Electrocardiogram Augmented Limb Leads (Unipolar)
In addition to 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 located on the left arm (aVL), the right arm (aVR), and the left leg (aVF). In practice, these are the same electrodes 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 to the figure using the axial reference system. The aVL lead is at -30° relative to the lead I axis; aVR is at -150° and aVF is at +90°. It is very important 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:
The 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 given instant in time. If a wave of depolarization is spreading from right-to-left along the 0° axis, then lead I will show the greatest positive amplitude. If the direction of the electrical vector for depolarization is directed downwards (+90°), then aVF will show the greatest positive deflection. If a wave of depolarization is moving from left-to-right at +150°, then aVL will show the greatest negative deflection according to the rules for ECG interpretation.