[Experimental principles of tolerance of the brain to ischemia]

The resistance of the brain to ischemia depends not only on the duration and severity of flow reduction but also on a number of pre- and post-ischemic metabolic and hemodynamic factors which are able to improve or impair the post-ischemic recovery process. Among pre-ischemic protective factors, the suppression of metabolic rate by drugs or hypothermia, the increase of brain tissue energy reserves and the inhibition of membrane permeability of cations are of particular importance. In contrast, increase of metabolic rate and increase of tissue acidosis induced by hyperglycemia or residual blood flow, reduce the ischemic tolerance of the brain. As long as cell membranes do not depolarize during ischemia, restitution of blood flow results in spontaneous recovery. After depolarization of membranes, however, numerous post-ischemic complications evolve, such as the no-reflow phenomenon, post-ischemic hypoperfusion, post-ischemic brain edema, disturbances of the coupling between metabolic activity and blood flow, etc. These complications require therapeutic intervention in order to prevent irreversible injury. By optimizing this therapy in a model of 1 hour complete normothermic brain ischemia in cat and monkeys, post-ischemic recovery of energy metabolism, protein synthesis, spontaneous and evoked electrocortical activity and even integrative neurological performance were observed. The resistance of the brain to ischemia, in consequence, is much higher than previously assumed and can be substantially improved by adequate post-ischemic treatment.