Electrocardiogram Interpretation

Recording the 12 lead ECG

So how was this ECG recording made?

9 leads are placed on the patient in the following positions:
Limb leads: aVL, aVR, aVF, neutral
Chest leads: V1, V2, V3, V4, V5, V6

These leads are now looking at the heart from different angles, or different points of view.

   

In order to simplify what is happening, we’re going to first look at the chest leads, then the limb leads.

Chest leads (V1-V6) are draped around the thorax and look at the
anterior (V1-2)
septal (V3-4)
and lateral (V5-6) heart

Hover your mouse over the leads below to see where to find them on the ECG

 

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Limb leads (aVR, aVL and aVF) can be simplified into the following diagram (note neutral lead on the right foot disappears as it does not provide a recording):

   

So where do leads aVF, aVR and aVL come from? Note that we placed a total of 10 electrodes on the body, one of which (neutral) doesn’t record. This leaves 9 electrodes recording the complete 12 lead ECG (aVF, aVR, aVL, I, II, III and V1-6).

Leads aVF, aVR, and aVL are vectors calculated by the machine from what is happening in I, II and III. You don’t need to know how the machine does this. Suffice to say it does the equivalent of joining up aVR, aVF and aVL:

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Now, the important thing to understand about the leads is the angle or DIRECTION at which they are viewing the heart, NOT their position in space (we’re talking about VECTORS for those who want to understand the maths). So we’re going to rearrange the above diagram and move the leads around in space WITHOUT changing the direction they’re pointing in.

 

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The result is a diagram that should look familiar to you, known as the axial reference system or Ebstein's triangle:

   

Now you should understand why leads II, III and aVF are called the INFERIOR leads –they are looking at the heart from below and hence tell us what is happening to the INFERIOR heart.

Hover your mouse over the leads below to see where to find them on the ECG

 

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Thinking Medicine
2007