ASSESSMENT OF TIME-DOMAIN ANALYSES FOR ESTIMATION OF LOW-FREQUENCY RESPIRATORY MECHANICAL-PROPERTIES AND IMPEDANCE SPECTRA

Time-domain estimation has been invoked for tracking of respiratory me chanical properties using primarily a simple single-compartment model containing a series resistance (R(rs)) and elastance (E(rs)). However, owing to the viscoelastic proper ties of respiratory tissues, R(rs) a nd E(rs) exhibit frequency dependence below 2 Hz. The goal of this stu dy was to investigate the bias and statistical accuracy of various tim e-domain approaches with respect to model properties, as well as the e stimated impedance spectra. Particular emphasis was placed on establis hing the tracking capability using a standard step ventilation. A simu lation study compared continuous-time versus discrete-time approaches for both the single-compartment and two-compartment models. Data were acquired in four healthy humans and two dogs before and after induced severe pulmonary edema while applying sinusoidal and standard ventilat or forcing. R(rs) and E(rs) were estimated either by the standard Fast Fourier Transform (FFT) approach or by a time-domain least square est imation. Results show that the continuous-time model form produced the least bias and smallest parameter uncertainty for a single-compartmen t analysis and is quite amenable for reliable on-line tracking. The di screte-time approach exhibits large uncertainty and bias, particularly with increasing noise in the flow data. In humans, the time-domain ap proach produced smooth estimates of R(rs) and E(rs) spectra, but they were statistically unreliable at the lower frequencies. In dogs, both the FFT and time-domain analysis produced reliable and stable estimate s for R(rs) or E(rs) spectra for frequencies out to 2 Hz in all condit ions. Nevertheless, obtaining stable on-line parameter estimates for t he two-compartment viscoelastic models remained difficult. We conclude that time-domain analysis of respiratory mechanics should invoke a co ntinuous-time model form.