Pulmonary diffusing capacity for CO (DCO) was measured in human subjects at various CO concentrations using a rebreathing procedure. Two stable CO isotopes, 12C18O and 13C18O, were used. These isotopes could be simultaneously and continuously recorded by a respiratory mass spectrometer. For 13C18O the initial concentration in the rebreathing bag was constant at 0.00016, whereas it was varied for 12C18O from 0 to 0.00224. DCO was calculated for both isotopes. In five normal subjects, both D12C18O and D13C18O were independent of the rebreathing CO concentration, with mean values of 31.0 and 30.2 ml . min-1 . Torr-1, respectively. These results, which are at variance with those of Mendoza et al. (J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 43: 880–884, 1977), are compatible with the assumption that diffusion is the sole mechanism of alveolocapillary CO transport; in particular, there is no evidence for facilitated transport.
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