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Cardiac Axis

So far the tutorial has foccused on limb lead II but the other leads are just as important. What they offer are different views of the spread of electrical activity through the heart and each lead has a characteristic appearence. 

 

In order to understand this more clearly we need to understand what is meant by cardiac axis. 

General Direction of Depolarisation

 

The red arrow in this diagram represents the general direction of depolarisation in a normal heart. This is called the cardiac axis and in a healthy person runs in the direction from 11 o'clock to 5 o'clock.

 

 

Look how the axis runs close to parallel with limb lead II and aVR. This means those leads will have the largest amplitudes. 

 

However the cardiac axis is going in the opposite direction to aVR. This means that depolarisation going from 11 o'clock to 5 o'clock will appear as a negative deflection in the aVR reading. This is what happens in a normal heart

 

As long as the vector of depolarisation runs between lead I and aVF then the axis is considered normal 

The Views From the Leads

 

 

  • Depolarisation running along parallel to the line of a lead appears as a deflection on the tracing for that lead

 

  • Depolarisation going at 90 degrees to a lead's line will not show on that lead

 

  • Depolarisation going in the opposite direction to the arrows will appear as a negative deflection in that lead (e.g. aVR)

Hover Over The Different Leads To Reveal How a Normal Tracing Appears In Each 

 

Use this outline if you have no prior knowlege of ECGs, how they work or how to set them up. This map will take you through the basic physics and anatomy, set up of the ECG and will touch on interpretation. There is a test at the end to examine your understanding.

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Use this outline if you have some prior knowlege of how an ECG works, its set up and a good understanding of the anatomy of the heart's conducting system. This will focus on interpreting the readings on different leads of the ECG.

 

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Use this map if you already have a thourough understanding of heart anatomy, different arrythmias and their appearance on ECGs. This map focuses on testing your existing knowledge of interpreting ECGs.

 

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