Cerebrovascular dynamics of autoregulation and hypoperfusion - An MRI study of CBF and changes in total and microvascular cerebral blood volume during hemorrhagic hypotension

Background and Purpose-To determine how cerebral blood flow (CBF), total an d microvascular cerebral blood volume (CBV), and blood oxygenation level-de pendent (BOLD) contrast change during autoregulation and hypotension using hemodynamic MRI. Methods-Using arterial spin labeling and steady-state susceptibility contra st, we measured CBF and changes in both total and microvascular CBV during hemorrhagic hypotension in the rat (n = 9). Results-We observed CBF autoregulation for mean arterial blood pressure (MA BP) between 50 and 140 mm Hg, at which average CBF was 1.27 +/- 0.44 mL.g(- 1).min(-1) (mean +/- SD). During autoregulation, total and microvascular CB V changes were small and not significantly different from CBF changes. Cons istent with this, no significant BOLD changes were observed. For MABP betwe en 10 and 40 mm Hg, total CBV in the striatum increased slightly (+7 +/- 12 %, P<0.05) whereas microvascular CBV decreased (-15 +/- 17%, P<0.01); on th e cortical surface, total CBV increases were larger (+21 +/- 18%, P<0.01) a nd microvascular CBV was unchanged (3 +/- 22%, P>0.05). With severe hypoten sion, both total and microvascular CBV decreased significantly. Over the en tire range of graded global hypoperfusion, there were increases in the CBV/ CBF ratio. Conclusions-Parenchymal CBV changes are smaller than those of previous repo rts but are consistent with the small arteriolar fraction of total blood vo lume. Such measurements allow a framework for understanding effective compe nsatory vasodilation during autoregulation and volume-flow relationships du ring hypoperfusion.