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The investigators have created and validated a device that allows us to accurately measure how many calories a baby consumes every day. The purpose of this study is to compare how many calories a baby consumes with how many calories are provided in a population of newborn infants and children. This may help clinicians to better understand how to feed babies during times of critical illness.

Inclusion Criteria:

1. Mechanically ventilated neonates, infants and children, regardless of age, up to 10 kg
2. Inpatients in the cardiac (8S), medical-surgical (7S) or neonatal (7N) intensive care unit
3. Assent of patient's intensive care attending physician
4. Written parental informed consent.
5. Patient on conventional mechanical ventilation on a Servo I or Servo U ventilator with fraction of inspired oxygen <80%.

Exclusion Criteria:

1. Clinically significant tracheo-esophageal fistula
2. Measured ETT leak >20% (i.e. difference between inspiratory and expiratory tidal volumes)
3. Patient on high frequency oscillatory ventilation or high frequency jet ventilation.

Mechanically ventilated children.