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What is Cardiac Axis

The electrical activity of the heart starts at the SA node then spreads to the AV node

It then spreads down the bundle of his & then purkinje fibres to cause ventricular contraction

So when viewing the heart from the front, the direction of depolarisation is 11 o’clock to 5 o’clock

The general direction of depolarisation is known as the cardiac axis


Normal Cardiac Axis

In healthy individuals you would expect the normal 11 o’clock to 5 o’clock spread

Therefore the spread of depolarisation would be heading towards leads I,IIIII

As a result you would see a positive deflection in all of these leads

With lead II been the most positive (it’s at 5 o’clock)

You would expect to see the most negative deflection in aVR

This is due to aVR looking at the heart in the opposite direction to lead II


Normal Cardiac Axis

Right axis deviation

Right axis deviation (RAD) is usually caused by right ventricular hypertrophy

In right axis deviation the direction of depolarisation is distorted to the right (1-7 o’clock)

Extra heart muscle causes a stronger signal to be generated by the right side of the heart

This causes the deflection in lead I to become negativedeflection in lead aVF & III to be more +ve

RAD is associated with pulmonary conditions as they put strain on the right side of the heart

It can be a normal finding in very tall individuals

Right Axis Deviation

Left axis deviation

In left axis deviation (LAD) the general direction of depolarisation becomes distorted to the left

This causes the deflection in lead III to become negative

It is only considered significant if the deflection of Lead II also becomes negative

LAD is usually caused by conduction defectsnot by increased mass of the left ventricle


Left axis deviation

Recommended reading

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