- 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.
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