TIME CONSTANT VOLUME RELATIONSHIP OF PASSIVE EXPIRATION IN MECHANICALLY VENTILATED ARDS PATIENTS

Since the adult respiratory distress syndrome (ARDS) lung is known to be inhomogeneous, one could expect an uneven distribution of expirator y time constant during uninterrupted mechanical ventilation. We invest igated the time constant/volume relationship of passive expiration, an d their modification by external resistive elements. In 12 paralysed i ntubated ARDS patients, we determined the expiratory time constant (ta u E) as a function of the expired volume (VE) during uninterrupted mec hanical ventilation, Mean expiratory time was 2.9+/-0.3s (+/-SD), VE w as divided into five equal volume slices (portions) and a mean tau E c alculated from the expiratory tidal volume/flow curve for each slice. The mean values of tau E for each volume slice did not differ signific antly throughout expiration, averaging 690+/-218 ms (mean+/-SD of five slices and 12 patients), We show that the flow-dependent resistance o f the endotracheal tube (RETT) is mainly responsible for the observed time constant homogeneity. We conclude that in ARDS patients during un interrupted mechanical ventilation the time constants of passive expir ation are markedly modified by the now-dependent resistance of the end otracheal tube (RETT), and also by the external resistance of tubing a nd ventilator (REX), RETT and REX render tau E about three times large r than the time constant of the patient's respiratory system alone.