Fig. 1. A typical cerbral oxygen saturation (Sco2) recording in a subject during the study protocol. At baseline the subject was ventilated to normocapnia with 21% fraction of inspired oxygen (Fio2). This subject then received 17% Fio2as the first treatment condition for 10 min, after which the subject was returned to baseline (21% Fio2) for 10 min. This was followed by 3% fraction of inspired carbon dioxide (Fico2) for 10 min and then back to baseline (21% Fio2). Sco2values during the last 1 min of each period were averaged by the computer to obtain representative Sco2value for that period. Baseline Sco2value was obtained by averaging the Sco2value before a treatment and that obtained immediately before the next treatment (allowing maximum time for a return to baseline conditions). Delta was calculated as the difference of treatment value and the averaged baseline.