Date of Award


Degree Type


Degree Name

Doctor of Philosophy


In this study, the responsiveness of the cerebrovasculature to hypercapnia provided an indication of the residual capacity for dilation available (ie. the dilative reserve). The relationship between cerebral blood flow (CBF) and pial vessel caliber responses and the dependence of these responses on the dilative reserve were examined in three groups of rabbits: 31 control animals; 21 phenoxybenzamine treated animals (alpha adrenoreceptor blockade); and 21 animals subjected to carotid artery occulusion. The effects of hemorrhagic hypotension on both CBF (H(,2) clearance) and pial vessel caliber (image splitting technique) and on the cerebrovascular responsiveness to hypercapnia (the CO(,2) response) were investigated in each group.;Three characteristic autoregulatory regions were identified. (1) During moderate reductions in perfusion pressure (PP) the pial vessels (< 200 (mu)m) dilated progressively and CBF autoregulation was complete. The dilative reserve was functionally intact as the CBF CO(,2) response was constant and that of the pial vessels increased. (2) At intermediate PP, autoregulatory pial vessel dilation continued and CBF declined gradually (incomplete autoregulation). Both the pial vessel and CBF CO(,2) responses decreased, indicating a reduced dilative reserve. (3) The lower limit of autoregulation occurred at a cerebral PP of approx. 35 mm Hg. Both CBF and pial vessel caliber decreased pressure passively and there was a complete loss of CO(,2) reactivity--a depletion of the dilative reserve. Phenoxybenzamine infusion resulted in an increase in the dilative reserve and a relative improvement in CBF autoregulation. These changes were related to the decrease in CBF and metabolism caused by phenoxybenzamine. Carotid artery occulsion reduced the pressure in the Circle of Willis. This decreased the dilative reserve and shifted the lower limit of autoregulation to a PP of 45 mm Hg. The dilative reserve and the efficiency of CBF autoregulation were strongly interrelated. Changes in total precapillary resistance closely paralleled alterations in pial vascular resistance whereas the large inflow arteries were unreactive. The pial vessel responses appeared to be qualitatively representative of those occurring in the intraparenchymal vasculature.



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