556 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



Barium fitanate 

 crystols 



Y///////////A 



9/ 



flow 



d^" 



velo city 



v///////////////7^:^^ 



60 cc/sec - 



o- 



FLOW WAVEFORM 



_A_IWV 



FIG. 3. Short pulses of ultra sound waves are transmitted 

 diagonally across the aortic flow profile from one barium 

 titanate crystal and received by the other. Immediately a 

 second pulse originates in the second crystal and is received 

 by the first. The average velocity of flow is derived from the 

 time diflTerence of sound transmission up and downstream. 

 Since the mount maintains the aorta at a constant diameter 

 the volume flow can be known from the cross area of the 

 stream and the average velocity of flow. Below is the pattern 

 of flow velocity from beat to beat. [From Franklin et al. (38).] 



entering the arterial ciiamber, produces a given pres- 

 sure rise is not a direct one. It depends upon the 

 capacity of each segment of the artery at the beginning 

 of tile pulse. Pulse wave velocity then depends upon 

 the relative distensibility of arteries and not upon their 

 absolute distensibility. The relationship may be ex- 

 pressed as follows [after Bramwell & Hill (9)] PWV = 



0-357\/''A/'/Ar' in which AP is the increment in 

 pressure in mm Hg corresponding to \V, the in- 

 crement in volume in cubic centimeters starting from 

 V, the initial volume of the tube. PWV is the pulse 

 wave velocity in meters per second. 



It is .seen that the initial volume of the tube is a 

 very important variable in the relationship between 

 distensibility and pulse wave velocity. Thus if pulse 

 wave velocity remained constant and diastolic volume 

 were doubled, AV, AP (absolute distensibility) would 

 be doubled. Now the diastolic capacity is very hard to 

 evaluate. The size of the aorta has been measured in 

 specimens from fresh cadavers at all physiological 

 ranges of pressure (112) (cf. fig. 4). The extreme varia- 

 bility ranging over twofold in people whose history 

 was not significantly different makes it impossible to 

 predict the size of the aorta at diastolic pressure in 

 any given individual. Actual measurement of aortic 

 size by aortography has not proved useful. 



Another reason for believing that pulse wave 

 velocity is not related quantitatively to absolute dis- 

 tensibility is that the expansion of the initial upstroke 

 of the pulse wave is made against a preset stiffness in 

 the wall of the artery that makes it "reluctant" to 

 yield. This "reluctance to stretch" does not vary with 

 different rates of rapid stretch and is therefore not 

 viscosity as the term is usually used. 



In other words, when stretched initially it is effec- 

 tively more rigid than when the stretch is maintained 

 through systole, and these forces have time to dissipate. 

 The pulse wave velocity is set by the initial upstroke 

 against preset resistance (65), while the amount of 

 blood which has been taken up by the arterial tree is 

 governed by the maintained stretch under circum- 

 stances that make the artery yield more and be inore 

 distensible than it is at the initial stretch. 



Authors who have assessed arterial distensibility 

 from pulse wave velocity have used tables of aortic 

 size from measurements made post mortem of the 

 undistended vessel (6). These figures are much less 

 than those measured at diastolic pressure (112) and 

 hence are not applicable in the calculation of absolute 

 distensibility. Bazett et al. (6) have made a detailed 

 study of different segments of the arterial tree and 

 have calculated the distensibilities of these segments 

 and hence of the arterial tree as a whole. The Munich 

 school have used measurements of the aortic resonant 

 or standing wave (see \'olume II) to assess arterial 

 distensibility (i i, 142). All of these have shown agree- 

 ment between their method and respiratory methods 

 (see below) of measuring the cardiac output. OnK 



