Mechanisms of postischemic brain edema: contribution of circulatory factors

Deep hypothermic circulatory arrest during cardiac surgery: effects on cerebral blood flow and cerebral oxygenation in children

Deep hypothermic circulatory arrest has become an essential technique to allow repair of complex congenital cardiac lesions in children. The arrested state has concerned the surgeon, cardiologist, and anesthesiologist alike, and yet deep hypothermic circulatory arrest has been highly successful with a low incidence of neurologic sequelae. Studies of cerebral blood flow and cerebral oxygenation demonstrate that the arrest hypothermic brain does not develop the immediate postarrest hyperemia or hyperoxia seen in normothermic ischemic brain models. However, both hypothermic and normothermic ischemic brains exhibit hypoperfusion beyond the immediate recirculation period, likely coupled with a reduced cerebral metabolic rate. That the hypothermic arrested brain likely becomes anoxic and recovery of the anoxic brain depends in large part on recirculatory hemodynamics suggests that the lack of hyperemia and hyperoxia may play more major roles than was previously believed. The mechanism of protection may be related to suppression of oxygen free radicals.

The role of neuroeffector mechanisms in cerebral hyperperfusion syndromes

Cerebral hyperperfusion, a state in which blood flow exceeds the metabolic needs of brain, may complicate a number of neurological and neurosurgical conditions. It may account for the propensity with which hemorrhage, cerebral edema, or seizures follow embolic stroke, carotid endarterectomy, or the excision of large arteriovenous malformations, and for some of the morbidity that accompanies acute severe head injury, prolonged seizures, and acute severe hypertension. Hyperperfusion syndromes have in common acute increases in blood pressure, vasodilatation, breakdown of the blood-brain barrier, and the development of cerebral edema. These common features suggest the possibility that they share the same pathogenic mechanisms. It was believed until recently that reactive hyperemia was caused primarily by the generation of vasoactive metabolites, which induced vasodilatation through relaxation of vascular smooth muscle. However, the authors have recently established that the release of vasoactive neuropeptides from perivascular sensory nerves via axon reflex-like mechanisms has a significant bearing upon a number of hyperperfusion syndromes. In this article, the authors summarize their data and discuss possible therapeutic implications for blockade of these nerves or their constituent neuropeptides.

A new model of global postischemic reperfusion in rabbit

An ideal model of global ischemia in rabbits has not yet been developed. The present study describes a new model of global postischemic reperfusion (GPIR) in the rabbit, characterized by lack of systemic hypotension. The experimental procedure involves reversible occlusion of the bilateral internal carotid arteries (ICA) and bilateral external carotid arteries (ECA) for 60 min combined with permanent ligation of bilateral vertebral arteries (VA). This grouping is called 6-artery occlusion (6AO). Sixty minutes after the occlusion, bilateral ICA and bilateral ECA were released for 120 min at which time the experiment was terminated. The results revealed severely depressed EEG activity; Water content of brain tissue increased to 80.33 +/- 1.20% (control 78.28 +/- 0.59%, p less than 0.01); K, Mg, and Zn decreased (p less than 0.05 or p less than 0.01), and were negatively correlated with tissue water content. Na increased (p less than 0.05) and correlated with water content of brain. No significant changes were observed in lipid peroxide (LPO) levels, but the activity of superoxide dismutase (SOD) of brain tissue decreased (p less than 0.01), and was negatively correlated with water content (r = -0.5808, p less than 0.05). These results were compared with those obtained with the model of 4-artery (bilateral common carotid arteries (CCA) and VA) occlusion (4AO) and suggested that the brain damage be more severe with 6AO than with 4AO.

Effect of aminophylline on postischemic edema and brain damage in cats

We attempted to ameliorate postischemic edema and brain tissue injury in cats by administering aminophylline to reduce the reactive hyperemia that supposedly aggravates both these sequelae. Forty-one cats were subjected to 1 hour of middle cerebral artery occlusion and were killed after 3 hours, 3 days, or 14 days of recirculation; one half of the cats received 0.916 ml/kg of a 25 mg/ml solution of aminophylline by infusion at a constant rate via the femoral vein starting 10 minutes before release of the occlusion and continuing for 5 minutes after initiation of recirculation; the other half received saline. Regional cerebral blood flow was monitored by the hydrogen clearance method and water content was evaluated by specific gravity measurements after 3 hours of recirculation; the status of the blood-brain barrier was assessed with Evans blue tracer. Morphologic observations were carried out in cats killed after 3 or 14 days of recirculation. Aminophylline-treated cats killed after 3 hours of recirculation showed significantly reduced hyperemia and edema and no leakage of Evans blue, which was present in all untreated cats killed after 3 hours or 3 days of recirculation. Morphologic observations revealed conspicuously more severe ischemic brain tissue damage in the untreated than in the aminophylline-treated cats after 3 and 14 days of recirculation. Our studies indicate the beneficial effect of administration of aminophylline in the amelioration of postischemic edema and brain tissue injury, which is presumably achieved by reduction of reactive hyperemia.

Disruption of blood-brain barrier following bilateral carotid artery occlusion in spontaneously hypertensive rats. A quantitative study

The present study was designed to clarify the relationship of cerebral blood flow (CBF) to blood-brain barrier (BBB) in the ischemic brains with or without recirculation, which were produced by clipping of both common carotid arteries in spontaneously hypertensive rats. CBF was measured by the hydrogen clearance method and BBB function was evaluated by the permeability of 131I-albumin and Evans blue dye. Cortical CBF was reduced from 48.8 +/- 9.5 to 4.0 +/- 1.2 ml/100 gm/min during 1 hr ischemia and further to 2.6 +/- 0.3 ml/100 gm/min during 3 hrs ischemia, while thalamic CBF was reduced much less from 50.0 +/- 3.6 to 17.9 +/- 6.5 ml/100 gm/min and to 17.5 +/- 11.0 ml/100 gm/min, respectively. There was no increase in permeability to protein tracers observed in such 1 hr or 3 hrs ischemic brain. Both cortical and thalamic CBF were markedly increased 2.5 to 6 fold of resting values at 5 min after recirculation in the 1 hr ischemic brain. In the 3 hrs ischemic brain, however, both CBF were only slightly increased but never restored to the resting level even at 30 min after recirculation. In such reperfused brains, exudation to Evans blue dye was observed in none of 16 animals with 1 hr ischemia, but in 18 of 23 with 3 hrs ischemia. Disruption of BBB was twice more frequent in the cortex (77.8%) than in either thalamus (33.3%) or hippocampus (33.3%). Permeability index of 131I-albumin (brain albumin/blood albumin) was significantly higher in the ischemic areas stained with blue dye (2.07 +/- 0.45%) than in non-ischemic control brain (0.10 +/- 0.01%).(ABSTRACT TRUNCATED AT 250 WORDS)

Relationships between perfusion defects and static brain scan positivity in patients with ischaemic completed stroke: considerations about the origin of the increased uptake

The relation between perfusion defects shown by radionuclide angiography and static brain scan positivity was evaluated in patients with ischaemic completed stroke at various intervals from the onset of symptoms. An inverse relation between radionuclide angiography and static scan positivity was found for the period within 15 days of the onset of symptoms. The possible relation between changes in perfusion and static brain scan positivity is discussed.

The role of hydrostatic pressure in ischemic brain edema

The mechanisms responsible for early prenecrotic ischemic brain edema were investigated in rats by comparing brain metabolism, tissue water (HOH) content, and sodium and potassium ion concentration in brain during ischemia induced by decapitation, by the Pulsinelli-Brierley technique, and by carotid embolization. Although brain metabolic functions were similarly disturbed in all three groups, an increase in brain HOH occurred only in the embolism model, which allowed collateral perfusion. Early ischemic brain edema is therefore dependent upon (1) impaired energy-dependent ion pumps and (2) a hydrostatic pressure gradient from patient vascular lumens. Elevated perfusion pressure increases the extent of this early edema. Induced hypertension causes impairment of blood-brain barrier function, as evidenced by extravasation of Evans blue dye 5 minutes after embolic ischemia, and strikingly increases the extent of macromolecular extravasation 4 hours after ictus. This increased protein leakage is accompanied by elevated HOH content and sodium concentration, as compared to findings in normotensive animals. It is concluded that the use of induced hypertension as a therapeutic modality in patients with acute stroke may be harmful.

Clinical monitoring of intracranial pressure in fulminant hepatic failure

Cerebral oedema is the commonest immediate cause of death in fulminant hepatic failure and an investigation was carried out to determine the value of monitoring intracranial pressure (ICP) and to examine the effects of ICP of dexamethasone therapy and mannitol administration. ICP values in 10 patients at the time of insertion of a subdural pressure transducer (grade IV encephalopathy) averaged 15.5 +/- SD 14.8 mmHg. Despite dexamethansone therapy, which had been started on admission, rises in ICP were subsequently observed in seven of the eight patients who died. In the two patients who survived, the highest reading were 47 and 35 mmHg. Mannitol consistently reversed or arrested ICP rises when pressure was < 60 mmHg. ICP monitoring provides additional information in the managment of patients and is essential if mannitol therapy is to be used.

Local cerebral blood flow following transient cerebral ischemia. I. Onset of impaired reperfusion within the first hour following global ischemia

Using the hydrogen clearance technique, local cerebral blood flow (LCBF) in 22 dogs was estimated at 6 parietal sites prior to and following 5 min of total global ischemia. Ischemia was immediately followed by an initial reactive hyperemia during which the electrocorticogram (ECoG) usually began to recover, and within the first 30 min, most of the LCBF's decreased to subnormal values. This onset of hypoperfusion was accompanied by a concomitant decrease in ECoG activity. Two animals that maintained normal local perfusion after the initial hyperemia recovered ECoG activity quickly. These results suggest that the subsequent poor reperfusion was caused by an increased microvascular resistance rather than by blood aggregates, increased blood viscosity, or a variety of other mechanism which have been proposed. Increased vascular tonus was, at least, partly responsible for the increased vascular resistance. This report supports the hypothesis that impaired reperfusion (which occurs some time after an initial hyperemia) may be responsible for ultimate neuronal death, rather than the period of global ischemic hypoxia per se.

Cerebral blood flow immediately following brief circulatory stasis

Cerebral blood flow was studied in rabbits immediately following complete circulatory stasis of varying duration. Systemic arterial pressure was measured continuously. The postischemic circulation was examined both by an infusion of carbon black and, in separate experiments, by injection of 14C-antipyrine into the blood. We examined the relationship between the duration of stasis, the postischemic arterial pressure, and the amount of cerebral reperfusion. As stasis increased from 5 to 30 min the pressure required to achieve reperfusion of the entire brain rose from 20 to 100 torr. Following even temporary exposure to arterial pressures above 110 torr all areas of the brain were generally reperfused. Blood flow in reperfused brain varied directly with arterial pressure, indicating failure of autoregulation. At normal (preischemic) arterial pressure, postischemic cortical flow was twice the normal rate. The data indicate that the pressure required to initiate flow in ischemic brain increases as the duration of stasis is lengthened and that once flow occurs there will be a significant hyperperfusion unless systemic arterial pressure is lowered to the low normal or hypotensive range.

Survival of rabbits after prolonged cerebral ischemia

Cerebral ischemia was produced by a combination of vascular occlusion and mild systemic hypotension in 2 groups of rabbits. Arterial blood pressure, arterial pH, arterial blood gases, blood glucose and PCV were monitored and recorded before, during and for 3 hours after reperfusion. Return of EEG activity, vasomotor control, spontaneous ventilation and corneal reflex were also recorded. At 4, 8, 12, 24 and 48 hours after reperfusion, the rabbits' neurologic status was assessed according to an arbitrary scale based on motor function. The 2 groups differed in return of reflexes and motor function. Eighty percent of the rabbits ischemic for 20 minutes and 75% of the rabbits ischemic for 30 minutes survived. The graduated response of motor function to cerebral ischemia is attributed to the ventilatory and circulatory support given the rabbits for the first 3 hours after reperfusion. The graduate response of motor function to ischemia supports the suggestion that motor function can be used as an index of neurologic damage.

Total cerebral ischemia: a new model system for the study of post-cardiac arrest brain damage

The pathophysiology of post-cardiac arrest brain damage is not well understood. Many of the model systems presently used to study global ischemia have serious limitations. A new model system for total cerebral ischemia (TCI), using aortic and inferior vena caval occlusion balloons, is described. This model system produces verifiable TCI and avoids surgical invasion of the thorax or the use of vasoactive drugs. It does not impede cerebral venous return and protects the cardiopulmonary system from damage. This model system can be used to study the efficacy of various therapeutic interventions following a standardized CNS global ischemic insult.

Pathophysiological mechanisms of brain edema development: role of tissue factors

In experiments carried out on adult rabbit "chest-head" preparations the volume changes of the exposed brain (BrV) were determined in repeated tests during a controlled increase of the systemic venous pressure (SVP) of about 13 mm Hg. The changes of both SVP and BrV were usually parallel at the onset of the experiments, but when the brains became preedematous hysteresis appeared in the plots of their relationships. The hysteresis increased gradually (sometimes with periods of partial decrease) thus indicating a delay in the draining of blood from the brain's venous system and in the removal of excess extracellular fluid from the cebral tissue. Evidence for water filtration through the capillary walls during increase of the SVP, and, thus, of brain intravascular pressure, was obtained by detecting the dynamics of [Na+] and [K+] in the extracellular fluid of the cerebral cortex by ion-selective electrodes. This process appeared reversible in normal brains while in the preedematous ones the excessive water filtration resulted in brain edema. The preedematous state of the brain is believed to be caused by changes of the mechanical properties of brain tissue and/or by changes in osmolarity.

Relationships among intracranial pressure, blood pressure, and superficial cerebral vasculature after experimental occlusion of one middle cerebral artery

A cranial window conforming to the contours of the underlying cerebral cortical surface was implanted successfully in 18 cats. Subsequently the left middle cerebral artery (MCA) was occluded inside the sealed cranium and changes in the superficial cortical vasculature were related to measurements of intracranial pressure (ICP), measured extradurally, and to the resulting infarcts. Vascular changes early after MCA occlusion were not predictive of the outcome of the occlusion, except for aggregation of formed elements of the blood in arterioles, which was a bad prognostic sign. Secondary reactive hyperemia was not beneficial; increases of ICP suggested that hyperemia led to increased cerebral edema as well as to swelling.

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.