How does exogenous surfactant really work?

The finding of a lag of up to 24 h between the response to exogenous surfactant in infants with respiratory disease syndrome as assessed by blood gases and improvements in respiratory system compliance has generated considerable interest. Studies have shown that there is rapid increase in functional residual capacity after natural surfactant and a less dramatic rise in effective pulmonary blood flow. These changes in blood flow are not associated with a sustained fall in pulmonary artery flow or pressure indicating that the main improvement in oxygenation results from a reduction in intrapulmonary shunting. Recent studies have shown that natural surfactant does produce rapid increases in lung compliance but only if the expired gases are vented to ambient rather than to positive end expiratory pressures. Thus the mechanical effects of natural surfactant are initially largely limited to the part of the tidal volume which is close to the functional residual capacity.