The Dependency of Cerebral Blood Flow on End-tidal CO2 Pressure

TARGET AUDIENCE – Neuroscientists using ASL/BOLD MRI for CO2 cerebrovascular reactivity measurements or quantitative BOLD PURPOSE – Cerebrovascular reactivity (CVR) due to CO2 inhalation is believed to be a measure of the capacity of the cerebral blood vessels to match the cerebral blood flow (CBF) to the metabolic needs of the brain. CO2 CVR has been shown to be disturbed in many neurological diseases, e.g. steno-occlusive diseases of the brain. ASL and BOLD MRI are increasingly used for the measurement of regional cerebrovascular reactivity to CO2. The acquired data are often treated in the same way as fMRI data, i.e. a linear relationship is assumed between the end-tidal CO2 pressure (PEtCO2) and the CBF or BOLD signal. However, already in 1964 Reivich showed that the CBF CO2 reactivity curve in monkeys has a sigmoidal shape and more recently the sigmoidal relationship between CO2 and CBF velocity (CBFv), measured via transcranial doppler (TCD), has been reported in humans. Furthermore, Bhogal et al. have recently showed that BOLD CO2 reactivity curve has a sigmoidal shape. Thus the assumption of a linear relationship between PEtCO2 and CBF/BOLD could potentially confound any reported findings. The purpose of this study was to accurately measure the relationship between PEtCO2 and both CBF and BOLD reactivity. Additionally, it is tested whether these relationships differ between the anterior and posterior cerebral circulations.