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  • Title: Carbon isotope fractionation between blood and expired CO2 at rest and exercise.
    Author: Panteleev N, Péronnet F, Hillaire-Marcel C, Lavoie C, Massicotte D.
    Journal: Respir Physiol; 1999 Jun 01; 116(1):77-83. PubMed ID: 10421035.
    Abstract:
    Carbon isotope fractionation occurs between bicarbonates and gaseous CO2. Accordingly, expired CO2 could be impoverished in 13C vs. blood CO2 (approximately 90% bicarbonates). The ratio 13C/12C in expired and blood CO2 was measured in six healthy subjects at rest and at the end of exercise (90 min; 65+/-5% VO2max), with ingestion of water (300 ml) without or with glucose (30 g) naturally or artificially enriched in 13C, in order to study a range of 13C/12C in blood (-17.5+/-0.3 to 3.4+/-0.6% delta 13C PDB-1). At rest, 13C/12C in expired CO2 was 4.7+/-0.2% delta 13C PDB-1 lower than in blood CO2. This difference was not modified in response to exercise with ingestion of water or 13C-glucose (average difference 4.6+/-0.4 % delta 13C PDB-1). Carbon isotope fractionation across the lung was approximately 30% lower than predicted from the fractionation factor between bicarbonates and gaseous CO2 (1.00674 at 37 degrees C, or a approximately 6.6% delta 13C PDB-1 difference). This is consistent with the fact that approximately 40% of expired CO2 is released from carbamates and dissolved CO2. From a methodological point of view, these results indicate that 13C/12C in expired CO2 adequately tracks 13C/12C in blood CO2 with a constant approximately 4.6 % delta 13C PDB-1 difference.
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