Effects of Carbon Dioxide on Cerebral Hemodynamics in Normal Subjects and in Cerebrovascular Disease Studied by Carotid Injection of Radioalbumin.

• Research has provided ample evidence that the cerebral vascular system possesses a marked degree of autonomy and is largely unaffected by changes in the systemic circulation. This autonomy is made possible, first of all, by minimal effect on cerebral vessels of those factors which act on the general circulation, such as the autonomic innervation and, second, by the extreme sensitivity of cerebral vessels to intrinsic regulatory mechanisms which ensure a large margin of compensation for variations in arterial pressure or cardiac output. The stimuli which have proved most effective in eliciting vasoactive responses are: 1) variations in the blood levels of oxygen and, particularly, of carbon dioxide; and 2) changes of perfusion pressure. The action of CO2 on cerebral circulation appears to involve an active vasodilatation" which reduces the cerebrovascular resistance to blood flow. CO2 concentrations of 5% to 7% in inspired air prodiice peripheral vasoconstriction and a slight increase of arterial pressure. As a result of this combined action (cerebral vasodilatation and increase of perfusion pressure), cardiac output is redistributed in favor of the brain. Cerebral blood flow increases by about 54% in normal young subjects during inhalation of 5% CO2. Car-