Changes in cerebral arterial, tissue and venous oxygenation with evoked neural stimulation: Implications for hemoglobin-based functional neuroimaging

Little is known regarding the changes in blood oxygen tension (P O2) with changes in brain function. This work aimed to measure the blood P O2 in surface arteries and veins as well as tissue with evoked somato-sensory stimulation in the anesthetized rat. Electrical stimulation of the forepaw induced average increases in blood flow of 44% as well as increases in the tissue P O ₂ of 28%. More importantly, increases in P O₂ throughout pial arteries (resting diameters59 to 129 m) and pial veins (resting diameters62 to 361 m) were observed. The largest increases in vascular P O₂ were observed in the small veins (from 33 to 40 mm Hg) and small arteries (from 78 to 88 mm Hg). The changes in oxygen saturation (S O₂) were calculated and the largest increases were observed in small veins (Δ11%) while its increase in small arteries was small (Δ4%). The average diameter of arterial vessels was observed to increase by 4 to 6% while that of veins was not observed to change with evoked stimulation. These findings show that the increases in arterial P O₂ contribute to the hyper-oxygenation of tissue and, mostly likely, also to the signal changes in hemoglobin-based functional imaging methods (e.g. BOLD fMRI).