All together the Leads I, II, III, aVL aVR and aVF are known as the ‘limb leads’ as they are determined from information gathered by leads attached only to the limbs.

They can be more easily understood if placed onto a circle. Leads I, II and III form an equilateral triangle (Einthoven’s triangle) and so bisect the circle at 60 degrees.

The augmented leads aVR, aVL and aVF can be thought to divide between these 60 degree angles as they would divide up Einthoven’s triangle.

So now you can see how all of the limb leads correspond to the cardiac electrical signal.

Different leads represent different areas of the heart:

• Leads I and aVL are referred to as a ‘lateral’ leads

• Leads II, III and aVF as ‘inferior’ leads as these are the areas of the heart represented in the trace