It is widely assumed in fMRI that the relationship between cerebral blood flow (CBF) and volume (CBV) changes observed during end-tidal CO(2) (PETCO(2)) perturbations is equivalent to that elicited by neuronal activation. This assumption has been validated in PET studies insofar as relating total flow to total CBV changes, but remains unconfirmed for venous CBV changes, which pertains to the primary vascular compartment modulating the BOLD signal. In this study, we measured CBF and venous CBV changes in healthy subjects in response to graded hypercapnia and hypocapnia, induced using computerized end-tidal CO(2) targeting, with a DeltaPETCO(2) range of between -6 and +9 mm Hg. Hypercapnia was found to elicit robust increases in CBF and venous CBV, while hypocapnia produced decreases in both. We used steady-state flow and volume changes to estimate the power-law relationship for cortical and subcortical brain regions, and did not observe significant difference between the two. The combined fit resulted in a power coefficient of 0.18+/-0.02, substantially lower than Grubb's coefficient of 0.38, but comparable to previous observations during neuronal activation. These results confirm that the BOLD-specific flow-volume relationship during CO(2) challenges is similar to that characterizing neuronal activation.
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