ESTIMATION OF CO2 PRODUCTION IN ENTERALLY FED PRETERM INFANTS USING AN ISOTOPE-DILUTION STABLE TRACER TECHNIQUE

Background: Estimates of the rate of CO2 production may be useful in p reterm infants, but assessment of the rate of respiratory excretion of CO2 (Vco(2)) may not always be practical in infants requiring constan t care. We hypothesized that the rate of dilution of (CO2)-C-13 (Raco( 2)) would be a valid index of CO2 production in preterm infants. Metho ds: Twelve studies of Raco(2) and Vco(2) were performed in six enteral ly fed preterm infants. Raco(2) was measured using a 2-hour, primed, c onstant, orogastric infusion of (NaHCO3)-C-13 with formula and an asse ssment of the plateau C-13 enrichment of expired CO2. Vco(2) was measu red over two 10-minute intervals during the infusion using a flow-thro ugh system. Energy expenditure was estimated from these data and the f ood quotient. Results: Mean (+/- SD) rate of CO2 production using Raco (2) (348 +/- 32 mu mol/kg/min) was 114% of that estimated using Vco(2) (304 +/- 51 mu mol/kg/min). The ratio of Vco(2)/Raco(2) is equal to t he fractional recovery of tracer CO2 in the expired air during the cou rse of the tracer infusion. In studies of short duration, this ratio i s generally less than 100% because of isotope exchange. For five pairs of studies performed on consecutive days, each individual value of Ra co(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 mu mo l/kg/min compared with 307 +/- 59 mu mol/kg/min for Vco(2). Conclusion s: Thus, Raco(2), particularly when corrected for isotope recovery, ma y be a useful index of group mean CO2 production and energy expenditur e in preterm infants.