J. H. Garcia

Transient Focal Ischemia in Subhuman Primates: Neuronal Injury as a Function of Local Cerebral Blood Flow

Unilateral, transient (30, 60, and 120 minutes (min)) middle-cerebralartery (MCA) occlusion was induced via transorbital craniotomy in 11 waking subhuman primates. Local cerebral blood flow (LCBF) was calculated from hydrogen clearance curves obtained through the use of intracerebral platinum microelectrodes. Unilateral MCA occlusion decreased LCBF in the territory of the ipsilateral MCA. Within minutes of the arterial occlusion all monkeys developed contralateral neurologic deficits that began disappearing three hours (h) after reopening the MCA. Regional ischemia, followed by 24 h of reperfusion, produced varying degrees of tissue vacuolation which correlated (r=0.60, p<0.01, n=49) with the percent reduction in LCBF multiplied by the occlusion time. Neurons were classified according to the structural features of their perikaryon. A plot of neuron types versus percent vacuolation suggested that normal neurons become increasingly scalloped under increasingly severe ischemic conditions. The number of scalloped neurons decreased precipitously in areas of marked sponginess coincident with the appearance of irreversibly damaged neurons. Local tissue edema values exceeding 30% correlated with irreversible injury to all neurons in the same area. Regional cerebral ischemia of increasing severity was acompanied by increasing numbers of lethally injured neurons.

The Contribution of Reoxygenation to Ischemic Brain Damage

This study examined the hypothesis that the level of postischemic reperfusion affects the severity of the resulting neuronal necrosis. In rats, tissue Po2% was monitored as an index of flow (reoxygenation) at four cortical sites by chronically implanted platinum electrodes. Twenty minutes of total global cerebral ischemia was followed by 30 min of reoxygenation. The level of reoxygenation was controlled to maintain the Po2 nearly constant at one or more of the cortical electrodes. Tissue from within 400 μm of each of 19 electrode sites among seven rats was evaluated histologically. There was a positive correlation between reoxygenation level and severity of neuronal damage. Perineuronal lucent halo formation, probably representing astrocyte foot process swelling, was negatively correlated with reoxygenation level. This study demonstrates that ischemic neuronal damage was aggravated by increased reoxygenation but that perineuronal swelling, as evidenced by halo formation, was somewhat ameliorated.

The role of tissue acidosis in ischaemic tissue injury: the conpept of the pH integral

Cerebral cortical tissue pH was monitored with an extracellular glass electrode in 32 rats subjected to total global cerebral ischaemia produced by ligation of the basilar and carotid arteries with systemic hypotension for periods of 8 to 60 min. The totality of the ischaemia, and its duration were confirmed by monitoring with a brain tissue O2 electrode. Reperfusion was induced by hypertension and maintained thereafter to exclude delayed ischaemia during 3 h survival after which the rats were sacrificed by perfusion fixation. The severity of tissue pH change was varied by inducing hyperglycaemia in some of the rats. Quantitative counts were made of neurons demonstrating changes reflecting severe ischaemic injury within 500 microns of the electrode tip. For the criterion of an ischaemically injured neuron count greater than 20%, there appeared to be a threshold at about 30 min, and more than 0.8 units change in pH. For quantitative assessment of the ischaemic insult a more satisfactory index was found by combining both time and acidosis as the integral of the pH change during the period of ischaemia. This was found to have a strong correlation with the histologic changes. There was a less strong correlation between the acidosis during reperfusion and the histologic change. Comparing these results with those for 3 rats subjected to 215 min of ischaemia without reperfusion, it appears that most of the effect of acidosis in aggravating ischaemic injury takes place during the first hour of ischaemia with little further aggravation for longer periods.

Experimental Hypoglycemia: Correlation Between EEG Abnormalities and Structural Alterations

Ischemic injury is characterized by a combination of decreased arterial flow, decreased tissue oxygen, limited availability of energy substrates, anaerobic glycolysis, retention of metabolic end-products (such as lactic acid), and depressed function [5]. The latter, in the case of the brain, is usually reflected in disappearance of delta waves in the electroencephalogram [1].