Analysis of the dynamic characteristics of pressure transducers for studying respiratory mechanics at high frequencies.

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Differential pressure transducers are commonly used to study respiratory mechanics at physiological frequencies as well as during external forcing at high frequencies. In the latter condition, measuring errors could occur if the input impedance of the pressure transducers is not sufficiently large with respect to that of the respiratory system. In this work we analysed the input impedance Z and the transfer function H of two common pressure transducers (Validyne MP-45 and Celesco LCVR) equipped with membranes of different sensitivities and with connecting tubes of different lengths. Z was measured by the tube method and H was measured by comparison with a flat-response pressure transducer. In agreement with the predictions based on a simple lumped-parameters model, we found that Z reached very low values, especially at the frequencies where H had a resonance peak. For instance, for the widespread Validyne MP-45 transducer (200 Pa) with connecting tubes of 16 cm length and 3.8 mm internal diameter a minimum of Z of 8300 Pa s litre-1 at 96 Hz was measured; at that frequency the amplitude of H attained a value of 3.1. Using the above transducer model we simulated the measurement of a rat input impedance up to 128 Hz using Validyne and Celesco transducers. With the Validyne MP-45 (200 Pa), equipped with the same connecting tubes as above, the computed error reached up to 50 per cent for the real part and 140 per cent for the imaginary part.