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 CO 2 . For example, the clearance rates of the oxygen-labeled gas are about twice as fast as those of carbon-labeled CO 2 . 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 CO 2 , 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 CO 2 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.
Submitted on December 7, 1960
The binding property of ionic indium (In3+) with plasma transferrin was utilized for determination of plasma volumes (PV) of whole body and individual organs in small animals. Plasma transferrin from a donor rat was labeled with 15-17 muCi 113mIn/ml plasma and injected into the tested rats. PV were determined either by extrapolation to the dilution at time zero (for whole animals) or by calculation of the ratio, organ radiation: radiation of a plasma unit volume (for organs). The reliability of the method for determination of whole-body PV was ascertained by comparing the results obtained with those obtained simultaneously by the Evans blue dilution method. Whole body PV values obtained by the two methods were similar, with a correlation coefficient (r) of 0.997. The short half life of 113mIn enables it to be used with other nuclides which have similar or different energies in the same sample; indium radiation was counted first and after it had disappeared the activity of the other nuclide could be measured.