Left bundle branch block (LBBB)

• QRS duration of 120 ms (3 small squares) or more
• No secondary R wave in lead V1
• No Q waves in the lateral leads (V5-6, I and aVL)
• Secondary ST-T changes:
• ST segment changes, opposite to the dominant (terminal) QRS component
• T wave changes in the same direction as the ST segments
• These changes can mask the primary changes of acute myocardial infarction

Features of this ECG

• Sinus rhythm, 66 b.p.m.
• Diagnostic features of LBBB:
• Broad QRS, 135 ms
• No secondary R wave in V1
• No Q waves in the lateral leads

• Other features of LBBB:
• ST elevation in leads V1-V4
• T wave inversion leads I and aVL

• Left axis deviation -30 degrees

Causes of LBBB, left anterior or posterior hemiblock

• Ischaemic heart disease
• Hypertension
• Fibrotic degeneration
• Calcific aortic stenosis
• Congestive or hypertrophic cardiomyopathy
• Congenital heart disease
• Following cardiac surgery

LBBB itself does not cause a change of QRS axis. LBBB with left axis deviation implies more extensive conduction system disease involving the main bundle proximally and the left anterior fascicle distally. It therefore carries a poorer long-term prognosis.