The Velocity of the Pulse Wave in Man. 



305 



We see here the same effect of pressure as was shown by the calculation 

 from Eoy's curves : the velocity increases comparatively slowly as the pres- 

 sure rises to about 80 mm., then more rapidly, and finally at high pressures 

 very considerable velocities are observed. As regards the absolute value, 

 we may compare the velocity given in the above Table with that found, at 

 the same (diastolic) pressure, in a normal living subject. According to 

 Gallavardin (5) the average normal value of the diastolic pressure in man 

 is 70 to 75 mm. Much higher values, however, are given by the use of the 

 Pachon oscillometer (80 to 110 mm.). In normal healthy young men our 

 observations (to be described elsewhere) have given velocities from 5*8 to 

 7'4 metres per second. Compared with the velocity interpolated in the above 

 Table, for a pressure of 70 to 75 mm., these velocities are high : accepting 

 the higher estimate of the diastolic pressure in man, the observed velocities 

 agree well with those given in the Table ; the velocities 5 - 8 and 7*4 m.p.s. 

 correspond roughly there to pressures of 85 and 102 mm., respectively. On 

 the whole, therefore, we may be satisfied that the pulse-wave has a velocity 

 in the living man not far different from that in an isolated artery, and that 

 its transmission is a mechanical phenomenon depending only on the elastic 

 properties of the vessels. 



Summary. 



The theory of the transmission of the pulse-wave in a blood vessel 

 is considered, and it is shown that its velocity, in metres per second, is 

 given by 



v — 3 - 57/ •/ (percentage increase in volume of artery per millimetre 

 of Hg increase of pressure). 



This velocity is relative to the blood in the vessel, and must have a small 

 correction applied for the velocity' of the blood itself. An observation of the 

 velocity, therefore, gives directly the degree of extensibility of the vessel, 

 and is shown to be one criterion of an efficient circulation. The experiments 

 of Eoy (1880) on the extensibility of vessels may be used to calculate the 

 velocity of the pulse- wave : the calculation shows : (a) that pressure has a 

 considerable effect on the velocity, a fact which is confirmed by experiments 

 on an isolated human artery, filled with mercury in order to slow the 

 transmission of the wave ; and (b) that the velocity so calculated is lower 

 than observed in man, a fact which is attributed to the phenomenon of elastic 

 " after-action," which affected Eoy's measurements. The experiments on an 

 isolated human artery gave a velocity comparable with that observed in man. 

 and it is concluded that the transmission of the pulse -wave is a purely 



vol. xciii. — b. z 



