Rapid magnetic resonance measurement of global cerebral metabolic rate of oxygen consumption in humans during rest and hypercapnia

The effect of hypercapnia on cerebral metabolic rate of oxygen consumption (CMRO₂) has been a subject of intensive investigation and debate. Most applications of hypercapnia are based on the assumption that a mild increase in partial pressure of carbon dioxide has negligible effect on cerebral metabolism. In this study, we sought to further investigate the vascular and metabolic effects of hypercapnia by simultaneously measuring global venous oxygen saturation (S_v O₂) and total cerebral blood flow (tCBF), with a temporal resolution of 30 seconds using magnetic resonance susceptometry and phase-contrast techniques in 10 healthy awake adults. While significant increases in S_v O₂ and tCBF were observed during hypercapnia (P<0.005), no change in CMRO₂ was noted (P<0.05). Additionally, fractional changes in tCBF and end-tidal carbon dioxide (R 2 0.72, P<0.005), as well as baseline S_v O₂ and tCBF (R₂ 0.72, P<0.005), were found to be correlated. The data also suggested a correlation between cerebral vascular reactivity (CVR) and baseline tCBF (R₂ 0.44, P<0.052). A CVR value of 6.1%1.6%/mm Hg was determined using a linear-fit model. Additionally, an average undershoot of 6.7%4% and 17.1%7% was observed in S_v O₂ and tCBF upon recovery from hypercapnia in six subjects.