The cardiac axis is the general direction followed by all depolarization vectors of the heart. To determine the heart axis, you look at the extremity leads I, II, and aVF. Electricity going towards one of these leads will create a positive deflection of the QRS-complex of that lead. Combining the degrees of deflection of the different leads will give you the vector of the ventricular depolarization. The heart can have several of these axes:
- -30° to +90°: normal axis
- – 30° to -90°: left axis deviation
- +90 to +180° : right axis deviation
- +180° to -90°: extreme axis deviation
A normal axis means that the direction of the ventricular depolarization goes down and to the left following the anatomic shape of the heart within the chest. A deviation in the axis can occur after a change in the orientation or physical shape of the heart, or because of abnormal depolarization by the ventricles because of a fault in the electrical conduct system.