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The hydraulic load presented to the left ventricle by the systemic circulation was characterized by expressing pressure-flow relationships in the ascending aorta as input impedance. This was determined by spectral or Fourier analysis of simultaneously recorded pressure and flow waves in 1 unanesthetized and 27 anesthetized dogs. Impedance modulus fell steeply from its value at zero frequency (the peripheral resistance) and its value was lowest (less than 1/20th of the peripheral resistance) over that band of frequencies (usually between 1.5 and 10 cycle/sec) which contained most of the energy of the left ventricular ejection (flow) wave. The patterns of modulus and phase of ascending aortic impedance were found to result from the presence of two functionally discrete reflecting sites in the systemic circulation, one in the upper part of the body and the other in the lower. The presence of these two sites appears to be an important factor in maintaining a low impedance modulus between 1.5 and 10 cycle/sec, and so in providing a favorable impedance to pulsatile flow from the heart. Both modulus and phase of impedance in the ascending aorta showed changes similar to those seen in other arteries when blood pressure was altered and when vasodilation occurred in the vascular bed.
O’Rourke et al. (Sat,) studied this question.