PHYSIOLOGY OF AORTA AND MAJOR ARTERIES 



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funnel, where the distensibility is reduced. The 

 speed at which the wave front would move would 

 be dependent in part upon the force of the drive. 

 For under these conditions, the ascending aorta 

 would be "driving" the fluid through the various 

 exit branches, one of which would be the thoracic 

 aorta. These statements are similar to those made 

 previously in the discussion of the relation between 

 the pressure curve and the fluid displacement. It 

 remains for future work to reconcile evidence which 

 seems to favor the presence of a fluid surge with that 

 which supports the proposed model, having wave 

 propagation based on fluid displacement from one 

 tiny vessel segment to the next. 



A study of a great number of pulse forms leaves 

 the impression that the volume uptake of the aorta 

 in the period when the pressure shows this initial 

 fast rise is not so large as would be expected from 

 volume-pressure relations taken from a static stretch 

 curve. This impression has not been proven. A rapid 

 pressure rise at a time when the volume input is 

 small was a pillar of the acceleration transient story 

 of Peterson (90). One would like to explain an excess 

 pressure height, if present, on the basis of wall hystere- 

 sis. If, in studies with rapid stretches of isolated 

 vessels, there had ever been a considerable overfling 

 of pressure at the end of a stretch, I would feel happier 

 about this possible answer. If the impression is correct 

 that pressure rise exceeds the expected volume gain, 

 then it could also be true that in the interval of the 

 shoulder of the pulse, the volume gain would con- 

 tinue, and thus "catch up" with the pressure. 



When the pulse enters either the arm system or the 

 aorta-leg system, the height of the shoulder is in- 

 creased. In the human arm system, this elevation of 

 the shoulder takes place largely in the subclavian 

 arteries. The brachial pulse then shows two systolic 

 waves, one representing the shoulder, and the other 

 the later systolic part of the entering wave (12, 69, 

 108) (fig. 13). Very often the first is higher than the 

 second, and hence sets the pulse pressure. This is 

 particularly true when the anacrotic rise formed in 

 the ascending aorta is steep and the shoulder is 

 high. Late in a Valsalva maneuver, for example, the 

 aortic pulse shows a steep anacrotic rise and high 

 shoulder, but the rest of the pulse tends to collapse 

 toward a low incisura. This contour is in keeping with 

 the much reduced stroke volume. But in the brachial 

 pulse the shoulder may remain at almost the normal 

 height, which means that a pulse pressure measured 

 from this height would have no relation to the stroke 

 volume (108). 



fig. 13. Transformation of pulse in subclavian-arm system 

 (subject 1). A: arterial pressure pulses recorded at 5-cm inter- 

 vals during withdrawal of catheter through subclavian-upper 

 brachial system of normal subject. Curve 1 is the aortic pulse, 

 recorded from the thoracic aorta through a catheter inserted via 

 right femoral artery. It has been set back by 0.03 sec. Curve 2 

 (starting at o time) is the pulse in most proximal point in sub- 

 clavian artery reached by catheter inserted via left radial 

 artery. B: continued withdrawal of catheter into lower brachial 

 and radial system. Heavy solid line represents pulse from most 

 proximal position in subclavian (o-cm withdrawal in A). The 

 30-cm withdrawal curve is last pulse from proximal unit (A), 

 which still showed the simultaneous initial pressure peak. The 

 70-cm withdrawal is point at which catheter again entered 

 needle in left radial artery. Pulse from right radial artery, re- 

 corded through a similar needle itself, is labeled "radial." 

 [From Remington & Wood (108). j 



Conversely, when the pressure is well supported 

 in the upper aorta in late systole, the second wave 

 of the brachial pulse becomes higher, and may then 

 determine the pulse pressure. The maintenance of 

 high, late systolic pressure in the upper aorta may be 

 related to a large stroke volume. Hence in aortic 

 regurgitation, where the ventricle is large and the 

 total stroke volume much increased, the second wave 

 of the brachial pulse becomes quite large. In the 

 aortic pulse, the ejection interval is prolonged and 



