Introduction In patients assisted by mechanical ventilation, the Work Of Breathing (WOB) is shared between the patient and the ventilator. During weaning from mechanical ventilation, the WOB performed by the patient must be adequate and efficient to sustain spontaneous ventilation after extubation. The monitoring of WOB during weaning might allow a better management of the weaning process. Esophageal pressure (PES) is the reference technique to measure WOB but alternate tools have been proposed. The main hypothesis is that Indirect Calorimetry (IC) is valid to track the changes in energy expenditure due to the changes in WOB in mechanically ventilated children during weaning from mechanical ventilation. The primary objective of this study is to assess the validity of IC method for the WOB assessment when compared to PES measurement and Electrical Activity of the diaphragm (EAdi) during a spontaneous breathing trial (SBT) in continuous positive airway pressure, which is a routine extubation readiness test which generally induces an increase in WOB.

Methods This is a prospective single center study. All intubated and mechanically ventilated children >1 months and <18 years old, hospitalized in the pediatric intensive care unit will be eligible.

Simultaneous recordings of Energy Expenditure, PES and EAdi will be performed during 3 steps: before, during and after the SBT. Then outcome of patients will be collected.

The investigators plan to study a sample of 15 patients to be representative.

Relevance to the importance of child health in Canada The investigators expect that the IC-based less invasive method will provide an accurate estimation of WOB assessment. Once this tool is validated, the interest of IC to (i) early detect an increase in WOB during mechanical ventilation in children, (ii) to assess the ability to extubate them and (iii) to optimize nutritional support will be assessed in future studies.

Inclusion Criteria:

All intubated and mechanically ventilated children >1 months and <18 years old, hospitalized in the pediatric intensive care unit will be eligible.

The patient is deemed to be ready for an extubation readiness test as per the attending team. In particular, the following criteria should be met:

• Improvement in the underlying condition that led to intubation;
• Presence of spontaneous breathing, and adequate oxygenation:, FiO2 ≤ 0.6 (to obtain a SpO2 between 92 and 97%), with Positive End Expiratory Pressure < 8 cmH2O;
• Adequate mental status: Arousal;
• Effective cough;
• No planned operative procedure requiring heavy sedation in the next 12 hours.

Exclusion Criteria:

• Contraindications to the placement of a new nasogastric tube (e.g. trauma or recent surgery in cervical, esophageal, or nasopharyngeal regions, severe coagulation disorder);
• Hemodynamic instability requiring milrinone ≥ 0.5µg/kg/min, dopamine ≥ 5µg/kg/min, epinephrine ≥ 0.03µg/kg/min, norepinephrine ≥ 0.03µg/kg/min, or dobutamine ≥ 5µg/kg/min;
• Severe respiratory instability, and in particular PaCO2 > 80 mmHg on the last blood gas in the last 4 hours;
• Axillary temperature >38°;
• Cuff leaks >10%, calculated by the ventilator as mean inspired tidal volume minus mean expired tidal volume divided by inspired tidal volume;
• Absence of parental or tutor consent;
• Patient for whom a limitation of life support treatments is discussed or decided.