Recovery of [13C]bicarbonate as respiratory 13CO2 in mechanically ventilated patients

Measurement of the nutrient oxidation rate with 13C as a tracer requires knowledge of the value of its coefficient of fractional recovery in the expired gas (FR). We measured FR in nine intensive care patients who were mechanically ventilated and received total parenteral nutrition. NaH13CO3 was administered at a priming dose (3.75 mumol.kg-1.min-1) followed by a continuous infusion (0.05 mumol.kg-1.min-1). Metabolic rate and pulmonary carbon dioxide elimination (VCO2) were measured by using a mass-spectrometer system. The 13C-12C ratio was measured in the expired gas with an isotopic-ratio mass spectrometer and FR was calculated by using standard equations. The average value of FR was 0.899 +/- 0.026 (means +/- SE) and remained stable for each patient on 2 consecutive days. Between patients, the coefficient of variation of FR was 8.6%. Metabolic rate was the only physiological factor found to affect the FR value.