Left bundle branch block

Content last updated 2026-10-07 · 2 min read ·

This is the date the draft was written, not a medical review date.

In left bundle branch block, activation of the left ventricle is delayed because it spreads cell to cell from the right ventricle. The QRS is wide with a characteristic broad, notched R wave in the lateral leads, and the ST-T changes are secondary to abnormal depolarisation.

ECG criteria

QRS duration 120 ms or more (complete LBBB).
Broad, notched or slurred R wave in I, aVL, V5 and V6, usually with no septal Q waves in these leads.
rS or QS complex in V1, sometimes with a small r wave.
Secondary ST-T changes: ST depression and T inversion in leads with a dominant R wave, ST elevation with upright T in leads with a dominant S wave.
Delayed R-wave peak time (above 60 ms) in V5-V6; left axis deviation is common but not required.

Mechanism

Block in the left main bundle or both fascicles forces the left ventricle to be activated through slow myocardial conduction after the septum depolarises abnormally from right to left. This reverses the normal direction of septal activation, removes the septal Q waves and lengthens the QRS.

Common causes

Ischaemic heart disease and previous myocardial infarction.
Hypertensive heart disease and left ventricular hypertrophy; dilated cardiomyopathy.
Aortic valve disease (especially aortic stenosis) and degenerative conduction disease.
Myocarditis, infiltrative disease, cardiac surgery or TAVI; rarely in healthy people.

Management principles

LBBB itself is not treated but marks underlying heart disease, so assess for coronary disease, cardiomyopathy and valvular disease and for symptoms of heart failure or syncope. Cardiac resynchronisation therapy may be considered in selected patients with heart failure, reduced ejection fraction and wide QRS; follow local guidelines.

High-level principles for study. Treatment decisions follow local guidelines and the clinical situation.

Pitfalls and mimics

LBBB hides the ST-segment changes of acute myocardial infarction; interpretation requires the modified Sgarbossa criteria or comparison with a prior ECG.
A wide-complex tachycardia in a patient with known LBBB may still be ventricular tachycardia.
Ventricular pacing from the right ventricular apex produces an LBBB-like pattern; look for pacing spikes.

Practise in the ECG trainer — Recognise this pattern on generated tracings.

References

AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, Parts I to VI. American Heart Association / American College of Cardiology Foundation / Heart Rhythm Society (2009) — Tier 2 (guidelines / primary literature)
Wagner GS, Strauss DG. Marriott's Practical Electrocardiography. Wolters Kluwer, 12th edition (2013) — Tier 3 (textbook)
2023 ESC Guidelines for the management of acute coronary syndromes. European Society of Cardiology (2023) — Tier 2 (guidelines / primary literature)

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