How it works
A 12-lead ECG uses ten electrodes: six on the chest and one on each limb. The first drawing shows the chest positions V1 to V6 on the ribs. The second shows the limb electrodes and leads I, II and III.
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The chest electrodes
V1 and V2 sit in the fourth intercostal space, the gap under the fourth rib, on either side of the sternum. V4 is in the fifth space on the midclavicular line, and V3 lies midway between V2 and V4. V5 and V6 are level with V4, on the anterior axillary and midaxillary lines.
Electrodes and leads
An electrode is a pad on the skin. A lead is one view of the heart, measured between electrodes. Lead I runs from the right arm to the left arm, lead II from the right arm to the left leg, and lead III from the left arm to the left leg. Together they form Einthoven's triangle.
- The person faces you, so their right side is at the left of the picture.
- Ten electrodes give twelve leads: I, II and III, three augmented limb leads (aVR, aVL, aVF) and the six chest leads.
Questions and answers
Where do the electrodes of a 12 lead ECG go?
Six go on the chest: V1 and V2 beside the sternum in the fourth intercostal space, V4 in the fifth space on the midclavicular line, V3 between V2 and V4, and V5 and V6 level with V4 toward the side. Four go on the limbs.
Why are there 12 leads but only 10 electrodes?
A lead is not a wire. It is a measurement between electrodes. The limb electrodes give six leads and the six chest electrodes give six more.
Is this page medical advice?
No. It names the standard positions for a class. It does not show how to record or read an ECG.