Microviscosity of amniotic fluid phospholipids, and its importance in determining fetal lung maturity

Fluorescence polarization measurements of microviscosity (apparent viscosity within the hydrophobic center of lipid bilayers) of amniotic fluid correlate well with lecithin/sphingomyelin ratios determined by thin-layer chromatography. In addition to lecithin, phosphatidylglycerol and phosphatidylinositol are important for determining fetal lung maturity, but the lecithin/sphingomyelin ratio gives no information concerning these other phospholipids. The microviscosity of sphingomyelin significantly exceeded that of lecithin over the temperature range 25--37 degrees C; values for phosphatidylglycerol, phosphatidylinositol, and phosphatidylserine were lower. Phosphatidylglycerol, phosphatidylinositol, and phosphatidylserine, added individually, significantly decreased the microviscosity of dispersions with lecithin/sphingomyelin ratios corresponding either to fetal lung immaturity or maturity. Phosphatidylglycerol caused the greatest decrease in both. Mixtures of the three phospholipids in the proportions found in term amniotic fluid decreased the microviscosity of fluids with either mature or immature lecithin/sphingomyelin ratios by 23--27%. When each was present in the proportion found in tracheal aspirate (twice that of term amniotic fluid), the decreases uere 46--50%. This technique quickly and precisely indicates not only fetal lung maturity but also the presence of important phospholipids other than lecithin and sphingomyelin.