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 and microvascular cerebral blood volume (CBV), and blood oxygenation level-dependent (BOLD) contrast change during autoregulation and hypotension using hemodynamic MRI. Methods-Using arterial spin labeling and steady-state susceptibility contrast, 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 (MABP) 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 CBV changes were small and not significantly different from CBF changes. Consistent with this, no significant BOLD changes were observed. For MABP between 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 the cortical surface, total CBV increases were larger (+21 +/- 18%, P<0.01) and microvascular CBV was unchanged (3 +/- 22%, P>0.05). With severe hypotension, both total and microvascular CBV decreased significantly. Over the entire range of graded global hypoperfusion, there were increases in the CBV/CBF ratio. Conclusions-Parenchymal CBV changes are smaller than those of previous reports but are consistent with the small arteriolar fraction of total blood volume. Such measurements allow a framework for understanding effective compensatory vasodilation during autoregulation and volume-flow relationships during hypoperfusion.