Effect of cardiac output on pulmonary gas exchange: role of diffusion limitation with VA/Q mismatch

We studied the effect of the interaction between diffusion limitation and alveolar ventilation to perfusion ratio ((V) over dot(A)/(Q) over dot) mismatch in the relation between blood gas partial pressures and cardiac output ((Q) over dot). 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 O-2, CO2 and N-2 and Bohr integration for O-2 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 ((V) over dot(A)/(Q) over dot) mismatch and diffusion impairment limited the increase of Pa-O2 with (Q) over dot. A maximum in the Pa-O2 vs. (Q) over dot curve can be attained, further (Q) over dot increases lead to reductions in Pa-O2. The effect was accentuated by increasing ((V) over dot(A)/(Q) over dot) and diffusion to perfusion heterogeneity. Combination of ((V) over dot(A)/(Q) over dot) mismatch and diffusion limitation was synergistic leading to greater reductions in Pa-O2 than expected from simple addition of their independent influences. The findings are compatible with experimental data. (C) 1998 Elsevier Science B.V. All rights reserved.