Mechanical ventilation interacts with endotoxemia to induce extrapulmonary organ dysfunction

Abstract

Introduction: Multiple organ dysfunction syndrome (MODS) is a common complication of sepsis in mechanically ventilated patients with acute respiratory distress syndrome, but the links between mechanical ventilation and MODS are unclear. Our goal was to determine whether a minimally injurious mechanical ventilation strategy synergizes with low-dose endotoxemia to induce the activation of pro-inflammatory pathways in the lungs and in the systemic circulation, resulting in distal organ dysfunction and/or injury.

Methods: We administered intraperitoneal Escherichia coli lipopolysaccharide (LPS; 1 microg/g) to C57BL/6 mice, and 14 hours later subjected the mice to 6 hours of mechanical ventilation with tidal volumes of 10 ml/kg (LPS + MV). Comparison groups received ventilation but no LPS (MV), LPS but no ventilation (LPS), or neither LPS nor ventilation (phosphate-buffered saline; PBS).

Results: Myeloperoxidase activity and the concentrations of the chemokines macrophage inflammatory protein-2 (MIP-2) and KC were significantly increased in the lungs of mice in the LPS + MV group, in comparison with mice in the PBS group. Interestingly, permeability changes across the alveolar epithelium and histological changes suggestive of lung injury were minimal in mice in the LPS + MV group. However, despite the minimal lung injury, the combination of mechanical ventilation and LPS resulted in chemical and histological evidence of liver and kidney injury, and this was associated with increases in the plasma concentrations of KC, MIP-2, IL-6, and TNF-alpha.

Conclusion: Non-injurious mechanical ventilation strategies interact with endotoxemia in mice to enhance pro-inflammatory mechanisms in the lungs and promote extra-pulmonary end-organ injury, even in the absence of demonstrable acute lung injury.

Figure 1

Physiological response to mechanical ventilation. Peak airway pressures (a) and mixed end-expiratory CO₂ (b) in mice treated with intraperitoneal PBS followed 14 hours later by 6 hours of mechanical ventilation (MV), and in mice treated with intraperitoneal lipopolysaccharide (LPS; 1 μg/kg) followed 14 hours later by 6 hours of mechanical ventilation (LPS + MV). The tidal volume was 10 ml/kg and the fraction of inspired oxygen was 0.21. *p < 0.05 compared with the MV group.

Figure 2

Cellular response. Lung homogenate myeloperoxidase activity (a), bronchoalveolar lavage fluid (BALF) total neutrophils (b), and BALF total cells (c) in mice treated with intraperitoneal PBS followed 14 hours later by 6 hours of spontaneous breathing (PBS) or mechanical ventilation (MV), and in mice treated with intraperitoneal lipopolysaccharide (LPS; 1 μg/kg) followed 14 hours later by either spontaneous breathing (LPS) or 6 hours of mechanical ventilation (LPS + MV). In all groups receiving mechanical ventilation, the tidal volume was 10 ml/kg and the fraction of inspired oxygen was 0.21. *p < 0.05.

Figure 3

Lung cytokine response. Lung homogenate concentrations of KC (a), macrophage inflammatory protein-2 (MIP-2) (b), and IL-6 (c) in mice treated with intraperitoneal PBS followed 14 hours later by 6 hours of spontaneous breathing (PBS) or mechanical ventilation (MV), and in mice treated with intraperitoneal lipopolysaccharide (LPS; 1 μg/kg) followed 14 hours later by either spontaneous breathing (LPS) or 6 hours of mechanical ventilation (LPS + MV). In all groups receiving mechanical ventilation, the tidal volume was 10 ml/kg and the fraction of inspired oxygen was 0.21. *p < 0.05.

Figure 4

Tissue response. Representative lung tissue sections stained with hematoxylin and eosin, from mice treated with intraperitoneal PBS followed 14 hours later by 6 hours of spontaneous breathing (PBS) (a, b) or mechanical ventilation (MV) (c, d), and from mice treated with intraperitoneal lipopolysaccharide (LPS; 1 μg/kg) followed 14 hours later by either spontaneous breathing (LPS) (e, f) or 6 hours of mechanical ventilation (LPS + MV) (g, h). The arrows show neutrophil in the alveolar walls. Note the slight thickening of the alveolar walls in (h). The right column shows magnifications of the indicated areas in the left column. Magnifications: left column, ×200; right column, ×400. In all groups receiving mechanical ventilation, the tidal volume was 10 ml/kg and the fraction of inspired oxygen was 0.21.

Figure 5

Plasma cytokine response. Plasma concentrations of KC (a), macrophage inflammatory protein-2 (MIP-2) (b), IL-6 (c), and TNF-α (d) in mice treated with intraperitoneal PBS followed 14 hours later by 6 hours of spontaneous breathing (PBS) or mechanical ventilation (MV), and in mice treated with intraperitoneal lipopolysaccharide (LPS; 1 μg/kg) followed 14 hours later by either spontaneous breathing (LPS) or 6 hours of mechanical ventilation (LPS + MV). In all groups receiving mechanical ventilation, the tidal volume was 10 ml/kg and the fraction of inspired oxygen was 0.21. *p < 0.05.

Figure 6

Markers of organ dysfunction. Plasma concentrations of alanine aminotransferase (ALT) (a), aspartate aminotransferase (AST) (b), and creatinine (c) in mice treated with intraperitoneal PBS followed 14 hours later by 6 hours of spontaneous breathing (PBS) or mechanical ventilation (MV), and in mice treated with intraperitoneal lipopolysaccharide (LPS; 1 μg/kg) followed 14 hours later by either spontaneous breathing (LPS) or 6 hours of mechanical ventilation (LPS + MV). In all groups receiving mechanical ventilation, the tidal volume was 10 ml/kg and the fraction of inspired oxygen was 0.21. *p < 0.05.

Figure 7

Liver histopathology. Representative samples from liver tissue sections stained with H&E, from mice treated with intraperitoneal PBS followed 14 hours later by 6 hours of spontaneous breathing (a) or mechanical ventilation (b), and from mice treated with intraperitoneal lipopolysaccharide (LPS; 1 μg/kg) followed 14 hours later by either spontaneous breathing (c) or 6 hours of mechanical ventilation (d). Livers from mice in the PBS group showed normal liver architecture, and cytoplasmic accumulation of glycogen (a, inset). Livers from mice in the mechanical ventilation (MV) group also showed normal architecture and glycogen depletion (b). Mice from the LPS group had accumulation of microvesicles in the cytoplasm (inset), which predominated in the periportal area (c). This microvesicular injury (inset) was also present in the LPS + MV group, but was markedly more extensive (d).