Fig. 1.

Time course of step changes in O₂ saturation ($\mathrm{S}{\mathrm{a}}_{{\mathrm{O}}_{\mathrm{2}}}$; top) and $\mathrm{P}\mathrm{E}{\mathrm{T}}_{\mathrm{C}{\mathrm{O}}_{\mathrm{2}}}$ (middle) for hypoxic and hypercapnic responses; bottom: time courses of ventilation and CBF responses. The paradigm is described in detail in Sato et al. (50). The initial 5 min is poikilocapnic ventilation while subjects acclimatize to the breathing apparatus. The rebreathing circuit is then connected, and after a further 5 min of hyperoxia, a small increase in $\mathrm{P}\mathrm{E}{\mathrm{T}}_{\mathrm{C}{\mathrm{O}}_{\mathrm{2}}}$ (~3 Torr) is introduced to achieve isocapnic ventilation. The response measurements start once steady-state ventilation (or steady-state CBF) is achieved (indicated as 0 time on the timeline). The changes following $\mathrm{S}{\mathrm{a}}_{{\mathrm{O}}_{\mathrm{2}}}$ step changes define the hypoxic response. Subjects are then again returned to hyperoxia, and after a 10-min period to remove any residual hypoxic drive, the hypercapnic and combined hypoxic hypercapnic responses are measured.