|
Respiratory distress syndrome (RDS) is caused by pulmonary surfactant deficiency in the lungs of neonates, most commonly in those born at < 37 wk gestation. Risk increases with degree of prematurity. Symptoms and signs include grunting respirations, use of accessory muscles, and nasal flaring appearing soon after birth. Diagnosis is clinical; prenatal risk can be assessed with tests of fetal lung maturity. Treatment is surfactant therapy and supportive care.
Etiology
Surfactant is not produced in adequate amounts until relatively late in gestation; thus, risk of RDS increases with greater prematurity. Other risk factors include multifetal pregnancies, maternal diabetes, and being male and white.
Risk decreases with fetal growth restriction, preeclampsia or eclampsia, maternal hypertension, prolonged rupture of membranes, and maternal corticosteroid use.
Rare cases are hereditary, caused by mutations in surfactant protein (SP-B and SP-C) and ATP-binding cassette transporter A3 (ABCA3) genes.
Pathophysiology
Pulmonary surfactant is a mixture of phospholipids and lipoproteins secreted by type II pneumocytes (see Perinatal Physiology: Pulmonary function). It diminishes the surface tension of the water film that lines alveoli, thereby decreasing the tendency of alveoli to collapse and the work required to inflate them.
With surfactant deficiency, the lungs become diffusely atelectatic, triggering inflammation and pulmonary edema. Because blood passing through the atelectatic portions of lung is not oxygenated (forming a right-to-left intrapulmonary shunt), the infant becomes hypoxemic. Lung compliance is decreased, thereby increasing the work of breathing. In severe cases, the diaphragm and intercostal muscles fatigue, and CO2 retention and respiratory acidosis develop.
Complications
Complications of RDS include intraventricular hemorrhage, periventricular white matter injury, tension pneumothorax, bronchopulmonary dysplasia, sepsis, and neonatal death. Intracranial complications have been linked to hypoxemia, hypercarbia, hypotension, swings in arterial BP, and low cerebral perfusion (see Perinatal Problems: Intracranial Hemorrhage; see Miscellaneous Disorders in Infants and Children: Hemorrhagic Shock and Encephalopathy Syndrome (HSES)).
Symptoms and Signs
Symptoms and signs include rapid, labored, grunting respirations appearing immediately or within a few hours after delivery, with suprasternal and substernal retractions and flaring of the nasal alae. As atelectasis and respiratory failure progress, symptoms worsen, with cyanosis, lethargy, irregular breathing, and apnea.
Neonates weighing < 1000 g may have lungs so stiff that they are unable to initiate or sustain respirations in the delivery room.
On examination, breath sounds are decreased. Peripheral pulses may be decreased with peripheral extremity edema and decreased urine output.
Diagnosis
Diagnosis is by clinical presentation, including recognition of risk factors; ABGs showing hypoxemia and hypercapnia; and chest x-ray. Chest x-ray shows diffuse atelectasis classically described as having a ground-glass appearance with visible air bronchograms; appearance correlates loosely with clinical severity.
Differential diagnosis includes group B streptococcal pneumonia and sepsis, transient tachypnea of the newborn, persistent pulmonary hypertension, aspiration, pulmonary edema, and congenital cardiopulmonary anomalies. Neonates typically require cultures of blood, CSF, and possibly tracheal aspirate. Clinical diagnosis of group B streptococcal pneumonia is extremely difficult; thus, antibiotics usually are started pending culture results.
RDS can be anticipated prenatally using tests of fetal lung maturity, which measure surfactant obtained by amniocentesis or collected from the vagina (if membranes have ruptured) and which can help determine the optimal timing of delivery. These are indicated for elective deliveries before 39 wk when fetal heart tones, human chorionic gonadotropin levels, and ultrasound measurements cannot confirm gestational age and for nonelective deliveries between 34 wk and 36 wk. Risk of RDS is low when lecithin/sphingomyelin ratio is > 2, phosphatidyl glycerol is present, foam stability index = 47, surfactant/albumin ratio (measured by fluorescence polarization) is > 55 mg/g, or a combination.
Treatment
Prognosis with treatment is excellent; mortality is < 10%. With adequate ventilatory support alone, surfactant production eventually begins, and once production begins, RDS resolves within 4 or 5 days. However, in the meantime, severe hypoxemia can result in multiple organ failure and death.
Specific treatment is intratracheal surfactant therapy. This therapy requires endotracheal intubation, which also may be necessary to achieve adequate ventilation and oxygenation. Less premature infants (those > 1 kg) and those with lower O2 requirements (fraction of inspired O2 [Fio2] < 40 to 50%) may respond well to supplemental O2 alone or to treatment with nasal continuous positive airway pressure. A treatment strategy of early (within 20 to 30 min after birth) surfactant therapy is associated with significant decrease in duration of mechanical ventilation, lesser incidence of air leak syndromes, and lower incidence of bronchopulmonary dysplasia.
Surfactant hastens recovery and decreases risk of pneumothorax, interstitial emphysema, intraventricular hemorrhage, bronchopulmonary dysplasia, and neonatal mortality in the hospital and at 1 yr. However, neonates who receive surfactant for established RDS have an increased risk of apnea of prematurity. Options for surfactant replacement include beractant (a lipid bovine lung extract supplemented with proteins B and C, colfosceril palmitate, palmitic acid, and tripalmitin) 100 mg/kg q 6 h prn up to 4 doses; poractant alfa (a modified porcine-derived minced lung extract containing phospholipids, neutral lipids, fatty acids, and surfactant-associated proteins B and C) 200 mg/kg followed by up to 2 doses of 100 mg/kg 12 h apart prn; and calfactant (a calf lung extract containing phospholipids, neutral lipids, fatty acids, and surfactant-associated proteins B and C) 105 mg/kg q 12 h up to 3 doses prn. Lung compliance can improve rapidly after therapy. The ventilator peak inspiratory pressure may need to be lowered rapidly to reduce risk of a pulmonary air leak. Other ventilator parameters (eg, Fio2, rate) also may need to be reduced.
Prevention
When a fetus must be delivered between 24 wk and 34 wk, giving the mother 2 doses of betamethasone 12 mg IM 24 h apart or 4 doses of dexamethasone 6 mg IV or IM q 12 h at least 48 h before delivery induces fetal surfactant production and reduces the risk of RDS or decreases its severity. (See also Abnormalities and Complications of Labor and Delivery: Preterm Labor.)
Prophylactic intratracheal surfactant therapy given to neonates that are at high risk of developing RDS (infants < 30 wk completed gestation especially in absence of antenatal corticosteroid exposure) has been shown to decrease risk of neonatal death and certain forms of pulmonary morbidity (eg, pneumothorax).
Last full review/revision March 2009 by Anand D. Kantak, MD; John T. McBride, MD
|
