Oxygen isotope fractionation in healthy subjects and in patients with COPD

We determined the oxygen isotope fractionation degree for oxygen utilized (δU) in expired alveolar gas relative to inspired air in patients with chronic obstructive lung disease (COPD) and, for comparison, in two groups of healthy subjects, old and young. In addition, we determined ΔrelRvent and ΔrelRtot. These determinants of δU (=ΔrelRtot − ΔrelRvent) are related to the oxygen isotope fractionation which occurs in the first part of the O2 pathway by ventilation of alveolar gas (ΔrelRvent) and by O2 transport and utilization in the rest of the O2 pathway from the alveolar space (ΔrelRtot). Mean δU values for the three groups of subjects were close: 9.0, 9.0 and 9.9‰, respectively, with no significant differences between groups. Mean ΔrelRvent for patients with COPD was substantially larger than for young, healthy subjects, 4.0‰ versus 0.94‰, with P < 10−3. This result indicates that the contribution of intrapulmonary gas transport by diffusion to ΔrelRvent is larger for patients with COPD than for young, healthy subjects. Mean ΔrelRtot for patients with COPD was also larger than for young, healthy subjects, 13.0‰ versus 10.84‰, but this difference was not significant (P = 0.06). Further, ΔrelRtot was much larger than ΔrelRvent for all groups of subjects (P < 10−7).