Modeling of the Transfer Function of the Flow Transducer Used in Ventilatory Impedance Measurements

The frequency response of the ventilatory system can be studied by applying pressure oscillations at the mouth and measuring the total respiratory impedance. Flow is measured by a transducer made of a pneumotachograph connected to a differential pressure transducer. To characterize amplitude and phase distortion introduced by the flow transducer, its transfer function is described by a second-order equation. Determination of the model parameters is performed by a nonlinear minimization technique, in both the time and frequency domains. Time and frequency approaches give nearly identical values of the parameters, provided appropriate filtering is applied to pressure and flow according to their bandwidth and degree of coherence. The use of these dual techniques is likely to improve reliability of the model identification procedure.