Final answer:
Substernal retractions in preterm infants are caused by insufficient pulmonary surfactant, which is necessary to reduce surface tension in the alveoli and facilitate lung inflation. Without enough surfactant, the lungs resist inflation, causing infants to exert more effort to breathe and leading to the visible retraction of the chest wall below the sternum. The critical period for surfactant production typically occurs later in gestation, after the time many preterm infants are born.
Step-by-step explanation:
Why Are Substernal Retractions Prominent in Preterm Infants?
Substernal retractions are a noticeable feature of respiratory difficulties in preterm infants due to underdeveloped lungs and a deficiency in pulmonary surfactant. Premature infants, particularly those born before 28 to 30 weeks of gestation, have lungs that are not fully prepared for the air-breathing environment outside the womb. The secretion of surfactant, which reduces surface tension in the alveoli to facilitate lung inflation, does not reach sufficient levels until later in gestation. When a preterm infant attempts to breathe, the lack of surfactant leads to higher resistance in the lungs and difficulty in maintaining inflated alveoli. This causes a phenomenon known as respiratory distress syndrome (RDS), where the infant must exert extra effort to breathe, leading to visible substernal retractions as the chest wall pulls inward below the sternum during inspiration.
Fetal development indicates that significant surfactant production starts around 20 weeks, but is inadequate for lung inflation upon birth. The lungs of a preterm infant also may not have fully formed gas exchange surfaces. Moreover, complications like meconium aspiration can exacerbate respiratory issues, as the inhaled meconium can block airways, strip surfactant from the lungs, lead to alveolar collapse, and cause pulmonary hypertension and inflammation.