1913.] 



Electrical Axis of the Human Heart. 



511 



synchronises the three records, in order to calculate from their corrected value 

 the magnitude of the angle a that must be formed with the horizontal by a 

 potential difference passing through the centre of the triangle. From this 

 basis Einthoven has calculated that in a particular subject (" Bak ") the 

 heart had rotated in the chest around a sagittal axis during the movement 

 of expiration by an angle of —36° {i.e. from « = 76° to « = 40°, referred to 

 the horizontal). 



2. Determination of the Electrical Axis. 



I have approached this problem of the angle from a different standpoint, 

 namely, from the point of view of my first observations of 1887-9, and the 

 distinction between " favourable " and " unfavourable " leads, or, as I now call 

 them, " strong " leads and " weak " leads. In order to calculate the obliquity 

 of the cardiac current-axis, I took values between a mesial point and the two 

 sides of the body, in fulfilment of the rough notion of a balance of which 

 the two unequally loaded arms, R and L, give an angular deflection of the 

 indicator in relation with a weight-difference between R and L. 



In electrical analogy with this idea, I took for calculation the values of the 

 potential differences between leads from the mouth and right hand and 

 between the mouth and left hand. According to this picture, the electrical 

 pivot or zero is an electrode in the mouth, and the weights are the potential 

 differences between the electrode M and the right hand R on one side, and 

 the left hand L on the other, to form the other electrode. These two leads 

 may be referred to as the right superior and left superior respectively, in 

 distinction from analogous leads between hands and feet, which will be 

 referred to as right and left lateral. The formula for calculating the angle a. 

 between current-axis and vertical is very simple: tan « = (L — R)/(L + R)„ 

 where L and R represent respectively observed magnitudes of potential 

 difference at the outset of systole by the left and right superior leads 

 respectively ; for example, with L = 9 and R = 3, 



9-3 6 



tan a = 



9 + 3 



= T2= °' 5 ' 



Similar considerations apply to the inferior (or posterior) half of the body, 

 the feet (or either foot if we admit that the small P.D. existing between the 



galvanometrische Registrirung des menschlichen Elektrocardiogrammes, zugleich eine 

 Beurtheilung der Anwendung des Capillar-Elektrometers in der Physiologie," ' P Auger's 

 Archiv,' 1903, vol. 99, p. 472 ; " Le Telecardiogramme," ' Archives Internat. de Physiol.,' 

 1906, vol. 4, p. 132 ; " Weiteres liber das Elektrocardiogramni," 'Pfliiger's Archiv,' 1908, 

 vol. 22, p. 517; "The different Forms of the Human Electrocardiogram and their 

 Signification," 'Lancet,' March, 1912, p. 853. 



* Or, more precisely, 26° 34' but in this connection a will be given to the nearest 

 degree only. 



