Middle age exacerbates acute respiratory distress syndrome in a double hit murine model

In a recent systematic review, aging has been identified as the only factor independently associated with mortality during human acute respiratory distress syndrome (ARDS). We explored this age-dependent severity in a clinically relevant double hit murine ARDS model.

Young adult (Y, 10-12weeks) and middle-old (O, 12-13months) male C57BL6 mice underwent an aspiration of Escherichia coli lipopolysaccharide (LPS) or control saline vehicle. Twenty hours later, four groups of mice were sacrificed [Y_(control), O_(control), Y_(LPS) and O_(LPS)]. Four other groups of mice underwent 3h of low tidal volume (8mL/kg) mechanical ventilation (MV) [Y_(MV), O_(MV), Y_(LPS+MV) and O_(LPS+MV)]. Lung mechanics were assessed hourly during MV. Right ventricular pressure and cardiac output were measured at the end of the MV. After sacrifice, lung inflammation, edema and injury were explored with bronchoalveolar lavage (BAL) and histology.

After saline aspiration, middle-old mice had a higher respiratory system compliance than young adult mice. LPS aspiration dramatically altered the baseline compliance in middle-old (O_(LPS)), but not in young adult (Y_(LPS)) mice. Middle-old mice had a more pronounced alteration in lungs mechanics during MV as compared to young adult mice. Lung inflammation (as assessed by the total cell count, IL-6, TNFα and MIP-2 concentrations in BAL fluid), systemic inflammation (as assessed by plasma IL-6 concentration) and alveolocapillary leak (as assessed by the total protein concentration of BAL fluid) were higher in O_(LPS) and O_(LPS+MV) mice as compared to Y_(LPS) and Y_(LPS+MV) mice, respectively. The combination of LPS+MV induced a higher lung injury as compared to LPS alone in middle-old mice but not in young adult mice. Hemodynamics (systemic blood pressure, cardiac output and pulmonary vascular resistances) were similar between Y_(MV) and O_(MV) on the one hand and between Y_(LPS+MV) and O_(LPS+MV) on the other hand.

Middle-old mice were more susceptible to both LPS alone and the combination of LPS and low tidal volume MV as compared to their young adult counterparts. The synergism between LPS and MV was amplified in middle-old mice.