Step Hypocapnia to Separate Arterial from Tissue Pco 2 in the Regulation of Cerebral Blood Flow

The change in cerebral blood flow was determined after a step decrease in the Pco 2 of arterial blood from 40 to 25 mm Hg in awake man. Subjects monitored their own end-tidal Pco 2 (infrared analyzer) and adjusted their voluntary ventilation to produce the step change, which they maintained for at least 1 hour. Cerebral blood flow relative to control was determined from the arterial-jugular venous oxygen saturation differences. After the step change, arterial Pco 2 fell in less than 30 sec to a plateau, cerebral blood flow fell with a time constant (to 1/e) of 0.3 min to a plateau of 68% of control, while jugular venous Pco 2 fell with a time constant for the fast component of 3.5 min. Base excess rose 1.2 mEq/liter within 1 min and remained at that level. It is concluded that CO 2 affects cerebral blood flow by direct diffusion into arteriolar walls, rather than by its effect on brain tissue Pco 2 or pH. It is postulated that the pH of the extracellular fluid of arteriolar smooth muscle is the common controlled variable through which CO 2 , and possibly hypoxia and blood pressure, determine vascular tone.