Dr. Grenache references the new CLSI guideline on assessment of FLM by the lamellar body count (endnote 3). Here is the Foreword to the document, slightly abridged and used by permission of CLSI. It provides good background into fetal lung development and the dangerous complications that can occur, particularly in premature neonates.
Development of the fetal lung can be divided into four stages: the pseudoglandular, canalicular, saccular, and alveolar stages. The first stage results in the development of three lung lobes on the right side and two on the left side. The second stage is remarkable for the differentiation of type I and type II pneumocytes and the first appearance of surfactant. The third stage involves formation of clusters of wide spaces in the peripheral airways. Finally, the fourth stage involves the formation of alveoli. It is during this stage that type II pneumocytes increase production of pulmonary surfactant. Lung development continues for approximately eight years.
Pulmonary surfactant functions to coat the alveolar epithelium and decrease the surface tension of the hydrated inner layer of alveoli. When surface tension is high and the alveolar radius is small, very high air pressure is needed to prevent alveolar collapse. Surfactant decreases the air pressure required to keep the alveoli from collapsing. Surfactant is composed of approximately 90% phospholipid and 10% protein, and is packaged into layered storage granules called lamellar bodies that begin to synthesize around 24 weeks of gestation. Lamellar bodies are secreted by the type II pneumocyte and unfold to form tubular myelin and other large aggregates that are absorbed onto the hydrated inner layer of the alveoli.
Respiratory distress syndrome (RDS) in premature infants is caused by developmental insufficiency of pulmonary surfactant production and structural immaturity of the lungs. Clinically, RDS presents with hypoxia, hypercapnia, and acidosis. Preventing premature birth is the most effective way to prevent RDS. Alternatively, administration of steroids to the mother can be used to accelerate lung surfactant production. Treatment of preterm newborns after birth with exogenous surfactant can be effective in preventing and treating RDS.
Fetal lung maturity (FLM) tests are used by physicians to weigh the risk of developing RDS if the newborn is delivered against the risk to the mother by continuing the gestation. To be clinically useful, FLM tests should possess high diagnostic sensitivity for RDS and a high predictive value of a mature result. Studies have indicated that the frequency of physician-ordered FLM testing is decreasing. This likely reflects a decrease in elective deliveries in response to studies that demonstrate more adverse outcomes in infants delivered before 39 weeks of gestation. However, despite the decreased use of FLM tests, physicians still report that they rely on them for clinical decision-making.
