Bilateral Failure of Cerebral Autoregulation is Related to Unfavorable Outcome After Subarachnoid Hemorrhage

Trial of the Cerebral Perfusion Response to Sodium Nitrite Infusion in Patients with Acute Subarachnoid Haemorrhage using Arterial Spin Labelling MRI

Aneurysmal subarachnoid haemorrhage (SAH) is a devastating subset of stroke. One of the major determinants of outcome is an evolving multifactorial injury occurring in the first 72 hours, known as early brain injury. Reduced nitric oxide (NO) bioavailability and an associated disruption to cerebral perfusion is believed to play an important role in this process. We sought to explore this relationship, by examining the effect on cerebral perfusion of the in vivo manipulation of NO levels using an exogenous NO donor (sodium nitrite). We performed a double blind placebo controlled randomised experimental medicine study of the cerebral perfusion response to sodium nitrite infusion during the early brain injury period in 15 low grade (World Federation of Neurosurgeons grade 1-2) SAH patients. Patients were randomly assigned to receive sodium nitrite at 10 mcg/kg/min or saline placebo. Assessment occurred following endovascular aneurysm occlusion, mean time after ictus 66h (range 34-90h). Cerebral perfusion was quantified before infusion commencement and after 3 hours, using multi-post labelling delay (multi-PLD) vessel encoded pseudocontinuous arterial spin labelling (VEPCASL) magnetic resonance imaging (MRI). Administration of sodium nitrite was associated with a significant increase in average grey matter cerebral perfusion. Group level voxelwise analysis identified that increased perfusion occurred within regions of the brain known to exhibit enhanced vulnerability to injury. These findings highlight the role of impaired NO bioavailability in the pathophysiology of early brain injury.

Cerebral autoregulation, spreading depolarization, and implications for targeted therapy in brain injury and ischemia

Cerebral autoregulation is an intrinsic myogenic response of cerebral vasculature that allows for preservation of stable cerebral blood flow levels in response to changing systemic blood pressure. It is effective across a broad range of blood pressure levels through precapillary vasoconstriction and dilation. Autoregulation is difficult to directly measure and methods to indirectly ascertain cerebral autoregulation status inherently require certain assumptions. Patients with impaired cerebral autoregulation may be at risk of brain ischemia. One of the central mechanisms of ischemia in patients with metabolically compromised states is likely the triggering of spreading depolarization (SD) events and ultimately, terminal (or anoxic) depolarization. Cerebral autoregulation and SD are therefore linked when considering the risk of ischemia. In this scoping review, we will discuss the range of methods to measure cerebral autoregulation, their theoretical strengths and weaknesses, and the available clinical evidence to support their utility. We will then discuss the emerging link between impaired cerebral autoregulation and the occurrence of SD events. Such an approach offers the opportunity to better understand an individual patient's physiology and provide targeted treatments.

Critical thresholds of long-pressure reactivity index and impact of intracranial pressure monitoring methods in traumatic brain injury

BACKGROUND

Moderate-to-severe traumatic brain injury (TBI) has a global mortality rate of about 30%, resulting in acquired life-long disabilities in many survivors. To potentially improve outcomes in this TBI population, the management of secondary injuries, particularly the failure of cerebrovascular reactivity (assessed via the pressure reactivity index; PRx, a correlation between intracranial pressure (ICP) and mean arterial blood pressure (MAP)), has gained interest in the field. However, derivation of PRx requires high-resolution data and expensive technological solutions, as calculations use a short time-window, which has resulted in it being used in only a handful of centers worldwide. As a solution to this, low resolution (longer time-windows) PRx has been suggested, known as Long-PRx or LPRx. Though LPRx has been proposed little is known about the best methodology to derive this measure, with different thresholds and time-windows proposed. Furthermore, the impact of ICP monitoring on cerebrovascular reactivity measures is poorly understood. Hence, this observational study establishes critical thresholds of LPRx associated with long-term functional outcome, comparing different time-windows for calculating LPRx as well as evaluating LPRx determined through external ventricular drains (EVD) vs intraparenchymal pressure device (IPD) ICP monitoring.

METHODS

The study included a total of n = 435 TBI patients from the Karolinska University Hospital. Patients were dichotomized into alive vs. dead and favorable vs. unfavorable outcomes based on 1-year Glasgow Outcome Scale (GOS). Pearson's chi-square values were computed for incrementally increasing LPRx or ICP thresholds against outcome. The thresholds that generated the greatest chi-squared value for each LPRx or ICP parameter had the highest outcome discriminatory capacity. This methodology was also completed for the segmentation of the population based on EVD, IPD, and time of data recorded in hospital stay.

RESULTS

LPRx calculated with 10-120-min windows behaved similarly, with maximal chi-square values ranging at around a LPRx of 0.25-0.35, for both survival and favorable outcome. When investigating the temporal relations of LPRx derived thresholds, the first 4 days appeared to be the most associated with outcomes. The segmentation of the data based on intracranial monitoring found limited differences between EVD and IPD, with similar LPRx values around 0.3.

CONCLUSION

Our work suggests that the underlying prognostic factors causing impairment in cerebrovascular reactivity can, to some degree, be detected using lower resolution PRx metrics (similar found thresholding values) with LPRx found clinically using as low as 10 min-by-minute samples of MAP and ICP. Furthermore, EVD derived LPRx with intermittent cerebrospinal fluid draining, seems to present similar outcome capacity as IPD. This low-resolution low sample LPRx method appears to be an adequate substitute for the clinical prognostic value of PRx and may be implemented independent of ICP monitoring method when PRx is not feasible, though further research is warranted.

Monitoring of cerebral blood flow autoregulation: physiologic basis, measurement, and clinical implications

Cerebral blood flow (CBF) autoregulation is the physiologic process whereby blood supply to the brain is kept constant over a range of cerebral perfusion pressures ensuring a constant supply of metabolic substrate. Clinical methods for monitoring CBF autoregulation were first developed for neurocritically ill patients and have been extended to surgical patients. These methods are based on measuring the relationship between cerebral perfusion pressure and surrogates of CBF or cerebral blood volume (CBV) at low frequencies (<0.05 Hz) of autoregulation using time or frequency domain analyses. Initially intracranial pressure monitoring or transcranial Doppler assessment of CBF velocity was utilised relative to changes in cerebral perfusion pressure or mean arterial pressure. A more clinically practical approach utilising filtered signals from near infrared spectroscopy monitors as an estimate of CBF has been validated. In contrast to the traditional teaching that 50 mm Hg is the autoregulation threshold, these investigations have found wide interindividual variability of the lower limit of autoregulation ranging from 40 to 90 mm Hg in adults and 20-55 mm Hg in children. Observational data have linked impaired CBF autoregulation metrics to adverse outcomes in patients with traumatic brain injury, ischaemic stroke, subarachnoid haemorrhage, intracerebral haemorrhage, and in surgical patients. CBF autoregulation monitoring has been described in both cardiac and noncardiac surgery. Data from a single-centre randomised study in adults found that targeting arterial pressure during cardiopulmonary bypass to above the lower limit of autoregulation led to a reduction of postoperative delirium and improved memory 1 month after surgery compared with usual care. Together, the growing body of evidence suggests that monitoring CBF autoregulation provides prognostic information on eventual patient outcomes and offers potential for therapeutic intervention. For surgical patients, personalised blood pressure management based on CBF autoregulation data holds promise as a strategy to improve patient neurocognitive outcomes.

Assessment of impaired cerebral autoregulation and its correlation with neurological outcome in aneurysmal subarachnoid hemorrhage: A prospective and observational study

Background

Cerebral autoregulation (CA) is crucial for the maintenance of cerebral homeostasis. It can be assessed by measuring transient hyperemic response ratio (THRR) using transcranial Doppler (TCD). We aimed at assessing the incidence of impaired CA (ICA) and its correlation with the neurological outcome in patients with aneurysmal subarachnoid hemorrhage (aSAH).

Methods

One hundred consecutive patients with aSAH scheduled for aneurysmal clipping were enrolled in this prospective and observational study. Preoperative and consecutive 5-day postoperative THRR measurements were taken. Primary objective of the study was to detect the incidence of ICA and its correlation with vasospasm (VS) postclipping, and neurological outcome at discharge and 1, 3, and 12 months was secondary objectives.

Results

ICA (THRR < 1.09) was observed in 69 patients preoperatively, 74 patients on the 1st and 2nd postoperative day, 76 patients on 3rd postoperative day, and 78 patients on 4th and 5th postoperative day. Significant VS was seen in 13.4% and 61.5% of patients with intact THRR and deranged THRR, respectively (P < 0.000). Out of 78 patients who had ICA, 42 patients (53.8%) at discharge, 60 patients (76.9%) at 1 month, 54 patients (69.2%) at 3 month, and 55 patients (70.5%) at 12 months had unfavorable neurological outcome significantly more than those with preserved CA.

Conclusion

Incidence of ICA assessed in aSAH patients varies from 69% to 78% in the perioperative period. The deranged CA was associated with significantly poor neurological outcome. Therefore, CA assessment using TCD-based THRR provides a simple, noninvasive bedside approach for predicting neurological outcome in aSAH.

Regional disparity in continuously measured time-domain cerebrovascular reactivity indices: a scoping review of human literature

Objective: Cerebral blood vessels maintaining relatively constant cerebral blood flow (CBF) over wide range of systemic arterial blood pressure (ABP) is referred to as cerebral autoregulation (CA). Impairments in CA expose the brain to pressure-passive flow states leading to hypoperfusion and hyperperfusion. Cerebrovascular reactivity (CVR) metrics refer to surrogate metrics of pressure-based CA that evaluate the relationship between slow vasogenic fluctuations in cerebral perfusion pressure/ABP and a surrogate for pulsatile CBF/cerebral blood volume.Approach: We performed a systematically conducted scoping review of all available human literature examining the association between continuous CVR between more than one brain region/channel using the same CVR index.Main Results: In all the included 22 articles, only handful of transcranial doppler (TCD) and near-infrared spectroscopy (NIRS) based metrics were calculated for only two brain regions/channels. These metrics found no difference between left and right sides in healthy volunteer, cardiac surgery, and intracranial hemorrhage patient studies. In contrast, significant differences were reported in endarterectomy, and subarachnoid hemorrhage studies, while varying results were found regarding regional disparity in stroke, traumatic brain injury, and multiple population studies.Significance: Further research is required to evaluate regional disparity using NIRS-based indices and to understand if NIRS-based indices provide better regional disparity information than TCD-based indices.

Blood pressure and the risk of rebleeding and delayed cerebral ischemia after aneurysmal subarachnoid hemorrhage

INTRODUCTION AND OBJECTIVE

Blood pressure is presumably related to rebleeding and delayed cerebral ischemia (DCI) after subarachnoid hemorrhage (aSAH) and could serve as a target to improve outcome. We assessed the associations between blood pressure and rebleeding or DCI in aSAH-patients.

MATERIALS AND METHODS

In this observational study in 1167 aSAH-patients admitted to the intensive care unit (ICU), adjusted hazard ratio's (aHR) were calculated for the time-dependent association of blood pressure and rebleeding or DCI. The aHRs were presented graphically, relative to a reference mean arterial pressure (MAP) of 100 mmHg and systolic blood pressure (sBP) of 150 mmHg.

RESULTS

A MAP below 100 mmHg in the 6, 3 and 1 h before each moment in time was associated with a decreased risk of rebleeding (e.g. within 6 h preceding rebleeding: MAP = 80 mmHg: aHR 0.30 (95% confidence interval (CI) 0.11-0.80)). A MAP below 60 mmHg in the 24 h before each moment in time was associated with an increased risk of DCI (e.g. MAP = 50 mmHg: aHR 2.59 (95% CI 1.12-5.96)).

CONCLUSIONS

Our results suggest that a MAP below 100 mmHg is associated with decreased risk of rebleeding, and a MAP below 60 mmHg with increased risk of DCI.

Optimal bispectral index level of sedation and cerebral oximetry in traumatic brain injury: a non-invasive individualized approach in critical care?

Background

Impaired cerebral autoregulation has been linked with worse outcomes, with literature suggesting that current therapy guidelines fail to significantly impact cerebrovascular reactivity. The cerebral oximetry index (COx_a) is a surrogate measure of cerebrovascular reactivity which can in theory be obtained non-invasively using regional brain tissue oxygen saturation and arterial blood pressure. The goal of this study was to assess the relationship between objectively measured depth of sedation through BIS and autoregulatory capacity measured through COx_a.

Methods

In a prospectively maintained observational study, we collected continuous regional brain tissue oxygen saturation, intracranial pressure, arterial blood pressure and BIS in traumatic brain injury patients. COx_a was obtained using the Pearson’s correlation between regional brain tissue oxygen saturation and arterial blood pressure and ranges from − 1 to 1 with higher values indicating impairment of cerebrovascular reactivity. Using BIS values and COx_a, a curve-fitting method was applied to determine the minimum value for the COx_a. The associated BIS value with the minimum COx_a is called BISopt. This BISopt was both visually and algorithmically determined, which were compared and assessed over the whole dataset.

Results

Of the 42 patients, we observed that most had a parabolic relationship between BIS and COx_a. This suggests a potential “optimal” depth of sedation where COx_a is the most intact. Furthermore, when comparing the BISopt algorithm with visual inspection of BISopt, we obtained similar results. Finally, BISopt % yield (determined algorithmically) appeared to be independent from any individual sedative or vasopressor agent, and there was agreement between BISopt found with COx_a and the pressure reactivity index (another surrogate for cerebrovascular reactivity).

Conclusions

This study suggests that COx_a is capable of detecting disruption in cerebrovascular reactivity which occurs with over-/under-sedation, utilizing a non-invasive measure of determination and assessment. This technique may carry implications for tailoring sedation in patients, focusing on individualized neuroprotection.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40635-022-00460-9.

Impact of Cerebral Autoregulation Monitoring in Cerebrovascular Disease: A Systematic Review

Cerebral autoregulation (CA) prevents brain injury by maintaining a relatively constant cerebral blood flow despite fluctuations in cerebral perfusion pressure. This process is disrupted consequent to various neurologic pathologic processes, which may result in worsening neurologic outcomes. Herein, we aim to highlight evidence describing CA changes and the impact of CA monitoring in patients with cerebrovascular disease, including ischemic stroke, intracerebral hemorrhage (ICH), and aneurysmal subarachnoid hemorrhage (aSAH). The study was preformed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analysis guidelines. English language publications were identified through a systematic literature conducted in Ovid Medline, PubMed, and Embase databases. The search spanned the dates of each database's inception through January 2021. We selected case-control studies, cohort observational studies, and randomized clinical trials for adult patients (≥ 18 years) who were monitored with continuous metrics using transcranial Doppler, near-infrared spectroscopy, and intracranial pressure monitors. Of 2799 records screened, 48 studies met the inclusion criteria. There were 23 studies on ischemic stroke, 18 studies on aSAH, 5 studies on ICH, and 2 studies on systemic hypertension. CA impairment was reported after ischemic stroke but generally improved after tissue plasminogen activator administration and successful mechanical thrombectomy. Persistent impairment in CA was associated with hemorrhagic transformation, malignant cerebral edema, and need for hemicraniectomy. Studies that investigated large ICHs described bilateral CA impairment up to 12 days from the ictus, especially in the presence of small vessel disease. In aSAH, impairment of CA was associated with angiographic vasospasm, delayed cerebral ischemia, and poor functional outcomes at 6 months. This systematic review highlights the available evidence for CA disruption during cerebrovascular diseases and its possible association with long-term neurological outcome. CA may be disrupted even before acute stroke in patients with untreated chronic hypertension. Monitoring CA may help in establishing individualized management targets in patients with cerebrovascular disease.

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.

Regulation of cerebral blood flow in humans: physiology and clinical implications of autoregulation

Brain function critically depends on a close matching between metabolic demands, appropriate delivery of oxygen and nutrients, and removal of cellular waste. This matching requires continuous regulation of cerebral blood flow (CBF), which can be categorized into four broad topics: 1) autoregulation, which describes the response of the cerebrovasculature to changes in perfusion pressure; 2) vascular reactivity to vasoactive stimuli [including carbon dioxide (CO2)]; 3) neurovascular coupling (NVC), i.e., the CBF response to local changes in neural activity (often standardized cognitive stimuli in humans); and 4) endothelium-dependent responses. This review focuses primarily on autoregulation and its clinical implications. To place autoregulation in a more precise context, and to better understand integrated approaches in the cerebral circulation, we also briefly address reactivity to CO2 and NVC. In addition to our focus on effects of perfusion pressure (or blood pressure), we describe the impact of select stimuli on regulation of CBF (i.e., arterial blood gases, cerebral metabolism, neural mechanisms, and specific vascular cells), the interrelationships between these stimuli, and implications for regulation of CBF at the level of large arteries and the microcirculation. We review clinical implications of autoregulation in aging, hypertension, stroke, mild cognitive impairment, anesthesia, and dementias. Finally, we discuss autoregulation in the context of common daily physiological challenges, including changes in posture (e.g., orthostatic hypotension, syncope) and physical activity.

Lateral Brain Displacement and Cerebral Autoregulation in Acutely Comatose Patients

OBJECTIVES

Lateral displacement and impaired cerebral autoregulation are associated with worse outcomes following acute brain injury, but their effect on long-term clinical outcomes remains unclear. We assessed the relationship between lateral displacement, disturbances to cerebral autoregulation, and clinical outcomes in acutely comatose patients.

DESIGN

Retrospective analysis of prospectively collected data.

SETTING

Neurocritical care unit of the Johns Hopkins Hospital.

PATIENTS

Acutely comatose patients (Glasgow Coma Score ≤ 8).

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Cerebral oximetry index, derived from near-infrared spectroscopy multimodal monitoring, was used to evaluate cerebral autoregulation. Associations between lateral brain displacement, global cerebral autoregulation, and interhemispheric cerebral autoregulation asymmetry were assessed using mixed random effects models with random intercept. Patients were grouped by functional outcome, determined by the modified Rankin Scale. Associations between outcome group, lateral displacement, and cerebral oximetry index were assessed using multivariate linear regression. Increasing lateral brain displacement was associated with worsening global cerebral autoregulation (p = 0.01 septum; p = 0.05 pineal) and cerebral autoregulation asymmetry (both p < 0.001). Maximum lateral displacement during the first 3 days of coma was significantly different between functional outcome groups at hospital discharge (p = 0.019 pineal; p = 0.008 septum), 3 months (p = 0.026; p = 0.007), 6 months (p = 0.018; p = 0.010), and 12 months (p = 0.022; p = 0.012). Global cerebral oximetry index was associated with functional outcomes at 3 months (p = 0.019) and 6 months (p = 0.013).

CONCLUSIONS

During the first 3 days of acute coma, increasing lateral brain displacement is associated with worsening global cerebral autoregulation and cerebral autoregulation asymmetry, and poor long-term clinical outcomes in acutely comatose patients. The impact of acute interventions on outcome needs to be explored.

Near-Infrared Spectroscopy in Neurocritical Care: A Review of Recent Updates

Neurocritical diseases and conditions are common causes of long-term disability and mortality. Early recognition and management of neurocritically ill patients is a significant challenge for neurosurgeons, neurologists, and neurointensivists. Although cerebral angiography, magnetic resonance imaging, computed tomography, and radionuclide imaging are useful in neuromonitoring and neuroimaging, they have several important limitations: they are not readily available, cannot be used for a continuous assessment of cerebral function, and frequently require patient transport to the radiological department. Near-infrared spectroscopy (NIRS) is an inexpensive, portable, noninvasive method that does not require advanced expertise and can be used at the bedside for critically ill patients without moving them to the radiology department. NIRS can detect and monitor multiple critical parameters, including cerebral oximetry, intracranial pressure, temperature, and cerebral blood flow. NIRS can be valuable for a wide variety of neurocritical diseases and conditions, such as ischemic and hemorrhagic strokes, severe traumatic brain injury, brain tumors, and perioperative neurosurgery. Although NIRS has been studied extensively in multiple neurocritical conditions, more evidence on its application is needed.

Association of transcranial Doppler blood flow velocity slow waves with delayed cerebral ischemia in patients suffering from subarachnoid hemorrhage: a retrospective study

Background

Cerebral vasospasm (VS) and delayed cerebral ischemia (DCI) constitute major complications following subarachnoid hemorrhage (SAH). A few studies have examined the relationship between different indices of cerebrovascular dynamics with the occurrence of VS. However, their potential association with the development of DCI remains elusive. In this study, we investigated the pattern of changes of different transcranial Doppler (TCD)-derived indices of cerebrovascular dynamics during vasospasm in patients suffering from subarachnoid hemorrhage, dichotomized by the presence of delayed cerebral ischemia.

Methods

A retrospective analysis was performed using recordings from 32 SAH patients, diagnosed with VS. Patients were divided in two groups, depending on development of DCI. Magnitude of slow waves (SWs) of cerebral blood flow velocity (CBFV) was measured. Cerebral autoregulation was estimated using the moving correlation coefficient Mxa. Cerebral arterial time constant (tau) was expressed as the product of resistance and compliance. Complexity of CBFV was estimated through measurement of sample entropy (SampEn).

Results

In the whole population (N = 32), magnitude of SWs of ipsilateral to VS side CBFV was higher during vasospasm (4.15 ± 1.55 vs before: 2.86 ± 1.21 cm/s, p < 0.001). Ipsilateral SWs of CBFV before VS had higher magnitude in DCI group (N = 19, p < 0.001) and were strongly predictive of DCI, with area under the curve (AUC) = 0.745 (p = 0.02). Vasospasm caused a non-significant shortening of ipsilateral values of tau and increase in SampEn in all patients related to pre-VS measurements, as well as an insignificant increase of Mxa in DCI related to non-DCI group (N = 13).

Conclusions

In patients suffering from subarachnoid hemorrhage, TCD-detected VS was associated with higher ipsilateral CBFV SWs, related to pre-VS measurements. Higher CBFV SWs before VS were significantly predictive of delayed cerebral ischemia.

Phase dynamics of cerebral blood flow in subarachnoid haemorrhage in response to sodium nitrite infusion

Aneurysmal subarachnoid haemorrhage (SAH) is a devastating subset of stroke. One of the major determinates of morbidity is the development of delayed cerebral ischemia (DCI). Disruption of the nitric oxide (NO) pathway and consequently the control of cerebral blood flow (CBF), known as cerebral autoregulation, is believed to play a role in its pathophysiology. Through the pharmacological manipulation of in vivo NO levels using an exogenous NO donor we sought to explore this relationship. Phase synchronisation index (PSI), an expression of the interdependence between CBF and arterial blood pressure (ABP) and thus cerebral autoregulation, was calculated before and during sodium nitrite administration in 10 high-grade SAH patients acutely post-rupture. In patients that did not develop DCI, there was a significant increase in PSI around 0.1 Hz during the administration of sodium nitrite (33%; p-value 0.006). In patients that developed DCI, PSI did not change significantly. Synchronisation between ABP and CBF at 0.1 Hz has been proposed as a mechanism by which organ perfusion is maintained, during periods of physiological stress. These findings suggest that functional NO depletion plays a role in impaired cerebral autoregulation following SAH, but the development of DCI may have a distinct pathophysiological aetiology.

Early Transcranial Doppler Evaluation of Cerebral Autoregulation Independently Predicts Functional Outcome After Aneurysmal Subarachnoid Hemorrhage

BACKGROUND

Cerebral autoregulation (CA) impairment after aneurysmal subarachnoid hemorrhage (SAH) has been associated with delayed cerebral ischemia and an unfavorable outcome. We investigated whether the early transient hyperemic response test (THRT), a transcranial Doppler (TCD)-based CA evaluation method, can predict functional outcome 6 months after aneurysmal SAH.

METHODS

This is a prospective observational study of all aneurysmal SAH patients consecutively admitted to a single center between January 2016 and February 2017. CA was evaluated within 72 h of hemorrhage by THRT, which describes the changes in cerebral blood flow velocity after a brief compression of the ipsilateral common carotid artery. CA was considered to be preserved when an increase ≥ 9% of baseline systolic velocity was present. According to the modified Rankin Scale (mRS: 4-6), the primary outcome was unfavorable 6 months after hemorrhage. Secondary outcomes included cerebral infarction, vasospasm on TCD, and an unfavorable outcome at hospital discharge.

RESULTS

Forty patients were included (mean age = 54 ± 12 years, 70% females). CA was impaired in 19 patients (47.5%) and preserved in 21 (52.5%). Impaired CA patients were older (59 ± 13 vs. 50 ± 9, p = 0.012), showed worse neurological conditions (Hunt&Hess 4 or 5-47.4% vs. 9.5%, p = 0.012), and clinical initial condition (APACHE II physiological score-12 [5.57-13] vs. 3.5 [3-5], p = 0.001). Fourteen patients in the impaired CA group and one patient in the preserved CA group progressed to an unfavorable outcome (73.7% vs. 4.7%, p = 0.0001). The impaired CA group more frequently developed cerebral infarction than the preserved CA group (36.8% vs. 0%, p = 0.003, respectively). After multivariate analysis, impaired CA (OR 5.15 95% CI 1.43-51.99, p = 0.033) and the APACHE II physiological score (OR 1.67, 95% CI 1.01-2.76, p = 0.046) were independently associated with an unfavorable outcome.

CONCLUSIONS

Early CA impairment detected by TCD and admission APACHE II physiological score independently predicted an unfavorable outcome after SAH.

Aerobic Training and Mobilization Early Post-stroke: Cautions and Considerations

Knowledge gaps exist in how we implement aerobic exercise programs during the early phases post-stroke. Therefore, the objective of this review was to provide evidence-based guidelines for pre-participation screening, mobilization, and aerobic exercise training in the hyper-acute and acute phases post-stroke. In reviewing the literature to determine safe timelines of when to initiate exercise and mobilization we considered the following factors: arterial blood pressure dysregulation, cardiac complications, blood-brain barrier disruption, hemorrhagic stroke transformation, and ischemic penumbra viability. These stroke-related impairments could intensify with inappropriate mobilization/aerobic exercise, hence we deemed the integrity of cerebral autoregulation to be an essential physiological consideration to protect the brain when progressing exercise intensity. Pre-participation screening criteria are proposed and countermeasures to protect the brain from potentially adverse circulatory effects before, during, and following mobilization/exercise sessions are introduced. For example, prolonged periods of standing and static postures before and after mobilization/aerobic exercise may elicit blood pooling and/or trigger coagulation cascades and/or cerebral hypoperfusion. Countermeasures such as avoiding prolonged standing or incorporating periodic lower limb movement to activate the venous muscle pump could counteract blood pooling after an exercise session, minimize activation of the coagulation cascade, and mitigate potential cerebral hypoperfusion. We discuss patient safety in light of the complex nature of stroke presentations (i.e., type, severity, and etiology), medical history, comorbidities such as diabetes, cardiac manifestations, medications, and complications such as anemia and dehydration. The guidelines are easily incorporated into the care model, are low-risk, and use minimal resources. These and other strategies represent opportunities for improving the safety of the activity regimen offered to those in the early phases post-stroke. The timeline for initiating and progressing exercise/mobilization parameters are contingent on recovery stages both from neurobiological and cardiovascular perspectives, which to this point have not been specifically considered in practice. This review includes tailored exercise and mobilization prescription strategies and precautions that are not resource intensive and prioritize safety in stroke recovery.

Cerebral venous circulation changes caused by aneurysmal subarachnoid hemorrhage

BACKGROUND

The aneurysmal subarachnoid hemorrhage (aSAH) is an acute severe hemorrhagic stroke with high morbidity and mortality with poor prognosis.

OBJECTIVE

This study aims to analyze the changes of cerebral venous circulation in patients with aneurysmal subarachnoid hemorrhage by digital subtraction angiography (DSA).

MATERIALS AND METHODS

Totally, 57 patients with aSAH, 48 patients with unruptured aneurysms, and 45 patients without aneurysms (control group) were enrolled. The microvascular cerebral circulation time (mCCT), venous cerebral circulation time (vCCT), cerebral arterioles and cortical veins were analyzed by DSA.

RESULTS

There were changes of cerebral microvessels and cortical veins in patients with aSAH. The mCCT (6.15±1.37 s) and vCCT (2.79±0.34 s) of aSAH patients significantly increased compared with control patients (3.74±0.50 s; 2.64±0.32 s) (P < 0.05). However, the mCCT increased more compared with vCCT in aSAH patients (P < 0.001), while the vCCT increased more compared with mCCT in severe aSAH cases (P < 0.01). There was no significant difference in mCCT and vCCT between patients with unruptured aneurysms and controls (P = 0.131; P = 0.621).

CONCLUSIONS

The mCCT increases in acute aSAH patients within 72 hours and vCCT increases in severe aSAH cases.

Can interhemispheric desynchronization of cerebral blood flow anticipate upcoming vasospasm in aneurysmal subarachnoid haemorrhage patients?

BACKGROUND

Asymmetry of cerebral autoregulation (CA) was demonstrated in patients after aneurysmal subarachnoid haemorrhage (aSAH). A classical method for CA assessment requires simultaneous measurement of both arterial blood pressure (ABP) and cerebral blood flow velocity (CBFV). In this study, we have proposed a cerebral blood flow asymmetry index based only on CBFV and analysed its association with the occurrence of vasospasm after aSAH.

NEW METHOD

The phase shifts (PS) between slow oscillations in left and right CBFV (side-to-side PS) and between ABP and CBFV (CBFV-ABP PS) were estimated using multichannel matching pursuit (MMP) and cross-spectral analysis.

RESULTS

We retrospectively analysed data collected from 45 aSAH patients (26 with vasospasm). Data were analysed up to 7th day after aSAH unless the vasospasm was detected earlier. A progressive asymmetry, manifested by a gradual increase in side-to-side PS on consecutive days after aSAH, was observed in patients who developed vasospasm (R_adj² = 0.14, p = 0.009). In these patients, early side-to-side PS was more positive than in patients without vasospasm (2.8° ± 5.6° vs -1.7° ± 5.7°, p = 0.011). No such a difference was found in CBFV-ABP PS. Patients with positive side-to-side PS were more likely to develop vasospasm than patients with negative side-to-side PS (21/7 vs 5/12, p = 0.0047).

COMPARISON WITH EXISTING METHOD

MMP, in contrast to the spectral approach, accounts for non-stationarity of analysed signals. MMP applied to the PS estimation reflects the cerebral blood flow asymmetry in aSAH better than the spectral analysis.

CONCLUSIONS

Changes in side-to-side PS might be helpful to identify patients who are at risk of vasospasm.

The Link Between Cerebrovascular Hemodynamics and Rehabilitation Outcomes After Aneurysmal Subarachnoid Hemorrhage

OBJECTIVE

The aim of the study was to assess the relation between cerebrovascular function early after aneurysmal subarachnoid hemorrhage onset and functional and rehabilitation outcomes.

DESIGN

Observational cohort study of subarachnoid hemorrhage patients (n = 133) admitted to rehabilitation (n = 49), discharged home (n = 52), or died before discharge (n = 10). We obtained hemodynamic markers of cerebral autoregulatory function from blood flow velocities in the middle cerebral artery and arterial pressure waveforms, recorded daily on days 2-4 after symptom onset, and functional independence measure (FIM) scores and FIM efficiency for those admitted to acute rehabilitation.

RESULTS

Compared to those discharged home, the range of pressures within which autoregulation is effective was lower in patients admitted to rehabilitation (4.6 [0.2] vs. 3.9 [0.2] mm Hg) and those who died (2.7 [0.4], P = 0.04). For those admitted to rehabilitation, autoregulatory range and the ability of cerebrovasculature to increase flow were related to discharge FIM score (R = 0.33 and 0.43, P < 0.01) and efficiency (R = 0.33 and 0.47 P < 0.01). The latter marker, along with subarachnoid hemorrhage severity and admission FIM, explained 84% and 69% of the variability in discharge FIM score and efficiency, respectively, even after accounting for age.

CONCLUSIONS

Early cerebrovascular function is a major contributor to functional outcomes after subarachnoid hemorrhage and may represent a modifiable target to develop therapeutic approaches.

TO CLAIM CME CREDITS

Complete the self-assessment activity and evaluation online at http://www.physiatry.org/JournalCME CME OBJECTIVES: Upon completion of this article, the reader should be able to: (1) Define cerebral autoregulation; (2) Explain the importance of the integrity of cerebral autoregulation for longer-term functional and rehabilitation outcomes after aneurysmal subarachnoid hemorrhage; and (3) Theorize why treatment strategies that may be effective in reducing large-vessel vasospasms after an aneurysmal subarachnoid hemorrhage might not always translate into improved functional outcomes.

LEVEL

Advanced ACCREDITATION: The Association of Academic Physiatrists is accredited by the Accreditation Council for Continuing Medical Education to provide continuing medical education for physicians.The Association of Academic Physiatrists designates this Journal-based CME activity for a maximum of 0.5 AMA PRA Category 1 Credit(s)™. Physicians should only claim credit commensurate with the extent of their participation in the activity.

Antioxidant Melatonin: Potential Functions in Improving Cerebral Autoregulation After Subarachnoid Hemorrhage

Subarachnoid hemorrhage (SAH) is a subtype of stroke with high mortality and morbidity. Impaired cerebral autoregulation following SAH has been reported owing to effects on sympathetic control, endothelial function, myogenic response, and cerebral metabolism. Impaired cerebral autoregulation is associated with early brain injury, cerebral vasospasm/delayed cerebral ischemia, and SAH prognosis. However, few drugs have been reported to improve cerebral autoregulation after SAH. Melatonin is a powerful antioxidant that is effective (easily crosses the blood brain barrier) and safe (tolerated in large doses without toxicity). Theoretically, melatonin may impact the control mechanisms of cerebral autoregulation via antioxidative effects, protection of endothelial cell integrity, suppression of sympathetic nerve activity, increase in nitric oxide bioavailability, mediation of the myogenic response, and amelioration of hypoxemia. Furthermore, melatonin may have a comprehensive effect on cerebral autoregulation. This review discusses the potential effects of melatonin on cerebral autoregulation following SAH, in terms of the association between pharmacological activities and the mechanisms of cerebral autoregulation.

Cerebral Autoregulation in Stroke

PURPOSE OF REVIEW

Cerebral autoregulation (CA) is a mechanism that maintains cerebral blood flow constant despite fluctuations in systemic arterial blood pressure. This review will focus on recent studies that measured CA non-invasively in acute cerebrovascular events, a feature unique to the transcranial Doppler ultrasound. We will summarize the rationale for CA assessment in acute cerebrovascular disorders and specifically evaluate the existing data on the value of CA measures in relation to clinical severity, guiding management decisions, and prognostication.

RECENT FINDINGS

Existing data suggest that CA is generally impaired in various cerebrovascular disorders. In patients with small vessel ischemic stroke, CA has been shown to be impaired in both hemispheres, whereas in large territorial strokes, CA impairment has been limited to the affected hemisphere. In these latter patients, impaired CA is also predictive of secondary complications such as hemorrhagic transformation and cerebral edema, hence worse functional outcome. In patients with carotid stenosis, impaired CA may also be associated with a higher ipsilateral hemispheric stroke risk. CA is also strongly linked to outcome in patients with intracranial hemorrhage. In patients with intraparenchymal hemorrhage, CA impairment correlated with clinical and imaging severity, whereas in those with subarachnoid hemorrhage, CA measures have a predictive value for development of delayed cerebral ischemia and radiographic vasospasm. Assessment of CA is increasingly more accessible in acute cerebrovascular disorders and promises to be a valuable measure in guiding hemodynamic management and predicting secondary complication, thus enhancing the care of these patients in the acute setting.

Sport-Related Concussion Alters Indices of Dynamic Cerebral Autoregulation

Sport-related concussion is known to affect a variety of brain functions. However, the impact of this brain injury on cerebral autoregulation (CA) is poorly understood. Thus, the goal of the current study was to determine the acute and cumulative effects of sport-related concussion on indices of dynamic CA. Toward this end, 179 elite, junior-level (age 19.6 ± 1.5 years) contact sport (ice hockey, American football) athletes were recruited for preseason testing, 42 with zero prior concussions and 31 with three or more previous concussions. Eighteen athletes sustained a concussion during that competitive season and completed follow-up testing at 72 h, 2 weeks, and 1 month post injury. Beat-by-beat arterial blood pressure (BP) and middle cerebral artery blood velocity (MCAv) were recorded using finger photoplethysmography and transcranial Doppler ultrasound, respectively. Five minutes of repetitive squat-stand maneuvers induced BP oscillations at 0.05 and 0.10 Hz (20- and 10-s cycles, respectively). The BP-MCAv relationship was quantified using transfer function analysis to estimate Coherence (correlation), Gain (amplitude ratio), and Phase (timing offset). At a group level, repeated-measures ANOVA indicated that 0.10 Hz Phase was significantly reduced following an acute concussion, compared to preseason, by 23% (-0.136 ± 0.033 rads) at 72 h and by 18% (-0.105 ± 0.029 rads) at 2 weeks post injury, indicating impaired autoregulatory functioning; recovery to preseason values occurred by 1 month. Athletes were cleared to return to competition after a median of 14 days (range 7-35), implying that physiologic dysfunction persisted beyond clinical recovery in many cases. When comparing dynamic pressure buffering between athletes with zero prior concussions and those with three or more, no differences were observed. Sustaining an acute sport-related concussion induces transient impairments in the capabilities of the cerebrovascular pressure-buffering system that may persist beyond 2 weeks and may be due to a period of autonomic dysregulation. Athletes with a history of three or more concussions did not exhibit impairments relative to those with zero prior concussions, suggesting recovery of function over time. Findings from this study support the potential need to consider physiological recovery in deciding when patients should return to play following a concussion.

Human cerebrovascular function in health and disease: insights from integrative approaches

Background

The marked increase in the size of the brain, and consequently, in neural processing capability, throughout human evolution is the basis of the higher cognitive function in humans. However, greater neural, and thus information processing capability, comes at a significant metabolic cost; despite its relatively small size, the modern human brain consumes almost a quarter of the glucose and oxygen supply in the human body.

Fortunately, several vascular mechanisms ensure sufficient delivery of glucose and oxygen to the active neural tissue (neurovascular coupling), prompt removal of neural metabolic by-products (cerebral vasoreactivity), and constant global blood supply despite daily variations in perfusion pressure (cerebral autoregulation). The aim of this review is to provide an integrated overview of the available data on these vascular mechanisms and their underlying physiology. We also briefly review modern experimental approaches to assess these mechanisms in humans, and further highlight the importance of these mechanisms for humans’ evolutionary success by providing examples of their healthy adaptations as well as pathophysiological alterations.

Conclusions

Data reviewed in this paper demonstrate the importance of the cerebrovascular function to support humans’ unique ability to form new and different interactions with each other and their surroundings. This highlights that there is much insight into the neural and cognitive functions that could be gleaned from interrogating the cerebrovascular function.

Transcranial Doppler: a stethoscope for the brain-neurocritical care use

Transcranial Doppler (TCD) ultrasonography is a noninvasive bedside monitoring technique that can evaluate cerebral blood flow hemodynamics in the intracranial arterial vasculature. TCD allows assessment of linear cerebral blood flow velocity, with a high temporal resolution and is inexpensive, reproducible, and portable. The aim of this review is to provide an overview of the most commonly used TCD derived signals and measurements used commonly in neurocritical care. We describe both basic (flow velocity, pulsatility index) and advanced concepts, including critical closing pressure, wall tension, autoregulation, noninvasive intracranial pressure, brain compliance, and cerebrovascular time constant; we also describe the clinical applications of TCD to highlight their utility in the diagnosis and monitoring of cerebrovascular diseases as the "stethoscope for the brain."

The change of plasma pituitary adenylate cyclase-activating polypeptide levels after aneurysmal subarachnoid hemorrhage

OBJECTIVE

Elevated circulating pituitary adenylate cyclase-activating polypeptide (PACAP) levels have been demonstrated to be associated with clinical outcomes of severe traumatic brain injury. The current study aimed to confirm whether elevated plasma PACAP levels are predictive of clinical outcomes of aneurysmal subarachnoid hemorrhage (aSAH).

MATERIALS AND METHODS

One hundred and eighteen aSAH patients and 118 controls were recruited. Plasma PACAP concentrations were determined using enzyme-linked immunosorbent assay. Patients were followed up until death or completion of 6 months after aSAH. An unfavorable outcome was defined as Glasgow Outcome Scale score of 1-3.

RESULTS

The admission PACAP levels were significantly elevated in all patients (296.6 ± 119.7 pg/ml) compared with controls (77.1 ± 17.9 pg/ml, P < 0.001). Plasma PACAP levels were independently associated with clinical severity indicated by World Federation of Neurological Surgeons (WFNS) score (t = 4.745, P < 0.001) and Fisher score (t = 4.239, P < 0.001) using a multivariate linear regression. PACAP was identified as an independent predictor for 6-month mortality [odds ratio (OR), 1.014; 95% confidence interval (CI), 1.005-1.030; P < 0.001] and 6-month unfavorable outcome (OR, 1.012; 95% CI, 1.006-1.028; P < 0.001) and 6-month overall survival (hazard ratio, 1.016; 95% CI, 1.008-1.023; P < 0.001) using a binary logistic regression analysis and a Cox's proportional hazard analysis, respectively. PACAP had similar predictive values compared with WFNS score and Fisher score according to the receiver operating characteristic curve analysis.

CONCLUSIONS

Higher plasma PACAP levels are associated with clinical severity and long-term prognosis of aSAH patients, and PACAP has potential to be a good prognostic biomarker of aSAH.

Pathophysiologic differences in cerebral autoregulation after subarachnoid hemorrhage

OBJECTIVE

To understand the physiologic basis of impaired cerebral autoregulation in subarachnoid hemorrhage (SAH) and its relationship to neurologic outcomes.

METHODS

The cohort included 121 patients with nontraumatic SAH admitted to a neurointensive critical care unit from March 2010 to May 2015. Vasospasm was ascertained from digital subtraction angiography and delayed cerebral ischemia (DCI) was defined as new cerebral infarction on high-resolution CT. Cerebral blood flow and beat-by-beat pressure were recorded daily on days 2-4 after admission. Autoregulatory capacity was quantified from pressure flow relation via projection pursuit regression. The main outcome was early alterations in autoregulatory mechanisms as they relate to vasospasm and DCI.

RESULTS

Forty-three patients developed only vasospasm, 9 only DCI, and 14 both. Autoregulatory capacity correctly predicted DCI in 86% of training cohort patients, generalizing to 80% of the patients who were not included in the original model. Patients who developed DCI had a distinct autoregulatory profile compared to patients who did not develop secondary complications or those who developed only vasospasm. The rate of decrease in flow was significantly steeper in response to transient reductions in pressure. The rate of increase in flow was markedly lower, suggesting a diminished ability to increase flow despite transient increases in pressure.

CONCLUSIONS

The extent and nature of impairment in autoregulation accurately predicts neurologic complications on an individual patient level, and suggests potentially differential impairments in underlying physiologic mechanisms. A better understanding of these can lead to targeted interventions to mitigate neurologic morbidity.

Hemodynamic variability and cerebrovascular control after transient cerebral ischemia

We investigated if hemodynamic variability, cerebral blood flow (CBF) regulation, and their interrelationships differ between patients with transient ischemic attack (TIA) and controls. We recorded blood pressure (BP) and bilateral middle cerebral artery flow velocity (MCAv) in a cohort of TIA patients (n = 17), and age-matched controls (n = 15). Spontaneous fluctuations in BP and MCAv were characterized by spectral power analysis, and CBF regulation was assessed by wavelet phase synchronization analysis in the very low- (0.02–0.07 Hz), low- (0.07–0.20 Hz), and high-frequency (0.20–0.40 Hz) ranges. Furthermore, cerebrovascular CO₂ reactivity was assessed as a second metric of CBF regulation by inducing hypercapnia with 8% CO₂ inhalation followed by hyperventilation driven hypocapnia. We found that TIA was associated with higher BP power (group effect, P < 0.05), but not MCAv power (P = 0.11). CBF regulation (assessed by wavelet phase synchronization and CO₂ reactivity) was intact in patients (all P ≥ 0.075) across both hemispheres (all P ≥ 0.51). Pooled data (controls and affected hemisphere of patients) showed that BP and MCAv power were positively correlated at all frequency ranges (R² = 0.20–0.80, all P < 0.01). Furthermore, LF phase synchronization index was a significant determinant of MCAv power (P < 0.05), while VLF and HF phase synchronization index, and TIA were not (all P ≥ 0.50). These results indicate that CBF stability and control is maintained in TIA patients, but BPV is markedly elevated. BPV attenuation may be an important therapeutic strategy for enhancing secondary stroke prevention in patients who suffer a TIA.

Noninvasive cerebral oximetry during endovascular therapy for acute ischemic stroke: an observational study

Implementing endovascular stroke care often impedes neurologic assessment in patients who need sedation or general anesthesia. Cerebral near-infrared spectroscopy (NIRS) may help physicians monitor cerebral tissue viability, but data in hyperacute stroke patients receiving endovascular treatment are sparse. In this observational study, the NIRS index regional oxygen saturation (rSO2) was measured noninvasively before, during, and after endovascular therapy via bilateral forehead NIRS optodes. During the study period, 63 patients were monitored with NIRS; 43 qualified for analysis. Before recanalization, 10 distinct rSO2 decreases occurred in 11 patients with respect to time to intubation. During recanalization, two kinds of unilateral rSO2 changes occurred in the affected hemisphere: small peaks throughout the treatment (n=14, 32.6%) and sustained increases immediately after recanalization (n=2, 4.7%). Lower area under the curve 10% below baseline was associated with better reperfusion status (thrombolysis in cerebral infarction ≥ 2b, P=0.009). At the end of the intervention, lower interhemispheric rSO2 difference predicted death within 90 days (P=0.037). After the intervention, higher rSO2 variability predicted poor outcome (modified Rankin scale > 3, P=0.032). Our findings suggest that bi-channel rSO2-NIRS has potential for guiding neuroanesthesia and predicting outcome. To better monitor local revascularization, an improved stroke-specific set-up in future studies is necessary.

Cerebral vasospasm affects arterial critical closing pressure

The effect of cerebral vasospasm (CVS) after aneurysmal subarachnoid hemorrhage (SAH) on critical closing pressure (CrCP) has not been fully delineated. Using cerebral impedance methodology, we sought to assess the behavior of CrCP during CVS. As CrCP expresses the sum of intracranial pressure (ICP) and vascular wall tension, we also explored its role in reflecting changes in vascular tone occurring in small vessels distal to spasm. This retrospective analysis was performed using recordings from 52 patients, diagnosed with CVS through transcranial Doppler measurements. Critical closing pressure was calculated noninvasively using arterial blood pressure and blood flow velocity. Outcome was assessed at both discharge and 3 months after ictus with the Glasgow Outcome Scale. The onset of CVS caused significant decreases in CrCP (P=0.025), without any observed significant changes in ICP (P=0.134). Vasospasm induced asymmetry, with CrCP ipsilateral to CVS becoming significantly lower than contralateral (P=0.025). Unfavorable outcomes were associated with a significantly lower CrCP after the onset of CVS (discharge: P=0.014; 3 months after SAH: P=0.020). Critical closing pressure is reduced in the presence of CVS in both temporal and spatial assessments. As ICP remained unchanged during CVS, reduced CrCP most probably reflects a lower wall tension in dilated small vessels distal to spasm.