Modelling of passive expiration in patients with adult respiratory distress syndrome.

The time-course of volume change during passive expiration preceded by an end-inspiratory hold was studied with a biexponential model in six adult respiratory distress syndrome (ARDS) patients. We measured the initial volumes and time constants of the fast (tau 1), and the slow (tau 2) compartments of expiration, as well as the static elastance of the respiratory system. The results were compared to those of 11 normal subjects. We observed that: 1) the biexponential model fitted closely the volume decay; 2) the fast compartment was responsible for 81 +/- 7% (ARDS) versus 84 +/- 10% (controls) of the total volume exhaled, with tau 1 = 0.35 +/- 0.11 s (ARDS) versus 0.50 +/- 0.22 s (controls); 3) the slow compartment contributed only 19 +/- 6% (ARDS) versus 16 +/- 7% (controls), with tau 2 = 4.67 +/- 2.38 s (ARDS) versus 3.27 +/- 1.54 s (controls); and 4) static elastance was higher in ARDS patients. The findings could be explained in terms of a four parameter viscoelastic model of the respiratory system.