Non-invasive assessment of fetal lung maturity: the role of pulmonary artery Doppler and AT/ET ratio in predicting respiratory distress syndrome

Abstract

Respiratory distress syndrome (RDS) remains a primary cause of neonatal morbidity and mortality. Traditional fetal lung maturity (FLM) assessment relies on invasive amniocentesis, which carries inherent risks. Fetal pulmonary artery Doppler, specifically evaluating the acceleration time to ejection time (AT/ET) ratio and peak systolic velocity (PSV), has emerged as a reliable non-invasive alternative to evaluate downstream pulmonary vascular resistance. A narrative review of current, peer-reviewed literature was conducted to evaluate the physiological basis, standardized ultrasonographic technique, and clinical applicability of the AT/ET ratio in predicting neonatal RDS, incorporating real-world clinical caveats. As fetal lungs mature, pulmonary vascular resistance decreases, significantly prolonging the pulmonary artery acceleration time and increasing the AT/ET ratio. Clinical evidence demonstrates that an AT/ET cut-off value near 0.30-0.31 provides optimal diagnostic accuracy in normally grown fetuses. However, values directly on the 0.30 threshold constitute a "gray zone" requiring integration with other biophysical markers, such as the PSV. Furthermore, conditions such as gestational diabetes mellitus and fetal growth restriction (FGR) independently alter these indices; notably, chronic hypoxia in FGR mandates significantly lower population-specific thresholds (e.g., AT/ET <0.19) to accurately predict neonatal respiratory complications. The fetal pulmonary artery AT/ET ratio is a highly reproducible and accessible bedside tool for assessing FLM. Integrating this parameter into routine obstetric ultrasound optimizes perinatal outcomes, safely guides the administration of antenatal corticosteroids, and reduces reliance on invasive diagnostic procedures, provided it is interpreted as an adjunct to primary maternal indications for delivery.