Variation of venous admixture, SF<inf>6</inf> shunt, Pa<inf>O2</inf>, and the Pa<inf>O2</inf>/FI<inf>O2</inf> ratio with FI<inf>O2</inf>

Background. Measures of impairment of oxygenation can be affected by the inspired oxygen fraction. Methods. We used a mathematical model of an inhomogenous lung to predict the effect of increasing inspired oxygen concentration (FlO2) on: (I) venous admixture (Qva/Qt); (2) arterial oxygen partial pressure (PaO2); (3) the PaO2/FlO2 index of hypoxaemia; and (4) sulphur hexafluoride (SF6) retention (often taken to be true right-to-left shunt). This model predicts whether or not atelectasis will occur. Results. For lungs with regions of low V/Q, increasing the inspired oxygen concentration can cause these regions to collapse. In the absence of atelectasis, the model predicts that Qva/Qt will decrease and arterial oxygen partial pressure increase as FlO2 is increased. However, when atelectasis occurs, Qva/Qt rises to a constant value, whilst PaO2 falls at first, but then begins to rise again, with increasing FlO2. The SF6 retention increased markedly in some cases at high FlO2. Conclusions. Venous admixture will estimate true right-to-left shunt at high FlO2, even when oxygen consumption is raised. This model can explain the way that the Pa/Fl ratio changes with increasing inspired oxygen concentration.