A model for the regulation of cerebral oxygen delivery

On the basis of the assumption that oxygen delivery across theendothelium is proportional to capillary plasmaPO₂, a model is presented that linkscerebral metabolic rate of oxygen utilization(CMR_O2) to cerebral blood flow(CBF) through an effective diffusivity for oxygen (D) of the capillarybed. On the basis of in vivo evidence that the oxygen diffusivityproperties of the capillary bed may be altered by changes in capillaryPO₂, hematocrit, and/or bloodvolume, the model allows changes in D with changes in CBF. Choice inthe 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 alimiting case of the present model in which  = 0. In contrast to thetrends predicted by the model of Buxton and Frank, in the current modelwhen > 0, the proportionality between changes in CBF andCMR_O2 becomes more linear, and similardegrees of proportionality can exist at different basal values ofoxygen extraction fraction. The model is able to fit the observedproportionalities between CBF and CMR_O2 for a large range ofphysiological data. Although the model does not validate any particularobserved proportionality between CBF andCMR_O2, generally values of(CMR_O2/CMR_O2)/(CBF/CBF) close to unity have been observed across ranges of graded anesthesia inrats and humans and for particular functional activations in humans.The model's capacity to fit the wide range of data indicates that theoxygen diffusivity properties of the capillary bed, which can bemodified in relation to perfusion, play an important role in regulatingcerebral oxygen delivery in vivo.