Estimation of Respiratory Impedance and Source Pressure Using a Thevenin Equivalent Circuit Model

Abstract

Equivalent linear network models of the resp iratory mechanics have been used to estimate the lung resistance and compliance for early diagnosis of the obstructive lung diseases (Butler et aI., 1960; Grimby et aI., 1968; Macklem, 1972; Pimmel et aI., 1978, Schwaber et aI. , 1967). Among those methods, the forced oscillation methods have been most extensively studied by various investigators(Dubois et aI., 1956; Franetzki et aI. , 1979; Goldman et aI. , 1970; Hyatt et. aI., 1976; Landser et a1.,1976; Michaelson et aI., 1975; Peslin et aI., 1975) . In the forced oscillation method, the effects of the active respiration force on the accuracy of computing respiratory impedance were neglected. However, one recent report(Delavault et. aI., 1980) has shown in a theoretical and simu lation study that the effect of the parasitic signals is an important source of erro r. In their study,the measured impedance value was shown to vary from the true respiratory impedance value of the subject to the impedance value of apparatus, depending upon the amplitude of the source signal produced by the subjec t. Also,the complexity of the procedure and instrumentation has limited clinical application of the forced oscillation method. Hence, it is desirable to Address: ByoungG. Min, Department of Biomedical Engineering, Seoul National University Hospital, 28, Yeongun-Dong Chongro-Koo, Seoul, Korea,llO. develop a new clinical method which can provide the estimated values of both the active respiration force and the lung impedance in a single clinical tes t. In this paper, we have used a Thevenin equivalent circuit model of the respiratory mechanics to estimate simultaneously the respiratory impedance and the source pressure during spontaneous and maximal breathing procedures. The computed respiratory resistances of six normal human subjects were compared with the measured airway resistance values using the plethysmographic method.