The physiology of the nervous system is strictly dependent on a close relationship between perfu sion, metabolism, and local function. Any impair ment of one of these variables induces a chain of events that leads to the derangement of the others and to the further secondary aggravation of the ini tial change. In stroke, the aetiopathological event is a temporary or permanent reduction in perfusion beyond the capacity of the cerebral tissue to com pensate and maintain adequate metabolism, and, therefore, function.