Cardiac axis

If the R-wave is greater than the S-wave it
suggests depolarisation is moving towards that lead

If the S-wave is greater than the R-waves it
 suggests depolarisation is moving away from that lead

If the R & S-waves are of equal size
it means depolarisation is travelling at exactly 90° to that lead

If the R-wave is greater than the S-wave it
suggests depolarisation is moving towards that lead

If the S-wave is greater than the R-waves it
 suggests depolarisation is moving away from that lead

If the R & S-waves are of equal size
it means depolarisation is travelling at exactly 90° to that lead

Normal Cardiac axis

In healthy individuals you would expect the axis to
lie between -30° and +90º

The overall direction of electrical activity is towards leads I,II & III (the
yellow arrow)

As a result you see a positive deflection in
all these leads, with lead II showing the most positive (it’s
looking in the most similar direction to the overall electrical activity)

You would expect to see the most negative deflection in aVR. This
is due to aVR looking at the heart in the opposite direction
to the overall electrical activity

Calculating Cardiac Axis

Left axis deviation

<-30°

Right axis deviation

> +90°

Normal axis

-30° to +90°

Example - Lead I

•Measure the height of the positive
deflection (R wave) +8

•Measure the height of the negative
deflection (S wave) -3.5

•Calculate the sum of the two
+4.5

Example - Lead AVF

•Measure the height of the positive
deflection (R wave) +4.5

•Measure the height of the negative
deflection (S wave) -0.5

•Calculate the sum of the two
+4

Calculating Cardiac Axis

Measure lead I + lead aVF deflections and plot
the RS sum on the graph shown

Normal axis

-30° to +90°

Left axis deviation

<-30°

Right axis deviation

> +90°