Estimation of respiratory impedance at low frequencies during spontaneous breathing using the forced oscillation technique

The forced oscillation technique (FOT) is a non-invasive method to measure the respiratory impedance Z, defined as the complex ratio of transrespiratory pressure P to the airflow at the airway opening Q as a function of frequency. FOT determines Z by superimposing small amplitude pressure oscillations on the normal breathing and measuring the resulting air flow. In this work a new approach for the analysis of the respiratory impedance Z at low frequencies (0.1-5 Hz) during spontaneous breathing is presented. When the respiratory impedance is measured in frequency ranges that overlap with the frequency of spontaneous breathing (0.1-1 Hz), the measured air flow will contain both the breathing of the patient and the response of the respiratory impedance to the pressure oscillations. A nonlinear estimator is developed which is able to separate the breathing signal from the respiratory response in order to obtain the respiratory impedance. The estimated results are used to obtain accurate estimates of airway and tissue components of a constant phase model.