Heart Model
Cardiovascular Physiology Concepts Richard E. Klabunde, PhD

Cardiovascular Physiology Concepts 3e textbook cover Cardiovascular Physiology Concepts, 3rd edition textbook, Published by Wolters Kluwer (2021)

CNormal and Abnormal Blood Pressure, Physiology, Pathophysiology and Treatment book cover Normal and Abnormal Blood Pressure, published by Richard E. Klabunde (2013)

Electrocardiogram Augmented Limb Leads (Unipolar)

ECG augmented lead axisBesides the three bipolar limb leads, there are three augmented unipolar limb leads. These are termed unipolar leads because there is a single positive electrode that is referenced against a combination of the other limb electrodes. The positive electrodes for these augmented leads are on the left arm (aVL), the right arm (aVR), and the left leg (aVF). These are the same electrode locations are used for leads I, II and III. (The ECG recorder does the actual switching and rearranging of the electrode designations). The three augmented leads are depicted as shown by the axial reference system. Lead aVL is at -30° relative to the lead I axis; aVR is at -150° and aVF is at +90°. It is essential to learn which lead is associated with each axis.

For a heart with a normal ECG and mean electrical axis of +60°, the augmented leads will appear as shown below:

ECG augmented lead tracings

ECG limb leads axisThe three augmented unipolar leads, coupled with the three standard bipolar limb leads, comprise the six limb leads of the ECG, as shown in the figure. These six leads record electrical activity along a single plane, termed the frontal plane relative to the heart. Using the axial reference system and these six leads, one can define the direction in the frontal plane of an electrical vector at any instant in time. If a wave of depolarization is spreading from right-to-left along the 0° axis, then lead I will have the greatest positive amplitude. Lead aVF will have the greatest positive deflection when the direction of the electrical vector for depolarization is directed downwards (+90°). If a wave of depolarization is moving from left-to-right at +150°, then aVL will have the greatest negative deflection according to the rules for ECG interpretation.

See also:

Revised 11/02/2023

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