Uptake of oxygen-15-labeled CO₂ compared with carbon-11-labeled CO₂ in the lung

¹ Department of Medicine, Postgraduate Medical School, Hammersmith Hospital, London, England

Carbon dioxide labeled with oxygen 15 has been used to measure regional blood flow in the lung by counting over the chest during a short breath-holding period following a single breath of the gas. This gas has been found to exhibit different physiological behavior from carbon-11-labeled CO₂. For example, the clearance rates of the oxygen-labeled gas are about twice as fast as those of carbon-labeled CO₂. Measurements on alveolar gas expired immediately after inspiration of active gas show that the oxygen-labeled gas disappears from the alveoli about ten times faster than its carbon-labeled counterpart. In addition, if venous blood is drawn shortly after a breath of oxygen-labeled CO₂, the activity cannot be removed from the blood by adding acid and shaking under vacuum, whereas the expected loss of activity occurs for carbon-labeled gas. These apparent discrepancies can be explained by exchange between the oxygen atoms of water and bicarbonate after the gas has been taken up by the blood. In vitro measurements show that the uptake of oxygen-labeled CO₂ into blood can be reduced by adding carbonic anhydrase inhibitor. The results affect the interpretation of the clearance rates of other radioactive gases in the lung.

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