

THE EVENTS OF THE HUMAN CARDIAC CYCLE 107 



auricular and venous curves are due to three causes : the increased 

 negative pressure in the chest, consequent on ventricular systole ; the 

 auricular relaxation dependent on the original intra-pleural pressure ; 

 and the stretching of the auricular walls as a result of ventricular 

 systole. Of these causes, the first is insignificant ; the second is 

 most active during the early phases, and the third most prominent 

 in the later phases of the depression. 



(e) The Third Negative or "y" Depression. With the 

 diastole of the ventricle, the pressure within it falls rapidly 

 and may become markedly negative. The fall of pressure is 

 accompanied by the opening of the auriculo-ventricular valves, 

 and the blood contained in the auricle passes into the ventricle. 

 As a consequence, the pressure in the auricle falls, and the depres- 

 sion in the auricular tracing timed to occur with the opening of 

 the valves 1 is universally attributed to this cause. The depression 

 may be due not only to a relief of the previous stasis, but to a 

 transmitted pressure wave of negative sign. To what extent the 

 negative pressure is thus transmitted to the auricle is uncertain. 

 Porter estimated the auricular pressure corresponding to this event 

 at '5 mm. Hg, and as a consequence concluded that the negative 

 pressure in the ventricle has little direct effect upon the pressure 

 in the auricle. Porter's experiments were carried out on dogs and 



1 The timing of this instant in man and the dangers which the methods involve 

 have been rejxjatedly referred to ; to emphasise them now entails tiresome but 

 necessary reiteration. The interpretation of the "y" depression maybe correct, 

 but it has not been arrived at by scrupulously accurate methods. It cannot be 

 said that the intra-auricular curve falls at the A.O. instant, while the curves of 

 Porter and those of Young and Hewlett and Rautenberg show it otherwise. 

 Moreover the drop in the jugular curve must be placed later, to allow for the 

 delay in transmission which undoubtedly occurs. So that if the drop in the 

 jugular curve is to be placed at the A.O. instant, the drop in the human intra- 

 auricular curve must be placed approximately ! sec. earlier, at a point at or near 

 the closure of the semilunar valves. 



Almost all writers, early and late, express the opinion that the drop occurs both 

 on auricular and jugular curves at the A.O. instant, or refer to unanimous state- 

 ments to that effect. And it is attributed by similar processes of reasoning to a 

 definite cause; In the above synoptic account the same line is taken, but for that 

 very reason this proviso is essential. For if the auriculo-ventricular valves ar<! 

 bulged into the auricle during early systole, they must remain bulged during the 

 plateau, and when the ventricular pressure begins to fall the tension in the valves 

 must decrease. Tin- fall in auricular PIV.-MIIV should therefore commence slightly 

 earlier than the A.O. point, namely, at or near the S.C. point. Porter, who took 

 differential curves of the pressures in the two cavities, actually found that the fall 

 in auricular pressure commenced at this instant. 





