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The change in cerebral blood flow was determined after a step decrease in the Pco2 of arterial blood from 40 to 25 mm Hg in awake man. Subjects monitored their own end-tidal Pco2 (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 Pco2 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 Pco2 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 CO2 affects cerebral blood flow by direct diffusion into arteriolar walls, rather than by its effect on brain tissue Pco2 or pH. It is postulated that the pH of the extracellular fluid of arteriolar smooth muscle is the common controlled variable through which CO2, and possibly hypoxia and blood pressure, determine vascular tone.

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

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