The four main causes of left axis deviation (LAD):

1. Left anterior fascicular block (LAFB):
   Diagnostic criteria include:
   
   • Mean QRS axis from - 45° to - 90° (possibly < -30°)
   • A qR pattern in lead aVL, with the R-peak time, i.e., the onset of the Q wave to the peak of the R wave > 45 msec (slightly more than one small box wide).
     
     
   • Clinical correlations: LAFB is a complication of hypertensive heart disease or CAD, or is a result of idiopathic conducting system disease.
2. Inferior MI:
   There is a pathologic Q wave > 30 msec in either leads aVF or II, in the absence of #3 below.
3. Ventricular pre-excitation (The W-P-W pattern):
   LAD is seen with posteroseptal accessory pathway locations. Mimics infero-posterior MI.
4. COPD:
   Seen in 10% of patients, in whom the axis shifts to the left because of the rotation of the right ventricle from its normal anterior position to a more inferior position.

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