Collateral gas transport by diffusion across tissue in the healthy, human lung; effects on dead space

The effects of collateral gas transport by diffusion across lung tissue on the Bohr and Fowler dead spaces were quantified for resting breathing conditions. The dead spaces (V d) of He, Xe and SF 6 were determined from expirograms obtained from the simultaneous washout of these test gases. The experiments were performed on seven healthy subjects. The contribution of collateral gas transport by diffusion on V d was obtained from the difference between V d SF 6 and V d Xe. These two gases have comparable diffusion coefficients (D) in residual gas but in lung tissue D Xe is roughly 25 times larger than D SF 6 due to the higher solubility of Xe in aqueous tissues. The data showed that the reducing effect of collateral gas transport by diffusion on V d Xe amounts to about 2 ml for both the Bohr and the Fowler dead space. The smallness of this effect means that the alveolar ventilation for Xe hardly benefits from this additional mechanism of intrapulmonary gas mixing.