Certified Respiratory Therapist Practice Exam

What is the interpretation of a 2:1 L/S ratio in a fetus?

1. Increased chances of fetal distress

2. Normal lung development

3. Beginning of mature surfactant production

4. Indicates respiratory failure at birth

The correct answer is: Beginning of mature surfactant production

A 2:1 lecithin-to-sphingomyelin (L/S) ratio is significant in assessing fetal lung maturity. Lecithin and sphingomyelin are types of phospholipids found in pulmonary surfactant, which is crucial for reducing surface tension in the alveoli and preventing their collapse. When a fetus reaches a 2:1 L/S ratio, it indicates that sufficient surfactant is being produced, suggesting the lungs are becoming mature enough for effective respiration outside the womb. This maturation typically occurs between 34 and 36 weeks of gestation, with a ratio of 2:1 being a marker that the fetus has a decreased risk of respiratory distress syndrome and is likely to fare better after birth. In the context of fetal development, lower L/S ratios point to immature lungs and a greater risk of complications. Therefore, a 2:1 L/S ratio does not reflect issues like fetal distress or respiratory failure; rather, it serves as a positive indication of advancing pulmonary maturity and the beginning of satisfying the pulmonary requirements necessary for a successful transition to breathing air at birth.