Effect of cardiac output on pulmonary gas exchange: Role of diffusion limitation with V̇a/Q̇ mismatch

We studied the effect of the interaction between diffusion limitation and alveolar ventilation to perfusion ratio (V̇a/Q̇) mismatch in the relation between blood gas partial pressures and cardiac output (Q). The analysis was based on a mathematical model of gas exchange involving two exchanging compartments and a right to left shunt. A system of equations describing alveolar-arterial mass conservation for O₂, CO₂ and N₂ and Bohr integration for O₂ and CO₂ was iteractively solved to find sets of alveolar and blood gas partial pressures fitting input data. Simulations used values compatible with patients in respiratory failure and neonate piglets. Association of (V̇a/Q̇) mismatch and diffusion impairment limited the increase of Pa(O(2)) with Q. A maximum in the Pa(O(2)) vs. Q curve can be attained, further Q increases lead to reductions in Pa(O(2)). The effect was accentuated by increasing (V̇a/Q̇) and diffusion to perfusion heterogeneity. Combination of (V̇a/Q̇) mismatch and diffusion limitation was synergistic leading to greater reductions in Pa(O(2)) than expected from simple addition of their independent influences. The findings are compatible with experimental data. Copyright (C) 1998 Elsevier Science B.V.