Measurement of the error in Wilson's central terminal: an accurate definition of unipolar leads.

Unipolar leads using Wilson's central terminal as an "indifferent" electrode have been in general clinical use for many years. The central terminal was not introduced as a theoretically correct reference for unipolar leads, but as a convenient reference that was at least better than using one of the limbs alone, as in the old CR, CL, and CF leads, in which the exploring electrode potentials were compared with those of electrodes placed respectively on the right arm, the left arm, or the left leg (Wilson et al., 1934). When unipolar leads are used to explore the potentials over the left chest at the level of the heart, the error in adopting the central terminal as a reference electrode is small because the potential variations of the exploring electrode are relatively large. A tendency to explore the right chest has developed in recent years. In this case, the error may be considerable because the smaller potential variations of the exploring electrode may be ofa magnitude comparable with those of the central terminal itself. Such records may be open to misinterpretation. A case has recently been described in which the unipolar lead tracings V3R were produced by potential variations of the central terminal rather than by those of the exploring electrode (Osborne and Dower, 1957), and resulted in a pattern resembling right ventricular hypertrophy for this lead. Methods of determining the error in the central terminal have been described by Eckey and Fr6hlich (1938), Burger (1939), Wilson et al. (1949), Dolgin et al. (1949), Bayley and Kinard (1954), and Frank (1955a), but none has been applicable to patients. For this reason, it has not been possible to study the error under clinical conditions. The method to be described makes this possible. The theoretical considerations that arose out of the development of this method have demanded a careful scrutiny of previous definitions of unipolar leads and this has led unexpectedly to the conclusion that a more accurate definition is required.