Excretion-retention data of steady state gas exchange in tidal breathing. II. Dependency on the diffusion coefficient

The steady state gas transfer properties of the lung can be derived from excretion-retention (E-R) data of inert tracer gases that are infused intravenously. E = PE/Pv and R = Pa/Pv, where PE, Pa and Pv represent the partial pressures of the tracer gases in mixed expired gas, arterial blood and mixed venous blood, respectively. In this paper, we investigate the influence of diffusive gas mixing in the lung on E and R. To that end, E-R data sets were simulated with a lung model that takes into account tidal breathing, the morphometric geometry of the airways, diffusion limited gas mixing in the alveolar space and gas dissolved in superficial lung tissue. The results show a linear relationship between R/E and D-0.5, where D represents the diffusion coefficient of the tracer gases in the alveolar gas mixture. This is in contrast to the results of simulations with a lung model that describes the different gas transport mechanisms, including diffusion, as a constant rate process, where a linear relationship between R/E and D-1 is predicted. It is further shown that E-R data of helium and sulphur hexafluoride cannot be used to demonstrate diffusion limited gas mixing in the lung, in particular, in the presence of a real shunt fraction. For that purpose, excretion data of pairs of tracer gases with different D but equal, medium blood-gas partition coefficients (1 less than lambda less than 30) should be used. For such pairs of tracer gases, the E values may differ by more than 10% when the D values for the two gases are 0.22 and 0.1 cm2s-1, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)