Assessment of cerebrovascular autoregulation: changes of highest modal frequency of cerebrovascular pressure transmission with cerebral perfusion pressure

The cerebrovascular response to norepinephrine: A scoping systematic review of the animal and human literature

Intravenous norepinephrine (NE) is utilized commonly in critical care for cardiovascular support. NE's impact on cerebrovasculature is unclear and may carry important implications during states of critical neurological illness. The aim of the study was to perform a scoping review of the literature on the cerebrovascular/cerebral blood flow (CBF) effects of NE. A search of MEDLINE, BIOSIS, EMBASE, Global Health, SCOPUS, and Cochrane Library from inception to December 2019 was performed. All manuscripts pertaining to the administration of NE, in which the impact on CBF/cerebral vasculature was recorded, were included. We identified 62 animal studies and 26 human studies. Overall, there was a trend to a direct vasoconstriction effect of NE on the cerebral vasculature, with conflicting studies having demonstrated both increases and decreases in regional CBF (rCBF) or global CBF. Healthy animals and those undergoing cardiopulmonary resuscitation demonstrated a dose-dependent increase in CBF with NE administration. However, animal models and human patients with acquired brain injury had varied responses in CBF to NE administration. The animal models indicate an increase in cerebral vasoconstriction with NE administration through the alpha receptors in vessels. Global and rCBF during the injection of NE displays a wide variation depending on treatment and model/patient.

Autoregulation assessment by direct visualisation of pial arterial blood flow in the piglet brain

Impairment of cerebrovascular autoregulation (CAR) is common after brain injury, although the pathophysiology remains elusive. The mechanisms of vascular dysregulation, their impact on brain function, and potential therapeutic implications are still incompletely understood. Clinical assessment of CAR remains challenging. Observational studies suggest that CAR impairment is associated with worse outcomes, and that optimization of cerebral blood flow (CBF) by individual arterial blood pressure (ABP) targets could potentially improve outcome. We present a porcine closed cranial window model that measures the hemodynamic response of pial arterioles, the main site of CBF control, based on changes in their diameter and red blood cell velocity. This quantitative direct CAR assessment is compared to laser Doppler flow (LDF). CAR breakpoints are determined by segmented regression analysis and validated using LDF and brain tissue oxygen pressure. Using a standardized cortical impact, CAR impairment in traumatic brain injury can be studied using our method of combining pial arteriolar diameter and RBC velocity to quantify RBC flux in a large animal model. The model has numerous potential applications to investigate CAR physiology and pathophysiology of CAR impairment after brain injury, the impact of therapeutic interventions, drugs, and other confounders, or to develop personalized ABP management strategies.

Astrocyte-produced carbon monoxide and the carbon monoxide donor CORM-A1 protect against cerebrovascular dysfunction caused by prolonged neonatal asphyxia

Neonatal asphyxia leads to cerebrovascular disease and neurological complications via a mechanism that may involve oxidative stress. Carbon monoxide (CO) is an antioxidant messenger produced via a heme oxygenase (HO)-catalyzed reaction. Cortical astrocytes are the major cells in the brain that express constitutive HO-2 isoform. We tested the hypothesis that CO, produced by astrocytes, has cerebroprotective properties during neonatal asphyxia. We developed a survival model of prolonged asphyxia in newborn pigs that combines insults of severe hypoxia, hypercapnia, and acidosis while avoiding extreme hypotension and cerebral blood flow reduction. During the 60-min asphyxia, CO production by brain and astrocytes was continuously elevated. Excessive formation of reactive oxygen species during asphyxia/reventilation was potentiated by the HO inhibitor tin protoporphyrin, suggesting that endogenous CO has antioxidant effects. Cerebral vascular outcomes tested 24 and 48 h after asphyxia demonstrated the sustained impairment of cerebral vascular responses to astrocyte- and endothelium-specific vasodilators. Postasphyxia cerebral vascular dysfunction was aggravated in newborn pigs pretreated with tin protoporphyrin to inhibit brain HO/CO. The CO donor CO-releasing molecule-A1 (CORM-A1) reduced brain oxidative stress during asphyxia/reventilation and prevented postasphyxia cerebrovascular dysfunction. The antioxidant and antiapoptotic effects of HO/CO and CORM-A1 were confirmed in primary cultures of astrocytes from the neonatal pig brain exposed to glutamate excitotoxicity. Overall, prolonged neonatal asphyxia leads to neurovascular injury via an oxidative stress-mediated mechanism that is counteracted by an astrocyte-based constitutive antioxidant HO/CO system. We propose that gaseous CO or CO donors can be used as novel approaches for prevention of neonatal brain injury caused by prolonged asphyxia. NEW & NOTEWORTHY Asphyxia in newborn infants may lead to lifelong neurological disabilities. Using the model of prolonged asphyxia in newborn piglets, we propose novel antioxidant therapy based on systemic administration of low doses of a carbon monoxide donor that prevent loss of cerebral blood flow regulation and may improve the neurological outcome of asphyxia.

Monitoring of intracranial pressure in patients with traumatic brain injury

Since Monro published his observations on the nature of the contents of the intracranial space in 1783, there has been investigation of the unique relationship between the contents of the skull and the intracranial pressure (ICP). This is particularly true following traumatic brain injury (TBI), where it is clear that elevated ICP due to the underlying pathological processes is associated with a poorer clinical outcome. Consequently, there is considerable interest in monitoring and manipulating ICP in patients with TBI. The two techniques most commonly used in clinical practice to monitor ICP are via an intraventricular or intraparenchymal catheter with a microtransducer system. Both of these techniques are invasive and are thus associated with complications such as hemorrhage and infection. For this reason, significant research effort has been directed toward development of a non-invasive method to measure ICP. The principle aims of ICP monitoring in TBI are to allow early detection of secondary hemorrhage and to guide therapies that limit intracranial hypertension (ICH) and optimize cerebral perfusion. However, information from the ICP value and the ICP waveform can also be used to assess the intracranial volume-pressure relationship, estimate cerebrovascular pressure reactivity, and attempt to forecast future episodes of ICH.

Pressure-derived versus pressure wave amplitude–derived indices of cerebrovascular pressure reactivity in relation to early clinical state and 12-month outcome following aneurysmal subarachnoid hemorrhage

Object

Indices of cerebrovascular pressure reactivity (CPR) represent surrogate markers of cerebral autoregulation. Given that intracranial pressure (ICP) wave amplitude–guided management, as compared with static ICP-guided management, improves outcome following aneurysmal subarachnoid hemorrhage (SAH), indices of CPR derived from pressure wave amplitudes should be further explored. This study was undertaken to investigate the value of CPR indices derived from static ICP–arterial blood pressure (ABP) values (pressure reactivity index [PRx]) versus ICP-ABP wave amplitudes (ICP-ABP wave amplitude correlation [IAAC]) in relation to the early clinical state and 12-month outcome in patients with aneurysmal SAH.

Methods

The authors conducted a single-center clinical trial enrolling patients with aneurysmal SAH. The CPR indices of PRx and IAAC of Week 1 after hemorrhage were related to the early clinical state (Glasgow Coma Scale [GCS] score) and 12-month outcome (modified Rankin Scale score).

Results

Ninety-four patients were included in the study. The IAAC, but not the PRx, increased with decreasing GCS score; that is, the higher the IAAC, the worse the clinical state. The PRx could differentiate between survivors and nonsurvivors only, whereas the IAAC clearly distinguished the groups “independent,” “dependent,” and “dead.” In patients with an average IAAC ≥ 0.2, mortality was approximately 3-fold higher than in those with an IAAC < 0.2.

Conclusions

The IAAC, which is based on single ICP-ABP wave identification, relates significantly to the early clinical state and 12-month outcome following aneurysmal SAH. Impaired cerebrovascular pressure regulation during the 1st week after a bleed relates to a worse outcome. Clinical trial registration no.: NCT00248690.

Autoregulatory model comparison and optimisation methodology

Cerebral pressure autoregulation (AR) is a process by which blood flow is kept constant over a specific cerebral perfusion pressure (CPP) range. There have been a number of advances in recent years in the monitoring and modelling of this physiological variable; however, there has been very little work done on the comparison or optimisation of some of the existing models in clinical use today: pressure reactivity index, highest modal frequency techniques and compartmental modelling. Presented here is a methodology for the comparison and optimisation results for these main AR models. By simple mathematical manipulation of the original modelling end points each model can be converted into a form that is directly comparable to the others. Using a standardised data set with known gold standard AR status indications, the models can then be readily assessed. As a consequence each of the models can then be optimised to maximise specificity and sensitivity.

Model-derived assessment of cerebrovascular resistance and cerebral blood flow following traumatic brain injury

The published guidelines point out the need for the development of methods that individualize patient cerebral perfusion management and minimize secondary ischemic complications associated with traumatic brain injury. A laboratory method has been developed to determine model-derived assessments of cerebrovascular resistance (mCVR) and cerebral blood flow (mCBF) from cerebrovascular pressure transmission, and the dynamic relationship between arterial blood pressure (ABP) and intracranial pressure (ICP). The aim of this two-fold study is to (1) evaluate relative changes in the model-derived parameters of mCVR and mCBF with the corresponding changes in the pial arteriolar vascular parameters of pial arteriolar resistance (PAR) and relative pial arteriolar blood flow (rPABF); and (2) examine the efficacy of the proposed modeling methodology for continuous assessment of the state of cerebrovascular regulation by evaluating relative changes in the model-derived parameters of CBF and cerebrovascular resistance in relation to changes of cerebral perfusion pressure prior to and following fluid percussion brain injury. Changes of ABP, ICP, PAR, relative arteriolar blood flow (rPABF) and the corresponding model-derived parameters of mCBF and mCVR induced by acute hypertensive challenge were evaluated before and following fluid percussion injury in piglets equipped with cranial windows. Before fluid percussion, hypertensive challenge resulted in a significant increase of PAR and mCVR, whereas both rPABF and mCBF remained constant. Following fluid percussion, hypertensive challenge resulted in a significant decrease of PAR and mCVR and consistent with impaired cerebrovascular regulation. Hypertensive challenge significantly increased both rPABF and mCBF, which approximately doubled with increased CPP with correlation values of r = 0.96 (P < 0.01) and r = 0.97 (P Collapse

Key Words

• cerebrovascular resistance
• cerebral blood flow
• traumatic brain-injury

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MESH Headings

• Animals
• Arterioles/physiology
• Arterioles/physiopathology
• Blood Pressure/physiology
• Brain Injuries/physiopathology
• Cerebrovascular Circulation/physiology
• Humans
• Intracranial Pressure/physiology
• Male
• Models, Cardiovascular
• Pia Mater/blood supply
• Regional Blood Flow
• Swine
• Vascular Resistance/physiology

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Grants

• R01 HL034059-25 NHLBI NIH HHS
• R01HL34059 NHLBI NIH HHS
• R01 HL034059 NHLBI NIH HHS
• R01 HL034059-24 NHLBI NIH HHS
• R37 HL042851-18 NHLBI NIH HHS
• R15NS051331 NINDS NIH HHS
• R37 HL042851 NHLBI NIH HHS
• R37HL042851 NHLBI NIH HHS
• R37 HL042851-17 NHLBI NIH HHS

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Affiliation(s)

Michael L. Daley Department of Electrical and Computer Engineering, The University of Memphis, Engineering Science Building, Rm. 208B, Memphis, TN 38152-3180, Phone: 901-678-3254, Fax 901-678-5469,
Nithya Narayanan Department of Electrical and Computer Engineering, The University of Memphis, Engineering Science Building, Rm. 208B, Memphis, TN 38152-3180, Phone: 901-678-4332, Fax 901-678-5469,
Charles W. Leffler Department of Physiology, The University of Tennessee Health Science Center, Memphis, TN 38163, Phone: 901-448-7122, Fax: 901-448-7126,

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Hypertension and cognitive function

Cumulative evidence implicates hypertension in the pathogenesis of Alzheimer disease. Although it may not presently be possible to completely differentiate the effects of treatment and control of hypertension itself from those of the medication used to achieve such treatment goals, efforts directed at the treatment and control of hypertension can have significant public health impact.

Assessment of cerebrovascular resistance with model of cerebrovascular pressure transmission

BACKGROUND

A two step modeling method of cerebrovascular pressure transmission, the dynamic relationship between arterial blood pressure (ABP) and intracranial pressure (ICP) has been developed as a means to continuously assess cerebrovascular regulation and resistance. Initially, system identification modeling was used to construct a numerical model of cerebrovascular pressure transmission. Next, the modal frequencies of the numerical model and the actual ABP recording were used to manipulate the parameters of a physiologically-based biomechanical model such that: (1) the actual and simulated ICP; and (2) the numerical and biomechanical model modal frequencies match.

MATERIALS AND METHODS

This study was designed to compare changes of cerebrovascular resistance of the biomechanical model with the expected changes of cerebrovascular resistance associated with the occurrence of either a plateau wave or refractory intracranial hypertension. Pressure recordings from five patients with plateau waves and five patients with intracranial hypertension were used.

FINDINGS

Vascular resistance decreased significantly during the plateau wave and was inversely related to CPP, indicating active vasoreactivity. In contrast, vascular resistance increased significantly during intractable intracranial hypertension and was directly related to CPP, indicating impaired cerebrovascular regulation.

CONCLUSIONS

Such results support the use of the modeling method as a means to continuously assess changes of cerebrovascular regulation and resistance.

Assessment of cerebrovascular resistance with a model of cerebrovascular pressure transmission

A method to assess continuous changes of cerebrovascular resistance based on a biomechanical model of cerebrovascular pressure transmission is developed. Such a method provides an end-point measure to assess new and/or existing management strategies during intensive-care management of patients with brain injury. Changes of both pial arteriolar resistance and cerebrovascular resistance derived by a physiologically based biomechanical model of cerebrovascular pressure transmission, the dynamic relationship between arterial blood pressure (ABP) and intracranial pressure (ICP), were compared to test the validity of the modeling procedure. Pressor challenge was administered to normoxic (N=5) and hypoxic (N=5) piglets equipped with closed cranial windows. Pial arteriolar diameters were used to compute arteriolar resistance. Percent change of pial arteriolar resistance (%DeltaPAR) and percent change of model-derived cerebrovascular resistance (%DeltasCVR) in response to pressor challenge were computed. During intact cerebrovascular regulation and during hypoxia-induced impairment of cerebrovascular regulation, changes in pial arteriolar resistance were accurately predicted by the proposed modeling method designed to assess changes of cerebrovascular resistance.

Influence of hypercapnic vasodilation on cerebrovascular autoregulation and pial arteriolar bed resistance in piglets

Changes in both pial arteriolar resistance (PAR) and simulated arterial-arteriolar bed resistance (SimR) of a physiologically based biomechanical model of cerebrovascular pressure transmission, the dynamic relationship between arterial blood pressure and intracranial pressure, are used to test the hypothesis that hypercapnia disrupts autoregulatory reactivity. To evaluate pressure reactivity, vasopressin-induced acute hypertension was administered to normocapnic and hypercapnic (N = 12) piglets equipped with closed cranial windows. Pial arteriolar diameters were used to compute arteriolar resistance. Percent change of PAR (%DeltaPAR) and percent change of SimR (%DeltaSimR) in response to vasopressin-induced acute hypertension were computed and compared. Hypercapnia decreased cerebrovascular resistance. Indicative of active autoregulatory reactivity, vasopressin-induced hypertensive challenge resulted in an increase of both %DeltaPAR and %DeltaSimR for all normocapnic piglets. The hypercapnic piglets formed two statistically distinct populations. One-half of the hypercapnic piglets demonstrated a measured decrease of both %DeltaPAR and %DeltaSimR to pressure challenge, indicative of being pressure passive, whereas the other one-half demonstrated an increase in these percentages, indicative of active autoregulation. No other differences in measured variables were detectable between regulating and pressure-passive piglets. Changes in resistance calculated from using the model mirrored those calculated from arteriolar diameter measurements. In conclusion, vasodilation induced by hypercapnia has the potential to disrupt autoregulatory reactivity. Our physiologically based biomechanical model of cerebrovascular pressure transmission accurately estimates the changes in arteriolar resistance during conditions of active and passive cerebrovascular reactivity.

Stroke with subarachnoid hemorrhage: assessment of cerebrovascular pressure regulation and simulated cerebrovascular resistance

BACKGROUND

Monitoring methods designed to assess cerebrovascular regulation and increased cerebrovascular resistance (CVR) of patients with subarachnoid hemorrhage (SAH) would facilitate therapeutic intervention and potentially reduce secondary complications. The aim of this study was to assess changes of cerebrovascular regulation and CVR by evaluating changes of cerebrovascular pressure transmission in patients with SAH.

METHODS

Admission Hunt-Hess grades, Fisher scores, Glasgow Outcome Scores (GOS) at 6 months, and pressure recordings were obtained from 20 patients. Biomechanical models of cerebrovascular pressure transmission were constructed over one-minute intervals for the initial and final two hours of post-hemorrhage monitoring.

FINDINGS

Classified according to the GOS score at 6 months, eight patients died (GOS 1), five were severely disabled (GOS 3), and seven patients were moderately disabled (GOS 4). During the initial monitoring period 100%, 80%, and 28.6% of groups with GOS 1, 3, and 4 demonstrated impairment of cerebrovascular regulation; whereas, in the final monitoring period 100%, 100%, and 14.3% respectively demonstrated impairment. Between monitoring periods, simulated CVR (sCVR) significantly increased (p < 0.001) for patients with GOS 1 and 3 and decreased for those with GOS 4 with mean resistance for the latter group significantly lower (p < 0.001) than other means.

CONCLUSIONS

Loss of cerebrovascular regulation and increased sCVR were observed in SAH patients with poor outcome.

Intracranial pressure and cerebral perfusion pressure as risk factors in children with traumatic brain injuries

OBJECT

The authors evaluated the initial intracranial pressure (ICP) and cerebral perfusion pressure (CPP) as prognostic factors in severe head injury in children and tried to determine the optimal CPP range.

METHODS

The authors performed a 9-year retrospective review of all patients with severe traumatic brain injuries (TBIs) who required invasive ICP monitoring and were admitted to the pediatric intensive care unit at their institution between January 1995 and December 2003. These patients had Glasgow Coma Scale scores lower than 8 and/or required ICP monitoring due to worsening neurological status or neuroimaging results suggestive of cerebral hypertension. Clinical summaries and imaging studies were reviewed. Data for 156 pediatric patients who ranged in age from 1 to 18 years were obtained. Half of these patients presented with normal initial ICPs (< 20 mm Hg), and a good outcome was achieved in 80% of these children. An unfavorable outcome was observed in more than 60% of patients with an initial CPP lower than 40 mm Hg. The proportion of patients with an unfavorable outcome decreased to 10% with initial CPPs higher than 60 mm Hg, but patients with initial CPPs higher than 70 mm Hg did not improve.

CONCLUSIONS

Initial ICP and CPP measurements were useful as prognostic factors in pediatric patients with severe TBIs: patients with initial CPPs between 40 and 70 mm Hg were found to have a better neurological prognosis than those with CPPs either higher or lower than that range.

Mode changes of cerebrovascular pressure transmission induced by cerebral vasodilation

Changes in the modes of cerebrovascular pressure transmission during cerebral vasodilation induced by hypercapnic challenge were examined as a means for developing the basis for a bedside method to evaluate regulation of cerebral blood flow. Recordings of arterial blood pressure (ABP) and intracranial pressure (ICP) obtained from a piglet preparation equipped with a cranial window were used to determine serial changes of the highest modal frequency (HMF) and dampening factor (DF) of a numerical system identification model of cerebrovascular pressure transmission. Resistance and compliance elements of a Windkessel model of ICP dynamics selected to provide the mathematical structure for the system identification modeling approach were also manipulated to obtain a match with HMF, DF, and the experimental and simulated recordings of ICP. During hypercapnic challenge, significant increases of ICP, pial arterial diameter (PAD) and partial pressure of arterial blood carbon dioxide increases, and a decrease of arterial pH were observed. Vasodilation changed the modes of the system identification model of cerebrovascular pressure transmission from a dominantly over-damped process to an under-damped one with a significant increase in HMF and decrease in DF. Simulations of the Windkessel circuit model required a decrease in the relative resistance and an increase in relative compliance of the arterial-arteriolar vascular bed consistent with the observed increases in PAD induced by vasodilation. Evaluation of changes in the modes of cerebrovascular pressure transmission may provide means of assessing the state of cerebrovascular vasodilation and autoregulation of cerebral blood flow in the clinical setting.

Exercise, cognition and Alzheimer's disease: More is not necessarily better

Regional hypoperfusion, associated with a reduction in cerebral metabolism, is a hallmark of Alzheimer's disease (AD) and contributes to cognitive decline. Cerebral perfusion and hence cognition can be enhanced by exercise. The present review describes first how the effects of exercise on cerebral perfusion in AD are mediated by nitric oxide (NO) and tissue-type plasminogen activator, the release of which is regulated by NO. A conclusion of clinical relevance is that exercise may not be beneficial for the cognitive functioning of all people with dementia if cardiovascular risk factors are present. The extent to which cardiovascular risk factors play a role in the selection of older people with dementia in clinical studies will be addressed in the second part of the review in which the effects of exercise on cognition are presented. Only eight relevant studies were found in the literature, emphasizing the paucity of studies in this field. Positive effects of exercise on cognition were reported in seven studies, including two that excluded and two that included patients with cardiovascular risk factors. These findings suggest that cardiovascular risk factors do not necessarily undo the beneficial effects of exercise on cognition in cognitively impaired people. Further research is called for, in view of the limitations of the clinical studies reviewed here.

Intracranial pressure monitoring: modeling cerebrovascular pressure transmission

OBJECTIVES

To examine changes in cerebrovascular pressure transmission derived from arterial blood pressure (ABP) and intracranial pressure (ICP) recordings by autoregressive moving average modeling technique.

METHODS

Digitized ICP and ABP recordings were obtained from patients with brain injury. Two groups were defined: Group A with 4 patients who demonstrated plateau waves, and Group B with 4 intracranial hypertensive, hypoperfused patients. For each 16.5 s interval, mean values of ICP, ABP, cerebral perfusion pressure (CPP), and corresponding highest modal frequency (HMF) of cerebrovascular pressure transmission were computed.

RESULTS

Mean values of CPP and HMF of 56.2 mmHg and 2.0 Hz for Group A were significantly higher (p < 0.005) than corresponding mean values of 31.9 mmHg and 0.744 Hz for Group B. The mean value of the slope of the regression line between HMF and CPP for group A of -0.034 Hz/mmHg was significantly different (p < 0.025) than the mean value of 0.0077 Hz/mmHg for Group B. Computations of HMF, pressure reactivity, and correlation pressure reactivity index on continuous pressure recordings are illustrated.

CONCLUSIONS

Values of HMF of cerebrovascular pressure transmission are inversely related to CPP when pressure regulation is thought to be intact, and directly related when regulation is likely lost.

Plateau waves: changes of cerebrovascular pressure transmission

OBJECTIVE

To test the validity of the hypothesis that active vasodilatation and vasoconstriction underlie the occurrence of intracranial pressure (ICP) plateau waves by evaluating corresponding changes of cerebrovascular pressure transmission of arterial blood pressure (ABP) to ICP.

METHODS

Digitized recordings of ICP and ABP sampled at 30 Hz were obtained from nine patients with traumatic brain injury. For each 16.5 s recording interval mean values of ICP, ABP, cerebral perfusion pressure (CPP), and the corresponding highest modal frequency (HMF) of cerebrovascular pressure transmission were calculated.

RESULTS

Mean ICP and HMF significantly increased (P < 0.003) and mean CPP decreased significantly (P < 0.00036) at onset of the wave. Conversely at termination, mean ICP and HMF significantly decreased (P < 0.026) and mean CPP significantly increased (P < 0.028). In addition, the strong negative correlations between mean ICP and mean CPP (r = -0.87) and mean HMF and CPP (r = -0.87) were demonstrated.

CONCLUSION

The findings that HMF increased at onset and decreased at the termination of plateau wave support the validity of the vasodilatatory/constriction cascade model that postulates active vasodilation at the onset and active vasoconstriction of the cerebrovascular bed at the termination of a plateau wave.