Relationship between baseline cerebral blood flow and vascular responses to changes in PaCO2 measured by positron emission tomography in humans:: implication of inter-individual variations of cerebral vascular tone

Aim: Inter-individual variations in normal human cerebral blood flow (CBF) at rest condition have been reported. Inter-individual variation of cerebral vascular tone is considered to contribute to this, and several determinants of cerebral vascular tone have been proposed. In the present study, the relationship between CBF and cerebral vascular tone to inter-individual variation at rest condition was investigated using positron emission tomography (PET). Methods: CBF was measured using PET with (H2O)-O-15 in each of 20 healthy subjects (20-28 years) under three conditions: at rest (baseline), during hypercapnia and during hypocapnia. The vascular response to change in PaCO2 was calculated as the percentage changes in CBF per absolute change in PaCO2 in response to hypercapnia and hypocapnia. Results: A significant negative correlation between baseline CBF and the vascular response to hypocapnia was observed in the thalamus, temporal cortex, parietal cortex, occipital cortex and cerebral cortex (P < 0.05). A trend towards negative correlation between baseline CBF and the vascular response to hypocapnia was observed in the cerebellum and putamen (P < 0.1). A significant negative correlation between baseline CBF and the vascular response to hypercapnia was observed in the occipital cortex (P < 0.05). No significant correlation was observed between baseline CBF and haemoglobin concentration, and PaCO2. Conclusion: These findings support the assumption that cerebral vascular tone might incline towards vasoconstriction and vasodilatation when baseline CBF is low and high between individuals respectively. Although several determinants of cerebral vascular tone have been proposed, the mechanism of such inter-individual differences in cerebral vascular tone is unknown.