THE MECHANISM OF THE HEART PUMP 



943 



On registering the endocardiac pressure by the optical method, it is 

 found that the curves vary in form according to the condition of the heart. 

 In order to interpret these curves, we must utilise the knowledge obtained 

 from a simultaneous record of the pressures in the auricle and ventricle, 

 or in the ventricle and aorta. Fig. 408 represents the different forms of 

 curve obtained from the left ventricle. Very often, as in A, the curve 

 is not unlike a single muscle twitch or the curve of contraction of the frog's 

 heart muscle. Nearly always however it is possible to see on the upstroke 

 one or two elevations, the most noticeable being the elevation marked Sj_. 

 This can be shown to correspond to the opening of the* aortic valves. This 

 is still better marked in B, where the heart was beating very forcibly and 



Aorta -. 



fS 



Ventricle 



Auricle 



345 

 FIG. 409. 



67 



rapidly under the influence of adrenalin, and is also very evident in c. In 

 some cases, as in A, the rise of pressure occurs distinctly more slowly after 

 S x than before. In B there is a further rapid rise of pressure after S A before 

 the curve begins to slope away and, at the change of velocity of rise, there 

 is a second wave at the point S 2 . This is also marked in c. The slope 

 of the curve after S x or S 2 varies considerably according to the amount 

 of blood the heart is sending out. In c the intra ventricular pressure curve 

 runs almost horizontal for a time, and this part of the curve is known as 

 the systolic plateau, but, as is evident from A, a plateau in the strict sense 

 of the term is not always present. At the end of the ' plateau ' the pres- 

 sure rapidly falls, and the period where the lines thin out, i. e. the point 

 at which the fall is occurring most rapidly, corresponds to the closure of the 

 aortic valves. 



The average course of the changes of pressure in the heart during each 

 beat is shown diagrammatically in Fig. 409 (Piper). The cardiac cycle 



