Use of a ventilated canopy for assessment of [13C]leucine oxidation in patients receiving total parenteral nutrition.

Rates of oxidation of infused 13C-labeled substrates are calculated from CO2 production and 13C enrichment in breath CO2. Breath sampling through a mouthpiece is not appropriate in severely ill patients; the authors therefore validated the use of direct air sampling from the ventilated canopy of an indirect calorimeter for measuring the oxidation of 13C-labeled substrates. Infusions of H13CO3Na or L-[1-13C]leucine were performed in four healthy postabsorptive adults and six malnourished patients receiving total parenteral nutrition (TPN). At each sampling point, air was collected from the canopy to compare with breath air sampled through a mouthpiece and 13CO2 enrichment determined by isotope ratio mass spectrometry. Despite five-fold dilution of expired air by room air within the canopy (a dilution required to maintain safe CO2 levels in inspired air): (1) Breath 13CO2 enrichment was accurately predicted using samples from the canopy, with a correction taking into account the measured CO2 fractions in canopy and room air; (2) the precision in isotopic determination was similar with both methods (SD/mean of 12 determinations = 2.5 +/- 1.0% vs 3.0 +/- 1.0%). These data demonstrate that the use of a ventilated canopy allows for combined assessment of energy expenditure and rates of oxidation of 13C-labeled substrates even in sick, debilitated patients receiving total parenteral nutrition.