276 THE CIRCULATION OF THE BLOOD 



travel from the sinoauricular to the auriculoventricular node and bundle. 

 In delayed transmission this interval becomes abnormally long. Obvi- 

 ously also conditions of heart-block, of auricular fibrillation, or of auric- 

 ular flutter will be immediately revealed by the electrocardiogram. The 

 interpretation of abnormalities in the contour of the ventricular portion 

 of the curve is, however, not so easy a matter, and should never be 

 undertaken unless curves from the three leads have been secured, for it 

 will be found that the corresponding electrocardiograms differ from 

 one another in detail; for example, the R-wave is usually most prominent 

 in lead 2, although sometimes it is more prominent in lead 3. T is always 

 upright in normal individuals in curves taken from lead 2, but it is not 

 infrequently inverted in those of lead 3, and may show partial inversion 

 in those from lead 1. The Q-R-S group is often of peculiar contour in 

 curves from lead 3. These variations are possibly dependent upon the 

 relative preponderance of the musculature in the left and right ven- 

 tricles, for it is evident that the amount of muscle included in the path- 

 way between the two leads will vary. 



Interpretation of Electrocardiograms by the Triangle Method 



Much light can be thrown on the interpretation of electrocardiograms 

 by following the change of direction and magnitude of the vector rep- 

 resenting the potential difference in the heart. 



An excellent account of the methods for working out these problems is given by 

 Fahr 51 who has made use in a modified form of the equilateral triangle method of 

 Einthoven. The triangle is formed by lines joining leads I, II, and III used in elec- 

 trocardiography (i. e., the two hands and the left foot). When a potential difference 

 is created somewhere about the center of such a triangle (as in the heart) and we lead 

 off to galvanometers from it through the corners of the triangle, then the potential 

 difference between any two corners is to that between any other two corners ' ' as the 

 projection of a line having the direction of the potential difference at the center on 

 to the side of the triangle lying between the first two corners is to the projection on the 

 side of the triangle between the second pair of corners. ' ' By ascertaining synchronous 

 heights of the electrocardiograms from two leads, we can, by this method, ascertain 

 the real direction of the electrical tension and the magnitude of the potential differ- 

 ence in the heart at any phase of its activity. The latter is proportional to a value 

 called the ''manifest value" which is found as follows: an angle of 60 is constructed 

 and a distance C, in centimeters, equal to the tenths of millivolts observed in the 

 electrocardiogram from lead I, is measured off on the horizontal side; a distance D 

 similarly obtained from the synchronous point on the electrocardiogram of lead II, is also 

 measured on the other side. Perpendiculars DB and CB are dropped from the ends 

 of these lines and their point of intersection B is joined to the vertex of the angle A. 

 This line AB is the manifest value for this moment of the cardiac cycle, its length be- 

 ing proportional to the potential difference and its direction giving the direction of the 

 resultant electromotive force set up in the heart. 



By ascertaining the manifest values at different phases of the cardiac 



