Effect of calcium on the surface properties of phospholipid monolayers with respect to surfactant formulations in respiratory distress syndrome.

The effects of calcium, in the form of calcium chloride, at concentrations of 5 and 20 mM, were studied on the surface properties of physiologic relevance to specialised biomaterials which replace lung surfactant in Respiratory Distress Syndrome. The dynamic surface pressure, re-spreading ratio, compressibility, hysteresis area and recruitment index of pure films of the main phospholipids of pulmonary surfactant namely dipalmitoyl phosphatidylcholine, phosphatidylglycerol, phosphatidylethanolamine and of binary mixed films of these phospholipids in the ratio of 2:3 were studied both in the presence and absence of calcium by in vitro analysis using a Wilhelmy balance. Surface excess films, of each of the surfactant systems, with initial concentration 15 A2 molecule were compressed at the rate of 50 seconds/cycle past collapse till a compression ratio of 4:1. The presence of 5 mM calcium caused a significant decrease in compressibility (p < 0.05 Mann-Whitney U test) of all the surfactant monolayer films. No further benefit was obtained by adding 20 mM calcium over that of 5 mM calcium. A significant beneficial effect of calcium (p < 0.05 Mann-Whitney U test) on film stability was observed when analysing the materials in a pulsating bubble surfactometer, in which liposomal suspensions of 1% concentration in the presence of 5 mM calcium were pulsated at a high frequency of 40 cycles per minute, corresponding to the respiratory frequency of neonates. The ultrastructure of the liposomal suspensions were also studied using cryogenic scanning electron microscopy and longitudinal micro-tubular structures were found on addition of 5 mM calcium, which could have resulted in the improved performance of the exogenous surfactants with respect to compressibility and stability.