To summarize, mean electrical axis of the heart can be estimated in a number of ways (Table 18).

Find the lead with greatest (+) deflection; it points to the axis.

Conversely, find the lead with the greatest (-) deflection - it is 180 degrees to the axis.

Another method is to find a lead with equal voltages in the R and the S waves; the axis is 90 degrees to the lead.

Axis deviation can be caused by a number of conditions: obesity, pregnancy, pulmonary vascular obstructive disease, left or right ventricle hypertrophy, and myocardial infarction. (Table 23)