828 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



seemed to be present, but it was small in magnitude 

 and short in duration. It was as though only the 

 first sudden acceleration of flow produced a reflected 

 wave. Whether this should be regarded as a common 

 finding, true for the whole arterial tree and also for 

 a closed-end rubber tube, is not clear. 



Hamilton & Dow (42) showed that when the aorta 

 was occluded, the frequency of the pressure oscilla- 

 tions seen on a pulse was more rapid than that usually 

 found in a normal dog. By moving the point of 

 occlusion distally, they concluded that the "end" of 

 the resonating system must lie outside the aorta. 

 Taking the time from the start of the central pulse 

 to the peak of a lower abdominal aortic pulse as 

 equal to half the total resonant wave length, they 

 calculated that the end should be near the knee, and 

 the node should be in the lower thoracic aorta. 

 Wezler & Boger (134) placed the end, which they 

 took as the point of negative reflection, in the femoral 

 artery near the inguinal ligament in the human. 

 Schmitt (115) located the node in the abdominal 

 aorta in man, and the end in the distal part of the 

 tibial artery. This was based simplv on transmission 

 times of the wave, for the time delay from the heart 

 to the node should also equal the time from the node 

 to the end, and equal a fourth of the total interval 

 between successive pressure peaks of a peripheral 

 pulse. Similar studies by Wetterer and co-workers 

 (58, 132) placed the end of the system beyond the 

 ankle in the foot. An occlusion by cuff inflation of 

 the legs shortened the interval between the systolic 

 and the postincisural pressure peaks, which they 

 reasoned could be true only if the cuffs were still 

 proximal to the end of the system (59). There are 

 two aspects of studies such as these that give room 

 for concern. First, the pressure peaks of pulses taken 

 from the leg arteries are not coincident with those 

 of the femoral artery (fig. 1 1), nor are they timed to 

 reciprocal oscillations of the central pulse. Of the 



Subclavian 



Popliteal 



Femoral 

 Tibial 



fig. 11. Pulse contours from peripheral arteries. [Redrawn 

 from Kapal et al. (59).] 



three criteria listed above for resonance, they satisfy 

 only one, i.e., the time interval between peaks re- 

 mains about the same as that seen more proximally. 

 It remains possible that the truly resonant pulse form 

 could be propagated with but little distortion of time 

 relations through the leg arteries. This seems to be 

 what happens in the arm vessels (108). Second, oc- 

 clusion of the aorta seems to make it behave as a 

 blind-end rubber tube would, and the waves which 

 are propagated back and forth in this occluded length 

 of vessel do not have the same characteristics as the 

 natural wave. It may be that leg occlusion could 

 introduce a reflection of the wave, and change the 

 timing between peaks, but that use of the occlusion 

 technique to identify the end of the resonant system 

 is not theoretically sound. It should be repeated that 

 our studies on aortic pulses in man (108) gave no 

 evidence of a standing peak. 



Alexander (3) believed that the node for the clog 

 was in the upper abdominal aorta where the large 

 visceral arteries exit. This is a point of sudden increase 

 in total cross-sectional area, where the flow rate, 

 relative to the vessel size, accounts for a large fraction 

 of the total cardiac output. When he occluded the 

 visceral arteries, the frequency of the resonant waves 

 seen in femoral pulses was increased, and the pre- 

 incisural trough of the pulse of the ascending aorta 

 became less conspicuous. Unfortunately, the pressure 

 was also raised by this maneuver, so that the changes 

 evoked are not indisputable evidence for his hy- 

 pothesis. Converselv, an intra-arterial injection of 

 histamine into the visceral arteries decreased the 

 frequency of oscillations (and also lowered the 

 systemic pressure). Ryan and co-workers (114) 

 repeated the occlusion experiments, and concluded 

 that the pressure rise might have been sufficient to 

 explain the changed frequency of the resonant waves 

 in the femoral pulse, but observed that the occlusion 

 did eliminate the preincisural trough of the central 

 pulse. In general, however, occlusion of exit arteries 

 has very little influence on the timing of the pressure 

 oscillations (57 ). 



Alexander (4) postulated that the aorta-femoral 

 system was essentiallv two open-end systems in 

 series, the region of visceral artery exit marking 

 an open end common to both. Arrival of the incident 

 wave at this area would be followed by a reflection 

 of a negative wave back toward the arch to produce 

 the preincisural trough. The incident wave would 

 also be propagated into the lower aorta as a positive 

 wave. Hence the two systems would be effectively 

 resonating with each other. 



