The aging brain and cerebrovascular reactivity.

Cerebrovascular reactivity (CVR) is a measure of vascular response to a vasoactive stimulus, and can be used to assess the health of the brain vasculature. In this current study we used different analyses of BOLD fMRI responses to CO ₂ to provide a number of metrics including ramp and step CVR, speed of response and transfer function analysis (TFA). 51 healthy control volunteers between the ages of 18-85 (26 males) were recruited and scanned at 3T field strength. Atlases reflecting voxel-wise means and standard deviations were compiled to assess possible differences in these metrics between four age cohorts. Testing was carried out using an automated computer-controlled gas blender to induce hypercapnia in a step and ramp paradigm, and monitoring end-tidal partial pressures of CO ₂ (P _ET CO ₂ ) and O ₂ (P _ET O ₂ ). No significant differences were found for resting P _ET CO ₂ values between cohorts. Ramp CVR decreased significantly with age in white matter frontal regions comprising the ACA-MCA watershed area, a finding that may be indicative of age related changes. Similarly, TFA showed that gain was reduced in the left white matter ACA-MCA watershed area as well as the posterior and anterior cingulate cortex, and superior frontal gyrus in the oldest compared to youngest cohort. These findings, detailing changes in cerebrovascular regulation in the healthy aging brain should prove useful in mapping areas of dysregulated blood flow in individuals with vascular risk factors especially those at risk for developing vascular dementia.
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