Figure 71-7 Posthyperventilation hypoxia and diffusion hypoxia. The upper (dashed) line represents normal PAO₂ during breathing of air. Posthyperventilation hypoxia is indicated by the upper solid line, assuming that the patient hyperventilated for an hour and was then allowed to accumulate CO₂ for 10 minutes before zero time. Diffusion hypoxia (after equilibration with 79% N₂ O) was added to posthyperventilation hypoxia as above, except that the inspired gas was changed to air at zero time. (Redrawn from Marshall BE, Wyche MQ: Hypoxemia during and after anesthesia. Anesthesiology 37:178, 1972.)