Fast-response whole body indirect calorimeters for infants

Portable whole body indirect calorimeters were constructed for full-term (2.5- to 8-kg) and preterm (1- to 2.5-kg) infants. A new calibration system significantly increased the accuracy of flowmeters and gas analyzers. Performance tests with N2 and CO2 infusions and butane combustion demonstrated that the error of individual measurements of O2 consumption and CO2 production were within +/- 2%. The measured error was close to the theoretical uncertainty of approximately +/- 1% calculated from test results of the flowmeters and gas analyzers. System response to a step change in butane combustion rate exceeded 90% within 2 min. Error of +/- 2% and response of 2 min are likely to be the practical lower limits for whole body infant indirect calorimeters with current technology. The calorimeters demonstrated a rapid increase in O2 consumption after feeding (preterm infants) and in the transition from non-rapid-eye-movement to rapid-eye-movement sleep stages (full-term infants).