Fig. 1.
Rebreathing-induced hypercapnia at rest. Top to bottom: partial pressure of transcutaneous CO2 (PtcCO2), fractional CO2, fractional O2, blood flow velocity of middle cerebral artery (MCAV), cerebral tissue O2 saturation (ScO2), arterial O2 saturation (SaO2), arterial blood pressure (ABP), and heart rate (HR). There is a time delay in PtcCO2 response to the rebreathing procedure, although PtcCO2 parallels end-tidal CO2 during a ramp hypercapnia. At the end of the rebreathing procedure, the inspired O2 remained ≥21%.