The ECG Leads

The heart occupies a position in the center of the thorax anda 12-lead ECG is simply a recordingof the current flux of cardiac depolarization and repolarization obtained from 12 different sites on the body surface.

There are six limb leads:

Back to contents Lead I records from the left at a coordinate of 0o.

Back to contents Lead II records from the foot at a coordinate of 60o.

Back to contents Lead III records from the foot at a coordinate of 120o.

Back to contents Lead aVR records from the right shoulder at a coordinate of -150o.

Back to contents Lead aVL records from the left shoulder at a coordinate of -30o.

Back to contents Lead aVF recordsdirectly from the feet below at a coordinate of 90o.

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There are also six chest leads with sensing electrodes positioned horizontally around the left anterior hemi-thorax between the 4th and 5th interspaces:

Back to contents Leads V1 and V2 record the current flux over the right ventricle directly.

Back to contents Leads V3 and V4 record directly the electrical activities of the ventricular septum and the anterior wall of the left ventricle.

Back to contents Leads V5 and V6 record the current flow generated by the left ventricle directly.

Irrespective of whether it is a limb lead or chest lead, a current surging directly in the direction of the recording electrode will cause a positive deflection on the ECG; a current flowing in the direction but not directly toward the recording electrode will be registered as a positive deflection of lower amplitude; a current running at right angle to the direction of the recording electrode will cause no deflection or a biphasic deflection; a current flowing away in a direction opposite to that of the recording electrode will be registered as a negative deflection; and a current flowing away but not directly will cause a negative deflection of smaller amplitude.

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