The acoustic impedance of the respiratory system can be inferred from oscillations that are generated in an airway of a subject. The impedance describes the frequency-dependent relation between the resulting oscillations in flow and pressure. When the impedance varies from inspiration to expiration, it has to be estimated with a high time resolution. A method is provided that reliably estimates the impedance in time intervals that are short enough for physiological purposes. A simple version of the uncertainty principle has been derived for discrete time and frequency. A discrete time-frequency transform has been developed that gives an optimal time-frequency resolution according to this principle. The transform is orthonormal, which permits an analysis of variance in the discrete time-frequency domain. The impedance follows from bivariate least-squares analysis in the time-frequency domain, under the assumption that noise is present in both flow and pressure.
