| Chapter 6: | Surfactant |   |
The active agent in the alveoli that cuts surface tension and reduces the need for high pressures to open the alveoli on inspiration is known as surfactant. Surfactant is also important for changing capillary and interstitial pressures, facilitating removal of fluids from the lungs and lowering pulmonary vascular resistance at birth.
A mixture of phospholipids (70-80%) and protein in relatively consistent proportions, surfactant production sharply decreases after 34 to 35 weeks gestation. Evaluation of the content of the surfactant provides valuable information regarding the surfactant-producing system's maturity. Just prior to term, the lungs' volume stabilizes, and the lipid composition of aspirates changes. After about 20 weeks gestation, phosphotidylcholine in surfactant is produced and saved.
By contrasting amount of lecithin with that of sphingomyelin in the amniotic sample, a ratio of lecithin/sphingomyelin (L/S) can be calculated. A ratio of greater than 2 indicates lung maturity, while ratios less than 1 can be suggestive of pulmonary immaturity and the potential for respiratory distress syndrome (RDS). However, the RCP should be aware of the potential for deceptively high L/S ratios in infants whose mothers are diabetic, or if there is blood or meconium in the amniotic fluid.
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The so-called shake-test can provide an estimate of the presence of surfactant. Mix the amniotic fluid with ethanol and shake the mixture approximately 15 seconds. Because surfactant produces stable bubbles, a closed ring of bubbles seen at the container's edge after 15 minutes indicates the presence of adequate surfactant. When no bubbles are seen after 15 minutes, an L/S ratio test should be conducted.
Surfactant production can be accelerated in premature (<34 weeks’ gestation) neonates by administering corticosteroids, which can also reduce the incidence and severity of RDS in some infants. However, since steroids can mask the presence of infections in infants, they should be used with caution.
Fetal lung maturation can also be accelerated by the release of catacholamines during birth. The secretion of surfactant secretion can be accelerated by: beta adrenergic drugs, methylxanthines, a decrease in PaC02, alveolar stretch, and cAMP. Inhibition of surfactant secretion can be caused by: decreased pulmonary blood flow, cholinergic stimulation, hypoxia, hyperoxia, and decreased pH levels.