RDS almost always occurs in newborns born before 37 weeks of gestation. The more premature the baby is, the greater is the chance of developing RDS. RDS is more likely to occur in newborns of diabetic mothers.
Surfactant, a mixture of phospholipids and lipoproteins, is secreted by lung cells. The air-fluid interface of the film of water lining the alveoli of the lung (where the exchange of oxygen and CO2 occurs) exerts large forces that cause the alveoli to close if surfactant is deficient. Lung compliance is decreased, and the work of inflating the stiff lungs is increased. The preterm newborn is further handicapped because his or her ribs are more easily deformed (compliant). Breathing efforts therefore result in deep sternal (breastbone) retractions but poor air entry if the ribs are compliant compared with the lungs. This results in diffuse atelectasis (collapse of the lungs).
Rapid, labored, grunting respirations usually develop immediately or within a few hours after delivery, with retractions above and below the breastbone and flaring of the nostrils. The extent of atelectasis (lung collapse) and the severity of respiratory failure progressively worsen.
Not all infants with RDS have signs of respiratory distress; extremely low birth weight newborns (i.e., < 1000 g) may be unable to initiate respirations at birth because their lungs are so stiff; they may fail to initiate breathing in the delivery room.