Diffusion-limited gas mixing in the lung and its consequences for transpulmonary gas transport

The ratio of alveolar ventilations of He and SF6 (VA,He/VA,SF6) was determined in 7 healthy male subjects at rest and at three different levels of exercise on a bicycle ergometer (75, 150 and 225 W). This ratio was calculated from the ratio of the specific ventilations for these gases which were obtained from the decay of the end-tidal partial pressures of He and SF6 during a simultaneous, multiple-breath washout. In all experiments, VA for He was larger than VA for SF6. On the average, VA,He/VA,SF6 was equal to 1.09, and the mean values of this ratio at rest and at the three levels of exercise were not significantly different. Therefore, the difference in VA for He and SF6 (VA,He-VA,SF6) increased with increasing work load. Further, we used the mean value obtained for VA,He/VA,SF6 to calculate the ratio of excretion values (E1/E2) for pairs of hypothetical tracer gases with equal blood-gas partition coefficients and with different diffusivities in the gas phase. E1/E2 ranged from unity for lambda = 0 to about 1.08 for lambda = 10. At a given lambda, E1/E2 decreased with increasing ventilation-perfusion ratio of the lung. Thus, the difference between the excretion values of light and heavy tracer gases will be most pronounced under rest conditions and for gases that are well soluble in blood.