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. Because no data correction procedure has been devised to account for transducer asymmetry, a highly symmetrical transducer is required to obtain reliable impedance data. Here, 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, a dynamic calibration and data correction procedure is defined. This was tested using highly asymmetrical transducers (common-mode rejection ratio between 45 and 27 dB) to measure the impedance of two respiratory analogs. Results obtained show that respiratory input impedance can be adequately measured if data are corrected for transducer asymmetry.