Estimation of CO2 Production in Enterally Fed Preterm Infants Using an Isotope Dilution Stable Tracer Technique

Background: Estimates of the rate of CO., production may be useful in preterm infants, but assessment of the rate of respiratory excretion of CO 2 (Vco 2 ) may not always be practical in infants requiring constant care. We hypothesized that the rate of dilution of 13 CO 2 (Raco 2 ) would be a valid index of CO 2 production in preterm infants. Methods: Twelve studies of Raco 2 and VCO 2 were performed in six enterally fed preterm infants. Raco 2 was measured using a 2‐hour, primed, constant, orogastric infusion of NaH 13 CO 3 with formula and an assessment of the plateau 13 C enrichment of expired CO 2 . VCO 2 was measured over two 10‐minute intervals during the infusion using a flow‐through system. Energy expenditure was estimated from these data and the food quotient. Results: Mean (± SD) rate of CO 2 production using Raco 2 (348 ± 32 μmol/kg/min) was 114% of that estimated using VCO 2 (304 ± 51 μmol/kg/min). The ratio of VCO 2 /RaCO 2 is equal to the fractional recovery of tracer CO 2 in the expired air during the course of the tracer infusion. In studies of short duration, this ratio is generally less than 100% because of isotope exchange. For five pairs of studies performed on consecutive days, each individual value of Raco 2 on day 2 was multiplied by the mean of the individual ratios of VCO 2 /RaCO 2 , on day 1 (0.78); corrected Raco 2 was 306 ± 19 μmol/kg/min compared with 307 ± 59 μmol/kg/min for Vco 2 . Conclusions: Thus, Raco 2 , particularly when corrected for isotope recovery, may be a useful index of group mean CO 2 production and energy expenditure in preterm infants. (journal of Parenteral and Enteral Nutrition 20 :389–393, 1996)