Carbon isotope fractionation between blood and expired CO2 at rest and exercise

Carbon isotope fractionation occurs between bicarbonates and gaseous CO2. A ccordingly, expired CO2 could be impoverished in C-13 vs. blood CO2 (simila r to 90% bicarbonates). The ratio C-13/C-12 in expired and blood CO2 was me asured in six healthy subjects at rest and at the end of exercise (90 min; 65 +/- 5% (V)over dot (O2max)), with ingestion of water (300 ml) without or with glucose (30 g) naturally or artificially enriched in C-13, in order t o study a range of C-13/C-12 in blood ( - 17.5 +/- 0.3 to 3.4 +/- 0.6 parts per thousand delta C-13 PDB-1). At rest, C-13/C-12 in expired CO2 was 4.7 +/- 0.2 parts per thousand delta C-13 PDB-1 lower than in blood CO2. This d ifference was not modified in response to exercise with ingestion of water or C-13-glucose (average difference 4.6 +/- 0.4 parts per thousand delta C- 13 PDB-1). Carbon isotope fractionation across the lung was similar to 30% lower than predicted from the fractionation factor between bicarbonates and gaseous CO2 (1.00674 at 37 degrees C, or a similar to 6.6 parts per thousa nd delta C-13 PDB-1 difference). This is consistent with the fact that simi lar to 40% of expired CO2 is released from carbamates and dissolved CO2. Fr om a methodological point of view, these results indicate that C-13/C-12 ex pired CO2 adequately tracks C-13/C-12 in blood CO2 with a constant similar to 4.6 parts per thousand delta C-13 PDB-1 difference. (C) 1999 Elsevier Sc ience B.V. All rights reserved.