Oxidative Stress and Inflammatory Response in Pulmonary Contusion: Temporal Analysis of Arterial Oxygen Partial Pressure, Reactive Oxygen Species, and Interleukin-6 with Associated Histopathological Changes

ABSTRACT Background: Pulmonary contusion represents a significant cause of respiratory morbidity following blunt chest trauma, characterized by complex pathophysiological mechanisms involving oxidative stress and inflammatory cascades. The temporal relationship between arterial oxygen partial pressure (PaO2), reactive oxygen species (ROS), interleukin-6 (IL-6), and histopathological changes including alveolar oedema, alveolar haemorrhage, and leukocyte infiltration remains incompletely understood. Objective: To investigate the temporal progression of oxidative stress markers, inflammatory cytokines, and histopathological alterations in an experimental pulmonary contusion model, with emphasis on the relationship between PaO2, ROS, IL-6, and pulmonary structural damage. Methods: Twenty-seven male Sprague-Dawley rats (8-12 weeks, 180-250g) were randomly allocated to three groups: control, 1-hour post-contusion, and 48-hour post-contusion. Pulmonary contusion was induced using a standardized blunt trauma model involving a 500-gram weight dropped from 50 cm height. Arterial oxygen partial pressure (PaO2) was measured using blood gas analysis and expressed in millimeters of mercury (mmHg). Blood samples were analyzed immediately after collection using a calibrated blood gas analyzer (ABL90 FLEX, Radiometer, Denmark) maintained at 37°C. Reactive oxygen species (ROS) levels were quantified using enzyme-linked immunosorbent assay (ELISA) and expressed as relative fluorescence units per milligram of protein (RFU/mg protein). Lung tissue samples were homogenized in phosphate-buffered saline (PBS) containing protease inhibitors, and protein concentration was determined using the Bradford assay. ROS levels were measured using the OxiSelect™ ROS Assay Kit (Cell Biolabs, Inc., San Diego, CA, USA) according to the manufacturer's protocol. Interleukin-6 (IL-6) concentrations were quantified using enzyme-linked immunosorbent assay (ELISA) and expressed in picograms per milliliter (pg/mL). Lung tissue homogenates were prepared as described above, and IL-6 levels were measured using the Rat IL-6 ELISA Kit (R&D Systems, Minneapolis, MN, USA). Histopathological examination was performed using hematoxylin-eosin staining to assess alveolar oedema, alveolar haemorrhage, and leukocyte infiltration. Statistical analysis employed one-way ANOVA with Tukey HSD post-hoc test for normally distributed data (PaO2, ROS, IL-6) and Kruskal-Wallis test with Mann-Whitney U post-hoc analysis for non-normally distributed data (histopathological parameters). Results: Arterial oxygen partial pressure demonstrated progressive deterioration from control levels (85.73 ± SD) to 76.89 ± SD at 1 hour post-contusion (p < 0.1) and 70.61 ± SD at 48 hours post-contusion (overall p < 0.001), indicating compromised gas exchange function. Reactive oxygen species levels showed significant elevation from baseline (874.0 ± SD) to 1314 ± SD at 1 hour and 1464 ± SD at 48 hours post-injury (overall p < 0.1), demonstrating sustained oxidative stress. Interleukin-6 concentrations increased dramatically from control values (7.378 ± SD) to 32.56 ± SD at 1 hour post-contusion (p < 0.1) and remained elevated at 32 ± SD at 48 hours (overall p < 0.1), indicating robust inflammatory activation. Alveolar oedema scores increased progressively from control (0.667 ± SD) to 1.9 ± SD at 1 hour (p < 0.1) and 2.78 ± SD at 48 hours post-contusion (overall p < 0.001). Alveolar haemorrhage demonstrated significant elevation from control levels (0.889 ± SD) to 2.3 ± SD at 1 hour (p < 0.05) and 2.78 ± SD at 48 hours post-contusion (overall p < 0.001). Leukocyte infiltration exhibited gradual increase from control (1.11 ± SD) through 1.8 ± SD at 1 hour to 2.33 ± SD at 48 hours post-contusion (overall p < 0.01). Conclusions: Pulmonary contusion triggers a biphasic pathophysiological response characterized by immediate oxidative stress and inflammatory activation followed by sustained tissue damage. The progressive decline in arterial oxygen partial pressure correlates with elevated ROS and IL-6 levels, accompanied by persistent alveolar oedema, alveolar haemorrhage, and leukocyte infiltration. These findings demonstrate the critical role of oxidative stress and inflammatory mediators in the pathogenesis of pulmonary contusion and provide valuable insights for developing targeted therapeutic interventions. Keywords: pulmonary contusion, arterial oxygen partial pressure, reactive oxygen species, interleukin-6, alveolar oedema, alveolar haemorrhage, leukocyte infiltration, oxidative stress, inflammatory response, acute lung injury