a pair of electrodes attached to a wire, used in recording changes in electric potential, created by activity of an organ, such as the heart (electrocardiography) or brain (electroencephalography). Also, the particular segment of the tracing produced by the potential registered through the specific electrodes; in electrocardiography, lead I records the potential differences between the two arms, lead II between the right arm and left leg, lead III between the left arm and left leg, and V leads from various sites over the heart and a composite reference point.
bipolar l. a configuration in which two electrodes are in contact with the organ being stimulated; this type is less susceptible to external electromagnetic interference than a unipolar lead is.
esophageal l. an electrode attached to a wire and inserted in the esophagus.
horizontal plane l. a unipolar precordial lead; it includes six of the 12 ECG leads whose positive electrodes are on the precordium and give information about right, left, anterior, or posterior current flow in the heart.
limb l's electrodes placed on the arms and left leg.
precordial l's leads recording electric potential from various sites over the heart, designated V with a subscript numeral indicating the exact site: V1, fourth intercostal space immediately to the right of the sternum; V2, fourth intercostal space immediately to the left of the sternum; V3, midway between V2 and V4; V4, fifth intercostal space in the midclavicular line (the imaginary vertical line on the anterior surface of the body), passing through the center of the nipple; V5, at the same horizontal level as V4, in the left anterior axillary line (the imaginary vertical line passing through the middle of the axilla); V6, left midaxillary line at the same horizontal level as V4 and V5.
unipolar l. a configuration of two electrodes, an active or therapeutic electrode in contact with the organ being stimulated and the other (the reference or indifferent electrode) at a distant site, usually the surface of the pulse generator.