Electrocardiogram Augmented Limb Leads (Unipolar)
In addition to the three bipolar limb leads described above, 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 machine does the actual switching and rearranging of the electrode designations). The three augmented leads, along with the three standard bipolar limb leads, are depicted as shown to the right 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.
The three augmented unipolar leads, coupled with the three bipolar leads, constitute the six limb leads of the ECG. These 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, it is simple to define the direction 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.
For a heart with a normal ECG and mean electrical axis of +60°, the augmented leads will appear as shown below: