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

1. Right ventricular hypertrophy: (the most common cause of RAD)
   Refer to diagnostic criteria (see section on RVH). However, one must first exclude acute occlusion of the posterior descending coronary artery causing LPFB, and also exclude items 2 & 3 below.
2. Extensive lateral and apical MI:
   Criteria include QS or Qr patterns in leads I &/or aVL & V4-6.
3. Ventricular pre-excitation (The W-P-W pattern)
   RAD seen with LV freewall accessory pathway locations. Mimics lateral MI.
4. Left posterior fascicular block (a diagnosis of exclusion)
   Diagnostic criteria include: Mean QRS axis from + 90° to + 180° with a qR complex in leads III and aVF an rS complex in leads aVL & I, with a Q wave < 40 msec in the inferior leads.

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