Clinical electrocardiography . erentiates the actual size of the deflections as they are repro-duced in the electrocardiogram, and their manifest size or differenceof electropotential. The manifest value varies in direct ratio tothe changes in magnitude of the actual potential differences pro-duced during each heart cycle. The string galvanometer is an instrument which measuresdifferences of potential, and because the resistance in its circuitis not infinite there is a slight tendency to decrease the difference 44 MATHEMATIC BASIS OF ELECTROCARDIOGRAPHY 45 of potential between the contact poin
Clinical electrocardiography . erentiates the actual size of the deflections as they are repro-duced in the electrocardiogram, and their manifest size or differenceof electropotential. The manifest value varies in direct ratio tothe changes in magnitude of the actual potential differences pro-duced during each heart cycle. The string galvanometer is an instrument which measuresdifferences of potential, and because the resistance in its circuitis not infinite there is a slight tendency to decrease the difference 44 MATHEMATIC BASIS OF ELECTROCARDIOGRAPHY 45 of potential between the contact points (derivations), but thisdifference is so small that it falls within the limits of experimentalerror. The height of the ordinate in Derivation II of the electro-cardiogram at any instant is the sum of the heights of the ordinatesin Derivations I and III; Derivation II equals Derivation I plusDerivation III. If it is desired to measure the difference of potentialof Derivation II at any instant, this can be accomplished by con-. Fig. 18.—Equilateral triangle using polar system of co-ordinates. (After Ein-thoven, Fahr, and De Waart.) necting the galvanometer with the right arm and the left potential difference can also be determined indirectly byobtaining the differences of Derivation I and Derivation III andadding them. Therefore, if the electrocardiogram elicits accuratevalue of potential differences, Derivation II = Derivation I +Derivation III at any and every instant during the heart in another way, Derivation II — Derivation I = Deri-vation III. 46 CLINICAL ELECTROCARDIOGRAPHY Einthoven used the polar system of co-ordinates in his compu-tations. In Fig. 18 the right and left arms and the left foot aredesignated R, L, F. They form the angles of the equilateraltriangle. The point H is the center and represents the arrow (E A) represents any given electric axis; the angleit forms with Derivation I (RL) is represented by a, and-any givenle
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