Measurement of surface tension in biological fluids by a pulsating capillary technique

Ever since the discovery of the antiatelectatic function of the pulmonary surfactant, the measurement of surface tension (ST) has been of increasing importance in respiration physiology and clinical research. For the determination of ST, the elevation of the level of a fluid artificially pulsated in a capillary glass tube was monitored, and ST was calculated with the digitalized video-computerized picture analysis program Surftens. The biological relevance of the method is given by the Gibbs-Thomson principle, according to which surface-active lipids stream towards the surface by an adsorptive process; on pulsation of the fluid in a capillary glass tube, therefore, ST is gradually decreased to a minimal value. ST values of 60 amniotic fluid samples collected from pregnancies with different gestation times were determined. A multiple regression analysis of the results, including other parameters (total protein content, total lipid content, phospholipid content and microviscosity), indicated that this method may enhance the precision of the determination of gestation time. Precision analysis of various samples proved that this technique gives well-reproducible results under the given standardized conditions. The main field of application of the method may be in clinical practice and in studies on as yet inadequately known factors affecting the ST of biological fluids.