Figure 2.

Schematic diagrams of the apparatus typically used to generate a fixed inspired hypercapnia challenge (a) and a fixed inspired hyperoxia challenge (b). In (a), a manually actuated valve enables the inspired gas to be switched between room air and a 5% CO₂–air mixture, whereas, in (b), during baseline subjects breathe room air through holes in the mask and hyperoxia is induced by allowing 100% O₂ to flow into the mask. Mixing with entrained room air reduces the inspired fraction of oxygen to approximately 50%.