The precordial electrocardiogram

Abstract Using a method which makes it possible to obtain electrocardiograms that represent the potential variations of a single electrode, a study of thirty normal subjects was carried out. The three standard leads and nine special leads were each taken simultaneously with Lead I. Three of the special leads recorded the potential variations of the right arm, left arm, and left leg, respectively; the others recorded the potential variations of a series of six precordial points. The special curves were taken in such a way that negativity of the exploring electrode, placed upon one of the extremities or upon the precordium, produced an upward deflection in the completed record. It is, however, pointed out that several advantages might be gained by making the connections in the opposite way. For this reason, and to avoid introducing a new nomenclature, our special leads have been labelled as if the direction of the deflections were reversed. In precordial leads the QRS group may normally show three distinct apices, which for convenience in description may be called "Q," "R," and "S." The R peak, which represents a positive variation in the potential of the exploring electrode and is therefore inverted in our records, is always present. This peak marks the beginning of a large and rapid excursion of the string which corresponds to the intrinsic deflection of direct leads from the ventricular surface. In leads from the right side of the precordium R is smaller and, on the average, reaches its apex about 0.02 second earlier than in leads from the left side. This indicates that, as a whole, the epicardial surface of the ventral wall of the right ventricle is activated earlier than the corresponding surface of the left. In our records Q and S are both summits. The former is always small in normal subjects and occurs only in leads from the left side of the precordium. The latter is larger on the right side of the precordium than on the left where it may be absent. When the exploring electrode is placed at or to the left of the sternal edge, its potential is invariably positive during the inscription of the T deflection. When the exploring electrode is placed to the right of the midline, the direction of T is variable. The leads which represent the potential variations of the extremities are related both to the standard leads and to precordial leads and are useful in comparing the one with the other. When the standard leads are of the normal type, the left leg curve usually resembles those obtained from the left side of the precordium, and the deflections of the left arm curve are small.