A correction procedure for the asymmetry of differential pressure transducers in respiratory impedance measurements.

The usual setup for measuring respiratory input impedance requires a differential pressure transducer attached to a pneumotachograph. As, up to now, no data correction procedure has been devised to account for transducer asymmetry, a highly symmetrical transducer is required to obtain reliable impedance data. In this communication, a general model for the measuring system is presented. Its main feature is that differential pressure transducers are modeled as two input-one output systems. From the theoretical model, we defined a dynamic calibration and data correction procedure. This was tested using highly asymmetrical transducers (common-mode rejection ratio between 45 and 27 dB) to measure the impedance of two respiratory analogs. The latter were linear resistance (R), inertance (I), compliance (C) series models simulating a normal subject (R = 3.47 hPa.s.l-1, I = 1.45 Pa.s2.l-1, C = 18.6 ml.hPa-1) and an obstructive patient (R = 11.15 hPa.s.l-1, I = 1.28 Pa.s2.l-1, C = 18.5 ml.hPa-1). Results obtained applying the devised procedure (errors in R, I, and C always less than 4 percent) show that respiratory input impedance can be adequately measured if data are corrected for transducer asymmetry.