Effect of tidal volume and positive end-expiratory pressure on expiratory time constants in experimental lung injury.

We utilized a multicompartment model to describe the effects of changes in tidal volume (VT) and positive end-expiratory pressure (PEEP) on lung emptying during passive deflation before and after experimental lung injury. Expiratory time constants (τE) were determined by partitioning the expiratory flow-volume (V˙EV) curve into multiple discrete segments and individually calculating τE for each segment. Under all conditions of PEEP and VT, τE increased throughout expiration both before and after injury. Segmented τE values increased throughout expiration with a slope that was different than zero (P < 0. 01). On average, τE increased by 45.08 msec per segment. When an interaction between injury status and τE segment was included in the model, it was significant (P < 0.05), indicating that later segments had higher τE values post injury than early τE segments. Higher PEEP and VT values were associated with higher τE values. No evidence was found for an interaction between injury status and VT, or PEEP. The current experiment confirms previous observations that τE values are smaller in subjects with injured lungs when compared to controls. We are the first to demonstrate changes in the pattern of τE before and after injury when examined with a multiple compartment model. Finally, increases in PEEP or VT increased τE throughout expiration, but did not appear to have effects that differed between the uninjured and injured state.