Fluid restriction reduces pulmonary edema in a model of acute lung injury in mechanically ventilated rats

Abstract

Experimental acute lung injury models are often used to increase our knowledge on the acute respiratory distress syndrome (ARDS), however, existing animal models often do not take into account the impact of specific fluid strategies on the development of lung injury. In contrast, the current literature strongly suggests that fluid management strategies have a significant impact on clinical outcome of patients with ARDS. Thus, it is important to characterize the role of fluid management strategies in experimental models of lung injury. In this study we investigated the effect of two different fluid strategies on commonly used outcome variables in a short-term model of acute lung injury, in relation to age. Infant (2-3 weeks) and adult (3-4 months) Wistar rats received intratracheal instillations of lipopolysaccharide and 24 hours later were mechanically ventilated for 6 hours. During mechanical ventilation, rats from both age groups were randomized to either a standard or conservative intravenous fluid strategy. We found that the hemodynamic response in infant and adult rats was similar in both fluid strategies. Lung wet-to-dry ratios were lower in adult, but not in infant rats receiving the conservative fluid strategy as compared to the standard fluid strategy. There were age-related differences in markers of alveolar capillary barrier disruption and alveolar fluid clearance, yet these were unaffected by fluid strategy. Finally, we found significantly higher IL-1β and TNF-α concentrations in the adult rats treated with the conservative as compared to the standard fluid regimen. In conclusion, the choice of fluid strategy in mechanically ventilated rats with experimental LPS-induced acute lung injury has a significant effect on pulmonary extravascular water, an important and well-recognized lung injury marker, and on the local pro-inflammatory cytokine profiles. We advocate the use of a more uniform, conservative, fluid strategy regimen in experimental models of acute lung injury.

Fig 1. Hemodynamic response is not affected by fluid strategy.

The mean arterial blood pressure per experimental group of LPS-inoculated and mechanically ventilated rats. Data are presented as mean + SEM; n = 6–8 animals per group. p = NS.

Fig 2. Representative images of lung pathology of each experimental group.

A: Adults with standard fluid strategy; B: Adults with conservative fluid strategy; C: Infants with standard fluid strategy; D: Infants with conservative fluid strategy. Magnification: 100X.

Fig 3. Lung and kidney wet-to-dry ratios.

Lung and kidney wet-to-dry weight ratios of each experimental group of LPS-inoculated and mechanically ventilated rats. Data are presented as median + interquartile range [IQR], the whiskers represent 1.5 IQR; n = 6–8 animals per group. *p<0.05, **p<0.01.

Fig 4. Neutrophil influx and activity is lower in infants than adults.

Total neutrophilic cell count in BALF and myeloperoxidase (neutrophil activity marker) in lung homogenate in each experimental group. Data are presented as median + interquartile range [IQR], the whiskers in represent 1.5 IQR; n = 6–8 animals per group. **p<0.01. BALF; bronchoalveolar lavage fluid.

Fig 5. Inflammatory cytokines in the lungs.

Inflammatory cytokines IL-6, MIP-2, IL-10 and TNF-α in BALF. IL-1β in lung homogenate. Data are presented as median + interquartile range [IQR], the whiskers in represent 1.5 IQR; n = 6–8 animals per group. p = NS. BALF: bronchoalveolar lavage fluid. *p<0.05, **p<0.01.

Fig 6. Alveolar permeability is lower in infants than adults.

Permeability markers total protein and alpha-2-macroglobulin in BALF in each experimental group. Data are presented as median + interquartile range [IQR], the whiskers in represent 1.5 IQR; n = 6–8 animals per group. **p<0.01. BALF: bronchoalveolar lavage fluid.

Fig 7. Epithelial cell sodium potassium ATPase pump activity in the lung.

Epithelial cell sodium potassium ATPase pump activity (Na⁺/K⁺/ATPase) was assessed in lung homogenate of rats in each experimental group. Data are presented as median + interquartile range [IQR], the whiskers in represent 1.5 IQR; n = 6–8 animals per group.

Fig 8. Static lung compliance.

Static lung compliance per experimental group of LPS-inoculated and mechanically ventilated rats. Data are presented as mean + SEM; n = 6–8 animals per group. p = 0.89.

Fig 9. PaO₂/FiO₂ ratios.

PaO₂/FiO₂ ratios at start (T = 0 hr), mid (T = 3 hr) and end (T = 6 hr) of the period of mechanical ventilation per experimental group of LPS-inoculated and ventilated rats. Data are presented as mean + SEM; n = 6–8 animals per group. p = 0.52.