Respiratory distress syndrome (RDS) was previously called hyaline membrane disease. 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 over 85 %.
A characteristic chest radiographic appearance (uniform reticulogranular pattern to lung fields with or without low lung volumes and air bronchogram) within the first 24 hours of life. The clinical course of the disease varies with the size of the infant, severity of disease, use of surfactant replacement therapy, presence of infection, degree of shunting of blood through the patent ductus arteriosus (PDA), and whether or not assisted ventilation was initiated.
Incidence of RDS is ∼91% at 23–25 weeks' gestation, 88% at 26–27 weeks' gestation, 74 % at 28–29 weeks' gestation, and 52% at 30–31 weeks' gestation. The incidence and severity of RDS are expected to decrease after the increase in use of antenatal steroids in recent years. After the introduction of exogenous surfactant, the survival from RDS is at >90%. During the surfactant era, RDS accounts for <6% of all neonatal deaths.
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 a surface-active material produced by airway epithelial cells called type II pneumocytes. This cell line differentiates, and surfactant synthesis begins at 24–28 weeks' gestation. Type II cells are sensitive to and decreased by asphyxial insults in the perinatal period. The maturation of this cell line 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, intrauterine growth restriction, and twin gestation. Surfactant, composed chiefly of phospholipid (75%) and protein (10%), is produced and stored in the characteristic 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 poorly compliant lungs.
Decreased intrathoracic pressure. The infant with RDS ...
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