Cardiac axis (on an ECG)

The cardiac axis is the predominant direction of ventricular depolarisation in the frontal plain. Thus the axis is parallel to that frontal lead with the largest QRS complex and at right angles to the lead in which the Q wave equals the R wave.

ÚÄÄÄÄÄÄÄÄÄÄÄ> Lead I, 0ø Lead I measures ³ in a horizontal ³ the normal heart has plane, aVF in ³ an axis between -30ø a vertical plane ³ and 90ø  aVF, 90ø

The axis is deduced using a vector diagram of the amount of deflection (squares above the line minus the squares below the line) in these two leads.

Really the area between the lines is important, but this is not often used.

Determination of the axis is useful in the diagnosis of:

• Right ventricular hypertrophy - when the axis is >90 degrees i.e. positive QRS in aVF and r<S in lead I.
• Left anterior hemiblock - when the axis is <-30 degrees i.e. positive QRS in aVL and r<S in lead II.