Electrical axis of ECG leads

The frontal plane (extremity or limb) ECG leads are represented on a hexaxial diagram. Each lead has a specific spatial orientation and polarity. The positive pole of each lead axis (red, solid line) and negative pole (blue, solid line) are designated by their angular position relative to the positive pole of lead I (0°). The QRS axis, which is normally between -30° and +100°, can be estimated as being at right angles to the positive pole of any extremity lead in which the positive and negative deflections are of roughly equal magnitude.