A high fraction of inspired oxygen does not mitigate atelectasis-induced lung tissue hypoxia or injury in experimental acute respiratory distress syndrome

Abstract

Although alveolar hyperoxia exacerbates lung injury, clinical studies have failed to demonstrate the beneficial effects of lowering the fraction of inspired oxygen (F_IO₂) in patients with acute respiratory distress syndrome (ARDS). Atelectasis, which is commonly observed in ARDS, not only leads to hypoxemia but also contributes to lung injury through hypoxia-induced alveolar tissue inflammation. Therefore, it is possible that excessively low F_IO₂ may enhance hypoxia-induced inflammation in atelectasis, and raising F_IO₂ to an appropriate level may be a reasonable strategy for its mitigation. In this study, we investigated the effects of different F_IO₂ levels on alveolar tissue hypoxia and injury in a mechanically ventilated rat model of experimental ARDS with atelectasis. Rats were intratracheally injected with lipopolysaccharide (LPS) to establish an ARDS model. They were allocated to the low, moderate, and high F_IO₂ groups with F_IO₂ of 30, 60, and 100%, respectively, a day after LPS injection. All groups were mechanically ventilated with an 8 mL/kg tidal volume and zero end-expiratory pressure to induce dorsal atelectatic regions. Arterial blood gas analysis was performed every 2 h. After six hours of mechanical ventilation, the rats were euthanized, and blood, bronchoalveolar lavage fluid, and lung tissues were collected and analyzed. Another set of animals was used for pimonidazole staining of the lung tissues to detect the hypoxic region. Lung mechanics, ratios of partial pressure of arterial oxygen (P_aO₂) to F_IO₂, and partial pressure of arterial carbon dioxide were not significantly different among the three groups, although P_aO₂ changed with F_IO₂. The dorsal lung tissues were positively stained with pimonidazole regardless of F_IO₂, and the HIF-1α concentrations were not significantly different among the three groups, indicating that raising F_IO₂ could not rescue alveolar tissue hypoxia. Moreover, changes in F_IO₂ did not significantly affect lung injury or inflammation. In contrast, hypoxemia observed in the low F_IO₂ group caused injury to organs other than the lungs. Raising F_IO₂ levels did not attenuate tissue hypoxia, inflammation, or injury in the atelectatic lung region in experimental ARDS. Our results indicate that raising F_IO₂ levels to attenuate atelectasis-induced lung injury cannot be rationalized.

Keywords: Acute respiratory distress syndrome; Alveolar hypoxia; Atelectasis; Hypoxia-induced inflammation; Oxygen.

Fig. 1

Schematic diagram of experimental design. BGA, blood gas analysis; F_IO₂, fraction of inspiratory oxygen; RM, recruitment maneuver.

Fig. 2

Computed tomography analysis. (A) Representative images of lung computed tomography at the T6–T7 vertebral level; and (B) aerated lung volumes at end-inspiratory phase. *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001. Data represent the means ± SEM.

Fig. 3

Physiological parameters and arterial blood gas analysis. (A) Dynamic driving pressure. (B) Partial pressure of arterial oxygen (P_aO₂). (C) P_aO₂ / fraction of inspiratory oxygen (F_IO₂) ratio (P/F ratio). (D) Partial pressure of arterial carbon dioxide (P_aCO₂). (E) Mean arterial blood pressure. (F) Blood lactate concentration. †p < 0.05 vs. Moderate F_IO₂ group, ‡p < 0.05 vs. High F_IO₂ group. Data represent the means ± SEM.

Fig. 4

Evaluation of lung tissue hypoxia. (A) Representative images; and (B) percent positive area of pimonidazole staining in lung tissue. (C) hypoxia-inducible factor (HIF)-1α protein levels in lung tissue. *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001. Data represent the means ± SEM.

Fig. 5

Analysis of inflammatory mediators and leukocytes in bronchoalveolar lavage fluids (BALF). (A) mRNA expressions of cytokines and chemokines in the lung tissues. (B) Protein concentrations of cytokines and chemokines in the BALF. (C) White blood cell counts and (D) myeloperoxidases (MPO) in the BALF. *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001. Data represent the means ± SEM.

Fig. 6

Analysis of tissue injury markers in bronchoalveolar lavage fluids (BALF) and histology. (A) total protein concentration; (B) soluble receptor for advanced glycation end products (sRAGE); and (C) intercellular adhesion molecule (ICAM)-1 in the BALF. (D) Representative images of lung sections stained with hematoxylin and eosin. (E) Histological scores assessed in a blinded manner. *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001. Data represent the means ± SEM.

Fig. 7

Analysis of liver and kidney injury markers in plasma. (A) glutamic oxaloacetic transaminase (GOT); (B) glutamic pyruvic transaminase (GPT); (C) Creatinine; and (D) Cystatin C in the plasma. *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001. Data represent the means ± SEM.