Assessment of fetal pulmonic maturity by measurement of the surface tension of amniotic fluid lipid extract.

Fetal pulmonic maturity is currently predicted by the lecithin/sphingomyelin (L/S) ratio which utilizes the changes with increased gestation in the lecithin component of amniotic fluid surfactant. The surface tension lowering property of lipids extracted from the amniotic fluid of 45 patients was measured directly. The surface tension (gamma) of various aliquots of the amniotic fluid lipid extract was measured by a modification of the Wilhelmy plate method. At 80 gamma of the extract, a gamma of 67 dynes/cm or less indicated pulmonic maturity, while a gamma of greater than 67 dynes/cm suggested immaturity, provided that the gamma at 200 gamma was greater than 56 dynes/cm. The fluid was termed transitional if the gamma was greater than 67 dynes/cm at 80 gama, but less than 56 dynes/cm at 200 gamma. A comparison was made with the L/S ratio; there were no false positive values and only 13.6% false negative surface tension values. Addition of blood to amniotic fluid, mature and immature, yielded the same results as with the L/S ratio. Two infants with respiratory distress syndrome had both immature L/S ratios and surface tension results. Results indicate that the surface tension of the amniotic fluid lipid extracts is a reliable and rapid means of antenatally predicting fetal pulmonic maturity.