Prenatal prediction of respiratory distress syndrome. Measurement of surface properties and Lecithin/Sphingomyelin ratio in human amniotic fluid.

118 samples of amniotic fluid were taken from 102 patients in the 25th to the 43th week of pregnancy by amniotomy or by transabdominal amniocentesis. 76 samples were obtained from patients with normal pregnancies without evidence for either induction or retardation of lung maturity; they served to establish normal values. In 37 cases samples were taken within 72 hours prior to delivery, the decisive period for the prediction of a respiratory distress syndrome. In seven cases a respiratory distress syndrome occured which was diagnosed by an independent pediatrician. The following tests were performed for all samples: 1. Surface tension was measured biomechanically using the Wilhelmy balance and a surface tension area diagram (Fig. 1) was made: A platimum plate is immersed into the liquid placed in the trough; the surface tension is measured via a transducer. The surface film of th liquid to be studied is compressed cyclically from an initial area of 100% to 20% by a moveable barrier and is expanded afterward. Platinum plate and barrier are connected to an X-Y recorder. The surface tension area diagrams are interpreted on the basis of the following criteria: see article 2. The determination of the L/S ratio (simultaneous comparison with the standard method): The method of GLUCK as modified by BORER(planimetric interpretation of the the spots) was employed. Both methods produced normal values. For the biomechanical measurements a classification into three groups was made (25th-30th, 31st-35th, 36th-43rd week of pregnancy) (Tab.I);the L/S ratio showed the usual steep increase from the 36th week of pregnancy onwards. In cases with RDS (Tab.II) surface tension-area-diagrams show high values for gamma max and a slight decrease in surface tension on compression (high gamma min values); the stability index is low, the hysteresis is reduced. Fig. 3. Up to the 34th week of pregnancy the 2:1L/S ratio assumed as the threshold for a sufficient surfactant concentration does not permit a clear interpretation; in subsequent weeks all cases fail to coincide with the normal pattern of distribution (Fig.2). Both the L/S ratio and the criteria of the surface tension area diagrams show significant differences in cases with and without RDS. The prognostic validity of both methods was tested for the probability of predicting occurrence of RDS, probability of RDS occurence and probability of non-occurrence of FDS. The prognostic value seems equal to the L/S Ratio.