Certified Respiratory Therapist Practice Exam

What L/S (lecithin/sphingomyelin) ratio indicates the onset of mature surfactant production in a fetus?

1. 1:1

2. 2:1

3. 3:1

4. 4:1

The correct answer is: 2:1

The lecithin/sphingomyelin (L/S) ratio is a significant measure used to assess fetal lung maturity and the production of surfactant, which is critical for proper lung function at birth. A ratio of 2:1 indicates that the fetus is beginning to produce mature surfactant in sufficient quantities to help reduce surface tension in the alveoli. This is vital for preventing atelectasis (lung collapse) after birth. Surfactant production begins to increase significantly around the end of the second trimester and into the third trimester; however, a ratio of 2:1 or higher is typically used as a clinical marker indicating that the risk of respiratory distress syndrome (RDS) is reduced and that the fetus is likely ready for delivery. A ratio lower than 2:1 suggests that the lungs may not yet be mature enough to function effectively at birth, increasing the risk of complications associated with prematurity. Thus, the identification of a 2:1 ratio serves as a critical threshold in assessing fetal lung maturity and guiding clinical decisions regarding the timing of delivery.