ECG: augmented limb leads

Principle

Unipolar leads and augmented limb leads

Basically a unipolar lead is an exploring electrode placed on a chosen site linked with an indifferent or reference electrode with a very small potential.

To obtain a common reference with ‘zero potential’, two limb electrodes and an electrode placed at the sternum (called central terminal, CT, also Wilson’s central terminal) are connected through 5 kΩ resistances to form the indifferent electrode, connected to the negative terminal of the ECG machine. The potential of the active electrode, connected to one the three limbs, creates one of the leads aVR, aVL, or aVF. The other two electrodes of the standard leads are connected with CT. These are the augmented limb leads. So, the only difference with the bipolar leads I, II, and III is the choice of the common reference. As with the standard leads, the right leg is never connected for a lead, but it can be used to attach an earth electrode (see EEG: 12-lead ECG).

The augmented limb leads can be presented as vectors with a certain angle in some plane and time varying amplitude.

Fig. 1 Recording of aVR. Since the vector is the source, the current which creates the voltage, flows in the body from negative to positive (as in a battery). The vector point to -150 degrees (up are negative angles, just as in the hexaxial reference system (see ECG: Hexaxial reference system).

The three augmented lead are:

Lead aVR or "augmented vector right" has the positive electrode (green in Fig. 1) on the right arm.

Lead aVL or "augmented vector left" has the positive (blue) electrode on the left arm.

Lead aVF or "augmented vector foot" has the positive (red) electrode on the left leg.

The augmented limb leads aVR, aVL, and aVF are amplified (augmented) compared to the signal when a unipolar limb lead is made with CT as the reference electrode. Such a lead is rather small to be useful. Together with leads I, II, and III, the augmented limb leads aVR, aVL, and aVF form the basis of the hexaxial reference system, which is used to calculate the heart's electrical axis in the frontal plane. See for an example of all six leads of a healthy subject Fig. 2 of EEG: 12-lead ECG.

Application

This three leads are frequently used in the clinic, for instance for intensive care and for routine non-cordial surgery, but then V5 is used in addition.

More info

When the limb electrodes are connected one by one with CT the leads VR (right arm), VL (left arm) and VF (left leg) are obtained. aVR is equal to:

aVR = VR – (VL + VF)/2 (1)

since (left arm) and VF (left leg) are connected, so the mean of both signals results. CT is the common reference and therefore it is not in the equation.

Eithoven’s triangle says that:

VR + VL + VF = 0 or VR = – (VL + VF). (2)

Substitution (2) in (1) yields:

aVR = 3VR/2. (3)

For the other two, aVL and aVF the same holds. So, recording the unipolar leads VR, VL and VF is a disadvantage. Therefore, the leads I, II and III are recorded together with the augmented leads.