Diaphragm Position on Chest Radiograph to Estimate Lung Volume in Neonates

Importance: Using chest radiographs to guide lung aeration during respiratory support in infants is common practice and recommended in neonatal intensive care unit (NICU) guidelines, but this practice has never been validated. Objective: To describe the association between diaphragm position on chest radiograph in infants and aerated lung volume calculated from computed tomography (CT). Design, Setting, and Participants: This was a retrospective cross-sectional study conducted at a tertiary children's hospital, the Royal Children's Hospital, in Melbourne, Australia. Included were infants without congenital lung pathology who received a chest CT in the first 30 days after birth between July 9, 2012, to December 31, 2022; infant data were retrieved from the Royal Children's Hospital Medical Imaging database. Study data were analyzed from December 2022 to September 2023. Exposure: Lung volume was calculated using CT semiautomated tissue segmentation and diaphragm position determined using a standardized definition. All investigators analyzing CTs were unaware of the chest radiograph measurements and vice versa. Main Outcomes and Measures: The primary outcome was the distribution and precision of total lung volume at each of the measured diaphragm positions (6th-11th posterior rib). Results: The imaging data of 218 infants (median [IQR] age, 11 [3-20] days old; mean [SD] age, 37.9 [1.9] weeks' gestation at birth; 119 male [55%]) were analyzed. Infants had a mean (SD) weight of 3055 (584) g at scan, and 132 (61%) had a primary cardiac diagnosis. The number of posterior ribs representing diaphragm position ranged from 6 to 11. There was only a weak association between diaphragm position and lung volume (Kendall τ = 0.23; 95% CI, 0.16-0.31). A similar weak association was observed by hemithorax (left, Kendall τ = 0.25; 95% CI, 0.15-0.34; right, Kendall τ = 0.21; 95% CI, 0.10-0.31), degree of consolidation (Kendall τ = 0.30; 95% CI, 0.21-0.38), apex-diaphragm distance (Kendall τ = 0.40; 95% CI, 0.28-0.51), and Hounsfield unit values (Kendall τ = -0.05; 95% CI, -0.15 to -0.06). Conclusions and Relevance: Results of this cross-sectional study suggest that despite long-standing acceptance in the NICU, the use of diaphragm position on chest radiograph lacked the precision required to assess aerated lung volume and guide clinical practice in infants.