QRS axis

The cardiac axis is just one way of summarising the overall direction of ventricular depolarisation.

You might have heard of other types of axis like the X and Y axis (in maths) or the Earth's axis (in geography). In electrocardiography, the axis is just another imaginary line.

We can calculate the direction of an axis by using several limb leads from the 12-lead ECG (I, II, III, aVR, aVL, aVF). We can also calculate different axes for the atria or the ventricles of the heart. There are a range of directions that can be normal for each axis.