Abstract
BACKGROUND: Nutrition and fluid balance are core components of supportive care in paediatric critical illness, yet their interaction remains poorly defined. We examined the relationship between fluid balance trajectories and nutritional delivery during the first week of paediatric intensive care. METHODS: We performed a retrospective cohort study of mechanically ventilated children (0–18 years) admitted for ≥ 24 h to a tertiary PICU between 2019 and 2024 who received furosemide. Patients with diagnoses requiring disease-specific fluid strategies were excluded. Fluid balance was calculated in 12-hour intervals and cumulatively. Nutritional delivery from enteral and parenteral sources was expressed as a percentage of predicted energy expenditure (DPEE). The primary outcome was cumulative nutritional delivery at ICU discharge or day 7, comparing patients with negative versus neutral/positive cumulative fluid balance. Multivariable linear regression adjusted for age, illness severity, vasoactive support, mechanical ventilation duration, and ICU length of stay. RESULTS: Among 511 included patients (median age 42 months), nutritional support increased progressively, with > 75% receiving nutrition by 72 h and > 90% by day 7; enteral feeding predominated. Fluid balance was initially positive, transitioning toward neutrality and negative balance after approximately 72 h. As cumulative fluid balance declined, cumulative nutritional delivery increased. In adjusted analyses, negative cumulative fluid balance was independently associated with a 50% reduction in delivered energy compared with neutral or positive balance (p = 0.002). Younger age was associated with lower nutritional delivery, while longer ICU stay was associated with modest increases. CONCLUSIONS: During early paediatric critical illness, fluid balance and nutritional delivery follow interdependent trajectories. Strategies aimed at achieving negative fluid balance are associated with substantially reduced energy delivery, independent of illness severity. These findings identify fluid and nutrition management as linked, modifiable targets and support integrated approaches to optimize supportive care in critically ill children. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s13054-026-05951-9.