IN industrialized societies, the major cause of death during the first week of life is clinical hyaline membrane disease (CHMD), an entity that occurs primarily in premature infants. Simply stated, the condition consists of an inability to keep the alveoli from collapsing at the end of expiration, which leads to progressive ventilation-perfusion abnormalities, insufficient exchange of respiratory gases, and, finally, to cardiorespiratory collapse. The etiology is not fully understood, but evidence suggests that CHMD results from insufficient production of surface-active phospholipids, collectively referred to as pulmonary surfactant. These substances are probably produced and stored within the type II alveolar lining cells and are then "secreted" into the lumen where they coat the alveolar surface, decreasing surface tension at end-expiration, and thereby preventing alveolar collapse.
Pulmonary surfactant is decreased in the lung of prematurely born fetuses and of those newborn infants who develop CHMD. As gestation progresses, increasing quantities can be
Rosenfeld CR, Eichenwald HF. The L-S Ratio: Its Use in the Prediction of Respiratory Maturity in Infants. JAMA. 1974;227(6):619–620. doi:10.1001/jama.1974.03230190011002
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