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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



This initial peak he labeled the acceleration transient, 

 and its force was equated to the small fluid mass 

 involved times the acceleration. Next, he added a 

 force which increased with the velocity, representing 

 the resistance offered to fluid displacement through 

 the tube. This reached significant proportions only as 

 the volume displacement did, which placed its con- 

 tribution later in time than the acceleration transient. 

 Finally, after a time lag, he added a straight line 

 increase in pressure to represent the force necessary 

 to prevent an elastic recoil of the walls as they were 

 stretched. 



Certainly the presence of these three forces in 

 constructing an aortic pulse cannot be denied. The 

 problem is to ascertain how large a role each of them 

 plays, and how much of a time delay between them 

 exists. Peterson's acceleration transient lasts for many 

 milliseconds. While wall hysteresis could, to use a 

 term employed long ago (46), make the vessel seg- 

 ment show a "reluctance to stretch," no studies on 

 isolated rings indicate that the reluctance could last 

 nearly this long. Just as crucial is his claim that the 

 same pressure excess which marks the acceleration 

 transient would persist through the whole systolic 

 period, so that all the pressure pulse would have a 

 higher value than would be predicted from a pressure- 

 volume diagram taken from stretch data. The fact 

 that he dealt with the whole arterial bed as a lumped 

 system has made it difficult to follow his argument. 



Rather than a discrete time lag of this sort, other 

 workers are supporting the presence of a sinusoidal 

 phase lag (34, 36, 83, 120, 128, 129). Using an 

 electrical analogue, the aorta is said to have an 

 inductive, capacitative, and a resistive impedance to 

 flow. Of these, the inductive and resistive factors 

 would be in phase, but the capacitative would lag up 

 to 90 degrees. In hydraulic terms, the first of these is 

 called inertance, which represents the mass of blood 

 displaced into the tube segment times its acceleration. 

 Opposing this inertance is the compliance (capaci- 

 tance) reflecting the volume taken to accomodate the 

 wall stretch, and the resistance, which represents all 

 fluid and wall factors that cause dissipation of energy 

 as heat. In an actual vessel subjected to pulsatile flow, 

 the interrelation of the three would be dependent 

 upon the rate of change in the driving pressure, 

 usually expressed in terms of the frequencies of the 

 harmonics. The better matched these frequencies are 

 to the inherent frequency of the vessel compliance, 

 which is a function of the visco-elastic properties of 

 the wall, the greater is the flow into and through a 



segment. The most proper match would be at the 

 "resonant" frequency of the vessel. 



If an isolated vessel is suddenly stretched and then 

 allowed to vibrate, it will show a definite period of 

 oscillation (77). This frequency will be different at 

 various pressure levels and with different parts of 

 the arterial system. It also can be changed by any 

 factor which influences the visco-elastic properties. 

 The matching frequency between a segment and the 

 driving pressure is therefore subject to considerable 

 variation. 



But it remains uncertain why such factors should 

 play a significant role in a distensible tube composed 

 of tiny segments. Certainly any final analysis of the 

 pressure-flow relation must reconcile the recent 

 data, based on the dictum that a phase lag must be 

 present, with the older descriptive work, which 

 includes evidence of a general absence of effect of 

 any physiological factor other than the diastolic 

 pressure level on pulse wave velocity, the details of 

 pulse contour change which takes place during 

 propagation (to be treated later), and the actual 

 time relation between flow and pressure curves. The 

 last of these has been least well covered. Records of 

 the flow pattern seen at different parts of the aorta 

 have been presented, and such records have, super- 

 ficially at least, much in common. But discrepancies 

 exist between them in regard to quantitation, timing 

 of peaks, and amount of end-systolic backflow. 

 (27, 53, 56, 84, 120). Unfortunately, a simultaneously 

 recorded pressure pulse is so rarely given that one 

 can never be sure whether the cardiodynamic con- 

 ditions were enough alike in the different experiments 

 that one should expect similar flow curves. 



In the ascending aorta, the flow rises sharply to a 

 peak reached in early systole and then falls more 

 gradually to reach a zero value, or below, at the 

 time of the aortic valve closure (fig. 7). The flow then 

 remains negligible throughout the diastolic period 

 (119), or may show a small sinusoidal increase in 

 diastole (131). In records taken from other parts of 

 the aorta, the amount of retrograde flow seen just 

 after the end of systole progressively increases as one 

 moves out the vessel, and the diastolic wave also 

 increases in magnitude (119). 



It may be well to digress into a semantic problem 

 that continually proves worrisome to students. The 

 point is frequently made or implied that there is a 

 clear distinction between the fluid displacement that 

 accompanies the movement of a pulse wave and the 

 "stream flow'" through the vessel. In the aorta there 

 really is no stream flow as such, and fluid displace- 



