Reperfusion of the brain after global ischemia: hemodynamic disturbances.

Recovery of the brain after a period of cerebral ischemia depends greatly on the restoration of nutritive blood flow, which, however, may be severely disturbed. Early post-ischemic deficits (no-reflow) multiply with increasing duration of ischemia. The pathophysiology is multifactorial and includes vascular factors (endothelial blebs, compression by swollen glial cells), blood factors (viscosity changes due to erythrocyte sludging, platelet aggregation, blood dehydration), and general cardiovascular factors (post-ischemic hypotension, venous congestion). Treatment of no-reflow requires a combination therapy (e.g., hypertensive flush, small volume hypertonic solutions, fibrinolysis) for interfering with as many of these factors as possible. Delayed post-ischemic hypoperfusion develops after a preceding phase of post-ischemic hyperemia and is characterized by increased vasotonus. Hypoperfusion is associated with a disturbed coupling between brain function, metabolism, and blood flow, and may lead to secondary stimulation of anaerobic metabolism. Causal factors include disturbed blood/vessel wall interactions (expression of adhesion molecules, generation of free radicals) and possibly down-regulation of endothelial nitric oxide synthase. Treatment of post-ischemic hypoperfusion includes neutrophil elimination and free radical scavengers but is still unsatisfactory. Improvement of reperfusion deficits is a challenging task that must be solved before proceeding to specific molecular interventions for the treatment of ischemic cell injury.