Calculated using the hexaxial reference system that shows the frontal view of the electrical activity of the heart via six limb leads (aVR, aVL, aVF, 1, 2, 3).
Easily remembered when represented as:
(The horizontal reference system uses the six precordial leads V1‐V6 (+/‐ V4R) to show the horizontal view of the electrical activity of the heart)
In lead 1, the R wave represents the leftward force, the S wave the rightward force.
In aVF, the R wave represents the downward force, the S wave the upward force.
A suggested method of axis calculation: the successive approximation method.
Use leads 1 and aVF to locate a quadrant
e.g. from panel 1 – the net QRS deflection seen from lead 1 is positive i.e. left ward, the net deflection of aVF is also positive i.e. downward, therefore the QRS axis is 0 ‐+90 degrees.
Find a lead with an equiphasic QRS complex (height of R equals depth of S). The QRS axis will be perpendicular to this in the previously determined quadrant.
Normal QRS axis varies with age
1 week – 1 month: + 110 (range +30 ‐ +180)
1 month – 3 months: + 70 (range +10‐ +125)
3 months – 3 years: + 60 (range +10 ‐ +110)
3 years +: + 60 (range +20 ‐ +120)
Adult: + 50 (range ‐30 ‐ +105)