Electrocardiographic Leads

IN the first paper of this series' it was shown that the relationship between the electromotive forces of the heart and the voltages produced by them in a given lead are completely determined by the electric field set up in the heart when a unit current is introduced into the lead. This means that to analyze the relationship between the electromotive forces and the lead voltage it is only necessary to determine the nature of the lead field. A brief discussion of various methods of studying these fields is given in this paper. When a current is introduced into a lead the resultant flow of current through the heart produces potential differences between various points in the muscle. If it were possible to measure these potential differences, this data would furnish the desired information about the field. Unfortunately, it is not possible to do this with human subjects. For this reason it is necessary to resort to other approaches, all essentially indirect. The simpler of these approaches furnish graphic patterns in which one component of the lead field current is taken as zero. Such twodimensional patterns are easy to obtain and to interpret, and frequently give an insight into the nature of the lead which would be quite difficult to obtain from three dimensional data, since the latter cannot be conveniently represented graphically. However, unless one component of the actual lead field is small