What are the electrocardiogram (ECG) findings and clinical presentation of a left bundle branch block (LBBB) in an older adult with a history of cardiovascular disease, hypertension, or coronary artery disease?

ECG Findings of Left Bundle Branch Block

Complete LBBB on ECG shows 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 leads, and an R peak time >60 ms in V5-V6. 1

Diagnostic Criteria for Complete LBBB

The following five criteria must be present for definitive diagnosis 1:

• QRS duration ≥120 ms in adults 1
• Broad notched or slurred R waves in leads I, aVL, V5, and V6 (occasionally an RS pattern may appear in V5-V6 due to displaced QRS transition) 1
• Absent Q waves in leads I, V5, and V6 (though a narrow Q wave may appear in aVL without indicating myocardial 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 are discernible 1
• ST segments and T waves directed opposite to the QRS complex (discordance pattern) 1

Expected ST-T Wave Patterns

The hallmark of LBBB is QRS-T wave axis discordance 1:

• Leads with predominantly negative QRS complexes (QS or rS patterns) typically show ST-segment elevation and prominent upright T-waves 1
• Leads with large monophasic R waves demonstrate ST-segment depression and inverted T-waves 1
• Loss of this normal discordance may indicate myocardial injury or ischemia and warrants urgent evaluation 1

Incomplete LBBB

Incomplete LBBB shares the same morphologic features but with important distinctions 1:

• QRS duration between 110-119 ms (not reaching the 120 ms threshold) 1
• Presence of left ventricular hypertrophy pattern 1
• R peak time >60 ms in leads V5, V5, and V6 1
• Absence of Q waves in leads I, V5, and V6 1

Clinical Significance and Red Flags

LBBB is very rare in healthy individuals and represents a strong ECG marker of underlying structural cardiovascular disease 1, 2:

• May be the earliest manifestation of ischemic heart disease or cardiomyopathy, appearing years before structural LV changes become detectable on imaging 1, 2
• Intermittent or rate-dependent LBBB carries the same clinical and prognostic significance as stable LBBB 1, 2
• Even asymptomatic LBBB mandates cardiology evaluation including exercise testing, 24-hour ECG monitoring, and echocardiography to exclude ischemic heart disease, cardiomyopathies, hypertensive heart disease, myocarditis, and other structural pathology 1, 2

Common Pitfalls

Diagnosing LVH in the presence of LBBB is unreliable unless specific criteria are met 2, 3:

• Standard voltage criteria should not be used 2, 3
• If LVH diagnosis is attempted, use QRS duration >155 ms combined with left atrial abnormality and precordial voltage criteria (SV2 + RV6 >4.5 mV) 2, 3, 4

Diagnosing acute MI in LBBB requires specific criteria 1:

• ST-segment elevation ≥1 mm concordant with QRS (sensitivity 73%, specificity 92%) 1
• ST-segment depression ≥1 mm in V1-V3 concordant with QRS (sensitivity 25%, specificity 96%) 1
• ST-segment elevation ≥5 mm discordant with QRS (sensitivity 19%, specificity 82%) 1

Additional ECG Features

The typical LBBB pattern shows 5, 6:

• No septal Q waves in leads I, V5, and V6 due to abnormal septal activation from right to left 5
• Notched or slurred R waves in at least one lateral lead 6
• R wave duration ≤20 ms in V1 when present 6
• QRS axis often shifts leftward, with median axis around -18° (approximately 18° more leftward than baseline) 6