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

Abstract

Background: Estimates of the rate of CO., production may be useful in preterm infants, but assessment of the rate of respiratory excretion of CO₂ (Vco₂) may not always be practical in infants requiring constant care. We hypothesized that the rate of dilution of ¹³CO₂ (Raco₂) would be a valid index of CO₂ production in preterm infants. Methods: Twelve studies of Raco₂ and VCO₂ were performed in six enterally fed preterm infants. Raco₂ was measured using a 2-hour, primed, constant, orogastric infusion of NaH¹³CO₃ with formula and an assessment of the plateau ¹³C enrichment of expired CO₂. VCO₂ 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 CO₂ production using Raco₂ (348 ± 32 μmol/kg/min) was 114% of that estimated using VCO₂ (304 ± 51 μmol/kg/min). The ratio of VCO₂/RaCO₂ is equal to the fractional recovery of tracer CO₂ 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 Raco₂ on day 2 was multiplied by the mean of the individual ratios of VCO₂/RaCO₂, on day 1 (0.78); corrected Raco₂ was 306 ± 19 μmol/kg/min compared with 307 ± 59 μmol/kg/min for Vco₂. Conclusions: Thus, Raco₂, particularly when corrected for isotope recovery, may be a useful index of group mean CO₂ production and energy expenditure in preterm infants. (journal of Parenteral and Enteral Nutrition 20:389–393, 1996)