Changes in arterioles, arteries, and local perfusion of the brain stem during hemorrhagic hypertension.

Cerebral arterioles have been regarded as the primary sites of autoregulatory responses, whereas the role of large arteries in the cerebral autoregulation is poorly understood. The goal of this study was to determine in vivo whether the basilar artery and its primary branches act as resistance vessels under hypotensive conditions by simultaneously measuring their diameters and local brain stem blood flow with laser-Doppler flowmetry. In 10 anesthetized rats, blood flow to the brain stem was well maintained during stepwise hemorrhagic hypotension when mean arterial blood pressure fell from 116 +/- 3 to 50 mmHg and decreased gradually between 50 and 30 mmHg. Diameter of the basilar artery (n = 10) and its large branches (n = 22), measured through an open cranial window, increased by 10% from the baseline value at 50 mmHg and reached their maximum at 30 mmHg (314 +/- 9 from 244 +/- 6 mum, and 149 +/- 4 from 117 +/- 3 mum, respectively). Small branches (n = 15) dilated to a larger extent compared with the larger arteries throughout hypotension and reached the maximum at 30 mmHg (69 +/- 3 from 48 +/- 2 mum). Below 30 mmHg, there was a steep fall in blood flow and reduction in diameter of all-sized arteries. Thus small vessels contribute to reductions in cerebrovascular resistance throughout the entire autoregulatory-range in the brain stem circulation. Large arteries, such as the basilar artery and its branches, also contribute to reductions in cerebrovascular resistance around the lower limits of cerebral blood flow autoregulation and may thus play a significant role in maintaining blood flow to the brain stem during severe systemic hypotension.