Gas density alters expiratory time constants before and after experimental lung injury

What is the central question of this study? Does the induction of a model of lung injury affect the expiratory time constant (τE) in terms of either total duration or morphology? Does ventilation with gases of different densities alter the duration or morphology of τE either before or after injury? What is the main finding and its importance? The use of sulfur hexafluoride in ventilating gas mixtures lengthens total expiratory time constants before and after lung injury compared with both nitrogen and helium mixtures. Sulfur hexafluoride mixtures also decrease the difference and variability of τE between fast- and slow-emptying compartments before and after injury when compared with nitrogen and helium mixtures. Acute lung injury is characterized by regional heterogeneity of lung resistance and elastance that may lead to regional heterogeneity of expiratory time constants (τE). We hypothesized that increasing airflow resistance by using inhaled sulfur hexafluoride (SF6) would lengthen time constants and decrease their heterogeneity in an experimental model of lung injury when compared with nitrogen or helium mixtures. To overcome the limitations of a single-compartment model, we employed a multisegment model of expiratory gas flow. An experimental model of lung injury was created using intratracheal injection of sodium polyacrylate in anaesthetized and mechanically ventilated female Yorkshire-cross pigs (n = 7). The animals were ventilated with 50% O2 and the remaining 50% as nitrogen (N2), helium (He) or sulfur hexafluoride (SF6). Values for τE decreased with injury and were more variable after injury than before (P < 0.001). Values for τE increased throughout expiration both before and after injury, and the rate of increase in τE was lessened by SF6 (P < 0.001 when compared with N2 both before and after injury). Altering the inhaled gas density did not affect indices of oxygenation, dead space or shunt. The use of SF6 in ventilating gas mixtures lengthens total expiratory time constants before and after lung injury compared with both N2 and He mixtures. Importantly, SF6 mixtures also decrease the difference and variability of τE between fast- and slow-emptying compartments before and after injury when compared with N2 and He mixtures.