Infant Respiratory Disease

Afflicting 10% of all premature infants, respiratory distress syndrome (RDS) results from a lack of pulmonary surfactant, a molecular substance that helps the lung's alveoli (air sacs) do their job of extracting carbon dioxide from the blood and replacing it with oxygen. The surfactant prevents the lung's alveoli from collapsing and helps keep them properly inflated by reducing their surface tension. The absence of surfactant prevents the alveoli from functioning properly.

In an in-vitro study that may improve treatment of this sometimes fatal condition, UC-Santa Barbara researchers (Joseph Zasadzinski, 805-893-4769, gorilla@engineering.ucsb.edu) measured the viscosity in single-molecule layers of human lung surfactant lipids similar to those used to treat premature infants with RDS. The viscosity of the single-molecule layer determines how quickly the lipids spread over the surface of an air sac.

The researchers found that the single-molecule layers consisted of islands of two-dimensional lipid crystals (a solid-like sheet of lipids arranged in a repeating pattern) floating in a continuous sea of lipids in a more liquid-like state. The relative fractions of area taken up by crystals and liquids change during the breathing cycle, as inhalation and exhalation changes the amount of surface area on which the surfactant can spread.

The researchers found that the viscosity of the single-molecule layer depends greatly on this relative fraction of crystals and liquids. Below a critical fraction of crystals, the molecule layer behaves like a liquid, with a low viscosity and low resistance to spreading. However, as a critical fraction of crystals is attained, the viscosity changes abruptly and the single-molecule layer becomes completely rigid and immobile.

Changing the lipid composition can alter the fraction of crystals in the molecule layer, so it is possible to engineer the surfactant to have the proper viscosity. The researchers believe that the rapid transition from liquid to almost rigid is important to keeping the alveoli open at the end of exhalation, when surface tension forces would like to empty them completely. These findings should help researchers formulate better replacement surfactants for treatment of premature infants with RDS. (Ding et al., Physical Review Letters, 22 April 2002.)