Anemia and cerebral outcomes: Many questions, fewer answers

A number of clinical studies have associated acute anemia with cerebral injury in perioperative patients. Evidence Of Such injury has been observed near the currently accepted transfusion threshold (hemoglobin [Hb] concentration, 7-8 g/dL), and well above the threshold for cerebral tissue hypoxia (Hb 3-4 g/dL). However, hypoxic and nonhypoxic mechanisms of anemia-induced cerebral injury have not been clearly elucidated. In addition, protective mechanisms which may minimize cerebral injury during acute anemia have not been well defined. Vasodilatory mechanisms, including h nitric oxide (NO), may help to maintain cerebral oxygen delivery during anemia as all three NO synthase (NOS) isoforms (neuronal, endothelial, and inducible NOS) have been shown to be up-regulated in different experimental models of acute hemodilutional anemia. Recent experimental evidence has also demonstrated an increase in an important transcription factor, hypoxia inducible factor (HIF)-1 alpha, in the cerebral cortex of anemic rodents at clinically relevant Hb concentrations (Hb 6-7 g/dL). This suggests that cerebral oxygen homeostasis may be in jeopardy during acute anemia. Under hypoxic conditions, cytoplasmic HLF-1 alpha degradation is inhibited, thereby allowing it to accumulate, dimerize, and translocate into the nucleus to promote transcription of a number of hypoxic molecules. Many of these molecules, including erythropoietin, vascular endothelial growth factor, and inducible NOS have also been shown to be Lip-regulated in the anemic brain. In addition, HIF-1 alpha transcription can be increased by nonhypoxic mediators including cytokines and vascular hormones. Furthermore, NOS-derived NO may also stabilize HIF-1a in the absence Of tissue hypoxia. Thus, during anemia, HIF-1a has the potential to regulate cerebral cellular responses under both hypoxic and normoxic conditions. Experimental studies have demonstrated that HIF-1 alpha may have either neuroprotective or neurotoxic capacity depending on the cell type in which it is up-regulated. In the current review, we characterize these cellular processes to promote I clearer understanding of anemia-induced cerebral injury and protection. Potential mechanisms of anemia-induced injury include cerebral emboli, tissue hypoxia, inflammation, reactive oxygen species generation, and excitotoxicity. Potential mechanisms of cerebral protection include NOS/NO-dependent optimization of cerebral oxygen delivery and cytoprotective mechanisms including HIF-1 alpha, erythropoietin, and vascular endothelial growth factor. The overall balance of these activated cellular mechanisms may dictate whether or not their up-regulation leads to cytoprotection or cellular injury during anemia. A clearer understanding of these mechanisms may help LIS target therapies that will minimize anemia-induced cerebral injury in perioperative patients.