Oxygen extraction fraction

CMRO2 with age, but they remain coupled so that the oxygen extraction fraction remains more or less constant (Blesa et al. 1997). 

It follows from the above that the ratio of cerebral blood flow to cerebral blood volume is a measure of cerebral perfusion reserve (Schumann et al. 1998). Below a ratio of approximately 6.0, even if cerebral blood flow is still normal, vasodilatation and cerebral blood volume are maximal and the reserve is exhausted, as shown by a rising oxygen extraction fraction on PET. 

Around acutely infarcted brain, there is an ischemic penumbra (Astrup et al. 1981). Here the blood flow is low, function depressed and the oxygen extraction fraction high. In other words, there is viable tissue with misery perfusion where the needs of the tissue are not being met The tissue may die or recover, depending on the speed and extent of restoration of blood flow. This concept opens up the possibility of a therapeutic time window during which restoration of flow or neuronal protection from ischemic damage might prevent both immediate cell death and the recruitment of neurons for apoptosis (see Ch. 21). 

During or following a cerebrovascular event, some brain areas may show relative or absolute hyperemia owing to good collateral flow, reperfusion after an occluded artery has been reopened and/or inflammation and vasodilatation in response to hypercapnia. In hyperemic areas, oxygen extraction fraction is low and there is luxury perfusion, indicating that flow is in excess of metabolic requirements, perhaps because the tissue has been irreversibly damaged. 

It is conceivable that patients with impaired cerebral reactivity and raised oxygen extraction fraction are at particular risk of stroke without surgery, and that this impairment can be corrected by carotid endarterectomy, but the studies have been too small to be sure (Schroeder 1988 Naylor et al. 1993b Yonas et al. 1993 Hartl et al. 1994 Yamauchi et al. 1996 Visser et al. 1997 Silvestrini et al. 2000 Markus and Cullinane 2001). Also, we do not know what proportion of strokes in patients with recently symptomatic severe carotid stenosis are actually caused by impaired cerebral reactivity, either as a direct result of low flow or perhaps indirectly as a result of an inadequate collateral circulation to compensate for acute arterial occlusion if it should occur. Nor do we know whether the risk of surgery is higher in these patients and so whether, on balance, carotid endarterectomy will indeed reduce stroke risk any more than in those without impaired reactivity. 

Jones T, Chesler DA, Ter-Pogossian MM (1976) The continuous inhalation of oxygen-15 for assessing regional oxygen extraction fraction in the brain of man.Br J Radiol 49 339-45... 

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