A precordial lead refers to one of the six standard electrocardiogram (EKG or ECG) leads, or electrodes, placed on the chest in order to obtain a 12-lead EKG report graphically showing the heart's electrical activity. A 12-lead EKG is made up of 12 different axis views of the heart obtained from 10 different leads: six precordial leads and one on each limb. Dr. Frank Wilson introduced the use of precordial leads in the 1940s in order to measure cardiac electrical activity more closely and this standard has remained the routine, except when modifications are necessary for exercise treadmill tests. Each precordial lead is designated "V" along with its electrode number, such as "V1" or "V3."
The six precordial leads are placed on the anterior chest in a standard configuration. V1 is placed on the right side of the sternum at the fourth intercostal, or rib space, while V2 is placed directly across the sternum on the left side of the chest. V4 is positioned at the fifth intercostal space below the middle of the left clavicle. Working backward, precordial lead V3 is applied directly between V2 and V4. V5 and V6 are placed at the same level as V4 at the left anterior axillary line and the left midaxillary line, respectively.
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Utilizing 10 electrodes allows for evaluation of the heart's electrical activity along 12 different axes and three separate planes. A 12-lead EKG measures cardiac electrical activity along the X axis that divides the heart into a right and left half; along the Y axis that divides the heart into a front and a back half; and along the Z axis that divides the heart into a top and bottom half. This degree of evaluation results in a great deal of data replication with eight independent leads and four redundant electrodes. According to Malmivuo and Plonsey's text, Bioelectromagnetism, all 12 leads are recorded, nonetheless, to enhance pattern recognition and thus improve the diagnostic value of the EKG evaluation.
The contribution of each precordial lead to all of this information along 12 axes and three planes dividing the heart allows for very close identification of the area of injury. Lack of oxygen, or ischemia, can cause temporary changes to specific EKG leads. A heart attack or myocardial infarction (MI) results in evolving changes to different precordial leads depending upon the specific area of infarction — anterior, posterior or inferior, for instance — as the electrical impulses are interrupted due to tissue injury and death.