Use of [13C]bicarbonate for metabolic studies in preterm infants: intragastric versus intravenous administration.

The metabolic fate of substrates in humans can be examined by the use of stable isotopes, one of which, [13C]bicarbonate, may serve to estimate CO2 production rate. In view of minimizing the burden of metabolic studies for preterm infants, the authors determined whether intragastric and intravenous infusions of [13C]bicarbonate would achieve the same 13CO2 enrichment in expired air during steady state. A second aim of this study was to determine the minimum time required to reach steady state during intragastric infusion. Ten preterm infants received a primed continuous [13C]bicarbonate infusion intragastrically, followed by an intravenous infusion the next day. Breath samples were obtained every 30 min by the direct sampling method. 13CO2 isotopic enrichment, expressed as atom percent excess, was measured by isotopic ratio mass spectrometry. Two-tailed t tests were used to detect statistically significant differences between the infusion routes. The isotopic enrichment at plateau did not differ between intragastric and intravenous infusion. A steady state of 13CO2 enrichment was achieved after 60 min of intravenous infusion and after 120 min of intragastric infusion. In conclusion, intragastric infusion of [13C]bicarbonate may serve to estimate the whole-body CO2 production rate in preterm infants. To reach 13CO2 steady state, a minimum of 120 min of bicarbonate administration is required.

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