Longitudinal Diaphragm Ultrasound Assessment Across Respiratory Support Modalities in Critically Ill Infants and Children: A Prospective Cohort Study

Background/Objectives: Diaphragm ultrasonography enables noninvasive bedside assessment of diaphragmatic structure and function. However, longitudinal changes in diaphragm ultrasound parameters across different respiratory support modalities remain insufficiently characterized in critically ill children. This exploratory study aimed to evaluate serial changes in diaphragm ultrasound parameters and to explore whether these changes differed according to the initial respiratory support modality or showed a relationship with respiratory outcomes. Methods: This prospective, single-center observational cohort study included children aged 1 month to 18 years who required respiratory support for acute respiratory failure between March 2025 and March 2026. Diaphragm excursion (DE), end-expiratory diaphragm thickness (DTee), end-inspiratory diaphragm thickness (DTei), and diaphragm thickening fraction (TFdi) were measured at baseline, 48 h, and immediately before discontinuation of respiratory support. Patients were grouped according to the initial respiratory support modality: high-flow nasal cannula, nasal noninvasive ventilation, mask noninvasive ventilation, or invasive mechanical ventilation. Results: A total of 100 children were included; the median age was 4 months, and 66% were male. No statistically significant longitudinal changes were observed in DE (p = 0.210), TFdi (p = 0.850), DTee (p = 0.492), or DTei (p = 0.467). Baseline values and longitudinal percentage changes did not differ significantly among the four respiratory support groups. No statistically significant relationship was observed between serial changes in DE, TFdi, or DTee and total respiratory support duration. Conclusions: No statistically significant longitudinal changes in diaphragm ultrasound parameters were detected during short-term respiratory support, and no statistically significant differences were identified among the initial respiratory support groups. These findings should not be interpreted as demonstrating physiological equivalence between respiratory support modalities. Serial diaphragm ultrasonography should be interpreted within the broader clinical context rather than as an isolated prognostic tool.