Use of a Ventilated Canopy for Assessment of [13C]Leucine Oxidation in Patients Receiving Total Parenteral Nutrition

Abstract

Rates of oxidation of infused ¹³C-labeled substrates are calculated from CO₂ production and ¹³C enrichment in breath CO ₂. 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 H¹³CO₃ Na 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% us 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. (Journal of Parenteral and Enteral Nutrition 15:65-70, 1991)