Bicarbonate kinetics and CO₂ production in VLBW babies

Background: Substrate oxidation studies with ¹³C tracers require estimation of fractional recovery (FR) of ¹³C in breath. A value of 0.8 is often assumed. We measured FR in VLBW babies by quantifying CO₂ production (VCO₂) using an indirect calorimeter(Deltatrac™) and rate of appearance of CO₂ (RaCO₂) during primed continuous infusion of ¹³C-labelled bicarbonate.

Method:6 continuously fed, self ventilating babies (gestational age 25-30 weeks; postnatal age 7-35 days; birth weight median 1125g, range 715-1379g) underwent primed continuous infusion of NaH¹³CO₃(bolus 0.25 mg/kg, maintenance 0.2 mg/kg/hr). VCO₂ was measured by indirect calorimetry; breath samples were obtained using a face mask.

Results:Isotopic steady state was reached by 90 minutes. Mean VCO₂ was 7.0 ml/kg/min(SD 1.4). Rate of appearance of CO₂(RaCO₂) was 8.1 ml/kg/min (SD 2.3). RaCO₂ exceeded VCO₂ by a mean of 1.1 ml/kg/min (SD 1.1). The mean FR was 0.89 (SD 0.1)

Conclusion:Mean FR is higher than previously reported. Limitations of indirect calorimetry make individual estimation of FR advisable.