Characteristic impedance: frequency or time domain approach?

TLDR

Characteristic impedance (Zc) is a key hemodynamic metric used to assess proximal arterial stiffness and pulse wave velocity, yet studies across species and conditions reveal that frequency‑ and time‑domain estimates often disagree. The study aims to investigate discrepancies between frequency‑ and time‑domain Zc estimates, considering experimental uncertainties and physiological conditions. The authors use published dynamic pressure‑flow data from humans, dogs, and mice to compare frequency‑ and time‑domain Zc calculations, examining effects of time delay, signal noise, flow uncertainties, and cardiac versus breathing periodicity. The study found that frequency‑ and time‑domain Zc estimates differ in sensitivity to phase lag, signal‑to‑noise ratio, and retrograde flow, and that these inherent mathematical differences preclude a single resolving criterion, suggesting the two estimates should be treated as distinct hemodynamic parameters.

Abstract

Characteristic impedance (Zc) is an important component in the theory of hemodynamics. It is a commonly used metric of proximal arterial stiffness and pulse wave velocity. Calculated using simultaneously measured dynamic pressure and flow data, estimates of characteristic impedance can be obtained using methods based on frequency or time domain analysis. Applications of these methods under different physiological and pathological conditions in species with different body sizes and heart rates show that the two approaches do not always agree. In this study, we have investigated the discrepancies between frequency and time domain estimates accounting for uncertainties associated with experimental processes and physiological conditions.We have used published data measured in different species including humans, dogs, and mice to investigate: (a) the effects of time delay and signal noise in the pressure-flow data, (b) uncertainties about the blood flow conditions, (c) periodicity of the cardiac cycle versus the breathing cycle, on the frequency and time domain estimates of Zc, and (d) if discrepancies observed under different hemodynamic conditions can be eliminated. Main results and Significance: We have shown that the frequency and time domain estimates are not equally sensitive to certain characteristics of hemodynamic signals including phase lag between pressure and flow, signal to noise ratio and the end of systole retrograde flow. The discrepancies between two types of estimates are inherent due to their intrinsically different mathematical expressions and therefore it is impossible to define a criterion to resolve such discrepancies. Considering the interpretation and role of Zc as an important hemodynamic parameter, we suggest that the frequency and time domain estimates should be further assessed as two different hemodynamic parameters in a future study.