Leads
- May show variable patterns but contribute to overall QRS morphology assessment 4
Additional ECG Characteristics
QRS axis deviation commonly accompanies LBBB:
- Left axis deviation (between -30° and -90°) occurs frequently but is not required for diagnosis 5
- The QRS axis typically shifts leftward by approximately 18° compared to baseline 3
- Left axis deviation in LBBB does not independently predict worse left ventricular function 5
ST-T wave changes are expected secondary repolarization abnormalities:
- ST segments and T waves are typically discordant (opposite direction) to the main QRS deflection 6
- These changes are a normal consequence of the conduction delay and do not indicate ischemia 6
Clinical Significance and Pitfalls
LBBB is rarely benign and demands evaluation. It is extremely uncommon in healthy individuals and strongly indicates underlying structural heart disease, including ischemic heart disease, cardiomyopathy, hypertensive heart disease, or diffuse myocardial pathology 6, 7. Autopsy studies show approximately 90% of LBBB patients have anatomic left ventricular hypertrophy 6.
QRS duration correlates inversely with left ventricular function. When QRS duration reaches ≥170 ms, the mean ejection fraction drops to approximately 24%, compared to 36% for QRS <170 ms 5. This relationship is significant (p<0.001) and clinically useful for risk stratification 5.
Diagnosing left ventricular hypertrophy in LBBB is unreliable unless all three criteria are met: QRS duration >155 ms, left atrial abnormality present, and precordial voltage criteria satisfied 6. Even then, sensitivity remains low 6.
Intermittent or rate-dependent LBBB carries identical clinical significance as persistent LBBB and requires the same thorough evaluation 6.
New LBBB in acute MI settings represents a positive ECG finding when the QRS duration increases by at least 20 ms from <120 ms to ≥120 ms 1. This pattern is associated with 64% increased odds of in-hospital mortality 6.
Recommended Evaluation
When LBBB is identified, comprehensive cardiology work-up should include 6:
- Exercise stress testing to detect exercise-induced arrhythmias or conduction worsening
- 24-hour Holter monitoring for intermittent conduction abnormalities
- Transthoracic echocardiography to assess for structural disease, cardiomyopathy, ischemic disease, or hypertensive heart disease