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12-Lead ECGs
Written by Tim Sheppard MBBS BSc. Created 2/7/11; last updated 2/7/11

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What is a 12 lead ECG?

The electrocardiogram is a great way of understanding the electrical activity in the heart, and is a cheap and quick investigation. However, you may hear people talking about the 12-lead ECG and wonder what people are talking about - why do you need 12 leads? What is the value of having so many?

The 12 lead ECG requires an individual to have three limb leads and six chest leads attached, and then compares the voltage between these leads in twelve different combinations to produce twelve different perspectives on the heart. A series of questions can then be used to interpret what they show.

The placement of the leads is very specific, and important if the ECG is to be interpreted properly. The first three leads should be placed on the right arm, left arm and left leg.

The remaining six leads need to be placed around the front of the heart, starting in the right fourth intercostal space (V1) to the right of the sternum, then on the opposite side of the sternum (V2), over the fourth rib (V3), then in the fifth intercostal space for V4, V5 and V6, evenly spread to the mid-axillary line on the left (i.e. immediately below the armpit) for V6.

Which parts of the heart do the leads look at?

The limb leads are leads I, II and III. They connect the right and left arms and the left leg. Lead I take the potential difference between the right arm and the left arm. Since it follows electrical activity flowing towards the left, it looks at the heart from the far left.

Lead II goes between the right arm and left leg, following electrical activity flowing down through the heart, looking in at the heart from the bottom left.

Lead III goes between the left leg and the left arm, following electrical activity flowing upwards and to the left. It looks at the heart from the bottom right.

The augmented limb leads are called that because they take the values from the limb leads and put them through an equation to give more information. These leads look at the heart from the feet (aVF), from the top right (aVR) and the top left (aVL). The leads aVF, II and III are said to be the inferior leads because they look at the heart from below.

The chest leads (or precordial leads) circle around the heart from the right side of the heart to the left, looking at it from the front. Because the right side of the heart is usually smaller (it only has to send blood to the lungs, unlike the left side which needs to get it all around the body), the leads coming from the right (V1 and V2) tend to give information about the septum.

Leads V3 and V4 are said to be the anterior leads because they look at the front of the heart.

Leads V5 and V6, along with aVL and I, are the lateral leads because they look at the heart from the side.

What are the posterior leads?

None of the leads on an ordinary 12-lead ECG look at the back side of the heart - the posterior side. This means that someone may have a heart attack and you wouldn't necessarily recognise it.

You could see it in part because you would get ST depression on the opposite side (the anterior leads). But another way is to add further leads around the back - V7, V8 and V9. These show the posterior side of the heart, and could give all the kinds of changes you would expect if anything happened on this side.

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