296 



THE MECHANICS OF THE HEAET 



Systolic, Diastolic and Mean Pressure. The pressure in the cham- 

 bers of the heart undergoes extreme variations during each cardiac 

 cycle. The lowest values are reached at the end of diastole and 

 the highest at the end of systole. Thus, the pressure in the left ven- 

 tricle rises in the course of 0.06 sec., from near zero to about 130 mm. 

 Hg. The former is called the diastolic and the latter the systolic 

 pressure. For ordinary purposes it suffices to calculate the average 

 pressure by simply obtaining the arithmetical mean between the 

 diastolic and systolic values. It is essential, however, to include a 

 considerable number of cardiac cycles in this calculation. 



The Membrane Manometer. The tendency has been in recent 

 years to procure an instrument which is capable of folloMdng the rapid 

 alterations in pressure without that its parts, when once displaced, 

 enter into vibrations of their own. It is desirable at all times to obtain 

 not only the extreme heights of the pressure, but also its intermediate 

 values; in other words, it is of importance to secure a complete trac- 



FIG. 151. DIAGRAM OF MEMBRANE MANOMETER. 



M, rubber membrane connected with writing lever (L). The drum (T) is connected 

 with the cannula in the blood vessel; R, rod to fasten manometer to stand. 



ing of the curve of pressure. This end has been attained with a fair 

 degree of accuracy by means of elastic manometers in which the pressure 

 is not counterbalanced by the weight of ordinary liquids, such as water 

 and mercury, but by the resistance resident in an elastic body. A 

 rubber disc or a metal spring are usually employed for this purpose. 



The simple membrane manometer designed by Hurthle, 1 consists of a metal 

 drum closed by a sheet of thin rubber, the excursions of which are transferred 

 directly to a writing lever (Fig. 151). The sensitiveness of this instrument has been 

 much increased by permitting the membrane to act against a steel spiral which in 

 turn is connected with a writing lever. This principle is embodied in the so-called 

 spring, torsion, and reflecting or optical manometers. In all of them the variations 

 in the pressure of the blood are transmitted through the column of the fluid con- 

 tained in the connecting tubes, to the rubber membrane of the manometer. The 

 displacement suffered by the latter in consequence of the transferred pressure is 

 recorded in magnified form upon the paper of a kymograph. An instrument of 

 this kind must be calibrated repeatedly, i.e., the excursions of its rubber membrane, 

 as indicated by the writing lever, must be compared with the movements of a 

 column of mercury so that they may be expressed in terms of millimeters of 

 mercury. 



1 Pfluger's Archiv, xlix, 1891, 29. The first elastic manometer was constructed 

 by Fick in 1864 in compliance with the metal manometer of Bourdon. 



