The effects of a fluctuating Fio2 on metabolic measurements in mechanically ventilated patients.

The determination of minute oxygen consumption (VO2) for metabolic studies on patients using either the Beckman Metabolic Cart (MMC) or the Respiratory Monitoring System (RMS) requires a stable fractional inspired oxygen concentration (FIO2). Measurement of VO2 on mechanically ventilated patients using the Bourns BEAR 1 and the Engstrom 311 and 312 was unsuccessful due to FIO2 variations greater than the acceptable limits. The inspiratory and expiratory O2 and CO2 waveforms were studied using various methods of supplying source gas to the ventilators. A method using an external air/oxygen blender was found that provided a stable FIO2 for metabolic measurements for each of the ventilators tested.