Errors in unipolar limb leads caused by unbalanced skin resistances, and a device for their elimination.

Abstract 1. 1. Unbalanced or dissimilar skin resistances may seriously alter deflections in the unipolar limb leads and lead to important clinical errors. These changes are due to potential variations at the so-called “indifferent” electrode and may occur in either augmented or nonaugmented unipolar extremity leads. 2. 2. Changes in amplitude, contour, and direction of P, QRS, and T waves were produced experimentally by altering skin resistances. These changes included the inversion of normally upright T waves and the production of Q waves not present with resistances balanced. 3. 3. These errors result from an inadvertent shift in axis of the unipolar leads clockwise or counterclockwise to a maximum of 30 degrees. The amount of shift in axis depends on the degree of imbalance. Thus, aV R (or V R ) may then erroneously resemble Lead I inverted or Lead II inverted; aV L (or V L ) may resemble Lead I or Lead III inverted; and aV F (or V F ) may resemble Lead II or Lead III. 4. 4. An electronic coupling device is described which prevents errors in unipolar limb leads due to dissimilar skin resistances even when the imbalance is extreme.