Unipolar limb leads

The conventional limb leads record potentials between two apices of the triangle. Lead 1 records potentials between the right arm and left arm, lead 2 records potentials between the right arm and left leg and lead 3 records potentials between the left arm and left leg. These are known as bipolar leads. In addition there are three unipolar leads attached to the limbs, which record potentials between the limb and a reference zero. 

The 12-lead ECG clinical diagnostic test is composed of the six limb leads and the six unipolar precordial leads an example is shown in Figure 10.4. 

Include three unipolar augmented limb leads (aVj, aVL, and aVp). 

Because this is an unsolvable problem, the ideal unipolar lead does not exist. Nonideal solutions are to use one electrode at some remote point (e.g., at a limb or an earlobe). Another solution is to add the voltages picked up by more than one electrode, such as two limb electrodes (augmented leads) or three limb electrodes (Wilson central terminal). 

Fig. 1.23 Three examples of fractured unipolar leads occurring at points of stress. A In the neck where the lead enters the external jugular vein. There is a sharp bend in the lead at the venous entry site and both limbs of the lead move differently with neck movement. B Within the right atrium. The lead is encased in a short endocardial tunnel, where it makes contact with the wall as it turns to enter the right ventricle. Again both limbs move differently with cardiac contractions. C At the lead connector site. There is a sharp bend in the lead after it emerges from the connector.

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