Respiratory distress syndrome (RDS) was previously called hyaline membrane disease (HMD). The Vermont Oxford Network definition for RDS requires that babies have:
An arterial oxygen tension (PaO2) <50 mm Hg and central cyanosis in room air, a requirement for supplemental oxygen to maintain PaO2 >50 mm Hg, or a requirement for supplemental oxygen to maintain a pulse oximeter saturation >85%.
A characteristic chest radiographic appearance (uniform reticulogranular pattern to lung fields and air bronchogram) within the first 24 hours of life. The clinical course of the disease has been changed because of advances in treatment practices, including the use of early continuous positive airway pressure (CPAP).
Incidence of RDS is about 85% at 28 weeks’ gestation and increases to 95% at 24 weeks’ gestation. However, by using early CPAP, approximately 50% of babies born at 26 to 29 weeks’ gestation can be managed without intubation or surfactant. The survival from RDS is generally >90%.
Surfactant deficiency is the primary cause of RDS, often complicated by an overly compliant chest wall. Both factors lead to progressive atelectasis and failure to develop an effective functional residual capacity (FRC). Surfactant is produced by airway epithelial cells called type II pneumocytes. Surfactant synthesis begins at 24 to 28 weeks’ gestation. Type II cells are sensitive to and decreased by perinatal asphyxial insults. The maturation of these cells is delayed in the presence of fetal hyperinsulinemia. The maturity of type II cells is enhanced by the administration of antenatal corticosteroids and by chronic intrauterine stress such as pregnancy-induced hypertension and intrauterine growth restriction. Surfactant, composed chiefly of phospholipid (75%) and protein (10%), is produced and stored in the lamellar bodies of type II pneumocytes. This lipoprotein is released into the airways, where it functions to decrease surface tension and maintain alveolar expansion at physiologic pressures.
Lack of surfactant. In the absence of surfactant, the small airspaces collapse; each expiration results in progressive atelectasis. Exudative proteinaceous material and epithelial debris, resulting from progressive cellular damage, collect in the airway and directly decrease total lung capacity. In pathologic specimens, this material stains typically as eosinophilic hyaline membranes lining the alveolar spaces and extending into small airways.
Presence of an overly compliant chest wall. In the presence of a chest wall with weak structural support secondary to prematurity, the large negative pressures generated to open the collapsed airways cause retraction and deformation of the chest wall instead of inflation of the lungs.
Decreased intrathoracic pressure. The infant with RDS who is <30 weeks’ gestational age often has immediate respiratory failure because of an inability to generate the intrathoracic pressure necessary to inflate the lungs without surfactant.
Shunting. The presence or absence of a cardiovascular shunt through a patent ductus arteriosus (PDA) and/or foramen ovale may change the course of the disease. Shortly after birth, the predominant shunting is right to left across the foramen ovale into the left atrium, which may result in worsening hypoxemia. After 18 to 24 hours, left-to-right shunting through ...