Bubble clicking in pharyngeal aspirate compared with lecithin/sphingomyelin ratio

Tracheal fluid flows into the pharynx, so surfactant can be measured in pharyngeal aspirate. The walls of bubbles in air-free water show rhythmic movements (bubble clicking) if they contain surfactant. Babies with respiratory distress syndrome (RDS) have very little lung surfactant which can be shown by a low lecithin/sphingomyelin (L/S) ratio. To investigate surfactant in newborn babies we analysed 125 pharyngeal aspirates for the L/S ratio and bubble clicking. Both tests became more positive towards term and were related to the incidence of RDS. Babies with birth asphyxia or mild respiratory difficulties had lower L/S ratios and less clicking than normal babies. Both investigations could be used to chart the course of RDS. The L/S ratio is a chemical measurement of surfactant. Bubble clicking, which takes only a few minutes, demonstrates surface tension effects.