Oxygen labelled CO2

Tests were carried out as to whether additional information concerning pulmonary gas exchange could be obtained from the application of oxygen labelled carbon dioxide. Single breath experiments were performed on two healthy subjects with 0.1 percent C16O18O and 2.8 percent C18O2 in the inspiratory gas. Breath-hold time was varied between 0.5-20s in different experiments. The 18O-concentration of the end-expired gas bi-exponentially decreased with increasing breath-hold time. The high and low rate constants 4s-1 and 0.12s-1 for C18O2 and 2.5s-1 and 0.87s-1 for C16O18O were derived, respectively. These results, together with model calculations, suggest: 1) the rapid disappearance of C18O2 from the alveolar space is primarily limited by diffusion, so that this isotopic species can be applied to quantify pulmonary diffusing conditions; 2) the lower disappearance rate of C16O18O is caused by a lower equilibration kinetics in blood, so that this isotopic species offers a possibility to study carbonic anhydrase activity of the red cells in vivo; 3) the slow phase of label decay is influenced by both alveolar dead space and carbonic anhydrase activity of the pulmonary tissues. Pathological dead spaces are expected to be sensitively detectable by C16O18O as well as by C18O2. (author). 4 refs.; 4 figs