Normal cardiac axis
Normal cardiac axis

What is Cardiac Axis?

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What is cardiac axis?

Cardiac axis represents the overall direction of electrical activity as it spreads through the cardiac conduction system. In other words, itΒ represents the net effect of all generated action potentials within the heart.

Whenever the net direction of electrical activity is towards a particular ECG lead you see a positive deflection in that lead on the ECG.

Whenever the net direction of electrical activity is away from a particular ECG lead you see a negative deflection in that lead on the ECG.

Β 
ECG Leads
ECG Leads

Normal cardiac axis

In healthy individuals, you would expect the cardiac axis to lie between -30Β° (aVL) and +90ΒΊ (aVF).

In the context of normal cardiac axis, the net direction of electrical activity spreads towards leads I, IIΒ andΒ III (the yellow arrow below). As a result, you see a positive deflection in all of these leads, with lead II showing the most positive deflection as it is the most closely aligned to the overallΒ direction of electrical spread.

You would expect to see the most negative deflection in aVR,Β due to aVR looking at the heart in the opposite direction.

Normal Cardiac Axis
Normal Cardiac Axis

Right axis deviation

Right axis deviation (RAD) involves the direction of depolarisation being distorted to the right (between +90ΒΊ and +180ΒΊ).

The most common cause of RAD is right ventricular hypertrophy. Extra right ventricular tissue results in a stronger electrical signal being generated by the right side of the heart. This causes the deflection inΒ lead I to becomeΒ negative and the deflection inΒ lead aVF/III to beΒ more positive.

RAD is commonly associated with conditions which result in the development of right ventricular hypertrophy such as pulmonary hypertension. RAD can, however, be a normal finding in very tall individuals.

Right Axis Deviation
Right Axis Deviation

Left axis deviation

Left axis deviation (LAD) involves the direction of depolarisation being distorted to the left (between -30ΒΊ and -90ΒΊ).

This results in the deflection of lead III becoming negative (this is only considered significant if the deflection of lead II also becomes negative).

LAD is usually caused by conduction abnormalities.

Left Axis Deviation (LAD)
Left Axis Deviation (LAD)

Reviewer

Dr Matthew Jackson

Consultant Interventional Cardiologist


 

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