Comparison of diaphragmatic thickness fraction between prone and supine positioning in preterm and sick term neonates: a prospective study.

UNLABELLED: Prone positioning provides advantages for respiration and is routinely performed in preterm and sick term infants admitted to neonatal intensive care units (NICUs). This study aimed to evaluate the effect of the prone position on the diaphragm in neonates compared with that of the supine position using diaphragmatic ultrasound (DU). This was a prospective, observational, single-centre study. We enrolled preterm and sick term infants admitted to the NICU between August 2025 and March 2026. We conducted DU from the right zone of apposition (8-9th intercostal spaces, midaxillary line), measured the diaphragm thickness, and calculated the diaphragmatic thickness fraction (DTF). DU was performed in three phases: prone (P1), supine (S), and prone again (P2). Measurements were obtained after confirming that the infant's posture and respiratory pattern had stabilised following each position change. We enrolled 11 neonates with a median gestational age of 36 (interquartile range [IQR], 34-37) weeks and median birth weight of 2,296 (IQR, 1,936-2,475) g. On day 4 (IQR, 3-9), the DTF (%) was P1: 23.5 (16.9-26.1), S: 38.0 (29.9-48.0), and P2: 22.3 (18.0-26.2), differing significantly among positions (Friedman test, p < 0.001). Post-hoc analysis with Bonferroni correction showed that P1 vs. S and S vs. P2 differed significantly at p < 0.01, but not P1 vs. P2. CONCLUSION: Compared with the supine position, prone position reduces DTF and prevents excessive diaphragmatic contraction in neonates admitted to the NICU. DU has the potential to serve as a non-invasive bedside tool for assessing respiratory effort. WHAT IS KNOWN: • Prone position enhances thoracoabdominal synchrony, increases rib cage motion, improves diaphragmatic efficiency, and optimises ventilation-perfusion mismatch, stabilizing percutaneous oxygen saturation and maintaining effective ventilation. • Diaphragmatic ultrasound is a valuable tool for real-time bedside assessment of diaphragmatic function in the ICU, and diaphragmatic thickening fraction (DTF) reflects respiratory effort. WHAT IS NEW: • Compared with the supine position, prone position reduces DTF and prevents excessive diaphragmatic contraction in preterm and sick term infants admitted to the NICU.