Step 3c: Calculate the electrical axis

Approximate the net QRS deflection for leads I and aVF. Remember that the mean QRS axis will be oriented towards the lead with the greatest positive net QRS deflection. If the net deflection is positive for both, the axis lies between leads I and aVF (0-90°) and is therefore normal.


  Lead 1   Lead aVF
Leads I and aVF equally positive. The axis will be midway between 0° and 90°.   Normal axis ~ 40°-50°

Leads I and aVF both positive. Lead I more positive than aVF. The axis will therefore be oriented more toward 0°.
  Normal axis ~ 20° - 40°

Lead I positive. Lead aVF almost equiphasic. Therefore, the axis will be approaching 0°. (Note: when a lead is equiphasic, the axis will be 90° to that lead.)   Normal axis ~ 0°

  Lead I positive. Lead aVF negative.The axis will be oriented negatively past 0°.   Left axis deviation ~ -30°
  Lead I negative. Lead aVF positive. The axis will be oriented positively past 90°.
  Right axis deviation ~ -120°
  Both leads I and aVF negative. The axis will be oriented between -90° and -180°.   Indeterminate axis ~ -135°