Respiratory distress syndrome is caused by a deficiency of surfactant, a phospholipid responsible for stabilizing alveolar surfaces and reducing surface tension. Surfactant is 70% lipid (phosphatidylcholine) combined with proteins. When surfactant is deficient, it is more difficult to generate the inspiratory pressure needed to inflate alveoli, resulting in progressive atelectasis. In addition, surfactant deficiency leads to lung inflammation, with resulting pulmonary edema and increased airway resistance. Diffuse atelectasis results in high resistance and low compliance in small airways. Hypoxemia results primarily from V/Q mismatch in atelectatic areas as blood flow continues through poorly ventilated regions of lung.
According to LaPlace’s law, the pressure (P) necessary to keep a sphere (alveolus) open is proportional to the surface tension (T) and inversely proportional to the radius (R) of the sphere, shown by the formula: P = 2T/R
If the alveolar volume is small (radius), as occurs at end expiration, and the surface tension is high, the pressure necessary to maintain the alveolus open is high. If this increased pressure cannot be generated, the alveolus collapses.
Prematurity, infants born to diabetic mothers, infants with mutations in the genes encoding surfactant proteins (SP-B and SP-C)
Respiratory distress and cyanosis occur at birth. Infants have tachypnea and labored breathing, as well as grunting. Grunting is a compensatory response to prevent end-expiratory alveolar collapse. Nasal flaring and intercostals retractions may also be seen. Chest x-ray is characterized by low long volumes. Atelectasis results in diffuse, ground glass appearance with air bronchograms.
Effects of ARDS
- Labored breathing, grunting (prevents alveolar collapse)
- Nasal flaring and intercostal retraction
Diagnosis is primarily based on clinical signs and symptoms discussed above. ABG shows hypoxemia that responds to supplemental oxygen, with normal to slightly elevated PaCO2. Hypercarbia can occur as the disease worsens.
Any intervention to prevent preterm birth is the best intervention for RDS. This includes cervical cerclage, use of tocolytic agents, smoking cessation, etc. If premature birth cannot be prevented, tests of fetal lung maturity performed on amniotic fluid prior to delivery can be used to assess the risk of development of RDS in a preterm infant. Lecithin and sphingomyelin are the primary phospholipids which make up surfactant. In early pregnancy, the concentration of lecithin is very small, while that of sphingomylin is much greater. Lecithin begins to be secreted into amniotic fluid by the developing fetal lung between 24 and 26 weeks gestation. At 32 to 33 weeks gestation, lecithin and sphingomyelin concentrations are about equal. Subsequently, lecithin begins to increase, with an abrupt rise around 35 weeks. In the mature lung, lecithin comprises 50-80% of the total surfactant lipid. Fetal lung maturity is present when the L/S ratio increases to 2.0 or more (or 3.5 or more for diabetic mothers).
Other options for assessment of fetal lung maturing include saturated phosphotidylglycerol (should be > 500 mg/dL in normal pregnancies, > 1000 mg/dL in diabetic women), and the simpler, faster, TDx-FLM. TDx-FLM is a fluorescent test accomplished by mixing a specific probe with amniotic fluid. It can be used quickly, including near immediate use in women with ruptured membranes. It is consistent across laboratories (< 40 mg/g indicates immaturity, > 55 mg/g predicts maturity) and does not need to be adjusted for diabetic women
Tests of Fetal Lung Maturity: Summary
L/S Ratio: 2 or higher in normal pregnancies, 3.5 or higher in diabetic women
Saturated phosphotidylglycerol > 500 mg/dL in normal pregnancies, > 1000 mg/dL in diabetic women
TDx-FLM < 40 mg/g indicates immaturity, > 55 mg/g predicts maturity, no adjustment for diabetics
Antenatal steroids reduce the risk of development of RDS. They are typically given to all women at risk for preterm delivery prior to 34 weeks gestation.
Initial management consists of supplemental oxygen and ventilator support, if needed. Often infants have increased levels of vasopressin, which results in low urine output, even if cardiac output is normal. In addition, lung injury may cause increased fluid filtration into the pulmonary circulation; therefore, fluid restriction is often part of the initial management. Surfactant improves survival in infants with RDS. Surfactant may be given prophylactically in the delivery room in infants at significant risk of RDS (those less than 30-32 weeks gestational age), or it may be given early within two hours of birth for infants who are intubated secondary to respiratory distress. It can be used later as a rescue therapy for infants who are not high risk for RDS at birth, but show signs and symptoms later. When mechanical ventilation is required, low tidal volumes should be used. High frequency oscillatory ventilation may also be used, although it does not seem to be superior to conventional ventilation with low tidal volumes.
There is a general concensus that infants less than 30 weeks gestation should be intubated at delivery and given prophylactic surfactant. If the infant is active and is breathing spontaneously, extubation and CPAP support can be considered.