Effect of acidosis on bilirubin-lipid extract surfactant interaction

The risk of bilirubin toxicity in newborn infants with respiratory distress syndrome and hyperbilirubinemia may depend on many factors including pH of the system. Biophysical activity and inhibition characteristics were studied in vitro for lipid extract surfactant (Curosurf, 0.25 mg/ml phospholipids), bilirubin (1.0 mg/ml dissolved in NaOH) and mixed solutions at different pH ranging from 5.0 to 7.4. It was found that unconjugated bilirubin modifies surface tension behavior of lipid extract surfactant films. Maximum and minimum surface tension levels were significantly higher in mixed solutions compared to experiments using pure Curosurf independently from pH. Film area compression for pure Curosurf was not influenced by pH and varied between 22 +/- 4% at pH 5.0 and 23 +/- 9% at pH 7.4. Adding bilirubin to lipid extract surfactant, area compression to achieve minimum surface tension increased significantly to 83 +/- 4% at pH 5.0 and 85 +/- 4% at pH 7.4 (p < 0.01). Bilirubin alone showed negligible surface activity independently from pH (83 +/- 7% at pH 5.0 and 78 +/- 9% at pH 7.4) (p > 0.5). We conclude that bilirubin has a detrimental effect on the surface tension properties of lipid extract surfactant in vitro and that this interaction is independent from pH. These data suggest no influence of acidosis on alveolar surfactant system in babies with respiratory distress syndrome and hyperbilirubinemia.