Effect of PaCO2 on hyperemia and ischemia in experimental cerebral infarction

The vascular steal phenomenon is an incomplete contributor to negative cerebrovascular reactivity in patients with symptomatic intracranial stenosis

'Vascular steal' has been proposed as a compensatory mechanism in hemodynamically compromised ischemic parenchyma. Here, independent measures of cerebral blood flow (CBF) and blood oxygenation level-dependent (BOLD) magnetic resonance imaging (MRI) responses to a vascular stimulus in patients with ischemic cerebrovascular disease are recorded. Symptomatic intracranial stenosis patients (n=40) underwent a multimodal 3.0T MRI protocol including structural (T1-weighted and T2-weighted fluid-attenuated inversion recovery) and hemodynamic (BOLD and CBF-weighted arterial spin labeling) functional MRI during room air and hypercarbic gas administration. CBF changes in regions demonstrating negative BOLD reactivity were recorded, as well as clinical correlates including symptomatic hemisphere by infarct and lateralizing symptoms. Fifteen out of forty participants exhibited negative BOLD reactivity. Of these, a positive relationship was found between BOLD and CBF reactivity in unaffected (stenosis degree<50%) cortex. In negative BOLD cerebrovascular reactivity regions, three patients exhibited significant (P<0.01) reductions in CBF consistent with vascular steal; six exhibited increases in CBF; and the remaining exhibited no statistical change in CBF. Secondary findings were that negative BOLD reactivity correlated with symptomatic hemisphere by lateralizing clinical symptoms and prior infarcts(s). These data support the conclusion that negative hypercarbia-induced BOLD responses, frequently assigned to vascular steal, are heterogeneous in origin with possible contributions from autoregulation and/or metabolism.

Mild hypothermia decreases the incidence of transient ADC reduction detected with diffusion MRI and expression of c-fos and hsp70 mRNA during acute focal ischemia in rats

The effects of mild hypothermia on the apparent diffusion coefficient of water (ADC) and expression of c-fos and hsp70 mRNA were examined during acute focal cerebral ischemia. Young adult rats were subjected to 60-min middle cerebral artery occlusion under either normothermia (37.5 degrees C) or hypothermia (33 degrees C). Diffusion-weighted echo-planar magnetic resonance imaging was used to monitor changes in ADC throughout the ischemic period. Perfusion MRI with dysprosium contrast was used at the end of the ischemic period to verify that the occlusion was successful. C-fos and hsp70 mRNA expression were examined with in situ hybridization at the end of the ischemic period. The results indicate that the size of the region that exhibited reduced ADC was smaller during hypothermia than during normothermia. Hypothermia also decreased the frequency of occurrence of transient ADC reductions, especially in dorsal aspects of cortex. Expression of both c-fos and hsp70 mRNA were markedly reduced by hypothermia. Transient ADC reduction and c-fos expression are associated with spreading depression, which is believed to contribute to lesion expansion during acute focal ischemia. The results suggest that part of the neuroprotective effect of hypothermia may be due to a reduced incidence of spreading depression.

Augmentation of blood flow through cerebral collaterals by inhibition of nitric oxide synthase

We examined the influence of nitric oxide (NO) on normal and collateral cerebral blood flow after occlusion of the middle cerebral artery (MCA). Effects of NG-nitro-L-arginine (nitroarginine), an inhibitor of NO synthase, were examined during normotension and hypotension (arterial pressure, 50 mm Hg) in 49 anesthetized dogs. Following a craniotomy, a branch of the MCA was cannulated, and collateral-dependent tissue was identified using the shadow-flow technique. Regional cerebral blood flow was measured with microspheres, and pial artery pressure was measured with a micropipette. Intravenous nitroarginine reduced blood flow to normal cerebrum by approximately 40% (p < 0.05) during normotension and hypotension, with aortic pressure maintained constant after nitroarginine administration. Injection of nitroarginine during hypotension, without control of pressor effects, increased aortic and pial artery pressure approximately twofold. Concurrently, blood flow to normal cerebrum decreased (p < 0.05), while flow to collateral-dependent cerebrum increased (p < 0.05). Phenylephrine was infused during hypotension to increase arterial pressure to values similar to those achieved following nitroarginine. Blood flow to collateral-dependent cerebrum increased (p < 0.05), but flow to normal cerebrum was not altered during infusion of phenylephrine. Thus, inhibition of NO synthase during hypotension increases arterial pressure, decreases blood flow to normal cerebrum, and increases blood flow to collateral-dependent cerebrum. Phenylephrine also increases perfusion pressure and blood flow to collateral-dependent cerebrum, but in contrast to nitroarginine, it does not redistribute blood flow from normal cerebrum.

Reductions in focal ischemic infarctions elicited from cerebellar fastigial nucleus do not result from elevations in cerebral blood flow

To determine whether the neuroprotection elicited from electrical stimulation of the cerebellar fastigial nucleus (FN) is attributable to the elevation in regional cerebral blood flow (rCBF), we compared the effects in spontaneously hypertensive rats of stimulation of the rostral ventrolateral medulla (RVL) or FN on (a) a focal ischemic lesion produced by middle cerebral artery (MCA) occlusion, and (b) the changes in rCBF, measured by laser-Doppler flowmetry for 1.5 h, over regions corresponding to the ischemic core (parietal cortex), penumbra (occipital cortex), and nonischemic area (contralateral parietal cortex). Stimulation of FN for 1 h following MCA occlusion reduced infarction 24 h later by 52%. Stimulation of RVL was ineffective. Changes in the lesion were confined to the penumbra. FN and RVL stimulation comparably and significantly increased rCBF up to 185% in unlesioned animals. Following MCA occlusion, stimulation of FN or RVL and hypercarbia failed to elevate rCBF in the ischemic area but did so in the nonischemic area, even though in the same animals only FN stimulation reduced infarction 24 h later. We conclude that (a) the neuroprotection elicited from FN is not the result of an increase in rCBF but results from another mechanism, possibly reduction of metabolism in penumbra, and (b) the pathways mediating central neurogenic vasodilation and neuroprotection are, in part, distinct.

Regional cerebral blood flow and focal cortical perfusion: a comparative study of 133Xe, 85Kr, and umbelliferone as diffusible indicators

We report that regional CBF determined by the initial slope technique using 133Xe and 85Kr in cats and rabbits can be significantly influenced by the size of the field of measurement. The clearance curves of umbelliferone, a lipid-soluble intracellular pH fluorescent indicator, from a visually avascular 80-micron field were used to cross-correlate rCBF with focal cortical perfusion. Our findings indicate that in the cat, as the gamma or beta detector's field of volume was reduced, regional CBF (rCBF) measured by intraarterially injected 133Xe or 85Kr decreased in value by 33% and 28%, respectively, and the slope of the rCBF-PaCO2 response curve became less steep by 56% and 45%, respectively. Umbelliferone, measuring a much smaller volume of tissue, showed a lower normocapnic flow and a more oblique PaCO2 response curve. In the rabbits studied, the normocarbic rCBFs and the rCBF-PaCO2 response curves measured with the three techniques corresponded to those measured in the cat. These results suggest that large field/volume measurements assess a measure of flow that is a weighted average of several distinct flow compartments and that these compartments differ in their response to changes in PaCO2.

The effect of spontaneous reperfusion on metabolic function in early human cerebral infarcts

Twelve patients were studied within 48 hours of stroke using positron emission tomography to determine cerebral blood flow (CBF), cerebral metabolic rate for oxygen (CMRO2), oxygen extraction fraction, cerebral blood volume, cerebral pH (CpH), and cerebral metabolic rate for glucose (CMRGlc), the last calculated using published normal rate constants (CMRGlc[N]) and those for severe ischemia. In these studies, a cortical region of severe ischemia (I) was outlined, its metabolic and perfusion properties evaluated, and its length measured. The contralateral uninvolved cortical rim (C) in these patients and the cortical rim in 5 older normal patients were used for comparison. The length of region I correlated with the neurological deficit measured independently by a clinical scoring method. The 12 patients fell into two groups: Group I (8 patients), whose CBF in I was 9.3 +/- 2.5 ml/100 gm/min (mean +/- SEM) and was in every patient lower than that in C (33.1 +/- 2.2), and Group 2 (4 patients), whose CBF in I was 42.1 +/- 8.5 ml/100 gm/min and was in every case higher than that in C (28.2 +/- 1.5). In Group I, region I showed a CMRGlc(N)/CMRO2 ratio of 0.22 +/- 0.06 and a CpH of 6.83 +/- 0.06. In Group 2, the same ratio in the region I was 0.58 +/- 0.09 and the CpH was 7.12 +/- 0.05. The CMRGlc (N)/CMRO2 ratio for the contralateral hemisphere was comparable in the two groups. Our data suggest that, within 48 hours of the clinical onset of stroke, the ischemic cortex is already reperfused in one third of patients. Those ischemic regions with persistent hypoperfusion appear acidotic, whereas in the reperfused regions, despite evidence of an increased CMRGlc/CMRO2 ratio, acidosis is not evident. The implications of these findings for therapies proposed in acute human cerebral ischemia are discussed.

Cerebral protection

Cerebral protection from an ischemic/hypoxic insult implies that tissue injury can be controlled or even prevented by certain therapeutic maneuvers. For example, physiological thresholds may be altered so that tissue vulnerability to the insult is reduced, or the intensity of an insult may be blunted by enhancing brain homeostasis. Such a therapeutic maneuver is carotid endarterectomy to improve blood flow in the disordered hemisphere. Alternatively, drugs with protective properties can be used before or even after the insult to "stabilize" injured tissue and prevent the harmful secondary effects that often follow. Various past and present approaches to cerebral protection employing physiological, pharmacological, and surgical intervention are reviewed. The mechanisms by which each allegedly protects the brain from ischemia and hypoxia are discussed briefly. Promising, but not always successful, approaches used in the past have pointed the way for new and more rational therapies. Truly effective protection of the brain from ischemia and hypoxia depends directly upon our capability to explore basic mechanisms of injury and our willingness to measure accurately and objectively the outcome of newly developed protective measures.

Reperfusion of focal ischemia of varying duration: postischemic hyper- and hypo-perfusion

Reperfusion into focal ischemia was studied in 25 cats after middle cerebral artery (mca) occlusion of 15 min to 2 hours duration. Changes in cerebral blood flow (CBF) were followed with the hydrogen clearance method in the center and periphery of the ischemic lesion expected. Postischemic hyperperfusion was found often after 15 and 30 min ischemia and regularly after 60 min mca occlusion. It was followed by normal flow after 15 and 30 min occlusion and by postischemic hypoperfusion after 1 hour ischemia. After 2 hours occlusion hypoperfusion generally was not preceded by hyperperfusion. After 60 min ischemia hyperperfusion could not prevent the development of severe hypoperfusion, but often was accompanied by a marked flow reduction in the periphery of the mca territory. The data indicate that hyperperfusion after ischemic periods lasting 60 min and more induces hypoperfusion in the area itself and in neighbouring regions by affecting perfusion pressure and thereby may enlarge ischemic damage.

Focal cerebral hyperemia in acute stroke. Incidence, pathophysiology and clinical significance

In a consecutive study comprising 41 patients with completed stroke of less than 72 hours duration, cerebral angiography and measurements of the regional cerebral blood flow (rCBF) were performed within 24 hours after admission. The rCBF study was done using the 133-Xenon intracarotid injection method and a 254 multi-detector camera. CT scan was done 24 hours after the rCBF study. Focal cerebral hyperemia was found in 16 patients. The study revealed 3 different types of hyperemia: Border-zone hyperemia, surrounding ischemic areas, was seen in patients with occluded arteries on angiography, presumably resulting from accumulation of acid metabolites in the border-zone of acute infarcts. Postischemic hyperemia was seen in patients without occlusion, presumably due to recanalization of a prior occluded artery. Remote hyperemia was found distant from the infarcted area, presumably due to local tissue pressure on brain tissue. Cortical infarcts (10 patients) all had extensive hyperemic areas. Because the 254 detector camera has an excellent resolution in the cortical surface, our findings strongly suggest that all acute cerebral infarcts are, in fact, associated with hyperemic areas. The hyperemic areas are often extensive and vascular reactivity is commonly impaired. It is suggested that treatment aimed at reducing blood flow in hyperemic areas might improve prognosis.

Survival of the ischemic brain: a progress report

The number of patients with cerebral infarctions increases as the population ages, despite campaigns against hypertension, the greatest risk factor. Cerebral ischemia initiates events that are presumed to defer the stage of irreversible injury. These events cause an increase of perfusion around the central ischemic zone and trigger the Bohr effect, both of which preserve tissue viability. Almost simultaneously, mitochondrial function fails, resulting in insufficient energy for the enzyme systems to control Na and K ion equilibrium. At the same time, protein synthesis slows and cellular respiratory enzymes decrease their activity, initiating an irreversible state of tissue change. Tissue fatty acids increase as a result of dissolution of cell membrane lipoprotein structure. Barbiturates reduce the extent of experimental infarction. Resperine and aminophylline are also effective, but there are no corroborative clinical trials. That ischemic brain damage may be the result of toxic substances in the ischemic tissue represents a new concept.

The effect of arterial hypertension of focal ischemic edema. An experimental study

The middle cerebral artery (MCA) of cats was occluded permanently for 24h to study the influence of arterial hypertension during the early phase of focal ischemia upon the development of endema and changes of the blood-brain barrier (BBB). In normotensive animals MCA occlusion results in a hemispheric weight increase of about 8% and marked water and electrolyte alterations in both the grey and white matter of the MCA territory. The RISA space increases mainly in the grey matter. Hypertension aggravates these changes significantly, whereby water and electrolyte changes in the grey matter are predominantly concerned, while there is a preferential increase of the RISA space in the white matter. It is suggested that arterial hypertension aggravates the ischemic edema and enhances a vasogenic type of edema in the white matter.

Failure of prolonged hypocapnia, hypothermia, or hypertension to favorably alter acute stroke in primates

The effects of induced hypocapnia, hypothermia, and hypertension were surveyed in a primate model of acute stroke during and following a 48-hour period of intensive care. The results were compared to a group of nine control animals previously studied. Hypocapnia (PaCO2=25 torr) was examined in five animals and did not appear to alter the expected mortality, degree of neurological deficit, or frequency of infarction. There was, however, a suggestion that the size of infarction may be reduced. Hypothermia (29 degrees C) in five animals had a detrimental effect in that no animals survived following the intensive care period and all had infarction with massive edema. We speculate that hypothermia caused a sufficient increase in blood viscosity as to compromise collateral flow, thereby accounting for this detrimental effect. Induced hypertension (to 20% above control levels) was abandoned after three animals because of severe systemic effects (cardiac failure and pulmonary edema) resulting in death during the period of intensive care.

Spinal cord blood flow after acute experimental cord injury in dogs

Spinal cord blood flow (SCBF) was measured in dogs before and following acute injury with 300 or 500 g-cm force (GCF). In addition, the responses to high and low PaCO2 and low PaO2 levels were studied. The hydrogen clearance technique was used and 0.3 mm platinum electrodes were placed in grey matter, central white matter or peripheral white matter of the L2 segment. The pre-trauma flows were: grey matter 12.5 +/- 2.7; central white matter 14.4 +/- 3.6 and peripheral white matter 15.1 +/- 4.2 ml/100g/min. Following a 300 GCF injury, a marked and progressive reduction in SCBF occurred in the grey and central white matter. This was present for the subsequent 4 hr of the study. The flow was lower than pre-trauma values during the second hour in the grey matter (9.0 +/- 1.4) and the third hour in the central white matter (10.8 +/- 1.8). By the fifth hour after trauma the flow in the grey matter was 5.0 +/- 3.5 and in the central white matter 9.7 +/- 1.5. In the peripheral white matter the SCBF was 10 +/-3.7 during the third hour but subsequently the flow increased to 11.5 +/- 3.9. Paired t-tests showed that this still significantly lower than pre-trauma levels. Two dogs showed a hyperaemic response which was persistent in one case but only temporary in the other dog. The vasodilatatory effect of CO2 was lost after trauma and in some cases a steal phenomenon was present. The sensitivity to an increase in CO2 was 0.48 +/- 0.23 ml/100g/min Hg before injury and this decreased to 0.0075 +/- 0.137 during the second hour after injury. The vasodilatation to hypoxia (30-40 mm Hg) was also absent but the vasoconstrictor effect to low PaCO2 appeared better preserved. These findings also applied to the peripheral white matter where the SCBF was not significantly reduced. The results were similar but more pronounced after 500 GCF injury. The results show that following injury the central areas of the cord become rapidly and progressively ischaemic. The peripheral white matter does retain a reasonably normal flow depending on the magnitude of the impact force. However, the vessels in all these areas lose their ability to respond to normal physiological stimuli.

Reduced nicotinamide adenine dinucleotide fluorescence and cortical blood flow in ischemic and nonischemic squirrel monkey cortex. 2. effects of alterations in arterial carbon dioxide tension, blood pressure, and blood volume

The fluorescence of reduced nicotinamide adenine dinucleotide (NADH) from cerebral cortex was measured before, during, and after middle cerebral artery (MCA) occlusion and then at death of the animal. In normal cortex, NADH remained constant throughout a wide range of variations in blood pressure and Paco2. In ischemic cortex, NADH levels were higher in hypovolemic hypotensive animals than in normotensive normovolemic animals. Neither hypercapnia nor hypocapnia was effective in decreasing NADH in regions of ischemia, but the latter was associated with a degree of hypotension that interfered with interpretation of data. NADH returned to normal with restoration of flow, supporting the reversibility of this degree of ischemia. The high levels of NADH at death, compared to those during ischemia, are consistent with incomplete ischemia in this model of cerebral infarction.

[Neurological complications of heart surgery. Review of their pathogenesis and bases for their treatment]

São relatadas as complicações neurológicas assinaladas em 320 pacientes submetidos a cirurgias cardíacas, com uma incidência de 7,8%. As etiologias mais comuns encontradas foram a embolia aérea (4,0%) e isquemia cerebral após hipotensão sistêmica (2,4%). Os autores analisam a patogenia das complicações cerebrais e tentam correlacioná-las com o fluxo sangüíneo cerebral, com o metabolismo cerebral e com a dinâmica dos pequenos vasos cerebrais. Uma revisão a respeito dos métodos para tratamento da isquemia cerebral é apresentada.

Cerebral apoplexy (stroke) treated with or without prolonged artificial hyperventilation. 1. Cerebral circulation, clinical course, and cause of death

Fifty patients with severe cerebral apoplexy were treated with artificial hyperventilation of three days' duration facilitated by general anesthesia (barbiturates and muscle relaxants) and instituted within the first day after onset of the attack. After a classification according to carotid angiographical findings, degree of consciousness and focal symptoms, a random allocation was performed so that 24 patients were subjected to moderate hypocapnia (Pa CO CO 2 about 25 mm Hg) and 26 patients to normocapnia (Pa CO CO 2 about 40 mm Hg). Afterward, 21 comparable patients not receiving ventilatory treatment were studied. The clinical course and the mortality rate showed no statistically significant differences between the three groups. All patients studied disclosed a longstanding (12 days) spontaneous hyperventilation.

The ventilation treatment was followed by a low cerebral perfusion pressure and a high rate of pulmonary complications. Autopsies from all groups studied typically showed tentorial herniation and pathological lung changes.

In 32 of the patients without occlusion of the carotid artery the regional cerebral blood flow was measured before hyperventilation treatment was applied. A low mean flow and focal abnormalities (ischemia, hyperemia, vasoparalysis) typically were found and in good correlation to the autopsy findings. In six of 13 patients tested an "inverse steal syndrome" following aminophylline was disclosed.

Cerebral vasodilator therapy in stroke

Cerebral vasodilating agents have been questioned as an effective form of therapy for the stroke patient and even have been considered by some to be harmful. Animal studies show that often vasodilating agents will cause an intracerebral steal, but such a reaction has rarely been demonstrated in stroke patients. Several studies measuring cerebral blood flow in man have shown that vasodilating agents will increase cerebral blood flow even in ischemic regions in some patients. Significant clinical studies have not been carried out to determine whether these agents will alter the natural history of the disease. On the basis of reported studies of the effect of vasodilating agents on the cerebral circulation, it is suggested that further laboratory and clinical studies be performed. These agents could potentially be an effective form of therapy in some patients with occlusive cerebrovascular disease.

Oxygen availability in ischemic brain following hypocarbia and hypercarbia. Polarographical depth electrode recordings in evolving and completed experimental stroke in the monkey

Acutely implanted oxygen-recording polarographical depth electrodes were used to appraise changes in oxygen availability (O 2 a) in the brain of the Rhesus monkey. Modifications of oxygen availability induced by hypocarbia and hypercarbia in the normal and the infarcted brain are described. The most consistent finding was a reversal of the expected effects of low and high Paco 2 values during the acutely ischemic period (evolving stroke) and an incomplete return to the normal response by five days (completed stroke).

Cerebral hemispheric "counter-steal" phenomenon during hyperventilation in cerebrovascular diseases

Use of hyperventilation (HV) was recently suggested for the treatment of acute cerebrovascular insufficiency. There is indeed no general agreement on the effectiveness of hyperventilation even though clinical and experimental findings could support its value in clinical use.

During carotid surgery, hyperventilation was used in order to attenuate cerebral ischemia effects during carotid clamping, and a counter-steal phenomenon affecting a whole cerebral hemisphere was demonstrated.

This suggests the role that hyperventilation may play in the treatment of cerebral ischemia.

Effects of acetazolamide on cerebral ischemia and infarction after experimental occlusion of middle cerebral artery

Acetazolamide was given to five of ten cats for 48 to 54 hours after extradural occlusion of a middle cerebral artery (MCA). At seven to eight days later, measurements of regional cerebral blood flow (CBF) and estimates of the sizes of the ischemic and infarcted areas of the brains were made. Neurological deficits were more severe and the ischemic and infarcted regions were larger in the cats given acetazolamide. Cerebral edema (brain swelling) was present and reactive hyperemia was common in the treated cats, even one week after MCA occlusion. The hypercapnia and decreases of pH of nonischemic brain tissue that are caused by acetazolamide are harmful for ischemic brain tissue, presumably because of vasodilatation in nonischemic brain tissue with resultant increases of intracranial pressure and decreases of CBF of ischemic regions.

Cerebral apoplexy (stroke): pathogenesis, pathophysiology and therapy as illustrated by regional blood flow measurements in the brain

Pathophysiological and pathogenetic concepts, particularly in occlusive cerebrovascular disease, are reviewed and discussed with emphasis on the results of current research. Therapy is discussed in the context of these concepts.

When focal ischemia, with or without cerebral infarction, is not associated with definable arterial occlusion, studies of regional cerebral blood flow strongly support the thromboembolic theory; the arterial defect is relatively transient and is caused by an embolus or thrombus which rapidly disappears (fragments or is lysed). The treatment of transient ischemic attacks by the administration of anticoagulant or surgical reconstruction of the appropriate artery is discussed.

When cerebral infarction is caused by arterial occlusion there is vasomotor paralysis (loss of autoregulation and of reactivity to carbon dioxide). In some instances hypercapnia apparently causes only the vessels in nonaffected brain to dilate so that an increased amount of blood streams to these parts while blood pressure falls in collaterals leading to the focus and blood flow to the infarct is decreased (steal syndrome). If blood flow is decreased to nonaffected brain, by vasoconstriction caused by hyperventilation, increased amounts of blood may be shunted into the infarct (reverse steal) where autoregulation is lost. Vasoconstriction, as a treatment, might be beneficial. However, in patients with severe cerebral infarction no convincing favorable affect has been noted. The potential therapeutic action of hyperventilation in patients with mild cerebral infarction has not been studied.

Extensive (global) changes in the cerebral blood flow in cerebral infarction, certain aspects of intracerebral hemorrhage, and the role of hypertension in cerebrovascular disease are also dealt with.