AJP - Heart and Circulatory Physiology, Vol 247, Issue 1 40-H51, Copyright © 1984 by American Physiological Society
Pial vessel caliber and cerebral blood flow during hemorrhage and hypercapnia in the rabbit
U. I. Tuor and J. K. Farrar
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.
