Reproducibility of O
manipulations using the new breathing circuit. Manipulations consisted of two runs of the 18-minute schedule described in ref . O2 (top) and CO2 (bottom) respiratory traces (grey) were modeled as described in Material and Methods to obtain end-tidal (ET) levels (black). Pure O2 was administered during 3 minutes in two different instances, inducing an approximate 600 mmHg increase in the fractional concentration of ETO2. A 5%CO2 mixture was also administered in two different instances, lasting 2 minutes each, which induced an approximate 10 mmHg increase in the fractional concentration of ETCO2. We also observed small changes in ETO2 and ETCO2 when the CO2 and, respectively, O2 were administered. When not breathing one of the above gas compositions, the subject breathed medical air.