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Departments of 1 Molecular Biophysics and Biochemistry and 2 Diagnostic Radiology, Yale University, New Haven, Connecticut 06510
On the basis of the assumption that oxygen delivery across the
endothelium is proportional to capillary plasma
PO2, a model is presented that links
cerebral metabolic rate of oxygen utilization
(CMRO2) to cerebral blood flow
(CBF) through an effective diffusivity for oxygen (D) of the capillary
bed. On the basis of in vivo evidence that the oxygen diffusivity
properties of the capillary bed may be altered by changes in capillary
PO2, hematocrit, and/or blood
volume, the model allows changes in D with changes in CBF. Choice in
the model of the appropriate ratio of 
(
D/D)/(CBF/CBF)
determines the dependence of tissue oxygen delivery on perfusion.
Buxton and Frank (J. Cereb. Blood Flow. Metab. 17: 64-72, 1997) recently presented a
limiting case of the present model in which = 0. In contrast to the
trends predicted by the model of Buxton and Frank, in the current model
when > 0, the proportionality between changes in CBF and
CMRO2 becomes more linear, and similar
degrees of proportionality can exist at different basal values of
oxygen extraction fraction. The model is able to fit the observed
proportionalities between CBF and CMRO2 for a large range of
physiological data. Although the model does not validate any particular
observed proportionality between CBF and
CMRO2, generally values of
(CMRO2/CMRO2)/(CBF/CBF) close to unity have been observed across ranges of graded anesthesia in
rats and humans and for particular functional activations in humans.
The model's capacity to fit the wide range of data indicates that the
oxygen diffusivity properties of the capillary bed, which can be
modified in relation to perfusion, play an important role in regulating
cerebral oxygen delivery in vivo.
brain mapping; positron emission tomography; blood oxygenation level dependent; functional magnetic resonance imaging; blood; glucose; lactate; metabolism; perfusion; permeability; diffusivity; conductivity
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