Modeling steady-state inert gas exchange in the canine trachea

The functional dependence between tracheal gas exchange and tracheal blood flow has been previously reported using six inert gases (sulfur hexafluoride, ethane, cyclopropane, halothane, ether, and acetone) in a unidirectionally ventilated (1 ml/s) canine trachea (J. E. Souders, S. C. George, N. L. Polissar, E. R. Swenson, and M. P. Hlastala. J. Appl. Physiol. 79: 918-928, 1995). To understand the relative contribution of perfusion-, diffusion- and ventilation-related resistances to airway gas exchange, a dynamic model of the bronchial circulation has been developed and added to the existing structure of a previously described model (S. C. George, A. L. Babb, and M. P. Hlastala. J. Appl. Physiol. 75: 2439-2449, 1993). The diffusing capacity of the trachea (in ml gas.s-1.atm-1) was used to optimize the fit of the model to the experimental data. The experimental diffusing capacities as predicted by the model in a 10-cm length of trachea are as follows: sulfur hexafluoride, 0.000055; ethane, 0.00070; cyclopropane, 0.0046; halothane, 0.029; ether, 0.10; and acetone, 1.0. The diffusing capacities are reduced relative to an estimated diffusing capacity. The ratio of experimental to estimated diffusing capacity ranges from 4 to 23%. The model predicts that over the ventilation-to-tracheal blood flow range (10-700) attained experimentally, tracheal gas exchange is limited primarily by perfusion- and diffusion-related resistances. However, the contribution of the ventilation-related resistance increases with increasing gas solubility and cannot be neglected in the case of acetone. The increased role of diffusion in tracheal gas exchange contrasts with perfusion-limited alveolar exchange and is due primarily to the increased thickness of the bronchial mucosa.