The Impact of Grey Matter Normalization on Cerebrovascular Reactivity

Abstract

Grey matter atrophy occurs with age; however, most studies on cerebral blood flow responses to hypercapnia and age do not normalize to grey matter volume. It is possible that adults with lower brain blood flow and lower brain blood flow responses simply have less grey matter. This is especially relevant in the context of aging, where grey matter declines 0.83% per year. Therefore, the purpose of this study was to determine the impact of grey matter volume (GM) normalization on the cerebrovascular reactivity quantification in young and older healthy adults. We recruited 15 young (YA: age = 26 ± 4 y; BMI = 23 ± 1 kg/m2) and 10 older (OA: age= 64 ± 3 y; BMI= 22 ± 3 kg/m2) habitually exercising men and women. Middle cerebral artery velocity (MCAv), mean arterial pressure (MAP), and end‐tidal carbon dioxide (ETCO2) were measured continuously at rest and during a stepped hypercapnia protocol. GM was calculated from a T‐1 weighted scan on a 3T MRI scanner and segmented using SPM12. Cerebrovascular conductance index (CVCi) was calculated as CVCi = MCAv/MAP. Reactivity was calculated as the linear relationship between the change in ETCO2 and the change in MCAv and CVCi. GM normalization was performed by dividing MCAv and CVCi by GM. GM was significantly higher in the young compared with the older adults (YA: 0.71±0.02 L; OA: 0.62±0.01 L; p<0.001). MCAv reactivity values normalized to GM were significantly higher than those of raw MCAv reactivity in both young (YA: Raw: 1.7±0.3 cm/s/mmHg; Norm: 2.3±0.4 cm/s/L/mmHg; p<0.001) and older adults (OA: Raw: 2.0±0.3 cm/s/mmHg; Norm: 3.3±0.4 cm/s/L/mmHg; p<0.001). However, MCAv reactivity was not different between groups for both the raw and GM normalized data (p>0.05). Similar results were found for CVCi reactivity such that CVCi reactivity values normalized to GM were higher than raw CVCi reactivity for both young (YA: Raw: 0.017±0.002 cm/s/mmHg2; Norm: 0.023±0.003 cm/s/L/mmHg2; p<0.001) and older adults (OA: Raw: 0.015±0.002 cm/s/mmHg2; Norm: 0.024±0.003 cm/s/L/mmHg2; p<0.001). However, there were no significant differences in CVCi reactivity between young and older adults for both raw and normalized data (p>0.05). These results suggest that GM normalization changes reactivity values similarly between young and older habitually exercising adults.Support or Funding InformationSupported by National Institute of Health grant HL118154.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.