Measurement of forced random noise respiratory impedance up to 200 Hz with a pneumotachograph

Respiratory impedance can be measured as the ratio of pressure to flow when forced random noise is applied at the mouth. With a screen pneumotachograph, flow is considered as proportional to the pressure drop Delta P=P/sub 1/-P/sub 2/ across a fixed resistance. Because of their limited in-phase rejection, most differential pressure transducers sense a drop that can noticeably differ from Delta P when frequency increases up to 200 Hz: flow is nonzero when the same pressure is applied to both sides of the transducer. Using simple physical models made of resistances and inertances the authors have tried to characterize the asymmetry of the transducer and to correct experimental impedances for this error. The results compare reasonably well with impedances obtained when two identical transducers, referenced to atmosphere, are used to sense separately P/sub 1/ and P/sub 2/.<>