THE ABILITY OF NEONATAL RAT LUNGS TO RECOVER AFTER EXPOSURE TO HYPEROXIA

Exogenous induction of hypercapnia is shown to improve pulmonary function during ventilator and endotoxin‐induced lung injury. These injuries, likewise hyperoxia‐induced lung injury, are partly mediated by neutrophils sequestration and production of reactive oxygen species. Therefore, we tested the hypothesis that hypercapnia may improve the morphological manifestations of prolonged hyperoxia in neonatal rats. One day‐old rats were randomly assigned for 96 hours of exposure to normoxia (21% O 2 ), hyperoxia (>98% O 2 ) and hypercapnic hyperoxia (95% O 2 plus 5% CO 2 ), (n=5/group). Pups were then euthanized, lungs cardiac lobe was dissected for histological preparations, and changes in the alveolar size (μ 2 ) were measured, using a computerized image analyzer. The results showed characteristic manifestations of pulmonary oxygen toxicity, such as substantial hypercellularity in both hyperoxic groups, as compared to normoxia. Although the alveolar density (number of alveoli/mm 2 ) was reduced, the alveolar size was increased by 21% (p>0.05) during hyperoxia, and by 60% during hypercapnic hyperoxia, as compared to normoxia (p<0.05; ANOVA followed by Tukey‐Kramer multiple comparisons test). These data suggest that exogenously induced hypercapnia does not improve the morphological manifestations of pulmonary oxygen toxicity.