Regional Cerebral Blood Flow in Dogs: Local and Remote Effect of Carbon Dioxide

THE RESPONSE of cerebral circulation to changes in arterial carbon dioxide pressure (Pco₂) is well known, but the mechanisms whereby carbon dioxide (CO₂) alters the tone of cerebral vessels are still uncertain. It has been generally accepted that CO₂ acts directly upon vascular smooth muscle, although some of the evidence for this is tenuous.¹ Recent studies by Severinghaus and Lassen,² as well as Betz and Heuser³ imply that CO₂ acts by altering the pH of vascular smooth muscle, and that an ion-impermeable barrier between intima and muscularis inhibits a similar effect by decreased arterial pH alone. Studies by Shalit et al^1,4,5 oppose the concept of a direct effect of CO₂ on vessel walls and suggest that one of the mechanisms of action of CO₂ on cortical vessels might be indirect through the brain stem. Certainly the hypothesis that the