Comparison of recoveries in breath carbon dioxide of H13CO3- and H14CO3- administered simultaneously by single 6 h constant unprimed intravenous infusion

Abstract

The aim of this study was to assess the bioequivalence of H13CO3- and H14CO3-, by administering both labels simultaneously by single infusion and comparing their recovery in breath CO2 and urinary urea. Six healthy male subjects (age range 24-41 years; weight 76.7 (SD, 18.6) kg; height 1.79 (SD 0.05) m) were infused with unprimed solutions of HCO3- (110.0 mmol/kg) labelled with 13C (0.76 mmol 13C/h) and 14C (48 Bq/h) at a constant rate for 6h, in a whole-body calorimeter (1400 litres) for measurement of CO2 production. Samples of breath were collected hourly in a Douglas bag and all urine was collected into two batches (0-4h and 4-6h) for estimating recovery of infused label by measurement of enrichment or specific activity. Recovery in breath CO2 of both labels increased from about 25% for the first hour to 88% and above for hours 3-4 onwards. Mean recovery of 13C in breath CO2 was slightly higher than that of 14C for all periods (mean difference always less than 1% of infused label) but was significant only for the first 3h (P<0.05). Recovery of 14C in urea was significantly higher (P<0.01) than 13C, but was confounded by substantial variability and uncertainties concerning 13CO2 background enrichments. These results suggest that there is no compelling need to alter factors currently used for recovery of 14C in breath when using 13C instead, and vice versa.