Surfactant therapy

Pulmonary surfactant is the evolutionary solution to the problem of surface tension and air breathing. Without surfactant, each breath would require inordinate energy expenditure to expose the huge intrapulmonary surface to inspired air, and life on land, at least as we know it, would be virtually impossible. Pulmonary surfactant exists in the alveolar hypophase in a complex microstructure of phospholipid-rich aggregates with incorporated four distinct surfactant proteins, each with their own function. Pulmonary surfactant serves two primary functions in the lungs. It is first and foremost a surface-active agent that lowers and varies surface tension to reduce the work of breathing, stabilize alveoli against collapse and over-distension, and lessen the hydrostatic driving force for edema fluid to transudate into the interstitium and alveoli. In addition, the specific apoprotein components of lung surfactant have been found to play an important role in the lung's innate immune response. The crucial physiological importance of lung surfactant in respiration is demonstrated by the fact that a lack of this material in premature infants contributes to the development neonatal respiratory distress syndrome, a potentially fatal disease process. Exogenous surfactant replacement is now standard of care in the treatment of premature infants, and can be argued as being the most important discovery in pediatric medicine in the past 30 years. Despite this breakthrough in the treatment of neonatal lung disease, it is clear that the pathophysiology of acute pulmonary injury outside of the neonatal period is much different, and multifactorial, including inflammation, surfactant dysfunction, vascular dysfunction, edema, oxidant injury, ventilation/perfusion mismatching, and injury to alveolar, capillary, and other pulmonary cells. Clinical studies of multiple surfactant preparations in multiple target populations have resulted in unequivocal results. Therefore, the use of exogenous surfactants for the treatment of acute lung disease outside of the neonatal period is much more uncertain and complex, and remains the subject of on-going research.