The electrocardiogram as generally taken consists of three leads, obtained by using the two arms and the left leg as the contact or leading off points. Einthoven1 has shown that these three leads bear a definite mathematical relationship to one another and has used an equilateral triangle to express this relationship graphically. For the better understanding of the succeeding parts of this paper we shall consider this mathematical relationship and its graphic representation at some length.
The mathematical relationship is expressed by the equation: lead II equals lead III plus lead I. This means that the height of the ordinate in lead II of the electrocardiogram at any instant is the sum of the heights of the ordinates in leads III and I. The reason for this relationship can be appreciated if we consider the string galvanometer simply as an instrument for measuring differences of potential.2 If now we wish
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