1913.] 



Electrical Axis of the Human Heart. 



513 



Where the spike is positive on the right side and negative on the left, the 

 positive maximum was reached O006 sec. earlier than the negative minimum, 

 and, as far as can be judged from the record, the two events appear to 

 commence synchronously. From the inspection of the left lateral spikes of 

 intermediate type, + — , it appears that the transition from 1 to 4 is by a 

 retrograde encroachment of — upon + . 



3. Proof of the Formulae : tan « = ^ — 5 tan « = 2 ? — ^ . 

 J J L + E E + L 



The normal difference between " strong " and " weak " leads is evidently 

 dependent upon the normal obliquity of the heart. The principal electrical 

 event of the beat, i.e. the systolic spike, is the resultant along a line BA 

 or CC representing the current-axis, of all the component differences of 

 potential existing in the heart muscle at the outset of systole. Comparing 

 the magnitudes of the spikes of the two sides it is obviously in correspon- 

 dence with the normal tilt of the heart's axis that the left superior is the 

 " strong " lead and the right superior the " weak " lead. 



From the observed magnitude of the two spikes, right and left, it is easy 

 to construct a geometrical figure by means of which the numerical value of 

 the obliquity can be expressed in terms of the angle made by the current 

 axis CC with the vertical line MV. 



In the triangle MRL, the sides ME, ML indicate the right and left 

 superior leads, M being the mouth, E the right hand, L the left hand. 

 Actually, in relation to the heart, these leading points are to be regarded 

 as sections through the neck and shoulders, and for convenience of calcu- 

 lation we shall take the angle at M = 90°, so that MV = \ EL = EY. 

 Taking, e.g. the right-hand spike = 1 and the left-hand spike = 3, we have 



at M the potential = 0, at E the potential = 1, at L the potential = 3. 

 Projecting the potential of M on to the horizontal line LE produced to 

 and taking the position of the point such that the length OE shall 

 represent the potential difference between M and E, and OL the potential 

 difference between M and L, we have MO as the line of zero potential. A 



