Autoregulation of cerebral blood flow. Studies during drug-induced hypertension in normal subjects and in patients with cerebral vascular diseases

Static autoregulation in humans

A recent publication in JCBFM, "Static autoregulation in humans" gives rise to critical comments. Autoregulation of cerebral blood flow implies that flow is rather constant within a certain blood pressure (BP) range with a lower and an upper pressure limit. The physiology at the two limits is very different, making comparison of the brain's vulnerability at the limits questionable. The recent publication in JCBFM claims a narrow autoregulatory range based on pooling of data from several studies. However, we argue that such pooling blurs the autoregulatory limits. We summarize the classical literature, therefrom we argue for a broad autoregulatory range.

Cerebral Pressure Autoregulation in Brain Injury and Disorders-A Review on Monitoring, Management, and Future Directions

The role of cerebral pressure autoregulation (CPA) in brain injury and disorders has gained increased interest. The CPA is often disturbed as a consequence of acute brain injury, which contributes to further brain damage and worse outcome. Specifically, in severe traumatic brain injury, CPA disturbances predict worse clinical outcome and targeting an autoregulatory-oriented optimal cerebral perfusion pressure threshold may improve brain energy metabolism and clinical outcome. In aneurysmal subarachnoid hemorrhage, cerebral vasospasm in combination with distal autoregulatory disturbances precipitate delayed cerebral ischemia. The role of optimal cerebral perfusion pressure targets is less clear in aneurysmal subarachnoid hemorrhage, but high cerebral perfusion pressure targets are generally favorable in the vasospasm phase. In acute ischemia, autoregulatory disturbances may occur and autoregulatory-oriented blood pressure (optimal mean arterial pressure) management reduces the risk of hemorrhagic transformation, brain edema, and unfavorable outcome. In chronic occlusive disease such as moyamoya, the gradual reduction of the cerebral circulation leads to compensatory distal vasodilation and the residual CPA capacity predicts the risk for cerebral ischemia. In spontaneous intracerebral hemorrhage, the role of autoregulatory disturbances is less clear, but CPA disturbances correlate with worse clinical outcome. Also, in community-acquired bacterial meningitis, CPA dysfunction is frequent and correlates with worse clinical outcome, but autoregulatory management is yet to be evaluated. In this review, we discuss the role of CPA in different types of brain injury and disease, the strengths and limitations of the monitoring methods, the potentials of autoregulatory management, and future directions in the field.

The importance of comorbidities in ischemic stroke: Impact of hypertension on the cerebral circulation

Comorbidities are a hallmark of stroke that both increase the incidence of stroke and worsen outcome. Hypertension is prevalent in the stroke population and the most important modifiable risk factor for stroke. Hypertensive disorders promote stroke through increased shear stress, endothelial dysfunction, and large artery stiffness that transmits pulsatile flow to the cerebral microcirculation. Hypertension also promotes cerebral small vessel disease through several mechanisms, including hypoperfusion, diminished autoregulatory capacity and localized increase in blood-brain barrier permeability. Preeclampsia, a hypertensive disorder of pregnancy, also increases the risk of stroke 4-5-fold compared to normal pregnancy that predisposes women to early-onset cognitive impairment. In this review, we highlight how comorbidities and concomitant disorders are not only risk factors for ischemic stroke, but alter the response to acute ischemia. We focus on hypertension as a comorbidity and its effects on the cerebral circulation that alters the pathophysiology of ischemic stroke and should be considered in guiding future therapeutic strategies.

Aggressive blood pressure treatment of hypertensive intracerebral hemorrhage may lead to global cerebral hypoperfusion: Case report and imaging perspective

Hypoperfusion injury related to blood pressure decrease in acute hypertensive intracerebral hemorrhage continues to be a controversial topic. Aggressive treatment is provided with the intent to stop the ongoing bleeding. However, there may be additional factors, including autoregulation and increased intracranial pressure, that may limit this approach. We present here a case of acute hypertensive intracerebral hemorrhage, in which aggressive blood pressure management to levels within the normal range led to global cerebral ischemia within multiple border zones. Global cerebral ischemia may be of concern in the management of hypertensive hemorrhage in the presence of premorbid poorly controlled blood pressure and increased intracranial pressure.

Myogenic and metabolic feedback in cerebral autoregulation: Putative involvement of arachidonic acid-dependent pathways

The present paper presents a mechanistic model of cerebral autoregulation, in which the dual effects of the arachidonic acid metabolites 20-hydroxyeicosatetraenoic acid (20-HETE) and epoxyeicosatrienoic acids (EETs) on vascular smooth muscle mediate the cerebrovascular adjustments to a change in cerebral perfusion pressure (CPP). 20-HETE signalling in vascular smooth muscle mediates myogenic feedback to changes in vessel wall stretch, which may be modulated by metabolic feedback through EETs released from astrocytes and endothelial cells in response to changes in brain tissue oxygen tension. The metabolic feedback pathway is much faster than 20-HETE-dependent myogenic feedback, and the former thus initiates the cerebral autoregulatory response, while myogenic feedback comprises a relatively slower mechanism that functions to set the basal cerebrovascular tone. Therefore, assessments of dynamic cerebral autoregulation, which may provide information on the response time of the cerebrovasculature, may specifically be used to yield information on metabolic feedback mechanisms, while data based on assessments of static cerebral autoregulation represent the integrated functionality of myogenic and metabolic feedback.

Zinc-finger nuclease knockout of dual-specificity protein phosphatase-5 enhances the myogenic response and autoregulation of cerebral blood flow in FHH.1BN rats

We recently reported that the myogenic responses of the renal afferent arteriole (Af-Art) and middle cerebral artery (MCA) and autoregulation of renal and cerebral blood flow (RBF and CBF) were impaired in Fawn Hooded hypertensive (FHH) rats and were restored in a FHH.1^BN congenic strain in which a small segment of chromosome 1 from the Brown Norway (BN) containing 15 genes including dual-specificity protein phosphatase-5 (Dusp5) were transferred into the FHH genetic background. We identified 4 single nucleotide polymorphisms in the Dusp5 gene in FHH as compared with BN rats, two of which altered CpG sites and another that caused a G155R mutation. To determine whether Dusp5 contributes to the impaired myogenic response in FHH rats, we created a Dusp5 knockout (KO) rat in the FHH.1^BN genetic background using a zinc-finger nuclease that introduced an 11 bp frame-shift deletion and a premature stop codon at AA121. The expression of Dusp5 was decreased and the levels of its substrates, phosphorylated ERK1/2 (p-ERK1/2), were enhanced in the KO rats. The diameter of the MCA decreased to a greater extent in Dusp5 KO rats than in FHH.1^BN and FHH rats when the perfusion pressure was increased from 40 to 140 mmHg. CBF increased markedly in FHH rats when MAP was increased from 100 to 160 mmHg, and CBF was better autoregulated in the Dusp5 KO and FHH.1^BN rats. The expression of Dusp5 was higher at the mRNA level but not at the protein level and the levels of p-ERK1/2 and p-PKC were lower in cerebral microvessels and brain tissue isolated from FHH than in FHH.1^BN rats. These results indicate that Dusp5 modulates myogenic reactivity in the cerebral circulation and support the view that a mutation in Dusp5 may enhance Dusp5 activity and contribute to the impaired myogenic response in FHH rats.

The adaptation of the cerebral circulation to pregnancy: mechanisms and consequences

The adaptation of the cerebral circulation to pregnancy is unique from other vascular beds. Most notably, the growth and vasodilatory response to high levels of circulating growth factors and cytokines that promote substantial hemodynamic changes in other vascular beds is limited in the cerebral circulation. This is accomplished through several mechanisms, including downregulation of key receptors and transcription factors, and production of circulating factors that counteract the vasodilatory effects of vascular endothelial growth factor (VEGF) and placental growth factor. Pregnancy both prevents and reverses hypertensive inward remodeling of cerebral arteries, possibly through downregulation of the angiotensin type 1 receptor. The blood-brain barrier (BBB) importantly adapts to pregnancy by preventing the passage of seizure provoking serum into the brain and limiting the permeability effects of VEGF that is more highly expressed in cerebral vasculature during pregnancy. While the adaptation of the cerebral circulation to pregnancy provides for relatively normal cerebral blood flow and BBB properties in the face of substantial cardiovascular changes and high levels of circulating factors, under pathologic conditions, these adaptations appear to promote greater brain injury, including edema formation during acute hypertension, and greater sensitivity to bacterial endotoxin.

Cerebral autoregulation and acute ischemic stroke

Cerebral autoregulation tightly controls blood flow to the brain by coupling cerebral metabolic demand to cerebral perfusion. In the setting of acute brain injury, such as that caused by ischemic stroke, the continued precise control of cerebral blood flow (CBF) is vital to prevent further injury. Chronic as well as acute elevations in blood pressure are frequently associated with stroke, therefore, understanding the physiological response of the brain to the treatment of hypertension is clinically important. Physiological data obtained in patients with acute ischemic stroke provide no clear evidence that there are alterations in the intrinsic autoregulatory capacity of cerebral blood vessels, except perhaps in infarcted tissue. While it is likely safe to modestly reduce blood pressure by 10-15 mm Hg in most patients with acute ischemic stroke, to date, there are no controlled trial data to indicate that reducing blood pressure is beneficial. There may be subgroups, such as those with persistent large vessel occlusion, large infarcts with edema causing increased intracranial pressure or local mass effect, or chronic hypertension, in which blood pressure reduction may lead to impaired cerebral perfusion in noninfarcted tissue.

Does hemodialysis hurt the brain?

Dialysis patients have a high prevalence of both structural brain abnormalities and cognitive impairment. The pathogenesis of the structural brain abnormalities and cognitive impairment as well the relationship between the two is however less clear. In this editorial, we question whether the hemodialysis procedure itself may be partly responsible for the structural abnormalities noted on imaging studies as well as for the cognitive impairment. We invoke several mechanisms whereby the procedure itself may "hurt" the brain including rapid changes in blood pressure, microembolization, microbleeds, and cerebral edema. We then summarize research questions whose answers may help move the field forward.

Intracranial pressure pulse amplitude during changes in head elevation: a new parameter for determining optimum cerebral perfusion pressure?

OBJECTIVE

During short-term postural changes, the factors determining the amplitude of intracranial pulse pressure (ICPPA) remain constant, except for cerebrovascular resistance (CVR). Therefore, it may be possible to draw conclusions from the ICPPA onto the cerebrovascular resistance (CVR) and thus the relative change in cerebral perfusion pressure (CPP).

METHODS

Age, sex, disease, Glasgow Coma Scale score, placement of ventricular drain, blood gas analysis, and parameters of airway management were prospectively recorded in 40 patients. The changes in intracranial pressure (ICP), CPP, mean arterial pressure (MAP), and ICPPA at head elevations of 0 degrees, 30 degrees, and 60 degrees were measured and analyzed online. Status of cerebrovascular autoregulation was checked using the pressure-reactivity index (PRx).

RESULTS

Altogether 36 subjects fulfilled the study conditions. Three patients had positive PRx indicating disturbed autoregulation and were excluded. Thus, 33 were left for analysis (18 females and 15 males). All of them were sedated and mechanically ventilated with Glasgow Coma scores ranging from 3-8. During change in head elevation from 0 degrees to 60 degrees, we found a significant (p < 0.05) improvement of the ICP, an increase of the ICCPA, a reduction of the MAP, and a decrease in the CPP. Increasing ICPPA was linked to decreasing CPP (0 degrees to 60 degrees, r = -0.42, p < 0.05).

CONCLUSIONS

Head elevation is an important part of the ICP and CPP therapy in neurointensive care. When searching for the patient-specific optimum upper body position, ICPPA may provide additional information. Providing that the cerebral autoregulation is intact, the lowest ICPPA of a patient corresponds to the individual upper body position with the highest CPP.

Autoregulation after ischaemic stroke

OBJECTIVES

Absent outcome data from randomized clinical trials, management of hypertension in acute ischaemic stroke remains controversial. Data from human participants have failed to resolve the question whether cerebral blood flow (CBF) in the peri-infarct region will decrease due to impaired autoregulation when systemic mean arterial pressure (MAP) is rapidly reduced.

METHODS

Nine participants, 1-11 days after hemispheric ischaemic stroke, with systolic blood pressure more than 145 mmHg, underwent baseline PET measurements of regional CBF. Intravenous nicardipine infusion was then used to rapidly reduce mean arterial pressure 16 +/- 7 mmHg and CBF measurement was repeated.

RESULTS

Compared with the contralateral hemisphere, there were no significant differences in the percent change in CBF in the infarct (P = 0.43), peri-infarct region (P = 1.00) or remainder of the ipsilateral hemisphere (P = 0.50). Two participants showed CBF reductions of greater than 19% in both hemispheres.

CONCLUSION

In this study, selective regional impairment of CBF autoregulation in the infarcted hemisphere to reduced systemic blood pressure was not a characteristic of acute cerebral infarction. Reductions in CBF did occur in some individuals, but it was bihemispheric phenomenon that likely was due to an upward shift of the autoregulatory curve as a consequence of chronic hypertension. These results indicate individual monitoring of changes in global CBF, such as with bedside transcranial Doppler, may be useful to determine individual safe limits when MAP is lowered in the setting of acute ischaemic stroke. The benefit of such an approach can only be demonstrated by clinical trials demonstrating improved patient outcome.

Blood pressure management in acute stroke

The optimal management of arterial blood pressure in the setting of an acute stroke has not been defined. Many articles have been published on this topic in the past few years, but definitive evidence from clinical trials continues to be lacking. This situation is complicated further because stroke is a heterogeneous disease. The best management of arterial blood pressure may differ, depending on the type of stroke (ischemic or hemorrhagic) and the subtype of ischemic or hemorrhagic stroke. This article reviews the relationship between arterial blood pressure and the pathophysiology specific to ischemic stroke, primary intracerebral hemorrhage, and aneurysmal subarachnoid hemorrhage, elaborating on the concept of ischemic penumbra and the role of cerebral autoregulation. The article also examines the impact of blood pressure and its management on outcome. Finally, an agenda for research in this field is outlined.

Anoxic injury-associated cerebral hyperperfusion identified with arterial spin-labeled MR imaging

BACKGROUND AND PURPOSE

Anoxic brain injury is a devastating result of prolonged hypoxia. The goal of this study was to use arterial spin-labeling (ASL) to characterize the perfusion patterns encountered after anoxic injury to the brain.

MATERIALS AND METHODS

Sixteen patients with a history of anoxic or hypoxic-ischemic injury ranging in age from 1.5 to 78.0 years (mean, 50.3 years) were analyzed with conventional MR imaging and pulsed ASL 1.0-13.0 days (mean, 4.6 days) after anoxic insult. The cerebral perfusion in each case was quantified by using pulsed ASL as part of the standard stroke protocol. Correlation was made among perfusion imaging, conventional imaging, clinical history, laboratory values, and outcome.

RESULTS

Fifteen of the 16 patients showed marked global hyperperfusion, and 1 patient showed unilateral marked hyperperfusion. Mean gray matter (GM) cerebral blood flow (CBF) in these patients was 142.6 mL/100 g of tissue per minute (ranging from 79.9 to 204.4 mL/100 g of tissue per minute). Global GM CBF was significantly higher in anoxic injury subjects, compared with age-matched control groups with and without infarction (F(2,39) = 63.11; P < .001). Three patients had global hyperperfusion sparing areas of acute infarction. Conventional imaging showed characteristic restricted diffusion in the basal ganglia (n = 10) and cortex (n = 13). Most patients examined died (n = 12), with only 4 patients surviving at the 4-month follow-up.

CONCLUSION

Pulsed ASL can dramatically demonstrate and quantify the severity of the cerebral hyperperfusion after a global anoxic injury. The global hyperperfusion probably results from loss of autoregulation of cerebral vascular resistance.

Blood pressure augmentation in acute ischemic stroke

Although control of hypertension is established as an important factor in the primary and secondary prevention of stroke, management of blood pressure in the setting of acute ischemic stroke remains controversial. Given limited data, the general consensus is that there is no proven benefit to lowering blood pressure in the first hours to days after acute ischemic stroke. Instead, there is concern that relative hypotension may lead to worsening of cerebral ischemia. For many years, the use of blood pressure augmentation ("induced hypertension") has been studied in animal models and in humans as a means of maintaining or improving perfusion to ischemic brain tissue. This approach is now widely used in neurocritical care units to treat delayed neurological deficits after subarachnoid hemorrhage, but its use in ischemic stroke patients remains anecdotal. This article reviews the cerebral physiology, animal models and human studies of induced hypertension as a treatment for acute ischemic stroke. Although there has not been a large, randomized clinical trial of this treatment, the available clinical data suggests that induced hypertension can result in at least short-term neurological improvement, with an acceptable degree of safety.

Blood Pressure Management in Acute Stroke

The optimal management of arterial blood pressure in the setting of acute stroke has not been firmly defined. The different types of stroke--ischemic, intracerebral hemorrhage, and subarachnoid hemorrhage--have different pathophysiologies and require different approaches in terms of blood pressure management in the acute setting. This article reviews the current literature and experience at the authors' institution.

Changes in hemodynamics during isoflurane and propofol anesthesia: a comparison study

OBJECTIVES

Volatile anesthetics are thought to impair cerebral autoregulation more than i.v. anesthetics. However, few comparative studies have been carried out in humans. The aim of our study was to evaluate the differences in cerebral hemodynamic changes after introduction of isoflurane (a volatile anesthetic) and propofol (an i.v. anesthetic).

METHODS

Eighteen consecutive patients submitted to laparoscopic cholecystectomy were selected. After the induction, anesthesia was maintained by isoflurane (one minimum alveolar anesthetic concentration) during the first part of the surgical operation, and then by propofol (5 mg/kg/hour i.v.). Ventilation was adjusted to maintain a constant end-tidal CO(2). Middle artery flow velocity was assessed by means of transcranial Doppler ultrasonography. Arterial blood pressure, heart rate (HR), capnometry, pulse oxymetry, inspired fraction of O(2), and body temperature, were monitored.

RESULTS

Cerebral artery velocity, HR, and mean arterial pressure all significantly increased from baseline after the introduction of isoflurane (p<0.05); the HR and mean arterial blood pressure showed no significant difference between the isoflurane and propofol phases. Isoflurane anesthesia induced a significant increase in cerebral blood velocity. Propofol introduction led to a significant decrease in cerebral artery velocity (p<0.05).

CONCLUSIONS

Propofol but not isoflurane decreased cerebral blood velocity thus restoring cerebral autoregulation and the coupling between cerebral blood flow and cerebral metabolism.

Assessing dynamic cerebral autoregulation after stroke using a novel technique of combining transcranial Doppler ultrasonography and rhythmic handgrip

OBJECTIVES

Dynamic cerebral autoregulation (CA) is impaired after stroke. Methods employed to assess this phenomenon usually involve deliberate alterations in blood pressure (BP) by physical means. We performed a pilot study to assess dynamic CA in acute stroke patients using a novel technique of combining transcranial Doppler (TCD) ultrasonography with rhythmic handgrip.

METHODS

Ten patients with ischaemic stroke in the middle cerebral artery (MCA) territory were studied. We performed continuous recordings of bilateral MCA velocities and used rhythmic handgrip to induce BP oscillations. Changes in autoregulation were indicated by changes in phase shift and gain of MCA velocity in relation to BP. Patients were examined at <7 days, 6 weeks, and 3 months after stroke.

RESULTS

There were no significant differences in phase shift or gain between the affected and unaffected cerebral hemispheres. Combining the results from both hemispheres, there was a trend of increasing phase shift (P=0.04) and decreasing gain (P=0.24) over the first three months after stroke, indicating improving CA. Rhythmic handgrip produced an average percentage change in BP (peak-to-trough) of 10% around the mean, and the frequency of the induced BP oscillations was very similar to that of the rhythmic handgrip.

CONCLUSIONS

Combining TCD with rhythmic handgrip appeared to be a useful technique for assessing dynamic CA and it deserves further studies. In this pilot study, there was some evidence that CA might improve up to 3 months after ischaemic stroke.

Effect of xenon on cerebral autoregulation in pigs

There are little data on the effect of anaesthetic concentrations of xenon on cerebral pressure autoregulation. In this study, we have investigated the effect of 79% xenon inhalation on cerebral pressure autoregulation and CO2 response in pigs. Ten pigs were randomly allocated to receive xenon 79% or halothane anaesthesia, respectively, in a crossover designed study. Halothane was used to validate the experimental set-up. Transcranial Doppler was performed to determine the mean flow velocities in the middle cerebral artery (vMCA) during defined cerebral perfusion pressures and during normo-, hyper- and hypoventilation. The results showed that the inhalation of 79% xenon preserved cerebral autoregulation during conditions of normo-, hyper- and hypoventilation and at different cerebral perfusion pressures in pigs. These results suggest that with the inhalation of xenon, in the highest concentration suitable for a safe clinical use, cerebral autoregulation is preserved.

Stroke precipitated by moderate blood pressure reduction

Rapid lowering of blood pressure can precipitate or worsen ischemic strokes. This usually has been observed in the setting of profoundly lowered pressure and hypotension. We report on six patients in whom ischemic neurologic injury ensued or worsened after moderate reduction of blood pressure by pharmacological treatment. The 6 patients suffered new or worsened ischemic neurologic deficits after receiving oral or intravenous antihypertensive medications, mostly after relatively small doses. Mean arterial blood pressure in these patients was decreased by 25 +/- 7.7%, or 37 +/- 16 mm Hg (mean +/- SD) without resultant hypotension. These cases emphasize the potential hazards of moderate blood pressure reduction by antihypertensive medications in the setting of an acute ischemic stroke or transient ischemic attack (TIA), as well as rapidly treated hypertension even in those who have not yet manifested ischemic symptoms.

Dynamic cerebral autoregulation in patients undergoing surgery for intracranial tumors

OBJECTIVE

Alterations of cerebral perfusion in brain parenchyma adjacent to tumors have been reported in the literature. The aim of this study was to test cerebral autoregulation in patients scheduled for tumor resection.

METHODS

Dynamic cerebral autoregulation was evaluated perioperatively using bilateral transcranial Doppler sonography and the thigh cuff method to alter arterial blood pressure in 50 patients (26 females and 24 males) with a mean age of 49.8 years (range 15-73 years). The alterations of cerebral autoregulation were correlated to size, location and histology of the tumor and the presence of accompanying diseases.

RESULTS

Mean cerebral autoregulation was normal before the induction of anesthesia, after intubation under normoventilation, after intubation under hyperventilation and after surgery on intensive care unit. Location, size or histological classification of the lesion was without influence on autoregulation. The patients with accompanying diseases, such as diabetes mellitus and/or hypertension had significantly lower autoregulation values prior to surgery and a significantly lower increase after hyperventilation.

CONCLUSION

Cerebral autoregulation is preserved in patients with intracranial tumors regardless of tumor size, if the patient's clinical status prior to surgery is good. The influence of accompanying diseases was demonstrable and should be considered in the perioperative patients management.

Impaired cerebral autoregulation 24 h after induction of transient unilateral focal ischaemia in the rat

Cerebral blood flow (CBF) and cerebral autoregulation have been investigated 24 h after transient focal ischaemia in the rat. Cerebral blood flow was measured autoradiographically before and during a moderate hypotensive challenge, to test autoregulatory responses, using two CBF tracers, (99m)Tc-d,l-hexamethylproyleneamine oxide and 14C-iodoantipyrine. Prior to induced hypotension, CBF was significantly reduced within areas of infarction; cortex (28 +/- 20 compared with 109 +/- 23 mL/100 g/min contralateral to ischaemic focus, P = 0.001) and caudate (57 +/- 31 compared with 141 +/- 32 mL/100 g/min contralaterally, P = 0.005). The hypotensive challenge (mean arterial pressure reduced to 60 mmHg by increasing halothane concentration) did not compromise grey matter autoregulation in the contralateral hemisphere; CBF data were not significantly different at normotension and during hypotension. However, in the ipsilateral hemisphere, a significant volume of cortex adjacent to the infarct, which exhibited normal flow at normotension, became oligaemic during the hypotensive challenge (e.g. frontal parietal cortex 109 +/- 15% to 65 +/- 15% of cerebellar flow, P < 0.01). This resulted in a 2.5-fold increase in the volume of cortex which fell below 50% cerebellar flow (39 +/- 34 to 97 +/- 46 mm3, P = 0.003). Moderate hypotension induced a significant reduction in CBF in both ipsilateral and contralateral subcortical white matter (P < 0.01). In peri-infarct caudate tissue, CBF was not significantly affected by hypotension. In conclusion, a significant volume of histologically normal cortex within the middle cerebral artery territory was found to have essentially normal levels of CBF but impaired autoregulatory function at 24 h post-ischaemia.

Impaired cerebral autoregulation after mild brain injury

BACKGROUND

Severe head injury may impair cerebral autoregulation, which can increase the risk of secondary neuronal injury. The likelihood of impairment in autoregulation is assumed to be low with mild head injury. We report here the absence of cerebral autoregulation in a patient who suffered a concussion from an automobile accident 6 days earlier.

METHODS

The patient participated in a clinical study approved by the institutional human subjects review committee, investigating the dose-effect relationship of anesthetics on cerebral autoregulation. The patient was scheduled to undergo repair of a knee injury suffered during a motor vehicle accident, during which she had a concussion. The screening evaluation revealed no evidence of neurologic disease. The test was to be performed three times in each patient: baseline autoregulation measurements during stable fentanyl-nitrous oxide anesthesia, second and third measurements during low dose and high dose of the anesthetic to which the patient was assigned. Autoregulation was tested by increasing the mean systemic blood pressure from 80 mm Hg-100 mm Hg using a phenylephrine infusion while simultaneously recording flow velocity from a middle cerebral artery using transcranial Doppler ultrasonography.

RESULTS

Static autoregulation testing during baseline testing demonstrated complete absence of this homeostatic mechanism and the study was canceled. Repeated testing in the recovery unit after the patient awoke showed identical results.

CONCLUSIONS

Trivial mild head injury may result in loss of cerebral autoregulation. A clinical study of a larger series to document the incidence is warranted.

Disturbance of autoregulation in patients with ruptured intracranial aneurysms: mechanism of cortical and motor dysfunction

Serial measurements of local cerebral blood flow were obtained with inhalation of stable xenon and computed tomography prior to and during induced hypertension with continuous infusion of dopamine (7-15 micrograms/kg/min) in 34 patients who underwent surgery for treatment of subarachnoid hemorrhage due to ruptured intracranial aneurysm. Cerebral vasospasm was detected angiographically in all but one of the patients studied. Vasospasm was not symptomatic in 19 patients, but was in the other 15 patients. Disturbance of autoregulation was observed just after surgical operation. In the latter group, local cerebral blood flow in the territory of the middle cerebral artery and the corona radiata on the craniotomy side reached their lowest values, 25.1 +/- 6.8 mL/100 g/min and 15.7 +/- 1.8 mL/100 g/min, respectively, on days 10-14, and each subsequently increased significantly, to 34.3 +/- 7.3 mL/100 g/min and 19.9 +/- 2.0 mL/100 g/min, respectively, during induced hypertension. In conclusion, cortical dysfunction and motor palsy in the patients studied here were thought to be due to significant reduction in local cerebral blood flow in the cortical territories of the middle cerebral artery and corona radiata, respectively.

Acute cerebral blood flow response to dopamine-induced hypertension after subarachnoid hemorrhage

The effects of dopamine-induced hypertension on local cerebral blood flow (CBF) were investigated in 13 patients suspected of suffering clinical vasospasm after aneurysmal subarachnoid hemorrhage (SAH). The CBF was measured in multiple vascular territories using xenon-enhanced computerized tomography (CT) with and without dopamine-induced hypertension. A territorial local CBF of 25 ml/100 gm/min or less was used to define ischemia and was identified in nine of the 13 patients. Raising mean arterial blood pressure from 90 +/- 11 mm Hg to 111 +/- 13 mm Hg (p < 0.05) via dopamine administration increased territorial local CBF above the ischemic range in more than 90% of the uninfarcted territories identified on CT while decreasing local CBF in one-third of the nonischemic territories. Overall, the change in local CBF after dopamine-induced hypertension was correlated with resting local CBF at normotension and was unrelated to the change in blood pressure. Of the 13 patients initially suspected of suffering clinical vasospasm, only 54% had identifiable reversible ischemia. The authors conclude that dopamine-induced hypertension is associated with an increase in flow in patients with ischemia after SAH. However, flow changes associated with dopamine-induced hypertension may not be entirely dependent on changes in systemic blood pressure. The direct cerebrovascular effects of dopamine may have important, yet unpredictable, effects on CBF under clinical pathological conditions. Because there is a potential risk of dopamine-induced ischemia, treatment may be best guided by local CBF measurements.

Evidence for adaptive autoregulatory displacement in hypotensive cortical territories adjacent to arteriovenous malformations. Columbia University AVM Study Project

We hypothesized that chronic hypotension in normal vascular territories fed by arteriovenous malformation pedicles may reset the lower limit of autoregulation and allow flow to remain constant over a lower pressure range. We studied the effect of increasing systemic mean arterial pressure (SMAP) with phenylephrine on cerebral blood flow using a novel technique. Fourteen patients undergoing 15 procedures were studied before endovascular embolization of arteriovenous malformations under neuroleptic conscious sedation. Mean pressures were transduced via a 1.5-F intracranial microcatheter, which was passed under fluoroscopic guidance into the target feeding artery. The microcatheter was positioned (unwedged) at a point that was relatively hypotensive to systemic pressure but that irrigated normal cortex on angiography; feeding mean arterial pressure (FMAP) and SMAP were recorded. A bolus of 133Xe in saline was injected into the microcatheter, and washout was recorded for 3 minutes by a scintillation detector placed over the vascular territory of the injected pedicle. SMAP was then increased approximately 25 mm Hg by phenylephrine infusion, a second bolus was given, and washout was recorded. After exclusion of the shunt spike, initial slope was calculated. The SMAP (mean +/- standard error) increased from 65 +/- 3 to 89 +/- 2 mm Hg (P < 0.0001), and FMAP increased from 46 +/- 3 to 63 +/- 3 mm Hg (P < 0.0001); cerebral blood flow did not change (40 +/- 2 to 40 +/- 2 ml/100 g per min, P = 0.9199). Dividing the cases on the basis of the baseline FMAP into a "severe" hypotensive group (FMAP = 38 +/- 2; n = 7) and a "moderate" hypotensive group (FMAP = 54 +/- 3; n = 8), cerebral blood flow did not change in either group during phenylephrine challenge. Chronic hypotension does not necessarily result in "vasomotor paralysis" with loss of the ability to vasoconstrict to acute increases in perfusion pressure. Instead, it appears to displace adaptively the lower limit of autoregulation in affected vascular territories by a shift of the autoregulatory curve to the left, conceptually analogous to the adaptive displacement seen with chronic hypertension and its treatment.

Hyperdynamic therapy for focal cerebral ischemia of rats: use of colloidal volume expansion and dobutamine

The effects of hyperdynamic therapy with colloidal volume expansion and pharmacological augmentation of cardiac function with dobutamine on local cerebral blood flow (CBF) and the size of ischemic injury were investigated in rats subjected to 6 h of middle cerebral artery (MCA) occlusion. At 45 min after MCA occlusion, each rat was randomly assigned to one of the following treatment groups: (1) control; (2) hetastarch infusion (HES); and (3) hetastarch plus dobutamine (12 micrograms/kg/min) infusion (HES/DOB). In both the HES and HES/DOB groups, cardiac output and local CBF in ischemic brain markedly increased after treatment and infarction volumes were significantly reduced as compared to the control group. There were, however, no significant differences between both groups apart from a dobutamine-induced tachycardia. Colloidal volume expansion augmented cardiac output, increased CBF in ischemic brain, and substantially modified the extent of ischemic injury. However, the addition of dobutamine did not bring about adjunctive beneficial effects of cardiac performance, CBF, or the degree of ischemic brain damage in the rat focal ischemic model.

Loss of cerebral regulation during cardiac output variations in focal cerebral ischemia

Focal cerebral ischemia was induced in anesthetized macaque monkeys by unilateral middle cerebral artery occlusion. The effect of blood volume expansion by a colloid agent and subsequent exsanguination to baseline cardiac output (CO) on local cerebral blood flow (CBF) was measured by the hydrogen clearance technique in both ischemic and nonischemic brain regions. Cardiac output was increased to maximum levels (159% +/- 92%, mean +/- standard error of the mean) by blood volume expansion with the colloid agent hetastarch, and was then reduced a similar amount (166% +/- 82%) by exsanguination during the ischemic period. Local CBF in ischemic brain regions varied directly with CO, with a correlation coefficient of 0.89 (% change CBF/% change CO), while CBF in nonischemic brain was not affected by upward or downward manipulations of CO. The difference in these responses between ischemic and nonischemic brain was highly significant (p less than 0.001). The results of this study show a profound loss of regulatory control in ischemic brain in response to alterations in CO, thereby suggesting that blood volume variations may cause significant changes in the intensity of ischemia. It is proposed that CO monitoring and manipulation may be vital for optimum care of patients with acute cerebral ischemia.

Safety of hypercapnic challenge: cardiovascular and neurologic considerations

The hemodynamic, cerebrovascular, and neurologic effects of hypercapnia with 4% and 6% CO2 were retrospectively reviewed in 217 patients referred for regional CBF (rCBF) procedures. Inhalation of CO2 significantly increased rCBF, blood pressure, and pulse from baseline. The findings suggest a higher incidence of side effects with 6% CO2 concentration and an equivalent vasoreactivity to 4%. We recommend the use of 4% CO2 for hypercapnic stimulation, and present safety guidelines for its use.

Thiopental loading during controlled hypotension for intracranial aneurysm surgery

In this study we report our clinical experience with supplementary thiopental loading, based on 30 patients undergoing surgery for intracranial aneurysm after a recent episode of subarachnoid haemorrhage. As standard procedure we used pentobarbitone induction, pancuronium relaxation, endotracheal intubation, maintenance with halothane 0.5%, N2O 66% in oxygen, fentanyl, and moderate hypocapnia. A thiopental load of up to 20 mg X kg-1 was supplied while the aneurysm was approached. Satisfactory and well-controlled hypotension was obtained in five cases after thiopental alone, and after thiopental and sodium nitroprusside (SNP) (means +/- s.d.) 1.3 +/- 0.9 microgram X kg-1 X min-1 in the remaining 25 patients. No ECG sign of myocardial ischaemia was observed. One disadvantage was a prolonged recovery period, which in some cases necessitated controlled ventilation for some hours. We conclude that thiopental loading can be used safely as a supplement to neuroanaesthesia for aneurysm surgery.

Beneficial effects of induced hypertension on experimental stroke in awake monkeys

The authors performed a controlled study of induced hypertension therapy for treatment of experimental stroke in unanesthetized monkeys. Ten control and 10 treated animals were subjected to a 4-hour occlusion of the middle cerebral artery (MCA) by an implanted tourniquet. Neurological status and local cerebral blood flow (CBF) were monitored serially. Local CBF was determined by hydrogen clearance in and around the elevated 20% to 40% by intravenous infusion of phenylephrine hydrochloride. Neuropathological evaluation was performed after about 2 weeks. A 4-hour occlusion of the MCA in control animals caused moderate stable neurological deficits, moderate stable decreases in local CBF, and medium-sized infarcts. With induced hypertension, five of 10 treated animals showed neurological improvement, and eight exhibited increased CBF in the ischemic zone. Average infarct size tended to be smaller in the treated group, although the difference did not reach statistical significance. Hemorrhagic infarcts were not observed. In four animals, phenylephrine caused cardiac dysrhythmias and hypotension which were reversed by appropriate measures. In this unanesthetized primate model of moderate experimental stroke, induced hypertension had beneficial effects on neurological status, local CBF, and infarct size without causing hemorrhagic infarction. Induced hypertension may be beneficial for some clinical cases of focal cerebral ischemia.

Regional cerebral blood flow in patients with vertebrobasilar disease

We have investigated the changes in regional cerebral blood flow (rCBF) in patients with occlusive vertebrobasilar disease. Fifty patients who showed severe stenosis or occlusion of the vertebrobasilar artery territory were studied by angiography. Lesions in 13 of these patients were limited to the vertebrobasilar artery (limited VB group), and 37 patients showed combined vertebrobasilar/carotid artery lesions (combined VB-C group). Measurements of rCBF in the group of 50 patients using the intravenous xenon 133 technique showed that there was a significant decrease in the mean rCBF (47.5 +/- 1.4 ml/100 gm/min) as compared with 19 age-matched normal subjects (54.9 +/- 1.8 ml/100 gm/min. There was also a significant difference in mean rCBF between the combined VB-C group (45.6 +/- 1.5 ml/100 gm/min) and a group of 153 patients with limited carotid artery lesions (C group, 49.8 +/- 1.0 ml/100 gm/min). However, no significant difference in mean rCBF was found between the limited VB group and normal subjects. There were no differences in rCBF when evaluated according to the presence or absence of angiographic collateral circulation via the posterior communicating artery (Pcom). Good correlation was found between mean rCBF and direction of collateral flow via the Pcom (p less than 0.05). The value of hemispheric rCBF of patients with carotid system transient ischemic attacks (TIAs) was significantly lower than that of patients with vertebrobasilar system TIAs (p less than 0.02). In cerebral autoregulation tests of 24 patients with occlusive vertebrobasilar disease (7 patients from the limited VB group and 17 patients from the combined VB-C group), 13 of these 24 patients (54%) showed an impairment of autoregulation.

Effect of hypertension on blood-brain barrier. Change after restoration of blood flow in post-ischemic gerbil brains. An electronmicroscopic study

The effect of induced hypertension on the blood-brain barrier (BBB) change in Mongolian gerbils exposed to various periods of ischemia was studied. Evans blue dye was used to determine the BBB change in animals subjected to different levels of hypertension after 3 h ischemia. Horseradish peroxidase (HRP) was used in electronmicroscopic studies of animals subjected to 30 min, 1, 3 or 6 h ischemia and subsequently exposed for 30 min to varying periods and sequences of normo- and hypertension. Furthermore, HRP-labeled vesicle counts were performed in animals from the 30-min ischemia group. Our findings revealed that hypertension, after blood flow restoration following ischemia, induces and/or accelerates BBB damage by enhancing endothelial vesicular and/or tubulo-channel transport.

Angiographic changes to induced hypertension in cerebral vasospasm. Case report

A case of cerebral vasospasm complicating intracranial aneurysm surgery is presented. Angiographic findings under hypertension and normotension revealed a paradoxical response of involved vessels suggesting that normal autoregulation is either lost or overcome by spasm.

Cerebral autoregulation in unconscious patients with brain injury

In 18 unconscious patients with traumatic brain injury, the cerebral autoregulation was tested during the first 2-3 weeks after the acute trauma. Regional cerebral blood flow (rCBF) was measured by the intra-arterial 133xenon washout method before and after an increase of about 20% in the mean arterial blood pressure (MABP) by angiotensin. The difference between MABP and intraventricular pressure (IVP) was used as cerebral perfusion pressure (PP). Simultaneously, ventricular fluid pH, lactate and pyruvate were measured. Regional loss of autoregulation indicated by a 20% flow increase was observed in 29 out of 35 studies (83%), while hemispheric loss of autoregulation was observed in only one study. The results of the autoregulation tests were unrelated to the clinical outcome, the presence of brain-stem lesion, and the ventricular fluid pH, lactate and lactate/pyruvate ratio. In repeated studies, a gradual normalization of the autoregulation was observed about 5 days after the acute trauma.

Cerebral blood flow autoregulation in the rat

Cerebral blood flow autoregulation (CBFA) to changes in perfusion pressure has not been previously reported in the rat. A modification of the Kety and Schmidt technique employing 133Xenon was used to measure cerebral blood flow (CBF) in paralyzed adult Sprague Dawley rats passively ventilated with 70% nitrous oxide and 30% oxygen. At a mean arterial blood pressure (MABP) of 121 +/- 19 mm Hg, and a mean arterial PCO2 of 36.2 +/- 2.9 mm Hg, mean CBF was 103 +/- 22 ml/min/100 gm of brain. CBF responses to hypercarbia were 4.9 ml/min/100 gm per mm Hg change in arterial PCO2. CBF was measured during steady state levels of hypo- and hypertension induced by phlebotomy, or by intravenous metaraminol, over the MABP range of 48-205 mm Hg. From a MABP of 80 to 160 mm Hg. CBF remained nearly constant, indicating the presence of CBFA. However, when MABP exceeded 160 mm Hg, CBF became pressure dependent, indicating a "breakthrough" of autoregulation in acute severe hypertension.

Hyperemia, CO2 responsiveness, and autoregulation in the white matter following experimental spinal cord injury

The authors present the results of a controlled, randomized study of alterations in spinal cord blood flow, CO2 responsiveness, and autoregulation following experimental spinal cord injury in cats. Permanent paraplegia is shown to be associated with persistent hyperemia, loss of CO2 responsiveness, and impaired autoregulation in the white matter at the injury site. Probable mechanisms underlying these changes in spinal cord vasomotor control are discussed. Marked similarities between vascular responses of injured spinal cord and luxury perfusion of the brain are pointed out.

Cerebral arterial blood flow and aneurysm surgery. Part 2: Induced hypotension and autoregulatory capacity

A study of 21 patients was conducted to clarify the autoregulatory capacity in patients subjected to induced hypotension during intracranial surgery for saccular aneurysms. Trimethaphan camsylate (Arfonad) was used for induced hypotension and arterial blood flow was measured with an electromagnetic flow probe on the internal carotid artery or one of its main intracranial branches. In Grade I and II patients the control arterial blood pressure (ABP) ranged from a mean of 90 to 135 mm Hg (average 110 mm Hg), with a lower level of autoregulation (LLAR) from 35 to 85 mm Hg (average 62 mm Hg). Grade III patients had a control ABP of between 105 and 145 mm Hg (average 124 mm Hg) and the LLAR was found to be between 60 and 95 mm Hg (average 76 mm Hg). There was a significant difference between the two groups with regard to both the control ABP and the LLAR. A surprising result obtained from these data was that the average lower autoregulatory range (the difference between control ABP and LLAR) is practically the same in the two groups. A systematic investigation of the upper limit of autoregulation was not possible for ethical reasons. In those few patients in whom spontaneous increase in the ABP made such observations possible, upper limits up to 150 mm Hg with a total autoregulatory capacity of about 75 mm Hg were observed. In some patients, however, lower limits and cing that the upper limit of autoregulation is markedly influenced by several factors.

Correlation between the clinical picture, the EEG and cerebral blood flow after partial occlusion of the middle cerebral artery in man

Quantitative measurements of global and regional cerebral blood flow were performed in 18 patients 1 to 3 days after the onset of symptoms of acute cerebral ischemia due to partial occlusion of the middle cerebral artery and were repeated 21 days thereafter. The first rCBF measurement revealed either an extensive ischemic focus or an ischemic focus with simultaneous reduction of the global blood flow within the corresponding hemisphere in all patients. The later measurements indicated no change in blood flow in the ischemic focus in any case, but there was a slight increase of global cerebral blood flow in 1/3 of the cases. The measurements of cerebral blood flow were correlated with the neurological, psychopathological and EEG findings and in spite of an unaltered cerebral blood circulation, all patients showed a distinct clinical imrovement. Various hypotheses for the return of neurological functions in patients with persistent cerebral ischemia are discussed.

Effect of carotid artery ligation on regional cerebral blood flow in normotensive and spontaneously hypertensive rats

Regional cerebral blood flow (rCBF) was measured in normotensive rate (NTR) and spontaneously hypertensive rats (SHR), in a lightly anesthetized state and with control of PaCO2 by artificial ventilation. Without carotid artery ligation, NTR and SHR showed almost identical rCBF values and distribution, despite significantly elevated levels of blood pressure in SHR. Bilateral carotid artery ligation, however, caused much more pronounced decreases of rCBF (ischemia) in SHR than NTR, in regions supplied by the carotid artery. The reduction of rCBF in SHR was rather homogenous and symmetrical. Mechanisms causing the differences between NTR and SHR are discussed.

Autoregulation in acute focal ischemia. An experimental study

The autoregulatory capacity of areas of the cerebral circulation subjected to ischemia by acute middle cerebral occlusion has been assessed in experimental primates. Autoregulation was tested to a rise in blood pressure induced by aramine, and to a fall in blood pressure induced by exsanguination. Whole hemisphere autoregulation was substantially disturbed due to both increased blood pressure and lowered blood pressure, but fractionation of this response indicated that autoregulation to increased blood pressure was preserved in the parasagittal and intermediate zones of the hemisphere, and totally lost in the region of the sylvian opercula where middle cerebral occlusion had produced the most dense ischemia. In relation to reduced perfusion pressure, autoregulation was again widely impaired and assessment of the degree of impairment by areas indicated no significant difference between the areas of the sylvian opercula and the remainder of the lateral aspect of the hemisphere studied. Where the degree of ischemia in each individual electrode was assessed, however, it appeared that the degree of auto-regulatory loss to decreased perfusion pressure was dependent upon the intensity of ischemia, and autoregulation was partially preserved in electrodes whose immediate post-occulsion flow values were greater than 40% of basal flow. Retransfusion following exsanguination in animals with acute middle cerebral occlusion indicated that there was a linear relationship between the degreee of reperfusion achieved by retransfusion and the intensity of ischemia induced by exsanguination following middle cerebral occlusion. Thus there was some support for the no-reflow phenomenon in intensely ischemic areas.