PATHOPHYSIOLOGY OF ACUTE CEREBRAL-ISCHEMIA - PET STUDIES IN HUMANS

Using positron emission tomography (PET), the combined imaging of cerebral blood flow (CBF), cerebral blood volume (CBV), tissue oxygen tension (PtO2), oxygen extraction fraction (OEF), oxygen consumption (CMRO2) and glucose utilization (CMRglu) has allowed to demonstrate the 3 stages of hemodynamic compromise associated with acute arterial occlusion (1) autoregulation stage (hemodynamic reserve) with elevation of CBV: (2) oligemic stage (OEF reserve) with reduced CBF, elevated OEF and normal CMRO2, and (3) ischemic stage, with reduced CMRO2 and elevated CMRglu/CMRO2 ratio (anaerobic glycolysis). Sequential studies have demonstrated the transition from ischemia to tissue infarction, but also that from ischemia to eventual tissue integrity (penumbra ?); following the ischemic phase, the stage of reperfusion is well demonstrated by PET, as the luxury-perfusion syndrome. PET imaging of tissue pH has revealed that the initial stage of ischemic acidosis is followed by tissular alkalosis at reperfusion. PET has also demonstrated that functional impairment (hypometabolism) occurs at sites distant from that of ischemic injury, an effect that may partly underly the initial clinical expression of, and the ensuing functional recovery from, ischemic stroke.