Indirect calorimetry is considered the reference method for measuring energy expenditure in intensive care patients. However, in infants and small children weighing less than 10 kilograms, its clinical use has historically been limited due to technical challenges related to low tidal volumes, potential increases in dead space, and measurement precision.

The goal of this observational study is to evaluate the feasibility and performance of indirect calorimetry using the Q-NRG+ device in critically ill children weighing less than 10 kg who are admitted to a pediatric intensive care unit.

Indirect calorimetry measures oxygen consumption and carbon dioxide production to calculate resting energy expenditure and is considered the reference method in intensive care. In mechanically ventilated pediatric patients, resting energy expenditure is commonly used as a surrogate for total energy expenditure, as activity-related expenditure is minimal due to sedation and ventilation. Despite its clinical relevance, the use of indirect calorimetry in children weighing less than 10 kg has been limited by technical challenges related to low tidal volumes and measurement precision. The Q-NRG+ is an indirect calorimeter designed to measure resting energy expenditure in mechanically ventilated and spontaneously breathing patients.

This prospective, single-center observational study will be conducted in a pediatric intensive care unit. Critically ill children weighing less than 10 kg who require mechanical ventilation or are spontaneously breathing and able to tolerate indirect calorimetry measurement using a canopy hood will be eligible for inclusion. Indirect calorimetry measurements will be performed according to a predefined protocol during the course of critical illness. For the measurement, a sampling module will be connected to the ventilator circuit without increasing ventilator dead space. The system continuously samples inspired and expired gases, and energy expenditure is calculated based on measured oxygen consumption and carbon dioxide production in combination with ventilator parameters.

The primary objective is to evaluate the feasibility of indirect calorimetry using the Q-NRG+ in critically ill children weighing less than 10 kg, defined as the proportion of technically valid and clinically interpretable measurements. Secondary objectives are to describe measured resting energy expenditure (kcal/kg/day) in this population, to evaluate changes in measured energy expenditure over the course of critical illness, and to compare measured energy expenditure with predicted energy requirements calculated using standard predictive equations. Demographic and clinical variables relevant to metabolic status (age, weight, diagnosis, ventilator settings, sedation, and phase of illness) will be collected from medical records. Descriptive and comparative statistical analyses will be performed.

Inclusion Criteria: Newborns and infants admitted to the PICU with body weight from 0,5 up to 10 kilograms. Either requiring mechanically ventilation, with FiO2 < 70%, PIP<30 cmH2O and no leakage in the ventilation system. Or spontaneous breathing patients tolerating measurements with canopy.

Exclusion Criteria:

In mechanically ventilated patients' unstable ventilation including ventilation settings changed during the last 60 minutes (FiO2 >70%, PIP>30 cmH2O, or ventilation system leakage). Patients spontaneous breathing not tolerating measurements with canopy.

The study population consists of critically ill infants and children weighing 0.5-10 kg who are admitted to a pediatric intensive care unit and require mechanical ventilation or are spontaneously breathing and eligible for indirect calorimetry measurement. Participants are recruited consecutively during the study period (Q2 2026-Q4 2027).