Segmental vascular responses to acute hypertension in cerebrum and brain stem.

The goal of this study was to examine hemodynamic mechanisms that contribute to regional differences in autoregulation during acute hypertension. We measured blood flow (microspheres) and pressure in pial arteries (approximately 160 microns) of anesthetized cats and calculated resistance of large and small vessels in cerebrum and brain stem. Moderate elevation of aortic pressure increased resistance of both large and small vessels in cerebrum but only small vessels in brain stem. During severe hypertension, resistance of both large and small vessels in cerebrum decreased and blood flow increased markedly. In contrast, in the brain stem large artery resistance did not change, small vessel resistance increased, and blood flow increased only modestly during severe hypertension. Pial artery pressure was 20 mmHg higher in brain stem than cerebrum during control conditions and 30-50 mmHg higher during moderate and severe hypertension. We conclude that resistance of large arteries is less and thus pial artery pressure is higher in brain stem than cerebrum under control conditions. More effective autoregulation in the brain stem than cerebrum during severe hypertension is due to greater resistance of small, not large, cerebral vessels.