Threshold of Response of the Cerebral Vessels of Man to Increase in Blood Carbon Dioxide 1

The level of carbon dioxide in the blood is now generally accepted as one of the major factors in the regulation of the cerebral circulation. It was first shown in experimental animals that increase in blood carbon dioxide exerts a vasodilator effect on cerebral blood vessels (1, 2). Similar effects were postulated in man by Lennox and Gibbs (3) on the basis of decrease in the cerebral arteriovenous oxygen difference. Conclusive evidence of the effects of carbon dioxide on cerebral blood flow in man was provided by the experiments of Kety and Schmidt with the nitrous oxide method. Reduction in arterial CO2 through hyperventilation was shown to produce a decrease in blood flow, (4) whereas increase in arterial CO2 through inhalation of 5 per cent and 7 per cent carbon diox- ide was found to cause a striking increase in blood flow (5).