The Cardiac Axis

The Cardiac Axis is a sum of general direction of travel of electrical conduction through the myocardium.

As we know, in a normal heart, the SA node impulses, electricity travels to the AV node, to the bundle of his, down the left and right bundle branches and into the purkinje fibres. This gives a general direction of travel of electricity as shown in the picture here:

Positive and Negative QRS Complexes

This image represents how the limb leads look at each part of the heart, which is important to how we work out the cardiac axis.

A mostly positive QRS complex means that electricity is travelling towards the lead, and a mostly negative QRS means that the electricity is travelling away from the lead.

The Cardiac Axis

The cardiac axis is given as one number, because it is a sum of the general direction of electricity through the heart.

Normal Axis: between -30° and +90°

Left Axis Deviation: Less than -30°

Right Axis Deviation: Greater than +90°

Extreme Axis Deviation: between -90° and 180°

Calculating the Axis

The Three Lead Analysis

Normal Axis

Lead I

To calculate the axis, you are going to look at the QRS complexes in Lead I, Lead II and Lead aVF. If they are all positive then it means this is a normal axis. Looking at the picture below, you can see that if all of these leads are positive, then this means that electricity generally goes towards these leads (red arrows) giving you a general direction of travel (yellow arrow).

This is a normal axis because the sum of general direction of electricity is between -30° and +90°

Left Axis Deviation

In left axis deviation, the direction goes towards, lead I, and away from lead II and lead aVF (red arrows). This gives you a general sum of direction of travel shown on the yellow arrow on the picture below.

This is left axis deviation because the sum of general direction of electricity is less than -30°

Right Axis Deviation

Lead I

In right axis deviation, conduction travels away from lead I, towards lead II and towards lead aVF (red arrows). This gives you a general sum of direction of travel shown on the yellow arrow on the picture below.

This is right axis deviation because the general direction of electricity is more than +90°

Ways to remember…

In a normal axis:

Lead I, lead II and lead aVF are all positive

In Left Axis Deviation:

Lead I is leaving away from lead II and aVF

Leaving - Left

In Right Axis Deviation:

Lead I is reaching towards lead II and aVF

Reaching – Right