Gas Density Alters Expiratory Time Constants Before And After Experimental Lung Injury

Acute lung injury is characterized by regional heterogeneity of lung resistance and elastance that may lead to regional heterogeneity of expiratory time constants ((E)). 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% O-2 and the remaining 50% as nitrogen (N-2), helium (He) or sulfur hexafluoride (SF6). Values for (E) decreased with injury and were more variable after injury than before (P<0.001). Values for (E) increased throughout expiration both before and after injury, and the rate of increase in (E) was lessened by SF6 (P<0.001 when compared with N-2 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 N-2 and He mixtures. Importantly, SF6 mixtures also decrease the difference and variability of (E) between fast- and slow-emptying compartments before and after injury when compared with N-2 and He mixtures.