Caffeine's effects on cerebrovascular reactivity and coupling between cerebral blood flow and oxygen metabolism

The blood-oxygenation-level-dependent (BOLD) signal is dependent on multiple physiological factors such as cerebral blood flow (CBF), local oxygen metabolism (CMRO2) and cerebral blood volume (CBV). Since caffeine affects both CBF and neural activity, its effects on BOLD remain controversial. The calibrated BOLD approach is an excellent tool to study caffeine because it combines CBF and BOLD measures to estimate changes in CMRO2. The present study used the calibrated BOLD approach with 5% CO2 to determine if a 2.5 mg/kg intravenous injection of caffeine changes the coupling between CBF and CMRO2 during motor and visual tasks. The results show that caffeine decreases n, the CBF:CMRO2 coupling ratio, from 2.58 to 2.33 in motor (p = 0.006) and from 2.45 to 2.23 in visual (p = 0.002) areas respectively. The current study also demonstrated that caffeine does not alter cerebrovascular reactivity to CO2. These results highlight the importance of the calibrated BOLD approach in improving interpretation of the BOLD signal in the presence of substances like caffeine.