Recovery of Brain Function Following Ischemia
Experimental evidence conveys clear suggestions that early reperfusion following at least focal cerebral ischemia in the primate is accompanied by a return of function demonstrably suspended during the ischemic period.
Complete and permanent arrest of the cerebral circulation has been known within seconds to lead to depression of brain electrical activity, and within minutes to gross disruption of the normal energy metabolism with failure of ionic homeostatic mechanisms. There is irreversible cell change and death within 5 to 10 minutes.
Very much more protracted periods of ischemia have been shown more recently to be associated with potential viability of neuronal function, and in clinical neurosurgery we have known for years that patients with established cerebral vascular occlusion and a dense neurological deficit may show quite evident improvement over months or years. In these protracted recoveries, the potential for relearning in nervous circuits may play a part, but in more acute circumstances, for example in the progressive recovery from vasospasm, re-learning is clearly not a factor, and this demonstrates quite evidently that neurons at one moment apparently non-functioning, can again within a few minutes recover function even after hours of apparent suppression.
The experimental evidence is fairly well known. In this symposium and elsewhere we have presented a model of experimental occlusion of the middle cerebral artery in primates demonstrating irreversible recovery of electrical function after some 20 minutes of middle cerebral artery occlusion, and reversible recovery of ionic homeostasis after periods of up to an hour. Measurements of regional blood flow has established thresholds for electrical function of around 16 ml/100 g × min and for ionic homeostasis of around 10 ml/100 g × min.
Interestingly, pH changes appear to occur in the region of the higher flow threshold for electrical failure, and the same applies to the early movements of water as assessed by accumulation in tissue.
Many of the biochemical experiments from Hossmann–s group demonstrate disintegration of biochemical aspects, such as protein synthesis during ischemia and the question of free radical generation remains uncertain.
This paper addresses a number of cases of careful monitoring during aneurysm surgery or during the recovery from sub-arachnoid hemorrhage associated with vasospasm in which protracted periods of dysfunction with definable occlusion as assessed by either direct operative observation, angiography, or bedside blood flow measurements with transcranial Doppler. The studies have shown unequivocal recovery in a time scale, which attests to the validity of the experimentally generated hypotheses in man.
KeywordsFocal cerebral ischemia ischemic flow thresholds recovery from ischemia
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