In left anterior fascicular block the current is conducted to the left ventricle via the left posterior fascicle, which makes the current travel downwards and rightwards producing small R waves in the inferior leads. Depolarisation then travels upwards and leftwards, producing large R waves in the left-sided leads and deep S waves in the inferior leads. The QRS is usually only slightly prolonged.

Features of LAFB:

left anterior fascicle block


There is Left Axis Deviation
- There are small Q waves with tall R waves in Lead I and aVL
- There are small R waves with deep S waves in leads II, III and aVF
- QRS duration should be essentially normal:
the fascicle blocks affect the direction but not the duration of the QRS
- R wave peak time is prolonged (over 45msec)
- Limb lead QRS voltage should be increased

in summary...

left anterior fascicle block summary
 

Causes of Left Anterior Fascicle Block


- Might just be a normal consequence of ageing
- …or, it could be severe LAD disease
- Its common after aortic valve surgery

Consequences of Left Anterior Fascicle Block


- This is an asymptomatic condition – usually doesn't amount to much

References

From "the ECG made easy", by Hampton (2003), and ECGs shamelessly stolen from Life in The Fastlane without any sort of permission, but in the non-commercial spirit of free education

2012 ACCF/AHA/HRS Focused Update of the 2008 Guidelines for Device-Based Therapy of Cardiac Rhythm Abnormalities A Report of the American College of Cardiology Foundation/American Heart Association Task Force on Practice Guidelines