PHYSIOLOGY OF AORTA AND MAJOR ARTERIES 



823 



120- 



fig. 8. Pressure and flow values given 

 by Spencer. Pulse a from Spencer & 

 Denison ( 1 20) ; pulses b and c from Spen- 

 cer el al. (119). Dotted line, summed 

 arterial bed uptake values, as described 

 in text. 



arbitrarily made to coincide with the presented 

 values. The agreement in contour between the curve 

 presented by Fry (solid line, fig. 9) and that calculated 

 (broken line) bears a good deal of resemblance to 

 those seen with the actual flow curves. It should be 

 mentioned that the differential pressure recording 

 presented by Fry indicates a peculiarly long delay 

 period between the two recording catheters, with a 

 slow transmission velocity through that particular 

 aorta (about 3 M/sec). 



These constructions provide presumptive evidence, 

 then, that there is a delay between the pressure and 

 fluid displacement curves. There will certainly con- 

 tinue to be interest in the factors which contribute to 

 this lag. Whether harmonic analysis of the pressure 

 pulse curves may be the most profitable tool for this 

 assessment remains to be decided. It is important 

 that we do not let sophisticated mathematics allow 

 us to lose sight of the basic processes by which a 

 distensible tube seems to be filled. Volume is displaced 

 from segment to segment, establishing and maintain- 

 ing a moving pressure wave. Since the distal parts of 

 the aorta are stiffer than the proximal, we would not 

 expect that the pattern of fluid displacement out of 

 the arch would be qualitatively similar to that of the 

 pressure curve, for the amount of fluid leaving the 

 upper aorta would be decreasing when the pressure 

 was rising. Toward the end of systole, when the wave 

 front has invaded the whole network of distensible 



vessels, flow would fall sharply to a low level which 

 represents mainly the drainage loss from the bed. 

 At this time, the aorta would be behaving more like 

 a rigid tube. 



Judging from cardiometer curves, ventricular ejec- 

 tion starts slowly, then rapidly attains a maximal and 

 constant rate which lasts through the first part of 

 systole. The outflow then slows, reaching a small 

 value some time before the valves actually close. 

 Because the first outflow, although slow, is confined 

 to the ascending aorta, the pressure rise produced 

 must be relatively large. As the wave moves through 

 the aorta, an even faster ejection rate will produce less 

 rise in ascending aorta pressure. This tendency is in 

 part offset by the stiffer walls of the more distal 

 vessels. But in any aortic segment a pressure rise 

 simply means that more blood is entering than is 

 leasing for the more distal regions. 



A pressure difference curve based upon a subtrac- 

 tion of pulses taken at two different sites can be mis- 

 leading. Even assuming no change in contour, until 

 the wave reaches the distal recorder the difference 

 will be but a replica of the proximal pulse. When the 

 pressure upstroke in the peripheral recording begins, 

 this difference curve must show a sharp inflection 

 and a fall, depending upon the duration of the first 

 steep pressure rise and the separation of the recorders. 

 As long as pressure is still rising in the proximal seg- 

 ment, the difference should remain slightly positive. 



