3. Objectives
• Define respiratory distress syndrome (RDS).
• Discuss the epidemiology, pathophysiology, and
diagnosis of RDS.
• List a differential diagnosis for respiratory distress in the
neonate.
• Describe the treatments for RDS.
• Discuss ventilation strategies that can be used in the
infant who has RDS.
• Describe long-term complications of RDS and its
treatments.
4. Definition
• Formerly known as hyaline membrane
disease
• Deficiency of pulmonary surfactant in an
immature lung
• Disease of prematurity
5. Epidemiology
• Major cause of morbidity and mortality in preterm infants
• 20,000-30,000 newborn infants each year
• Incidence and severity of RDS are related inversely to
gestational age of newborn infant
• 26-28 weeks' gestation : 50%
• 30-31 weeks' gestation : <30%
• Overall incidence in 501-1500 grams: 42%
• 501-750 grams: 71%
• 751-1000 grams: 54%
• 1001-1250 grams: 36%
• 1251-1500 grams: 22%
18. Surfactant Inactivation
• Meconium and blood can inactivate
surfactant activity (Full-term > Preterm)
• Proteinaceous edema and inflammatory
products increase conversion rate of
surfactant into its inactive vesicular form
• Oxidant and mechanical stress associated
with mechanical ventilation that uses large TV
23. Prevention
• Antenatal glucocorticoids
• Enhances maturational changes in lung architecture
and inducing enzymes
• Stimulate phospholipid synthesis and release of
surfactant
• All pregnant mothers at risk for preterm delivery at or
below 34 weeks gestation should receive ACS
Normal alveolar development occurs in 4 stages.
Embryonic period –
At about 26 days gestation, the embryonic stage begins with the first appearance of the fetal lung, which appears as a protrusion of the foregut. Initial branching of the lung occurs at 33 days gestation forming the prospective main bronchi, which begin to extend into the mesenchyme.
Further branching forms the segmental bronchi as the lung enters the next stage of development.
Pseudoglandular stage –
7th to 16th weeks of gestation, 15 to 20 generations of airway branching occur starting from the main segmental bronchi and ending as terminal bronchioles.
end of the pseudoglandular stage, airways are surrounded by a loosely packed mesenchyme, which includes a few blood vessels, and is lined by glycogen-rich and morphologically undifferentiated epithelial cells with a columnar to cuboidal shape.
In general, epithelial differentiation is centrifugal so the most distal tubules are lined with undifferentiated cells with progressive epithelial differentiation of the more proximal airways.
Canalicular stage –
16th and 25 weeks gestation, transition from previable to a potential viable lung occurs as the respiratory bronchioles and alveolar ducts of the gas exchange region of the lung are formed.
The surrounding mesenchyme becomes more vascular and condenses around the airways.
The closer vascular proximity ultimately results in fusion of the capillary and epithelial basement membranes.
After 20 weeks gestation, cuboidal epithelial cells begin to differentiate into alveolar type II cells with formation of cytoplasmic lamellar bodies [2]. The glycogen in these cells is used for surfactant production, which is stored in the lamellar bodies.
Saccular stage –
About 24 weeks gestation, there is potential for viability because gas exchange is possible due to the presence of large and primitive forms of the future alveoli.
In this stage, formation of alveoli (ie, alveolarization) occurs by the outgrowth of septae that subdivide terminal saccules into anatomic alveoli, where air exchange occurs.
The number of alveoli in each lung increases from zero at 32 week gestation to between 50 and 150 million alveoli in term infants and 300 million in adults.
Alveolar growth continues for at least two years after birth at term.
Surfactant components are synthesized from precursors in the endoplasmic reticulum and transported through the Golgi apparatus by multivesicular bodies.
Components are ultimately packaged in lamellar bodies, which are intracellular storage granules for surfactant before its secretion.
After secretion (exocytosis) into the liquid lining of the alveolus, surfactant phospholipids are organized into a complex lattice called tubular myelin. Tubular myelin is believed to generate the phospholipid that provides material for a monolayer at the air-liquid interface in the alveolus, which lowers surface tension.
Surfactant phospholipids and proteins are subsequently taken back into type II cells, in the form of small vesicles, apparently by a specific pathway that involves endosomes, and then are transported for storage into lamellar bodies for recycling.
Alveolar macrophages also take up some surfactant in the liquid layer.
A single transit of the phospholipid components of surfactant through the alveolar lumen normally requires a few hours.
The phospholipid in the lumen is taken back into type II cell and is reused 10 times before being degraded.
Surfactant proteins are synthesized in polyribosomes and extensively modified in the endoplasmic reticulum, Golgi apparatus, and multivesicular bodies.
Surfactant proteins are detected in lamellar bodies or secretory vesicles closely associated with lamellar bodies before they are secreted into the alveolus.
Amniotic fluid L/S ratio increases progressively with gestational age. L/S ratio greater than two signifies maturity of surfactant system of lung
- Surfactant deficiency
- Inflammation and Lung injury
- Pulmonary edema
- Surfactant inactivation
- Pulmonary function and gas exchange
Impaired surfactant synthesis and secretion atelectasis, V/Q inequality, hypoventilation hypoxemia and hypercarbia
Respiratory / metabolic acidosis pulmonary vasoconstriction impaired endothelial and epithelial integrity leakage of proteinaceous exudate and formation of hyaline membranes
Deficiency of surfactant decreases lung compliance and FRC, with increased dead space
Impair surfactant production and/or secretion
Hypoxia, acidosis, hypothermia, hypotension
Oxygen toxicity influx of inflammatory cell exacerbates vascular injury BPD
Antioxidant deficiency and free-radical injury worsen injury
Lungs with HMD require far more pressure than to achieve a given volume of inflation than do lungs obtained from an infant dying of a nonrespiratory cause.
Arrows indicate inspiratory and expiratory limbs of the pressure-volume curves.
Note the decreased lung compliance and increased critical opening and closing pressures, respectively, in the premature infant with HMD
line the alveoli (see the image below) may form within a half hour after birth.
In larger premature infants, the epithelium begins to heal at 36-72 hours after birth, and endogenous surfactant synthesis begins.
The recovery phase is characterized by regeneration of alveolar cells, including type II cells, with a resultant increase in surfactant activity.
low lung volume and the classic diffuse reticulogranular ground-glass appearance with air bronchograms
two doses of betamethasone administered 24 hours apart is currently the recommended steroid for antenatal use
Antenatal steroid administration has been shown to be beneficial if provided fewer than 24 hours before delivery
Furthermore, a reduction in RDS has been seen in infants born up to 7 days after the first dose of antenatal steroids was administered. (1) No benefit is seen in infants who receive the first dose of steroids more than 7 days before birth.
They recommend repeat doses of corticosteroids in women at risk for preterm birth when the first course of steroids was administered more than 7 days previously because of the short-term benefits to the fetal lungs. They do, however, warn about the possibility of decreased birthweight and head circumference at birth, which has been reported.
