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

Abstract

New Findings r What is the central question of this study?Does the induction of a model of lung injury affect the expiratory time constant (τ E ) in terms of either total duration or morphology? Does ventilation with gases of different densities alter the duration or morphology of τ E either before or after injury? r 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 τ E between fastand 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 (τ E ). We hypothesized that increasing airflow resistance by using inhaled sulfur hexafluoride (SF 6 ) 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 (SF 6 ). Values for τ E decreased with injury and were more variable after injury than before (P < 0.001). Values for τ E increased throughout expiration both before and after injury, and the rate of increase in τ E was lessened by SF 6 (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 SF 6 in ventilating gas mixtures lengthens total expiratory time constants before and after lung injury compared with both N 2 and He mixtures.

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

“…First, consistent with our previous work (Henderson et al. ), τ E increased throughout expiration, and injury increased the difference between late and early τ E segments compared to before injury. These changes were due to increases in both R RS and E RS .…”

“…, ; Henderson et al. ). When a segmented model was used to analyze our data, we were able to observe the mechanical properties of passive expiration with a higher degree of resolution than is afforded by the single‐compartment model.…”

“…, ; Henderson et al. ). The multisegment method allows a more nuanced description of the changes in τ E throughout expiration, and therefore facilitates better understanding of the physiology of passive expiration than does a single‐compartment model.…”

“…We have recently demonstrated that the τ E pattern of lung passive expiration in the same subject differs with injury status and that these patterns can be altered by manipulating the density of the ventilating gas (Henderson et al. ). Kondili and colleagues have examined changes in τ E due to manipulation of positive end‐expiratory pressure (PEEP) following lung injury (Kondili et al.…”

The Effect of Positive End-Expiratory Pressure on Lung Micromechanics Assessed by Synchrotron Radiation Computed Tomography in an Animal Model of ARDS

“…In fact, alveoli that have different time constants inflate at different speeds and maintain different volumes during the respiratory cycle [14]. Computation of time constants is very useful for a thorough assessment of lung mechanics [15], however their estimation in the clinical context must take into consideration also the effects of tubing resistance [16] and the density of the respiratory gases [17].…”