The measurement of how much oxygen a baby consumes provides important information about the health of the baby, and of how much energy they are consuming. Currently, there is no device which measures either oxygen consumption, or another variable that depends on oxygen consumption - resting energy expenditure - in neonates or infants. Our group has developed a new device which can attach to any ventilator and measures these two variables with accuracy in the preclinical setting, including in rodents as small as severely preterm infants. The purpose of this study is to compare measurements of oxygen consumption and energy expenditure in neonates using this device and comparing it with a gold standard which is rarely used, a Douglas bag method in which expired gas is collected and later analyzed.

Inclusion criteria.

1. Mechanically ventilated neonates and infants (<1 year of age).
2. Inpatients in the cardiac or neonatal intensive care unit at Boston Children's Hospital.
3. Assent of patient's intensive care attending physician, including agreement to place patient on 40% oxygen for up to 60 minutes.
4. Written parental informed consent.

Exclusion criteria.

1. Hemodynamic or respiratory instability.
2. Patients on extracorporeal membrane oxygenation support.
3. Clinically significant tracheo-esophageal fistula
4. Measured endotracheal tube leak >20% (i.e. difference between inspiratory and expiratory tidal volumes)

Patients will be selected from inpatients in the neonatal or cardiac intensive care unit who are mechanically ventilated.