cardiac axis

The cardiac axis is the overall direction in which the main wave of ventricular activation travels through the heart, viewed in the frontal plane. Picture all the little electrical currents of the ventricles adding up into one big arrow; the cardiac axis is the way that arrow points.

It is worked out by comparing how positive or negative the QRS complex looks across the limb leads, each of which views the heart from a different angle. In most healthy adults the arrow points down and to the left, toward the bulkier left ventricle, which is described as a normal axis. The reference frame is usually drawn as a circle of degrees around the chest.

Shifts in the axis can flag underlying changes. Left axis deviation may accompany certain conduction blocks or a thickened left ventricle, while right axis deviation can occur with right-heart strain, lung disease, or in slim, tall individuals as a normal variant. The axis is one clue among many and is most useful when combined with the rest of the ECG rather than read alone.