Changes in the Arterial Fraction of Human Cerebral Blood Volume during Hypercapnia and Hypocapnia Measured by Positron Emission Tomography

Hypercapnia induces cerebral vasodilation and increases cerebral blood volume (CBV), and hypocapnia induces cerebral vasoconstriction and decreases CBV. Cerebral blood volume measured by positron emission tomography (PET) is the sum of three components, that is, arterial, capillary, and venous blood volumes. Changes in arterial blood volume (Va) and CBV during hypercapnia and hypocapnia were investigated in humans using PET with H215O and 11CO. Arterial blood volume was determined from H215O PET data by means of a two-compartment model that takes Va into account. Baseline CBV and values during hypercapnia and hypocapnia in the cerebral cortex were 0.034 ± 0.003, 0.038 ± 0.003, and 0.031 ± 0.003 mL/mL (mean ± s.d.), respectively. Baseline Va and values during hypercapnia and hypocapnia were 0.015 ± 0.003, 0.025 ± 0.011, and 0.007 ± 0.003 mL/mL, respectively. Cerebral blood volume changed significantly owing to changes in PaCO2, and Va changed significantly in the direction of CBV changes. However, no significant change was observed in venous plus capillary blood volume (= CBV-Va). This indicates that changes in CBV during hypercapnia and hypocapnia are caused by changes in arterial blood volume without changes in venous and capillary blood volume.