FICK'S MANOMETER 



183 



Fig. 144. 



OTHER METHODS OF RECORDING BLOOD PRESSURE 



We have had occasion to note that the mercury manometer is in certain 

 respects deficient, owing to the great inertia of its moving parts. A small 

 force, therefore, which lasts a short time can only produce a very minute 

 effect, and this will be masked by 



any larger effects occurring simul- B D F 



taneously. Even variations in 

 pressure of as great amount as 

 those produced by a heart beat are 

 only recorded after a considerable 

 delay, so that before the mercury 

 has had time to reach the level 

 which represents the highest pres- 

 sure attained that pressure has 

 ceased to act, and is again falling. 

 Fig. 144 illustrates this point. 



Suppose the line a b c r> to represent the actual changes in pressure 

 in a liquid where vertical ordinates represent pressures, and horizontal 

 ordinates time. If the changes take place very slowly then the mercury 

 manometer is able to register them as the line abcd with absolute accu- 

 racy. If, on the other hand, the changes are carried out rapidly, the record 

 obtained is very different, and something like the line abed. The mean 

 value is the same in both cases, but the amplitude of the record is much less ; 

 the highest and lowest pressures are not recorded, and, moreover, there is a 

 delay : the highest points of the curve b and d lie more to the right, and there- 

 fore later than the in- 

 stants of time at which 

 the highest pressures 

 actually occurred. The 

 apices also are rounded 

 instead of sharp. 



To obtain, therefore, a 

 true record of the changes 

 of fluid pressure in an 

 artery, we see that one of 

 the first necessities is to 

 choose apparatus in which 

 the moving parts possess 

 the least possible inertia. 

 One of the first means 

 that suggests itself is that 

 we should record changes 

 in diameter of the artery 

 whose pressure we wish 

 to record. This will give 

 us a measure of the varia- 

 tions of blood pressure in 

 the artery, because the 

 artery wall being elastic, 

 the lateral pressure upon 

 the wall causes a stretch- 

 ing of the wall in propor- 

 tion to that pressure. Having recorded the variations we can subsequently 

 ■calibrate them by recording the diameters of the same piece of artery under 



145.— Pick's C-Spbikg Manojd n B. 



