Pial vessel caliber and cerebral blood flow during hemorrhage and hypercapnia in the rabbit

We examined the relationship between cerebral blood flow (CBF) and pial vessel caliber responses to graded hemorrhagic hypotension at both normocapnia and hypercapnia in 31 anesthetized rabbits. Changes in CBF (hydrogen clearance) and pial arteriolar diameter (image splitting) were predictably related at all perfusion pressures (PP). Three autoregulatory regions were identified. 1) At PP greater than 65 mmHg, autoregulation was complete as CBF and the CBF response to hypercapnia remained at control levels. The pial vessels dilated progressively, and their response to hypercapnia increased. 2) At PP between 65 and 35 mmHg autoregulation continued but was incomplete. CBF decreased proportionately less than the corresponding reductions in PP due to continued pial vascular dilatation. Both the CBF and pial vessel responses to hypercapnia diminished. 3) At PP less than 35 mmHg, autoregulation was abolished. Pial arteriolar caliber and CBF decreased pressure passively, and there were no responses to hypercapnia. A comparison of changes in pial vascular resistance and total precapillary resistance indicated that the responses of pial vessels (particularly those less than 50 micron) paralleled the responses of the intraparenchymal arterioles.