826 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



just about the same time in all pulses taken from the 

 lower part of the aorta. It is most difficult to time the 

 peak of a pulse exactly, but this approximate identity 

 was taken as evidence that this peak was "standing." 

 This suggests that the aorta was achieving a "reso- 

 nance" with the first transit of the pulse wave. 



Resonance and Standing (1 m < s 



To explain the resonance concept, let us visualize 

 a somewhat elongated rubber balloon, filled with 

 fluid, and connected at one end to a syringe. A sudden 

 imput of fluid would start the bag oscillating, due 

 to a sloshing of fluid from one end to the other with 

 a reversal of movement, or a reflection, taking place 

 at each blind end. The period of such oscillations 

 must reflect the time required for the fluid slosh to 

 traverse the balloon, and therefore is related to the 

 conduction velocity of the fluid wave and the length 

 of the bag. The first of these is a function of the 

 distensibility of the part ot the bag through which 

 the wave is moving, as described earlier. If the wave 

 length of the slosh is just twice that of the transmission 

 time through the bag (or a simple multiple of it), 

 we could say that the bag was resonating, for a) the 

 pressure changes at the ends would be just 180 degrees 

 out of phase; b) the peak pressure, produced by a 

 summing of the incident wave with the reflected 

 wave, would be reached at the same time through 

 half the length of the tube. This means that there 

 would be a point of minimal pressure oscillation, or 

 a "node," at the mid point of the tube, and all peaks 

 and pressure troughs seen on either side of this node 

 would be "standing" through half the tube; c) the 

 time interval between two successive pressure peaks, 

 as recorded from any point in the tube, should be a 

 constant, and be an index to the length of tube and 

 the wave velocity. 



The records of Hamilton and Dow suggest that 

 all three criteria can be met in the arterial system. 

 There are pressure oscillations at the two ends of 

 the dog aorta which seem 180 degrees out of phase, 

 and which maintain approximately (but not exactly) 

 the same period until they are damped out. The 

 amount of pressure change with such oscillations is 

 much smaller in the arch of the aorta than in the 

 abdominal aorta, just as the distensibility of the two 

 regions is different. There are times when all three 

 criteria are not met in the dog, but more recent map- 

 pings indicate that what seems to be a true resonance 

 very often is achieved (4, 101). 



The carotid artery (or whole head system?) shows 



no similar oscillations, or even any great change in 

 pulse pressure with outward propagation of the 

 pulse (39). In man, the arm system shows augmenta- 

 tion of the pulse pressure but not resonance as defined 

 above (108). Records made in this laboratory indicate 

 that the foreleg system of the dog produces pulse con- 

 tour changes similar to those seen in the human arm. 

 Further, the aorta-femoral system does not show such 

 resonance or even clear oscillations in very small 

 animals (140). There is question whether resonance 

 occurs in an animal as large as man (108). The 

 German workers do believe the human aorta to show 

 resonance, but, as will be discussed later, their con- 

 clusion is not based on a standing peak for the periph- 

 eral pulses. 



Attempts to design a model that could illustrate 

 the prompt achievement of resonance, as occurs in 

 the dog, have not been successful. Certainly, ex- 

 periments in which independent pulses were gen- 

 erated in a closed and moderately long rubber tube 

 (46) provided little insight into how it would be 

 possible to make a previously quiet bag resonate 

 with the first propagation of a pressure pulse through 

 it. Granted that if the time of volume injection was 

 made identical to the transmission time of the wave 

 peak through the tube, reciprocal oscillations at the 

 ends would be seen from the time of completion of 

 the injection. But if the injection period was ap- 

 preciably longer or shorter than this, there was no 

 such immediate resonance. Instead the formed wave 

 peak could be followed back and forth through the 

 tube, as it was propagated at a steady rate, and 

 reflected at each blind end. Because the wave length 

 changed during these propagations, the foot moving 

 more rapidly than the peak, which in turn moved 

 faster than the "tail," after several trips through the 

 tube the wave could finally achieve a length equal 

 to that which would make the tube resonate. Whether 

 a given wave ever attained such resonance would 

 depend upon the number of trips required to 1 hange 

 its wave length, and the number that were possible 

 because of incomplete reflection and continued 

 damping. The change in wave length attending 

 propagation was attributed to the hysteresis behavior 

 of the wall. 



Similar changes in the length of an artificial pulse 

 were seen in a tied off but in situ dog aorta (46). 

 This change is directly opposite to that predicted by 

 the Bramwell and Hill formula, which would have 

 the peak moving faster than the foot. Of course, 

 artificial waves never attained the same rate ot 



