A MODEL FOR THE REGULATION OF CEREBRAL OXYGEN DELIVERY

On the basis of the assumption that oxygen delivery across the endothe lium is proportional to capillary plasma Pot, a model is presented tha t Links cerebral metabolic rate of oxygen utilization (CMRO2) to cereb ral blood flow (CBF) through an effective diffusivity for oxygen (D) o f 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 Po,, hematocrit, and/or blood volume, the model allows ch anges in D with changes in CBF. Choice in the model of the appropriate ratio of Omega = (Delta D/D)/(Delta CBF/CBF) determines the dependenc e of tissue oxygen delivery on perfusion. Burton and Frank (J. Cereb. Blood Flow. Metab. 17: 64-72, 1997) recently presented a Limiting case of the present model in which Omega = 0. In contrast to the trends pr edicted by the model of Burton and Frank, in the current model when Om ega > 0, the proportionality between changes in CBF and CMRO2 becomes more Linear, and similar degrees of proportionality can exist at diffe rent basal values of oxygen extraction fraction. The model is able to fit the observed proportionalities between CBF and CMRO2 for a large r ange of physiological data. Although the model does not validate any p articular observed proportionality between CBF and CMRO2, generally va lues of (Delta CMRO2/CMRO2)/(Delta CBF/CBF) close to unity have been o bserved across ranges of graded anesthesia in rats and humans and for particular functional activations in humans. The model's capacity to f it the wide range of data indicates that the oxygen diffusivity proper ties of the capillary bed, which can be modified in relation to perfus ion, play an important role in regulating cerebral oxygen delivery in vivo.