Internal Jugular Vein Respiratory Variability as a Marker of Disease Severity in Infant Bronchiolitis

This prospective observational cohort study evaluates whether respiratory-cycle variation in internal jugular vein (IJV) diameter, measured by point-of-care ultrasound, correlates with clinical disease severity in infants 1-23 months of age presenting with acute bronchiolitis. Using a standardized M-mode protocol, the IJV Variability Index (\[IJVmax-IJVmin\]/IJVmax x 100) will be calculated and compared with the Wang Bronchiolitis Severity Score, four additional validated clinical severity instruments (Modified Tal Score, Respiratory Distress Assessment Instrument, Kristjansson Respiratory Score, Respiratory Assessment Change Score), oxygen saturation, respiratory rate, need for high-flow nasal cannula or supplemental oxygen, pediatric intensive care unit admission, and length of hospital stay. Concurrent lung and diaphragm point-of-care ultrasound will be performed to build a multimodal ultrasound severity model. No intervention is assigned; all ultrasound assessments are performed in addition to standard clinical care and do not alter treatment decisions.