The role of cerebral blood flow for the recovery of the brain after prolonged ischemia

Cerebral circulation after cardiac arrest. Microangiographic and protein tracer studies

The effects of ventricular fibrillation and subsequent resuscitation on the microcirculation of the verebral cortex were studied with microangiographic and fluorescent protein tracer techniques. Immediately after revival, a transient period of impaired cerebral perfusion occurred before complete recovery from circulatory arrest was obtained. A circulatory arrest of longer than ten minutes, followed by four to six hours of resuscitation, caused defects of cortical capillary filling in both microangiography and Trypan blue fluorescence. This was considered to represent impairment of cortical perfusion, most likely due to edema. Cardiac arrest up to 12 minutes and subsequent resuscitation per se caused no blood-brain barrier damage. Minimal blood-brain barrier damage occurred in one dog following uncomplicated revival from a 14-minute arrest and in animals with prolonged hypertension post-resuscitation. Up to three consecutive carotid angiographies did not cause blood-brain barrier damage in the postischemic brain.

Coagulopathy following experimental cerebral ischemia

In adult normothermic cats cerebral blood flow was interrupted for 1 hour by clamping the innominate and subclavian arteries. Following ischemia the brains were recirculated with blood, and the coagulation system was investigated by measuring coagulation times and blood content of fibrinogen and platelets. Ischemia induced progressive consumption coagulopathy with an increase in coagulation times and a decrease of platelets and fibrinogen by more than 40%. Coagulopathy was accompanied by a respiratory distress syndrome with a significant increase in the alveolar-arterial carbon dioxide gradient from --3.3 to --13.5 mm Hg. A correlation was found between plasma fibrinogen concentration, cerebral blood flow and electrophysiological function, indicating that a relationship exists between the severity of postischemic coagulopathy and functional recovery following prolonged cerebral ischemia.

Cation activities in reversible ischemia of the cat brain

In normothermic anesthetized cats cerebral blood flow was interrupted completely for one hour by arterial clamping and induced hypotension. The effect of ischemia on the ionic gradients of the cerebral cortex was assayed by determining total cortical electrolytes and by recording the activities of extracellular potassium ([K+i1e) and subarachnoid sodium ions ([Na+])s) with ion-sensitive electrodes. During ischemia [K+]e increased from 3.3+/-0.3 to 56+/-5.4 mEq per liter (means+/-SE) and [Na+]s decreased from 133+/-3.8 to 53+/-5.8 mEq per liter. When the brains were recirculated with blood after one hour's ischemia, [K+]e and [Na+]a gradually returned to normal within 45 minutes. The calculated intracellular uptake of sodium during ischemia amounted to 139 mEq per kilogram dry weight, whereas the intracellular release of potassium was only 64 mEq per kilogram. The increase in intracellular cation was accompanied by a movement of water from the extracellular into the intracellular compartment, causing a reversible shrinkage of the extracellular space from 18.9 to 8.5 vol %. The changes in ionic gradients were related to the development and resolution of ischemic brain swelling, and to the elctrophysiological events during and after ischemia.

Patterns of Changes of Blood Flow and Relationships to Infarction in Experimental Cerebral Ischemia

Regional cerebral blood flow (CBF) was measured in both middle ectosylvian gyri of ten cats by recording the clearance of molecular hydrogen (H 2 ) with implanted polarized electrodes 125 µ in diameter, before and up to seven days after occlusion of the left middle cerebral artery (MCA) with a device implanted in the intact cranium. Five patterns of changes of CBF were recorded with the leftsided electrodes. In eight cats MCA occlusion caused immediate decreases of CBF; in the other two cats CBF values were lowest two days after occlusion, presumably because of ischemic edema. Both persistent severe ischemia and early spontaneous postischemic hyperemia were associated with severe neurological deficits, marked swelling of the left cerebral hemispheres, and large infarcts. Late postischemic hyperemia was associated with less severe deficits, less swelling, and smaller infarcts, but the least severe deficits and smallest infarcts were noted in association with persistent moderate ischemia. No consistent patterns were recorded with the rightsided electrodes in this study. Hyperemia which develops spontaneously or is induced shortly after the onset of cerebral ischemia potentially may be harmful because of secondary increases of cerebral edema.

Cerebral Uptake of Glucose and Oxygen in the Cat Brain After Prolonged Ischemia

In cats, total cerebral ischemia of one hour's duration was produced by arterial clamping and lowering of the blood pressure. Following ischemia the brains were recirculated with blood for various periods between 30 minutes and six hours. CBF and uptake of O 2 and glucose were determined and correlated with the electrophysiological recovery of the brain.

In four of 20 cats recovery was absent because of respiratory or circulatory insufficiency. In 16 cats signs of progressive electrophysiological function returned. These animals exhibited postischemic hyperemia; electrical excitability of the brains recovered within 15 minutes, and the adenylate energy charge returned to more than 95% of the preischemic value within 30 minutes. O 2 uptake was significantly reduced for about one hour, but returned to or above normal as soon as spontaneous ECoG activity began to reappear. Glucose uptake was initially in the normal range until the cerebral glucose stores were replenished, but then followed a time course similar to that of O 2 .

It is concluded that aerobic glycolysis may return after one hour's ischemia if the brain is adequately resupplied with glucose and O 2 .

Recovery of the cortical evoked response following temporary middle cerebral artery occlusion in baboons: relation to local blood flow and PO2

The degree of recovery of the somatosensory cortical evoked response following a period (15 to 65 minutes) of partial ischemia, produced by temporary occlusion of the middle cerebral artery (MCA), was assessed in baboons and related to the local tissue blood flow and PO2 before, during and after the occlusion. Flow was measured using the technique of two-minute hydrogen clearance. Failure of complete recovery of the evoked response was associated with significantly greater depths of ischemia and tissue hypoxia during occlusion, and with significantly greater and persisting tissue hypoxia after occlusion, than complete recovery. Complete recovery of the evoked response also was associated with tissue hyperoxia after occlusion. The reduced postocclusive PO2 levels associated with incomplete recovery of the evoked response suggest that reduced perfusion during ischemia was sufficiently severe to cause some degree of irreversible anoxic damage. The effect of a brief (three to ten minutes) period of ventilation with air (instead of oxygen) under such low-flow conditions was to depress the evoked response significantly further; normally perfused brain, however, was unaffected by this procedure. This finding has clinical implications in regard to normobaric oxygen therapy.

Global ischemia in dogs: cerebrovascular CO2 reactivity and autoregulation

One hypothesis on the pathogenesis of post-ischemic-anoxic encephalopathy is impaired cerebral perfusion or the no-reflow phenomenon. Therapies aimed at preventing the development of this phenomenon are increased cerebral perfusion pressure (CPP) and hyperventilation or hypercapnia. Using a dog model in which we have described the progressive development of post-ischemic (PI) cerebral hypoperfusion after 15 minutes of global ischemia induced by aortic and vena cavae clamping, our aims in this study were to determine during the PI cerebral hypoperfusion period: (1) cerebrovascular reactivity to CO2, and (2) cerebral blood (CBF) autoregulation. Post-ischemic cerebral hypoperfusion to about 50% of normal was not accompanied by raised intracranial pressure (ICP) but cerebrovascular CO2 reactivity was markedly attenuated while maintaining some kind of autoregulatory phenomenon. Cerebral uptake of oxygen was not significantly affected by changing PACO2 from 20 to 60 torr at constant CPP or by changing CPP from 64 to 104 torr at constant PaCO2. These results suggest that increasing both CPP and hypocapnia/hypercapnia would not significantly attenuate PI neurological deficit after global cerebral ischemia. However, in two dogs inadvertently hemodiluted in the PI period, increasing CPP from 50 to 200 torr increased CBF by 200%, suggesting that hemodilution plus increased CPP may be effective therapy for amelioration of post-ischemic-anoxic encephalopathy. The significance of our findings on cerebrovascular CO2 reactivity and autoregulation with respect to the mechanism of the no-reflow phenomenon is discussed.