The various inclinations of the electrical axis of the human heart. Part Ia.―The normal heart: Effects of respiration

In my first account of the electrical action of the human heart,f I made no allusion to the influence upon the electrical pulse of the movements of respiration. I noticed that influence indeed which is especially well marked in my own case (where the heart happens to be of the complete horizontal type) but only as disturbing the demonstration, and in some cases rendering the direction of the pulse uncertain. I noticed in particular that when demonstrating the transverse effect from the two hands, best effects were shown by holding my breath in expiration, and that these effects were markedly diminished during deep inspiration. I imagined at that time that the effect was due to a disfavouring of current spread from the heart by reason of the distended lung, but was puzzled by the fact that with the axial lead (right hand and left foot) the electrical pulse was augmented during inspiration instead of diminished as was the case with the transverse lead. I did not, however, follow up the clue afforded by this discrepancy, and it was only much later, i.eafter the introduction by Einthoven of the stringgalvanometer and the observations of Einthoven, Kraus and Nicolai, Samojloff and others, that the meaning of the discrepancy and with it the whole mechanism of the respiratory effects became clear. The variations of ampli tude are, if not entirely, almost entirely due to the rise and fall of the diaphragm, raising and lowering the heart as a lever hinged at the aortic end and thus widening and narrowing the “ axial angle.” (By axial angle I mean the angle formed with the vertical by the current axis of the heart or line of greatest potential difference at right angles to the equator 00.) In 1889 I represented this angle as being 45° to the left (and 45° to the right in cases of situs viscerum inversu ), and drew two series of curved