The effects of ventilation pattern on carbon dioxide transfer in three computer models of the airways

We investigate the effects on arterial P_CO2 and on arterial-end tidal P_CO2 difference of six different ventilation patterns of equal tidal volume, and also of various combinations of tidal volume and respiratory rate that maintain a constant alveolar ventilation. We use predictions from three different mathematical models. Models 1 (distributed) and 2 (compartmental) include combined convection and diffusion effects. Model 3 incorporates a single well-mixed alveolar compartment and an anatomical dead-space in which plug flow occurs. We found that: (i) breathing patterns with longer inspiratory times yield lower arterial P_CO2; (ii) varying tidal volume and respiratory rate so that alveolar ventilation is kept constant may change both PA_CO2 and the PA_CO2-PET_CO2 difference; (iii) the distributed model predicts higher end-tidal and arterial P_CO2 than the compartmental models under similar conditions; and (iv) P_CO2 capnograms predicted by the distributed model exhibit longer phase I and steeper phase II than other models.