272 THE CIRCULATION OF THE BLOOD 



which the electromotive force is flowing at the instant a particular deflection occurs. 

 The resultant direction of the electromotive changes the electrical axis as it is 

 termed is not constant, but varies from instant to instant throughout the cardiac 

 cycle and may be determined mathematically by employing as a basis the triangle 

 of Einthoven. This is a geometrical representation of the comparative potential 

 values in the three leads and their relation to the potential changes created within 

 the heart. 



An imaginary equilateral triangle is constructed, embracing the heart, having 

 its base in a horizontal plane and directed upwards and its vertex directed down- 

 wards. (Fig. 84-J5.) The sides of such a triangle lie approximately in alignment with 

 the three leads, and represent the direction of the circuits in these leads. An 

 electromotive force produced within the heart will cause a deflection of the gal- 

 vanometer string, the magnitude and direction of the deflection being dependent 

 upon the angle which the electrical axis makes with the side of the triangle repre- 

 sentative of the particular lead taken at the time. When the axis is precisely at 

 right angles to the line of the lead the string is undeflected in that lead, and the 

 tracing follows the line of zero potential. When it forms an angle other than a 

 right angle, a movement of the string occurs in one or other direction. The more 

 nearly parallel the electrical axis comes to lie to the side of the triangle corre- 

 sponding to the respective lead, the greater will be the electromotive force flowing 

 along that lead, and consequently the greater will be the magnitude of the recorded 

 deflection. The direction of the wave in the tracing (above or below the zero line) 

 with a given electrical axis, will, of course, depend upon the manner in which the 

 electrodes are connected to the galvanometer as regards polarity. When the elec- 

 trodes are connected in the standard way, an upward deflection occurs in lead I if the 

 general direction of the electromotive force is from right to left, and in leads II 

 and III when its general direction is downwards. A downward deflection occurs in 

 the tracing when these directions are reversed. 



If a line be drawn through the centre of the triangle, then its projection upon 

 the side of the latter representing lead II will be found to equal (according to the 

 direction of the line) the sum of or the difference between its projections upon the 

 remaining sides. Taking such a line to represent the electrical axis of the heart, 

 Einthoven has formulated the rule that e 1 + e 3 =. e 2 where e 1 e 2 and e 3 represent the 

 potential values respectively in the three leads. For example, if, after synchronous 

 points in the records from the three leads have been superimposed, and the heights of 

 the R waves measured, it will be found that R n (R wave in lead II) equals 

 B + R m . The same will be true whatever deflections are compared in the different 

 leads provided the sign of their respective values be taken into consideration. 

 Knowing the potential values of a given wave in the three leads, the direction of the 

 electrical axis during the production of that wave may be calculated from a trigono- 

 metrical formula, which, however, need not be detailed here. The following is a 

 simple practical method for obtaining the direction of the electrical axis at any 

 instant. The heights (or depths) in millimeters equal to tenths of millivolts of the 

 particular deflection in leads I and II, are laid off on the corresponding sides of the 

 triangle. (Fig. 84- A.} Perpendicular lines AB, CD, EF, and GH are next drawn 

 from the ends of these measurements toward the centre of the triangle, where they 

 intersect at X and Y. The direction of XY represents the electrical axis, and its 

 length the manifest value of the electromotive force created within the heart. Since 

 the potential values in the three leads are as the projections of the line XY on to 

 the sides of the triangle, it follows that the manifest value will be the maximal 



