Changes in cerebral vasomotor reactivity to CO 2 and autoregulation following experimental subarachnoid hemorrhage

Regulation of the cerebral circulation: bedside assessment and clinical implications

Regulation of the cerebral circulation relies on the complex interplay between cardiovascular, respiratory, and neural physiology. In health, these physiologic systems act to maintain an adequate cerebral blood flow (CBF) through modulation of hydrodynamic parameters; the resistance of cerebral vessels, and the arterial, intracranial, and venous pressures. In critical illness, however, one or more of these parameters can be compromised, raising the possibility of disturbed CBF regulation and its pathophysiologic sequelae. Rigorous assessment of the cerebral circulation requires not only measuring CBF and its hydrodynamic determinants but also assessing the stability of CBF in response to changes in arterial pressure (cerebral autoregulation), the reactivity of CBF to a vasodilator (carbon dioxide reactivity, for example), and the dynamic regulation of arterial pressure (baroreceptor sensitivity). Ideally, cerebral circulation monitors in critical care should be continuous, physically robust, allow for both regional and global CBF assessment, and be conducive to application at the bedside. Regulation of the cerebral circulation is impaired not only in primary neurologic conditions that affect the vasculature such as subarachnoid haemorrhage and stroke, but also in conditions that affect the regulation of intracranial pressure (such as traumatic brain injury and hydrocephalus) or arterial blood pressure (sepsis or cardiac dysfunction). Importantly, this impairment is often associated with poor patient outcome. At present, assessment of the cerebral circulation is primarily used as a research tool to elucidate pathophysiology or prognosis. However, when combined with other physiologic signals and online analytical techniques, cerebral circulation monitoring has the appealing potential to not only prognosticate patients, but also direct critical care management.

Osmotic therapy: fact and fiction

This review examines the available data on the use of osmotic agents in patients with head injury and ischemic stroke, summarizes the physiological effects of osmotic agents, and presents the leading hypotheses regarding the mechanism by which they reduce ICP. Finally, it addresses the validity of the following commonly held beliefs: mannitol accumulates in injured brain; mannitol shrinks only normal brain and can increase midline shift; osmolality can be used to monitor mannitol administration; mannitol should be not be administered if osmolality is >320 mOsm; and hypertonic saline is equally effective as mannitol.

Persistent autoregulatory disturbance after angioplasty for cerebral vasospasm. A case report

SUMMARY

Hyperdynamic therapy, consisting of hypervolemia, haemodilution, and hypertension, is an established treatment for cerebral vasospasm following subarachnoid haemorrhage. Angioplasty has emerged as an additional, effective treatment for symptomatic vasospasm. Loss of autoregulation, however, can occur despite effective angioplasty, underscoring the need for treatment with hyperdynamic therapy in combination with angioplasty. A 43-year-old woman underwent endovascular coiling of a ruptured left posterior communicating artery aneurysm. The patient went on to develop symptomatic vasospasm and was treated with hyperdynamic therapy and angioplasty. Autoregulation was assessed with xenon CT cerebral blood flow (CBF) measurement. An initial CBF study was obtained when the patient received dopamine and dobutamine infusions to maintain systolic blood pressure at 160 mmHg. The vasopressor drips were then temporarily held for twenty minutes, allowing the patient's systolic blood pressure to drop to 140 mmHg, and a repeat CBF study was obtained. Several days after angioplasty, CBF decreased significantly when the patient was taken off vasopressors, indicating impaired autoregulation. Hyperdynamic therapy was continued, and another CBF study one week later showed a return of autoregulation and normalization of CBF without induced hypertension. Autoregulation is disturbed during vasospasm. Although angioplasty can improve large artery blood flow during vasospasm, hyperdynamic therapy is also needed to maintain cerebral perfusion, particularly in the face of impaired autoregulation. Quantitative CBF measurement permits the maintenance of optimal CBF and monitoring of response to therapy.

Cerebral blood flow autoregulation following subarachnoid hemorrhage in rats: chronic vasospasm shifts the upper and lower limits of the autoregulatory range toward higher blood pressures

We sought to determine whether chronic vasospasm following subarachnoid hemorrhage (SAH) would abolish the cerebral blood flow (CBF) autoregulation in anesthetized Sprague-Dawley rats. SAH was induced by intracisternal injection of autologous blood; in control animals saline was injected instead. CBF was measured 48 h after SAH, that is during chronic vasospasm, by laser-Doppler flowmetry over the frontal cortex under condition of hypertension (SAH, n = 6; control, n = 8) or hypotension (SAH, n = 6; control, n = 6). Hyper- and hypotension were induced by increasing mean arterial blood pressure (MABP) stepwise from 90 to 180 mmHg with phenylephrine (0.1-10 micrograms/min i.v.), or by decreasing it from 90 to 40 mmHg by controlled hemorrhage. An autoregulatory index (AI) expressed as delta CBF (%) per 10 mmHg increase or decrease in MABP was employed to analyze CBF response. CBF remained constant (-7 < AI < 7) at MABPs ranging from 60 to 130 mmHg in the control group and from 70 to 140 mmHg in the SAH group, showing CBF autoregulation. In the SAH group, that is, the upper and the lower limits of autoregulatory range were increased by 10 mmHg (p < 0.05). SAH did not increase intracranial pressure significantly (control 9.2 +/- 0.67 vs. SAH 10.0 +/- 1.05 mmHg, n = 5) 48 h after SAH was induced. These results indicate that, during chronic vasospasm, SAH does not abolish the autoregulation process but raises its lower and upper blood pressure limits. The capacity of spastic cerebral arteries to dilate in case of hypotension decreased, while their tolerance to hypertension increased.

The relationship of blood velocity as measured by transcranial doppler ultrasonography to cerebral blood flow as determined by stable xenon computed tomographic studies after aneurysmal subarachnoid hemorrhage

Transcranial doppler (TCD) ultrasonography is often used to guide the management of patients with subarachnoid hemorrhage (SAH). However, the correlation between increased blood velocity as measured by TCD ultrasonography and angiographic vasospasm was established before the routine use of hypervolemia/hemodilution and administration of nimodipine and did not address blood flow. The relationship of blood velocity as measured by TCD ultrasonography and local cerebral blood flow (LCBF) in SAH managed with these modalities is unknown. Patients presenting with aneurysmal SAH between January 1992 and September 1993 who underwent TCD ultrasonography and xenon computed tomographic (Xe/CT) LCBF studies within 12 hours were retrospectively studied. Fifty patients underwent a total of 94 paired studies, encompassing 709 vascular territories. All were treated with nimodipine and hypervolemia/hemodilution. Hematocrit, blood pressure, and partial carbon dioxide pressure were similar at the time of TCD ultrasonography and Xe/CT measurement of LCBF. When LCBF in the middle cerebral artery (MCA) was < or = 31 ml/100 g/min, the corresponding peak systolic velocity measured by TCD ultrasonography was 119 cm/s, whereas those > 31 ml/100 g/min had a velocity of 169 cm/s (P = 0.006). High LCBF was associated with high velocity in all vascular territories, reaching significance in all but the internal carotid artery. At the time of each study, 41 neurological examinations were focal and 53 were nonfocal. The Xe/CT measurement of LCBF in the MCA contralateral to a deficit was significantly less than in territories without corresponding clinical deficits (P = 0.01), whereas peak systolic velocities in the MCA were not significantly different (P = 0.71). Territories with increases in blood velocity in the MCA of > 50 cm/s/24 h did not have statistically different LCBF (P = 0.183). Our results suggest that increased blood velocity revealed by TCD ultrasonography correlates with increased LCBF and not with ischemia. No difference in LCBF was found in territories with and without rapid increases in blood velocity in the MCA. Furthermore, although focal neurological deficits corresponded with decreased contralateral LCBF in the MCA, increased velocity did not correlate with neurological findings. Therapeutic decisions based solely on blood velocity revealed by TCD ultrasonography might be inappropriate and potentially harmful. Xe/CT studies of LCBF are useful in guiding the management of SAH.

Cerebral oedema after subarachnoid haemorrhage. Pathogenetic significance of vasopressin

The authors report the frequency, characteristic clinical symptoms, laboratory alterations and diagnostic criteria of the syndrome of inappropriate secretion of antidiuretic hormone (SIADH) after subarachnoid haemorrhage. The data on 290 patients with subarachnoid haemorrhage (SAH) during a period of years at the Division of Neurosurgery, University Medical School, Szeged, are analysed. Twenty-seven (9.3%) patients developed SIADH. Thirteen (4.5%) patients had severe and 14 (4.8%) had mild SIADH. The problems of the treatment are discussed in detail and the different therapeutic methods are listed: NaCl infusion, water withdrawal and administration of Dilantin, diuretics, mineralocorticosteroids, lithium and demeclocycline. The undesirable side-effects observed accompanying various therapeutic regimen are analysed. The introduction of V2 antagonists into clinical practice appears to be a most perspective procedure. For study of the pathogenesis of SIADH following SAH, the possibility of treatment with V2 antagonists on an experimental model of SAH in rat was created. A significant water retention and increases in brain water and sodium content were observed in rats with SAH. Plasma AVP levels were also elevated after SAH. AVP plays an important role in the development of antidiuresis following water loading and disturbance of the brain water and electrolyte balance after SAH. Water retention and the higher brain water and sodium accumulation could be totally prevented by administration of a V2 antagonist. These results demonstrate that cerebral oedema generated by artificial cerebral bleeding in rats is significantly reduced following the administration of a highly specific V2 antagonist, suggesting a new approach to the treatment of SIADH.

Altered cerebrovascular CO2 reactivity following subarachnoid hemorrhage in cats

The authors tested the hypothesis that cerebral blood flow (CBF) reactivity to CO2 was blunted following subarachnoid hemorrhage (SAH). Subarachnoid hemorrhage was produced in five cats by performing four cisterna magna injections of blood in each (SAH Group). A second group of six cats was treated with an antifibrinolytic agent (AF) in addition to four cisterna magna blood injections (SAH+AF Group). Four cats received AF and four cisterna magna injections of saline (Control Group). The presence or absence of basilar artery vasospasm was determined by comparing baseline and follow-up selective angiograms. Cerebral blood flow reactivity was determined by randomly varying the concentration of inspired CO2 to alter PaCO2 from 20 to 75 mm Hg. Regional CBF was measured with radiolabeled microspheres. Basilar artery vasospasm was seen following subarachnoid injection of blood but not of saline. Normocapnic CBF was similar in all three groups in the brain stem (mean +/- standard error of the mean: SAH Group 46 +/- 6, SAH+AF Group 46 +/- 6, and Control Group 44 +/- 9 ml/min/100 gm) and in the supratentorial compartment (SAH Group 53 +/- 8, SAH+AF Group 61 +/- 9, and Control Group 51 +/- 13 ml/min/100 gm). At intermediate levels of hypercarbia (PaCO2 50 +/- 3 mm Hg), CBF increased similarly in all three groups (SAH Group 161% +/- 32%, SAH+AF Group 118% +/- 33%, and Control Group 174% +/- 19% compared to baseline); at higher levels of PaCO2 (60 +/- 3 mm Hg), CBF values were SAH Group 265% +/- 50%, SAH+AF Group 205% +/- 47%, and Control Group 159% +/- 30% of baseline. At the highest level of PaCO2 (75 +/- 6 mm Hg), supratentorial CBF did not increase as much in the SAH+AF Group as in the Control Group (179% +/- 59% vs. 463% +/- 58% of baseline, respectively). The authors conclude that, in this model of SAH, there is no change in normocapnic CBF; however, blood flow reactivity to hypercarbia is blunted. It is possible that this may result from a combination of narrowing of proximal large vessels and globally impaired reactivity of small vessels.

Time course of the impairment of cerebral autoregulation during chronic cerebral vasospasm after subarachnoid hemorrhage in primates

The time course of the impairment of cerebral autoregulation during chronic cerebral vasospasm after subarachnoid hemorrhage was studied in 18 monkeys. Changes in cerebral blood flow (CBF) at the regional level and central conduction times during either graded hypo- or hypertension were evaluated in these animals at three stages (3, 7, and 14 days) following the introduction of an autologous blood clot around the right middle cerebral artery (MCA). Angiograms revealed a reduction in vessel caliber (compared to the baseline level in the involved MCA) of 30% at 3 days, 50% at 7 days, and 10% at 14 days. At all stages, CBF remained constant at mean arterial blood pressures (MABP) of 60 to 160 mm Hg in the noninvolved hemisphere. In contrast, at the 3- and 7-day stages, there was an impairment of autoregulation in the involved hemisphere at MABP of 40 to 180 mm Hg. The right hemispheric CBF was significantly (p less than 0.05) lower than that in the left throughout the period of investigation at MABP below 120 mm Hg, but rose to exceed the left CBF at MABP above 180 mm Hg at the 7-day stage and 160 mm Hg at the 14-day stage. The right-sided central conduction time showed significant (p less than 0.05) prolongation at MABP below 60 mm Hg at the 3-day stage and 40 mm Hg at the 7-day stage. It is suggested that these results may help to develop guidelines for hemodynamic therapy for vasospasm in its various stages.

Cerebrovascular CO2 reactivity during delayed vasospasm in a canine model of subarachnoid hemorrhage

While the in vitro reactivity of cerebral conducting vessels following subarachnoid hemorrhage has been extensively studied, in vivo cerebrovascular CO2 reactivity has not been systematically investigated. We tested the hypothesis that, in the canine model of subarachnoid hemorrhage, the rise in cerebral blood flow normally seen with hypercapnia is blunted during delayed vasospasm. Four groups of animals were studied: one received two 4-ml subarachnoid injections of nonheparinized arterial blood into the cisterna magna (n = 8), one received three subarachnoid injections of 5 ml blood (n = 5), one received two subarachnoid injections of 4 ml saline (n = 5), and a control group (n = 5) had no subarachnoid injections or angiography. Basilar artery diameter was measured from baseline and follow-up angiography. We determined CO2 reactivity by randomly varying the concentration of inspired CO2 and measuring regional cerebral blood flow with radiolabeled microspheres. Basilar artery diameter was not affected by saline injection and was reduced by 26 +/- 2.9% in the two-hemorrhage group and 55 +/- 1.9% in the three-hemorrhage group. Baseline cerebral blood flow and CO2 reactivity were similar in all four groups. We conclude that, in this model of delayed vasospasm, regional cerebral vascular CO2 reactivity is intact and extrapolation of in vitro data regarding basilar artery diameter and reactivity to cerebral blood flow must be done cautiously.

Clinical vasospasm after subarachnoid hemorrhage: response to hypervolemic hemodilution and arterial hypertension

Delayed neurologic deterioration from vasospasm remains the greatest cause of morbidity and mortality following subarachnoid hemorrhage. The authors assess the incidence and clinical course of symptomatic vasospasm following subarachnoid hemorrhage using a uniform management protocol over a 24-month period. One hundred eighteen consecutive patients were admitted to the neurovascular surgery service within 2 weeks of subarachnoid hemorrhage not attributed to trauma, tumor, or vascular malformation (113 patients had aneurysms). Early surgery was performed whenever possible, and hypertensive hypervolemic hemodilution therapy was instituted at the first sign of clinical vasospasm. Forty-two patients (35.6%) developed characteristic signs and symptoms of clinical vasospasm with angiographic verification of spasm in 39 cases. All patients with clinical vasospasm received hypervolemic hemodilution therapy aiming for a hematocrit of 33-38%, a central venous pressure of 10-12 mm Hg (or a pulmonary wedge pressure of 15-18 mm Hg), and a systolic arterial pressure of 160-200 mm Hg (120-150 mm Hg for unclipped aneurysms) for the duration of clinical vasospasm. Over the course of treatment, 60% of patients with clinical vasospasm had sustained improvement by at least 1 neurologic grade, 24% maintained a stable neurologic status, and 16% continued to worsen. At the end of hypervolemic hemodilution therapy, 47.6% had become neurologically normal, 33.3% had a minor neurologic deficit, and 19% had a major neurologic deficit or were dead. There were 3 instances of cardiopulmonary deterioration (7%), all of which were in patients without Swan-Ganz catheters, and all resolved with appropriate diuresis. One patient rebled and died while on hypervolemic hemodilution therapy.(ABSTRACT TRUNCATED AT 250 WORDS)

Cerebrovascular and metabolic changes during the delayed vasospasm following experimental subarachnoid hemorrhage in baboons, and treatment with a calcium antagonist

A model has been designed in baboons for simulating the clinical situation during the late phase of vasospasm in patients with subarachnoid hemorrhage (SAH). A total amount of 14-33 ml autologous blood was injected into the cisternal system on 3 occasions in the course of 4 days. Neurological symptoms were seen, and the mortality rate was 29%. Angiography 3 days after the last injection showed arterial vasoconstriction amounting to 23% in the vertebro-basilar system, and 11% (right) and 18% (left) in the carotid system. Cerebral blood flow (CBF) measured by the intra-arterial 133Xe technique and the cerebral metabolic rate of oxygen (CMRO2) were reduced by 18% and 11%, respectively. The hypercapnic CBF response was significantly impaired, from a mean of 3.90 ml/100 g/min to 1.72 ml/100 g/min of flow increase for each mm Hg elevation of paCO2. Autoregulation, tested by administration of angiotensin II, was also significantly affected as evidenced by a pressure-dependent increment of CBF during hypertension in 5 out of 7 animals tested. The impaired autoregulation was reflected in the autoregulatory index, which in the whole group increased from 0.06 ml/100 g/min for each mm Hg increase in MABP in the pre-SAH animals to 0.29 ml/100 g/min per mm Hg post-SAH. Treatment with the calcium antagonist, nimodipine (0.5 microgram/kg/min i.v. during 45 min), enhanced CBF significantly by 17% before experimental SAH, whereas after SAH the effect was slight and did not reach statistical significance; CMRO2 was not significantly affected in either group. Intravenous nimodipine combined with hypertension resulted in a marked increase in the autoregulatory index to 1.58 ml/100 g/min per mm Hg in pre-SAH animals and a less pronounced increment to 0.58 ml/100 g/min per mm Hg following experimental SAH. The beneficial effect of nimodipine reported in SAH patients is therefore, in view of our findings, more likely due primarily to a protective mechanism at the cellular level than to an influence on the vascular bed.

The pathogenetic and prognostic significance of blood-brain barrier damage at the acute stage of aneurysmal subarachnoid haemorrhage. Clinical and experimental studies

In a retrospective study, pathological tissue enhancement was found in nearly two fifths of patients with acute SAH on contrast-enhanced cranial computed tomography. By means of absorption measurements with the region of interest technique over the basal ganglia, it was proved indirectly that pathological tissue enhancement should be brought about not only by hyperaemia, i.e., a blood volume increase, but also by extravasation of the contrast material, i.e., blood-brain barrier (BBB) disruption. A similar conclusion was drawn from the retrospective isotope brain scintigraphy study. It was further established that, although the pathological contrast enhancement was most obvious in the cortex, and particularly in the neighbourhood of the subarachnoid spaces, the phenomenon is probably widespread throughout the brain. Patients with abnormal enhancement are likely to be in less favourable clinical grades, have a high incidence of marked or diffuse spasm, have a poorer outcome independent of surgical or conservative treatment, and develop cerebral infarction more frequently. Systemic arterial hypertension was associated with an increased incidence of abnormal enhancement. Pathological tissue contrast enhancement or isotope accumulation in the first few days of SAH may serve as prognostic signs indicative of the late development of vasospasm and ischaemia. As ischaemic disruption of the capillary system is not prominent in the initial days following any stroke, vasoactive substances arising from the breakdown of the blood clot should play important part in the BBB damage in the acute stage of SAH. The "cortical SAH" model developed in the animal experiments ensured a constant subarachnoid blood volume with minimal local brain damage. The intracranial pressure and mean arterial blood pressure did not change significantly, and perfusion defects did not arise. Thus, this model proved suitable for studying the influence on the BBB of vasoactive blood breakdown products (responsible for arterial spasm) without the accompanying effects of pathological conditions such as raised intracranial pressure, systemic hypertension, non-reflow phenomena, which also disrupt the BBB. Measurements on the water, electrolyte, albumin contents of brain tissue, as well as the immunohistochemical localization of albumin, clearly indicated that the brain oedema developing at the acute stage of experimental SAH could be classified as having a primary vasogenic component in addition to the cytotoxic component. This increased capillary permeability was found to be brought about by opening of tight junctions and pinocytosis in the endothelial cells. The pathological capillary permeabilit

Sub-pial infiltration of blood products following experimental subarachnoid haemorrhage

The authors studied the trans-pial penetration of blood products into the cerebral cortex of cats within the first hour following subarachnoid hemorrhage (SAH). Twenty-two animals were subjected to SAH by cisternal injection of blood, and 13 were used as controls. The brains were fixed in situ and the histological sections were examined under the fluorescence microscope. The cerebral parenchyma of the control animals displayed a homogeneous greenish autofluorescence. In the animals with SAH, the subarachnoid blood showed a yellow autofluorescence that also penetrated into the superficial cortex. The possible clinical implications of the observation are discussed.

The blood-brain barrier following experimental subarachnoid hemorrhage. Part 2: Response to mercuric chloride infusion

Under controlled physiological conditions, fresh blood was injected into the cisterna magna of 10 adult cats to produce subarachnoid hemorrhage (SAH). Damage to the blood-brain barrier (BBB) was induced 30 minutes after SAH by the intracarotid injection of a 6 x 10(-5)M solution of mercuric chloride (HgCl2). A control series of five cats received the same injection of HgCl2. Intravenously injected Evans blue dye was used to indicate areas of BBB damage. The lesions were confirmed by fluorescence microscopy. All control animals showed BBB damage in the hemisphere injected with HgCl2. Of the animals in the test group with SAH, 90% were free from lesions. When lesions were present, the distribution differed from that in the control group. These results bear a similarity to the reported absence of HgCl2 lesions during the acute stages after total cerebral ischemia. This suggests that the cellular components of the BBB participate in a general metabolic inhibition following SAH.

The blood-brain barrier following experimental subarachnoid hemorrhage. Part 1: Response to insult caused by arterial hypertension

In three groups of cats, the authors studied the effect of subarachnoid hemorrhage (SAH) on the permeability of the blood-brain barrier (BBB) to the penetration of Evans blue-protein complex. One group received arterial hypertension alone, one group SAH alone, and one group SAH followed by arterial hypertension. Animals subjected to arterial hypertension alone showed areas of BBB breakdown. However, when cats were rendered hypertensive after SAH, there were no demonstrable BBB lesions. The SAH was produced by intracisternal injection of whole blood and hypertension by the intravenous injection of metaraminol. The preservation of the BBB after SAH is discussed. Vasospasm is considered as a possible hemodynamic variable responsible for the protection of the BBB from hypertensive damage. The need for a new model is proposed to further investigate the state of the BBB after SAH.

Experimental cerebral vasospasm: resolution by chlorpromazine

Vasospasm of the cat basilar artery was produced by electrical, mechanical, or chemical stimuli or by subarachnoid hemorrhage. The vasospasm induced by these stimuli was relieved by the topical application of chlorpromazine to the vascular wall. Chlorpromazine appears to be a nonspecific vasoparalytic agent. Unlike previously used substances, it is effective in resolving vasospasm caused by mechanical as well as chemical irritation.

Effects of experimental fluid-percussion injury of the brain on cerebrovascular reactivity of hypoxia and to hypercapnia

To test the hypothesis that concussive brain injury interferes with the normal vasodilator response of the cerebral circulation to hypoxemia, 30 cats were subjected to mild (PaO2 50 mm Hg) and severe (PaO2 30 mm Hg) hypoxemia while measurements were made of arterial and intracranial pressure, regional cerebral blood flow (CBF), and arterial blood gases. Ten cats served as controls, 10 were subjected to mild fluid-percussion injury of the brain (0.8 to 1.7 atmospheres (atm)), and 10 to severe injury (2.4 to 4.1 atm). The CBF response to hypercapnia (PaCO2 50 mm Hg) was also tested in most animals, and the response of CBF autoregulation to hemorrhagic hypotension was tested in four animals of each group. Trauma was found to severely attenuate the capacity of CBF to increase during hypoxemia. Responsiveness to hypoxemia appeared to be better preserved in traumatized animals than was autoregulation, but was less robust than the response to hypercapnia.

Autoregulation of cerebral blood flow after experimental fluid percussion injury of the brain

To test the hypothesis that concussive brain injury autoregulation of cerebral blood flow (CBF), 24 cats were subjected to hemorrhagic hypotension in 10-mm Hg increments while measurements were made of arterial and intracranial pressure, CBF, and arterial blood gases. Eight cats served as controls, while eight were subjected to mild fluid percussion injury of the brain (1.5 to 2.2 atmospheres) and eight to severe injury (2.8 to 4.8 atmospheres). Injury produced only transient changes in arterial and intracranial pressure, and no change in resting CBF. Impairment of autoregulation was found in injured animals, more pronounced in the severe-injury group. This could not be explained on the basis of intracranial hypertension, hypoxemia, hypercarbia, or brain damage localized to the area of the blood flow electrodes. It is, therefore, concluded that concussive brain injury produces a generalized loss of autoregulation for at least several hours following injury.

Delayed effects of subarachnoid haemorrhage on cerebral metabolism and the cerebrovascular response to hypercapnia in the primate

A technique is described for the production of subarachnoid haemorrhage in baboons and their subsequent recovery for chronic study of cerebrovascular reactivity. The baboons make complete neurological recoveries but the response of their cerebral circulation to hypercapnia is impaired one week later. Baseline values of cerebral blood flow and of cerebral oxygen consumption are unaffected at this time. There is no evidence of hypoxic brain damage.

Noninvasive measurement of cerebral vasopasm in patients with subarachnoid hemorrhage

Regional cerebral blood flow (rCBF) was measured as fast flow clearance (F1) and the initial slope index (ISI2) after inhalation of 133Xe in 30 patients with subarachnoid hemorrhage (SAH). Vasomotor responsiveness to reduction in end-tibal PECO2 was examined in those patients who could carry out this procedure satisfactorily as a test for the presence or absence of vasospasm. F1 and ISI2 were significantly reduced in patients with recent SAH compared to 35 age-matched normal volunteers. The degree of reduction of F1 and ISI2 correlated directly with severity of the neurological deficit graded according to the Hunt and Hess rating scale. Topographic reductions of rCBF correlated with angiographically demonstrated vasospasm or intracerebral hematoma. The degree of impairment of cerebral vasomotor responsiveness to reduction of PECO2 by hyperventilation also correlated with the severity of vasospasm demonstrated angiographically in 16 patients. The reductions of rCBF values were maximal during the first week after SAH but returned gradually toward normal by the 5th week. Individual patients with SAH whose lowest F1 values were above 50 ml/100 g brain/min tolerated surgical intervention best. Non-invasive measurements of rCBF after SAH appear to be helpful in estimating the presence and time course of vasospasm, in recognizing the development of normal pressure hydrocephalus, and in planning medical and surgical management.

Late effects of subarachnoid haemorrhage on the response of the primate cerebral circulation to drug-induced changes in arterial blood pressure

The ability of the cerebral circulation to maintain a constant level of cerebral blood flow with halothane-induced hypotension and angiotensin-induced hypertension was found to be impaired one week after subarachnoid haemorrhage in the baboon. No evidence of hypoxic brain damage was found. The clinical significance of these findings is discussed.

Cerebral arterial responses to induced hypertension following subarachnoid hemorrhage in the monkey

Regional cerebral blood flow (rCBF), angiographic cerebral arterial caliber, and cerebrospinal fluid (CSF) pressure were measured in rhesus monkeys to determine the effect of experimentally induced subarachnoid hemorrhage (SAH) on cerebral arterial responses to graded increases in blood pressure. These measurements were also performed in a control group of monkeys subjected to a mock SAH by injection of artificial CSF into the cerebral space. Before subarachnoid injection of blood or artificial CSF, graded increases in mean arterial blood pressure (MABP) to a level 40% to 50% above baseline values had no effect on rCBF. The major cerebral arteries constricted and CSF pressure remained unchanged. Similar responses were observed after injections of artificial CSF. When MABP was increased in animals that had been subjected to subarachnoid injection of blood, rCBF increased and was associated with dilatation of the major cerebral arteries and moderate increases in CSF pressure. These results demonstrate that cerebral arterial responses to increases in blood pressure may be abnormal in the presence of subarachnoid blood. The manner in which abnormal cerebral arterial reactivity, changes in blood pressure, and vasospasm combine to determine the level of cerebral perfusion following SAH is postulated.

Report of Joint Committee for Stroke Resources. IV. Brain edema in stroke

A classification of brain edema is provided as well as an extensive review of the animal models from which we have derived most of the basic information we have about the formation and resolution of edema. The clinical aspects of cerebral edema in stroke are discussed and also modern methods for identifying cerebral edema in the human. Attention is given to computed tomography and enhanced CT and advances in their application to this condition. Treatment of cerebral edema in the stroke patient using glycerol, dextran 40, mannitol, steroids, and other drugs is discussed and the need pointed out for controlled clinical trials of the therapeutic effectiveness of these agents.

Effect of intracisternal phentolamine on cerebral blood flow after subarachnoid injection of blood

The hydrogen clearance method was used to measure total and focal cerebral blood flow (CBF) in the monkey before and for 5 hours after a simulated subarachnoid hemorrhage (SAH). Some monkeys also received 0.2 to 1.0 mg/kg phentolamine intracisternally 3 hours after SAH. Results show that SAH did not change cerebrovascular resistance, but as cerebral perfusion pressure decreased, CBF fell transiently. Phentolamine injected intracisternally 3 hours after SAH produced a significant fall in arterial blood pressure; cerebrovascular resistance did not change but CBF decreased significantly. These data indicate that intracisternal phentolamine cannot be considered potentially useful to treat ischemic encephalopathy after SAH.

Neurogenic control of cerebral blood flow in the baboon

Cerebral chemical vasomotor reactivity and autoregulation were tested in normal baboons before and after the intravenous or intravertebral infusion of atropine sulfate (0.02 mg/kg). Atropine did not appreciably effect autoregulatory response, but intravertebral injection suppressed the increase of cerebral blood flow (CBF) by inhalation of 5% CO2 and enhanced the decrease of CEF induced by hyperventilation. These changes produced by intravertebral injection of atropine were not observed after intravenous injection. Since the vertebrobasilar arterial system supplies the brain stem and diencephalon, this suggests that a central vasodilator tonus of the cerebral vessels is maintained by the innervation of the cerebral vessels by cholinegic neurons which have their central origin in the brain sterm and diencephalic area.

The effect of a simulated subarachnoid hemorrhage on cerebral blood flow in the monkey

The hydrogen clearance method was used to measure local and total cerebral blood flow (CBF) in the rhesus monkey before and for five hours after a simulated subarachnoid hemorrhage (SAH). CBF remained stable after SAH unless SAH was associated with a fall in cerebral perfusion pressure. In addition, cerebrovascular resistance did not increase after SAH. These results suggest that vasoactive agents in fresh whole blood, and the arterial spasm they produce when added to cerebrospinal fluid (CSF), play only a limited role in the pathogenesis of ischemic encephalopathy that follows an SAH.

Neurogenic control of cerebral blood flow in the baboon. Effects of alpha adrenergic blockade with phenoxybenzamine on cerebral autoregulation and vasomotor reactivity to changes in PaCO2

Cerebral autoregulation was studied in the baboon by increasing and decreasing cerebral perfusion pressure (CPP) before and after intravenous administration (1.5 mg per kilogram) of a long-acting alpha adrenergic blocker, phenoxybenzamine (PBZ). Likewise, cerebral vasomotor reactivity to changes of arterial carbon dioxide tension (Pa CO 2 ) was examined before and after PBZ.

In order to permit quantitative analysis, cerebral autoregulation (A.I.) and chemical vasomotor reactivity (C.I.) were expressed as indices where

A.I.=δCBF/δCPP and C.I.=δCBF/δPa CO 2 .

Following the intravenous injection of PBZ, cerebral autoregulatory vasoconstriction was impaired as CPP was increased. Cerebral vasomotor reactivity to changes in Pa CO 2 was altered both during hyperventilation hypocapnia (HV) and hypercarbia induced by inhalation of 5% carbon dioxide if alterations of CPP brought about by these procedures were taken into consideration. During hypocapnia C.I. was reduced 30% and during hypercarbia C.I. was increased 10%.

It is concluded that PBZ reduces the vasoconstrictor tonus of cerebral vessels during hypocapnia and raised CPP. It also enhances the vasodilator response to CO 2 inhalation.