The average direction of spread of the depolariation wave through the ventricle as seen from the frontal plane is call the cardiac axis. The axis can be determined approximately measuring the net differences between the positive and negative peaks of the QRS. The normal direction of the mean QRS vector is generally said to be between -30 degree.to +110 degree. An axis beyond -30 degreeis considered as left axis deviation (LAD). A cardiac axis beyond +110 degree is right axis deviation (RAD).
Right Axis Deviation
Table 1: Estimation of the cardiac axis
| Lead I | Lead III | Cardiac Axis |
| Positive QRS | Positive QRS | Normal |
| Negative QRS | Positive QRS | Right axis deviation |
| Positive QRS | Negative QRS | Left axis deviation |