Regional differences in cerebral vascular response to P(a)co(2) changes inhumans measured by positron emission tomography

Hypercapnia and hypocapnia produce cerebral vasodilation and vasoconstricti on, respectively. However, regional differences in the vascular response to changes in PaCO2 in the human brain are not pronounced. In the current stu dy, these regional differences were evaluated. In each of the Il healthy su bjects, cerebral blood flow (CBF) was measured using O-15-water and positro n emission tomography at rest and during hypercapnia and hypocapnia. All CB F images were globally normalized for CBF and transformed into the standard brain anatomy, t values between rest and hypercapnia or hypocapnia conditi ons were calculated on a pixel-by-pixel basis. In the pens, cerebellum, tha lamus, and putamen, significant relative hyperperfusion during hypercapnia was observed, indicating a large capacity for vasodilatation. In the pens a nd putamen, a significant relative hypoperfusion during hypocapnia, that is , a large capacity for vasoconstriction, was also observed, indicating mark ed vascular responsiveness. In the temporal, temporo-occipital, and occipit al cortices, significant relative hypoperfusion during hypercapnia and sign ificant relative hypoperfusion during hypocapnia were observed, indicating: that cerebral vascular tone at rest might incline toward vasodilatation. S uch regional heterogeneity of the cerebral vascular response should be cons idered in the assessment of cerebral perfusion reserve by hypercapnia and i n the correction of CBF measurements for variations in subjects' resting Pa CO2.