Estimation of CO2 production in enterally fed preterm infants using an isotope dilution stable tracer technique.

Abstract

Estimates of the rate of CO2 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. 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. Mean (+/- SD) rate of CO2 production using RaCO2 (348 +/- 32 mumol/kg/min) was 114% of that estimated using VCO2 (304 +/- 51 mumol/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 mumol/kg/min compared with 307 +/- 59 mumol/kg/min for VCO2. Thus, RaCO2, particularly when corrected for isotope recovery, may be a useful index of group mean CO2 production and energy expenditure in preterm infants.