Impact of baseline CO2 on Blood-Oxygenation-Level-Dependent MRI measurements of cerebrovascular reactivity and task-evoked signal activation.

Neurovascular coupling describes the cascade between neuronal activity and subsequent Blood-Oxygenation-Level-Dependent (BOLD) signal increase. Based on this premise, the correlation of this BOLD signal increase with a particular task, such as finger-tapping, is used to map neuronal activation. This signal increase may be dampened in brain areas exhibiting impaired cerebrovascular reactivity (BOLD-CVR), leading to false negative activation. Blood-oxygenation-level-dependent (BOLD) cerebrovascular reactivity (CVR) has also been used to optimize task evoked BOLD signal changes. To measure BOLD-CVR, controlled BOLD-CVR studies have commonly been performed using a preset isocapnic carbon dioxide (CO2; ~40 mmHg) baseline, independent of subjects' resting CO2. This arbitrary baseline, however, may influence BOLD-CVR measurements. We therefore performed BOLD-CVR, as well as BOLD fMRI during a controlled bilateral finger-tapping task in two groups of ten subjects: group A at subject's resting CO2 and group B at a preset isocapnic CO2 baseline (40 mmHg). Whole brain BOLD-CVR was significantly decreased for group B (group A 0.26 (SD 0.05) vs group B 0.16 (SD 0.05), p < 0.001). For the predefined hand area in the precentral cortex, BOLD-CVR and BOLD fMRI signal changes were significantly lower for group B (group A 0.20 (SD 0.04) vs group B 0.13 (SD 0.05), p < 0.01; 1.19 (SD 0.31) vs 0.62 (SD 0.37), p < 0.01).CO2 levels significantly influence both BOLD-CVR and BOLD fMRI measurements. Hence, for an accurate interpretation, baseline CO2 levels and BOLD CVR should be considered complementary to task evoked BOLD fMRI.