The 12-lead ECG: electrodes, limb leads and cardiac axis
THE CONTEXT
The 12-lead electrocardiogram (ECG) uses 10 skin electrodes to generate recordings from pairs or sets of electrodes.
Electrodes and lead groups
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One electrode is attached to each limb and six are attached to the chest.
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The left arm, right arm and left leg electrodes connect to a central terminal that acts as a virtual electrode in the centre of the chest. The right leg electrode provides earthing.
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The 12 leads comprise three dipole limb leads, three augmented voltage limb leads and six unipole chest leads.
Dipole limb leads
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Lead I records between the right arm, which is negative, and the left arm, which is positive.
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Lead II records between the right arm, which is negative, and the left leg, which is positive.
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Lead III records between the left arm, which is negative, and the left leg, which is positive.
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These leads record electrical activity along three different axes in the frontal plane.
Augmented limb leads
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Leads aVR, aVL and aVF record between a limb electrode and a modified central terminal.
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For aVL, the left arm is positive and the negative central terminal is formed by connecting the right arm and left leg electrodes.
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Corresponding augmented recordings are obtained from the right arm in aVR and the left leg in aVF.
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The augmented leads record in the frontal plane, with each lead 120° apart. The aVF axis is +90° and the aVL axis is −30°.
Axis deviation patterns
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Left axis deviation is illustrated by a negative deflection in lead II and a positive deflection in lead I.
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Right axis deviation is illustrated by a negative deflection in lead I and a positive deflection in lead II.