Modeling diffusion limitation of gas exchange in lungs containing perfluorocarbon

We reportedchanges in alveolar-arterial PO₂ gradient,ventilation-perfusion heterogeneity, and arterial-alveolarPCO₂ gradient during partial liquid ventilation(PLV) in healthy piglets (E. A. Mates, P. Tarczy-Hornoch, J. Hildebrandt, J. C. Jackson, and M. P. Hlastala. In: OxygenTransport to Tissue XVII, edited by C. Ince. New York: Plenum,1996, vol. 388, p. 585-597). Here we develop two mathematicalmodels to predict transient and steady-state (SS) gas exchangeconditions during PLV and to estimate the contribution of diffusionlimitation to SS arterial-alveolar differences. In the simplest model,perfluorocarbon is represented as a uniform flat stirred layer and, ina more complex model, as an unstirred spherical layer in a ventilatedterminal alveolar sac. Time-dependent solutions of both models showthat SS is established for various inert and respiratory gases within5-150 s. In fluid-filled unventilated terminal units, all times toSS increased sometimes by hours, e.g., SF₆ exceeded 4 h. SSsolutions for the ventilated spherical model predicted minorend-capillary disequilibrium of inert gases and significantdisequilibrium of respiratory gases, which could explain a largeportion of the arterial-alveolar PCO₂ gradient measured during PLV (14). We conclude that, during PLV, diffusion gradients for gases are generally small, except for CO₂.