Autoregulation of White Matter Cerebral Blood Flow to Arterial Pressure Changes in Normal Subjects

Endothelial dysfunction causing impaired cerebrovascular vasodilatory capacity in response to reduced blood pressure has been proposed as a mechanism of white matter (WM) disease development. This study investigated autoregulation of CBF to blood pressure reduction in WM and gray matter (GM) in normal subjects recruited as controls for a study of cerebrovascular function in human immunodeficiency virus positive subjects. They underwent baseline CBF and oxygen extraction fraction measurement by MRI before and after mean arterial pressure (MAP) reduction. Autoregulatory Index (AI) was computed as CBF AI = -%CBF change/% MAP change. Thirty of 44 subjects achieved target MAP reduction. MAP was reduced -13.65 ± 2.35 (range 10 to 20) %. WM AI of -0.61 ± 1.23 was significantly more negative than GM AI of 0.02 ± 0.44 (paired t test, p= 0.016). WM CBF fell (paired Wilcoxon, p= 0.03) whereas GM CBF did not change (paired Wilcoxon, p=0.92). WM AI was different from 0 (p=0.011, one-sample t-test vs 0), whereas GM AI was not (p=0.913, one-sample t-test vs 0). These data demonstrate that maintenance of CBF to 10-20% reductions in MAP is less effective in WM than in GM. This may put WM at higher risk for ischemic damage.