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T Ti im me e c co on ns st ta an nt t/ /v vo ol lu um me e r re el la at ti io on ns sh hi ip p o of f p pa as ss si iv ve e e ex xp pi ir ra at ti io on n i in n m me ec ch ha an ni ic ca al ll ly y v ve en nt ti il la at te ed d A AR RD DS S p pa at ti ie en nt ts s The mean values of τE for each volume slice did not differ significantly throughout expiration, averaging 690±218 ms (mean±SD of five slices and 12 patients). We show that the flow-dependent resistance of the endotracheal tube (RETT) is mainly responsible for the observed time constant homogeneity.We conclude that in ARDS patients during uninterrupted mechanical ventilation the time constants of passive expiration are markedly modified by the flow-dependent resistance of the endotracheal tube (RETT), and also by the external resistance of tubing and ventilator (REX). RETT and REX render τE about three times larger than the time constant of the patient's respiratory system alone.

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In tracheally intubated or tracheostomized spontaneously breathing patients, adequate compensation of tube resistance (i.e. with minimal overcompensation and minimal undercompensation) is best done by the ATC mode.