388 THE ELECTROCARDIOGRAM 



that, in the heart, it is not constant, but varies in direction with 

 every phase of the cardiac cycle. 



The three leads are represented on paper by the three sides of an 

 equilateral triangle, vertex pointing downwards, and a drawing of 

 the heart is placed in the triangle, having its base on one side 

 (corresponding to lead I.) and its right side (on the left of the 

 drawing) = lead II. Now, if a line be drawn on the heart in the 

 triangle to represent the electrical axis at any moment, and the 

 line be projected by drawing perpendiculars on the three sides of 

 the triangle, then the algebraic sum of the projections on any two 

 sides is equal to the projection on to the remaining side. The 

 projection of the electrical axis will be greatest on that side of the 

 triangle which is more nearly parallel to it. From this mathe- 

 matical truth Einthoven has formulated the rule that the potential 

 values represented in the cardiogram from lead II. are equal to the 

 sum of the corresponding values obtained in the graphs from leads 

 I. and III. That is, the height of Rj, (R in cardiogram from 

 lead II.) is equal to the sum of the heights of R, and Rjj,. Knowing 

 the potential values of a given wave, P, R, S or T in the three 

 leads, the direction of the electrical axis during the production of that 

 wave can be calculated by use of a trigonometrical formula. 



Since the potential values in the three leads are the projections 

 of the line of the electrical axis on the three sides of the triangle, 

 it follows that the maximum manifest value for any wave will 

 appear on the cardiogram when the representation of the electrical 

 axis is parallel to the line representing the lead. It will have a 

 minimum value when the axis line is at right angles to the line of 

 the lead. The magnitude of any wave from any lead, therefore, 

 depends on the angle which the electrical axis makes with the side 

 of the triangle representative of the particular lead taken at that 

 time. That is, the deflection of the string will be greatest in 

 lead I. when the electrical axis is parallel to the base of the heart, 

 in lead II. when parallel to a line drawn from apex to right side of 

 base and making an angle of 60 degrees with it, and in lead III. with 

 a corresponding line on the left side of the heart. 



Effect of H ion Concentration. It is well known that the rate at 

 which the excitation is conducted over the heart muscle varies 

 considerably with the /?H of the fluid medium in a perfused heart. 

 Increase in alkalinity, for instance, increases the conduction rate 

 and decreases the refractory period. Acid, naturally, has the 

 opposite effect. It has been found that the current of injury of 

 skeletal muscle {q.v.) can be reversed by decreasing the /?H below 

 7*4, the critical level. It is, therefore, suggestive to find that the 

 P. R and T waves of the electrocardiogram can be reversed in sign 



