Predicting a Reflection Site from Impedance Analysis: Implications of the Time‐domain Windkessel

One of the most important implications of the impedance spectra is to predict the reflection sites in aorta via the minimum of modulus and the zero‐crossing of phase. The pattern having received the most attention is one in which modulus declines sharply and monotonically from its zero‐frequency value (SVR) to reach a minimum within the first few harmonics. Recently, we demonstrated that aortic pressure is the instantaneous sum of P ex (due to wave motion) and P Wk (Windkessel pressure, due to changes in arterial volume). The Fourier transform of P Ao /Q Ao equals the sum of Fourier transform of P ex /Q Ao plus P Wk /Q Ao . The power spectrum of each term was shown in figure; the modulus spectrum of P Wk /Q Ao declines monotonically from a zero‐frequency value near that of SVR. It is almost equal to that of P Ao /Q Ao over the low frequency range. The modulus of P ex /Q Ao seems to be independent of frequency and is not different in magnitude from the value of characteristic impedance. We conclude that the minimum modulus of P Ao /Q Ao may be the fortuitous result of the summation of a monotonically declining P Wk /Q Ao modulus and an essentially flat P ex /Q Ao modulus. If this is true, the significance of the apparent minimum needs to be re‐examined theoretically and experimentally: experiments should be performed to demonstrate that reflection sites correspond to those predicted from the apparent minimum.