PULSATILE BLOOD FLOW 



849 



0.5 sec 



H 



y I 26 mm Hg 



Abdominal Aorta 



table 2. Distribution of Arterial Flow Pulse 



Mean Peak Velocity 



fig. 12. Transformation of the aortic blood flow between the 

 ascending aorta and abdominal aorta. High frequencies are 

 attenuated and a resonant frequency is superimposed. 



cardiac output decreases the resonant frequency. 

 Presumably the over-all compliance changes more 

 than the inertance in these conditions. 



3) The amplitude of the resonant wave is increased 

 when, in tachycardia, the systolic flow pulse is in 

 phase (48) with the resonant wave. 



4) The pressures and movements in the arterial 

 system represent, at any steady state of the hormonal 

 and nervous controlling conditions, transient re- 

 sponses to the flow input from the left ventricle. 



5) The augmentation of the pressure pulse, as it is 

 transmitted to the abdominal aorta, results from the 

 lower gross compliance of the abdominal arterial bed 

 as compared to the aortic arch vascular bed. 



6) The 30-100 cps components prominent in the 

 central aortic pressure pulse, as in the anacrotic 

 wave and the incisura, result from the stiffness of the 

 arterial walls and are damped out as they proceed 

 away from the heart. The dicrotic wave so prominent 

 in the peripheral pulse does not arise from this 

 source but is an expression of the resonant wave 

 phenomenon. 



Transmission Line Model {Distributed System) 



This is a useful concept in the arterial system, as in 

 any hydraulic continuum. It is represented by van der 

 Tweel (55) in figure 16. No matter how short or how 

 long a given segment may be, there is always present 

 some combination of inertance, compliance, and 

 resistance which may be lumped in a close approxima- 

 tion of the behavior of that particular segment. 



The performance of the transmission line is greatly 

 affected by the relation of the terminating impedance 

 to the characteristic impedance (52) of the line. If 

 the terminal impedance is equal to the characteristic 

 impedance all the energy will be absorbed and no 

 reflections occur. The characteristic impedance, 

 however, is frequency dependent, increasing with 

 frequency. If the terminal Z is greater than the line 

 Z, positive reflections will occur. Negative reflec- 

 tions will occur if the terminal Z is less than the 

 characteristic Z. 



Distribution of the Blood in the Aortic Arch 



This is shown in figure 17. In a manner analogous 

 to Kirchoff's current law, the flow into the arch at 

 any given instant from the ascending aorta is equal 

 to the sum of the flows into the brachiocephalic and 

 left subclavaian arteries, and the flow into the de- 

 scending thoracic aorta plus the uptake rate of the 



