Enhancement Effects of Hypercapnia on the Acute Lung Injury Caused by Acid Aspiration

Abstract

Acid aspiration or intrapulmonary instillation of gastric particles causes lung inflammation leading to acute lung injury (ALI). Hypercapnia exerts different effects on ALI caused by various insults. The effects of hypercapnia on lung inflammation and injury due to acid aspiration are yet to be determined. The present study was designed to investigate the involvement of inducible nitric oxide synthase (iNOS) and other mediators in acid-aspiration-induced ALI. We also sought to evaluate the effects of hypercapnia on the lung and associated changes induced by acid aspiration. We used Spague-Dawley rats anesthetized with intraperitioneal pentobarbital (40 mg/kg). Gastric acid particles were prepared from the stomach contents of rats at necropsy. The rats were randomly assigned to receive intratracheal instillation of physiological saline solution (PSS) at pH 7.24 (Control group), PSS at pH 1.25 (Low pH, LPH group), gastric particles (GP group), and GP with low pH PSS (GPLPH group). There were 10 rats in each group. The animals were observed for 6 hrs. To evaluate the effects of hypercapnia, we carried out two series of experiments: one under normocapnia and the other under hypercapnia with alteration of CO2 fraction in inspired air. Arterial pressure (AP) was monitored from the femoral arterial catheter. Heart rate was obtained from AP traicing. We determined the blood gases and acid-base status. Lung weight to body weight (LW/BW) ratio, LW gain (LWG), protein concentration in bronchoalveolar lavage (PCBAL) and leakage of Evans blue dye tracer were measured. Plasma nitrate/nitrite, methyl guanidine (MG), myeloperoxidase (MPO), phospholipase A2 (PLA2), proinflammatory cytokines were assessed. Histopathological examination of the lung tissue was performed. We employed reverse-transcriptase polymerase chain reaction to detect the expression of iNOS mRNA. GP and GPLPH caused hypotension, decreases in PaO2, pH and SaO2, and an increase in PaCO2. The insults also elevated LW/BW, LWG, PCBAL and dye leakage, plasma nitrate/nitrite, MG, MPO, PLA2, tumor necrosis factor(alpha), interleukin-beta and interleukin-6. The lung pathology was characterized by alveolar edema and hemorrhage with inflammatory cells infiltration. Assessment of lung injury score revealed that GP and GPLPH caused ALI. Furthermore, hypercapnia significantly enhanced ALI and associated changes following LPH, GP and GPLPH. Intratracheal instillation of GP in normal or low pH PSS causes ALI accompanied with biochemical changes. The release of nitric oxide via iNOS isoform is detrimental to the lung. Hypercapnia tended to enhance ALI and associated changes induced by gastric acid instillation.