There are certain fundamental physical and physiologic principles concerned in the production of the human electrocardiogram, which, if thoroughly appreciated, are sufficient for an analysis of the various normal and pathologic forms of this curve. This paper will concern itself with a brief statement of these principles, their proof and their application in electrocardiography.
1. Lead II = Lead I plus Lead III. This is a law whose proof is found in the law of the conservation of energy. If this principle were not as stated, the law of the conservation of energy would be false and perpetual motion possible.
Leads I, II and III are the expressions of potential differences existing in the heart at any given moment. As these three leads lie in the frontal plane, they represent the frontal plane value of the heart's potential differences; that is, the value of cardiac potential differences situated in any
FAHR G. SOME FUNDAMENTAL PRINCIPLES OF ELECTROCARDIOGRAPHY. Arch Intern Med (Chic). 1921;27(1):126–130. doi:10.1001/archinte.1921.00100070129008