Although the 3-element Windkessel (Wk) is still a useful analogue of arterial hemodynamics, can the validity of the frequency-domain analysis continue to be assumed? Our alternative time-domain approach holds that measured pressure is the sum of a Wk (P_Wk) and an “excess” pressure (P_excess).

“Characteristic impedance” (Z₀) is critical. Originally called characteristic resistance by Westerhof, Z₀ was simulated like peripheral resistance in a hydraulic model but recently has been interpreted only in the frequency domain. We have shown that P_excess varies linearly with aortic inflow with a slope of Z₀. Bench-top experiments with canine peak flows and aortic dimensions yielded pressure drops equal to those measured physiologically, and a proximal resistance approximating Z₀. A bench-top experiment simulating Westerhof’s hydraulic circuit demonstrated a P_Wk waveform.

We calculated the frequency-dependent impedance of measured pressure, P_Wk and P_excess, under the influence of nitroprusside (NP) and methoxamine (Mtx). With NP, there was no impedance minimum and the modulus of P_excess was frequency-independent. With Mtx, an impedance minimum was demonstrated but was due entirely to P_Wk. Thus, the impedance minimum appears to be due only to the P_Wk and may not also be essentially related to wave reflection.

Finally, we used our approach to demonstrate positive and negative wave reflection in the canine aorta. However, if P_Wk was not initially subtracted, backward waves appeared first in the ascending aorta and they appeared to be propagated forward (figure).

These profoundly paradoxical results above seem to undermine the fundamental presuppositions of the frequency-domain analysis.