HEART-VECTOR AND LEADS.

The purpose of electrocardiographic investigation is to find out something about the heart itself by means of an accurate knowledge of its electric action. This may be represented by the so-called heart-vector, a directed quantity, indicating in which direction electricity is propagated by the heart. The amount of each lead at a given instant depends on the heart-vector. The latter changes during the heart beat in direction and magnitude and, consequently, the value of the lead, too, changes with time. The relation between heart disease and leads is, at the root, very complicated. One can, however, analyse this relation by dividing it into a relation between heart disease and heartvector and a relation between heart-vector and leads. The first relation is only to be found on the ground of medical experience. The second relation, on the contrary, is of a purely physical nature, and will be the subject of our discussion. A long time ago Einthoven et al. (1913) with his so-called triangle rule, tried to formulate the relation between the heart-vector (which he called the manifest potential difference) and the three limb leads. Einthoven himself realized very well that this rule gives only an approximation of the truth. What the significance is of this approximation can be decided only by measurement. We have tried to formulate the physical problem and next to obtain, by measurements on a phantom, the data necessary for the application of the fundamental solution of the physical problem.