Cerebral autoregulation of blood velocity and volumetric flow during steady-state changes in arterial pressure

In vivo phenotypic vascular dysfunction extends beyond the aorta in a mouse model for fibrillin-1 (Fbn1) mutation

In individuals with Marfan Syndrome (MFS), fibrillin-1 gene (FBN1) mutations can lead to vascular wall weakening and dysfunction. The experimental mouse model of MFS (Fbn1C1041G/+) has been advantageous in investigating MFS-associated life-threatening aortic aneurysms. It is well established that the MFS mouse model exhibits an accelerated-aging phenotype in elastic organs like the aorta, lung, and skin. However, the impact of Fbn1 mutations on the in vivo function and structure of various artery types with the consideration of sex and age, has not been adequately explored in real-time and a clinically relevant context. In this study, we investigate if Fbn1 mutation contributes to sex-dependent alterations in central and cerebral vascular function similar to phenotypic changes associated with normal aging in healthy control mice. In vivo ultrasound imaging of central and cerebral vasculature was performed in 6-month-old male and female MFS and C57BL/6 mice and sex-matched 12-month-old (middle-aged) healthy control mice. Our findings confirm aortic enlargement (aneurysm) and wall stiffness in MFS mice, but with exacerbation in male diameters. Coronary artery blood flow velocity (BFV) in diastole was not different but left pulmonary artery BFV was decreased in MFS and 12-month-old control mice regardless of sex. At 6 months of age, MFS male mice show decreased posterior cerebral artery BFV as compared to age-matched control males, with no difference observed between female cohorts. Reduced mitral valve early-filling velocities were indicated in MFS mice regardless of sex. Male MFS mice also demonstrated left ventricular hypertrophy. Overall, these results underscore the significance of biological sex in vascular function and structure in MFS mice, while highlighting a trend of pre-mature vascular aging phenotype in MFS mice that is comparable to phenotypes observed in older healthy controls. Furthermore, this research is a vital step in understanding MFS's broader implications and sets the stage for more in-depth future analyses, while providing data-driven preclinical justification for re-evaluating diagnostic approaches and therapeutic efficacy.

Effects of phenylephrine on systemic and cerebral circulations in humans: a systematic review with mechanistic explanations

We conducted a systematic review of the literature reporting phenylephrine-induced changes in blood pressure, cardiac output, cerebral blood flow and cerebral tissue oxygen saturation as measured by near-infrared spectroscopy in humans. We used the proportion change of the group mean values reported by the original studies in our analysis. Phenylephrine elevates blood pressure whilst concurrently inducing a reduction in cardiac output. Furthermore, despite increasing cerebral blood flow, it decreases cerebral tissue oxygen saturation. The extent of phenylephrine's influence on cardiac output (r = -0.54 and p = 0.09 in awake humans; r = -0.55 and p = 0.007 in anaesthetised humans), cerebral blood flow (r = 0.65 and p = 0.002 in awake humans; r = 0.80 and p = 0.003 in anaesthetised humans) and cerebral tissue oxygen saturation (r = -0.72 and p = 0.03 in awake humans; r = -0.24 and p = 0.48 in anaesthetised humans) appears closely linked to the magnitude of phenylephrine-induced blood pressure changes. When comparing the effects of phenylephrine in awake and anaesthetised humans, we found no evidence of a significant difference in cardiac output, cerebral blood flow or cerebral tissue oxygen saturation. There was also no evidence of a significant difference in effect on systemic and cerebral circulations whether phenylephrine was given by bolus or infusion. We explore the underlying mechanisms driving the phenylephrine-induced cardiac output reduction, cerebral blood flow increase and cerebral tissue oxygen saturation decrease. Individualised treatment approaches, close monitoring and consideration of potential risks and benefits remain vital to the safe and effective use of phenylephrine in acute care.

Assessment of cerebral autoregulatory function and inter-hemispheric blood flow in older adults with internal carotid artery stenosis using transcranial Doppler sonography-based measurement of transient hyperemic response after carotid artery compression

Unhealthy vascular aging promotes atherogenesis, which may lead to significant internal carotid artery stenosis (CAS) in 5 to 7.5% of older adults. The pathogenic factors that promote accelerated vascular aging and CAS also affect the downstream portion of the cerebral microcirculation in these patients. Primary treatments of significant CAS are eversion endarterectomy or endarterectomy with patch plasty. Factors that determine adequate hemodynamic compensation and thereby the clinical consequences of CAS as well as medical and surgical complications of carotid reconstruction surgery likely involve the anatomy of the circle of Willis (CoW), the magnitude of compensatory inter-hemispheric blood flow, and the effectiveness of cerebral microcirculatory blood flow autoregulation. This study aimed to test two hypotheses based on this theory. First, we hypothesized that patients with symptomatic and asymptomatic CAS would exhibit differences in autoregulatory function and inter-hemispheric blood flow. Second, we predicted that anatomically compromised CoW would associate with impaired inter-hemispheric blood flow compensation. We enrolled older adults with symptomatic or asymptomatic internal CAS (>70% NASCET criteria; n = 46) and assessed CoW integrity by CT angiography. We evaluated transient hyperemic responses in the middle cerebral arteries (MCA) after common carotid artery compression (CCC; 10 s) by transcranial Doppler sonography (TCD). We compared parameters reflecting autoregulatory function (e.g., transient hyperemic response ratio [THRR], return to baseline time [RTB], changes of vascular resistance) and inter-hemispheric blood flow (residual blood flow velocity). Our findings revealed that CAS was associated with impaired cerebral vascular reactivity. However, we did not observe significant differences in autoregulatory function or inter-hemispheric blood flow between patients with symptomatic and asymptomatic CAS. Moreover, anatomically compromised CoW did not significantly affect these parameters. Notably, we observed an inverse correlation between RTB and THRR, and 49% of CAS patients exhibited a delayed THRR, which associated with decreased inter-hemispheric blood flow. Future studies should investigate how TCD-based evaluation of autoregulatory function and inter-hemispheric blood flow can be used to optimize surgical techniques and patient selection for internal carotid artery revascularization.

In Vivo Phenotypic Vascular Dysfunction Extends Beyond the Aorta in a Mouse Model for Fibrillin-1 ( FBN1 ) Mutation

In individuals with Marfan Syndrome (MFS), fibrillin-1 gene ( FBN1 ) mutations can lead to vascular wall weakening and dysfunction. The experimental mouse model of MFS ( FBN1 C1041G/+ ) has been advantageous in investigating MFS-associated life-threatening aortic aneurysms. Although the MFS mouse model presents an accelerated-aging phenotype in elastic organs (e.g., lung, skin), the impact of FBN1 mutations on other central and peripheral arteries function and structure with the consideration of the impact of sex remains underexplored. In this study, we investigate if FBN1 mutation contributes to sex-dependent alterations in central and cerebral vascular function similar to phenotypic changes associated with normal aging in healthy control mice. In vivo ultrasound imaging of central and cerebral vasculature was performed in 6-month-old male and female MFS and C57BL/6 mice and sex-matched 12-month-old (middle-aged) healthy control mice. Our findings confirm aortic enlargement (aneurysm) and wall stiffness in MFS mice, but with exacerbation in male diameters. Coronary artery blood flow velocity (BFV) in diastole was not different but left pulmonary artery BFV was decreased in MFS and 12-month-old control mice regardless of sex. At 6 months of age, MFS male mice show decreased posterior cerebral artery BFV as compared to age-matched control males, with no difference observed between female cohorts. Reduced mitral valve early-filling velocities were indicated in MFS mice regardless of sex. Male MFS mice also demonstrated left ventricular hypertrophy. Overall, these results underscore the significance of biological sex in vascular function and structure in MFS mice, while highlighting a trend of pre-mature vascular aging phenotype in MFS mice that is comparable to phenotypes observed in older healthy controls.

Static autoregulation in humans

The process by which cerebral blood flow (CBF) remains approximately constant in response to short-term variations in arterial blood pressure (ABP) is known as cerebral autoregulation. This classic view, that it remains constant over a wide range of ABP, has however been challenged by a growing number of studies. To provide an updated understanding of the static cerebral pressure-flow relationship and to characterise the autoregulation curve more rigorously, we conducted a comprehensive literature research. Results were based on 143 studies in healthy individuals aged 18 to 65 years. The mean sensitivities of CBF to changes in ABP were found to be 1.47 ± 0.71%/% for decreased ABP and 0.37 ± 0.38%/% for increased ABP. The significant difference in CBF directional sensitivity suggests that cerebral autoregulation appears to be more effective in buffering increases in ABP than decreases in ABP. Regression analysis of absolute CBF and ABP identified an autoregulatory plateau of approximately 20 mmHg (ABP between 80 and 100 mmHg), which is much smaller than the widely accepted classical view. Age and sex were found to have no effect on autoregulation strength. This data-driven approach provides a quantitative method of analysing static autoregulation that can be easily updated as more experimental data become available.

Transcranial Doppler studies in Type 2 diabetes mellitus: A systematic review

BACKGROUND

Type II Diabetes mellitus (T2DM) patients are at the risk of developing cerebrovascular diseases, often contributed by altered cerebral haemodynamics. We present a systematic review of studies on cerebral haemodynamics assessment using transcranial Doppler (TCD) in T2DM.

REVIEW METHOD

A systematic review of the published articles in the English language between 1991 to 2021.

DATA SOURCES

Articles were retrieved via Pubmed and Cochrane library. We included Cross-sectional, prospective, retrospective, randomized controlled, and cross-over studies for this review.

RESULTS

A total of 25 articles met the inclusion criteria, which provided data for 3212 patients.

CONCLUSION

Cerebral autoregulation is often impaired among patients with T2DM. The risk increased with the duration of T2DM, related complications and presence of comorbidities.

Integrative physiological assessment of cerebral hemodynamics and metabolism in acute ischemic stroke

Restoring perfusion to ischemic tissue is the primary goal of acute ischemic stroke care, yet only a small portion of patients receive reperfusion treatment. Since blood pressure (BP) is an important determinant of cerebral perfusion, effective BP management could facilitate reperfusion. But how BP should be managed in very early phase of ischemic stroke remains a contentious issue, due to the lack of clear evidence. Given the complex relationship between BP and cerebral blood flow (CBF)-termed cerebral autoregulation (CA)-bedside monitoring of cerebral perfusion and oxygenation could help guide BP management, thereby improve stroke patient outcome. The aim of INFOMATAS is to 'identify novel therapeutic targets for treatment and management in acute ischemic stroke'. In this review, we identify novel physiological parameters which could be used to guide BP management in acute stroke, and explore methodologies for monitoring them at the bedside. We outline the challenges in translating these potential prognostic markers into clinical use.

A method to evaluate dynamic cerebral pressure-flow relationships in the conscious rat

The classic dogma of cerebral autoregulation is that cerebral blood flow is steadily maintained across a wide range of perfusion pressures. This has been challenged by recent studies suggesting little to no "autoregulatory plateau" in the relationship between cerebral blood flow and blood pressure (BP). Therefore, the mechanisms underlying the cerebral pressure-flow relationship still require further understanding. Here, we present a novel approach to examine dynamic cerebral autoregulation in conscious Wistar rats (n = 16) instrumented to measure BP and internal carotid blood flow (iCBF), as an indicator of cerebral blood flow. Transient reductions in BP were induced by occluding the vena cava via inflation of a chronically implanted intravascular silicone balloon. Falls in BP were paralleled by progressive decreases in iCBF, with no evidence of a steady-state plateau. No significant changes in internal carotid vascular resistance (iCVR) were observed. In contrast, intravenous infusions of the vasoactive drug sodium nitroprusside (SNP) produced a similar fall in BP but increases in iCBF and decreases in iCVR were observed. These data suggest a considerable confounding influence of vasodilatory drugs such as SNP on cerebrovascular tone in the rat, making them unsuitable to investigate cerebral autoregulation. We demonstrate that our technique of transient vena cava occlusion produced reliable and repeatable depressor responses, highlighting the potential for our approach to permit assessment of the dynamic cerebral pressure-flow relationship over time in conscious rats.NEW & NOTEWORTHY We present a novel technique to overcome the use of vasoactive agents when studying cerebrovascular dynamics in the conscious rat. Our method of vena cava occlusion to reduce BP was associated with decreased iCBF and no change in iCVR. In contrast, comparable BP falls with intravenous SNP increased iCBF and reduced iCVR. Thus, the dynamic cerebral pressure-flow relationship shows a narrower, less level autoregulatory plateau than conventionally thought. We confirm our method allows repeatable assessment of cerebrovascular dynamics in conscious rats.

Cerebral Autoregulation in Subarachnoid Hemorrhage

Subarachnoid hemorrhage (SAH) is a devastating stroke subtype with a high rate of mortality and morbidity. The poor clinical outcome can be attributed to the biphasic course of the disease: even if the patient survives the initial bleeding emergency, delayed cerebral ischemia (DCI) frequently follows within 2 weeks time and levies additional serious brain injury. Current therapeutic interventions do not specifically target the microvascular dysfunction underlying the ischemic event and as a consequence, provide only modest improvement in clinical outcome. SAH perturbs an extensive number of microvascular processes, including the “automated” control of cerebral perfusion, termed “cerebral autoregulation.” Recent evidence suggests that disrupted cerebral autoregulation is an important aspect of SAH-induced brain injury. This review presents the key clinical aspects of cerebral autoregulation and its disruption in SAH: it provides a mechanistic overview of cerebral autoregulation, describes current clinical methods for measuring autoregulation in SAH patients and reviews current and emerging therapeutic options for SAH patients. Recent advancements should fuel optimism that microvascular dysfunction and cerebral autoregulation can be rectified in SAH patients.

Hemodynamics in acute stroke: Cerebral and cardiac complications

Hemodynamics is the study of blood flow, where parameters have been defined to quantify blood flow and the relationship with systemic circulatory changes. Understanding these perfusion parameters, the relationship between different blood flow variables and the implications for ischemic injury are outlined in the ensuing discussion. This chapter focuses on the hemodynamic changes that occur in ischemic stroke, and their contribution to ischemic stroke pathophysiology. We discuss the interaction between cardiovascular response and hemodynamic changes in stroke. Studying hemodynamic changes has a key role in stroke prevention, therapeutic implications and prognostic importance in acute ischemic stroke: preexisting hemodynamic and autoregulatory impairments predict the occurrence of stroke. Hemodynamic failure predisposes to the formation of thromboemboli and accelerates infarction due to impairing compensatory mechanisms. In ischemic stroke involving occlusion of a large vessel, persistent collateral circulation leads to preservation of ischemic penumbra and therefore justifying endovascular thrombectomy. Following thrombectomy, impaired autoregulation may lead to reperfusion injury and hemorrhage.

Origin of the Time Lag Phenomenon and the Global Signal in Resting-State fMRI

The global mean signal of resting-state fMRI (rs-fMRI) shows a characteristic spatiotemporal pattern that is closely related to the pattern of vascular perfusion. Although being increasingly adopted in the mapping of the flow of neural activity, the mechanism that gives rise to the BOLD signal time lag remains controversial. In the present study, we compared the time lag of the global mean signal with those of the local network components obtained by applying temporal independent component analysis to the resting-state fMRI data, as well as by using simultaneous wide-field visual stimulation, and demonstrated that the time lag patterns are highly similar across all types of data. These results suggest that the time lag of the rs-fMRI signal reflects the local variance of the hemodynamic responses rather than the arrival or transit time of the stimulus, whether the trigger is neuronal or non-neuronal in origin as long as it is mediated by local hemodynamic responses. Examinations of the internal carotid artery signal further confirmed that the arterial signal is tightly inversely coupled with the global mean signal in accordance with previous studies, presumably reflecting the blood flow or blood pressure changes that are occurring almost simultaneously in the internal carotid artery and the cerebral pial/capillary arteries, within the low-frequency component in human rs-fMRI.

Powered Air Purifying Respirator (PAPR) restores the N95 face mask induced cerebral hemodynamic alterations among Healthcare Workers during COVID-19 Outbreak

Background and aim

COVID-19 pandemic has resulted in an unprecedented increased usage of Personal protective equipment (PPE) by healthcare-workers. PPE usage causes headache in majority of users. We evaluated changes in cerebral hemodynamics among healthcare-workers using PPE.

Methods

Frontline healthcare-workers donning PPE at our tertiary center were included. Demographics, co-morbidities and blood-pressure were recorded. Transcranial Doppler (TCD) monitoring of middle cerebral artery was performed with 2-MHz probe. Mean flow velocity (MFV) and pulsatility index (PI) were recorded at baseline, after donning N95 respirator-mask, and after donning powered air-purifying respirator (PAPR), when indicated. End-tidal carbon-dioxide (ET-CO2) pressure was recorded for participants donning PAPR in addition to the N95 respirator-mask.

Results

A total of 154 healthcare-workers (mean age 29 ± 12 years, 67% women) were included. Migraine was the commonest co-morbidity in 38 (25%) individuals while 123 (80%) developed de-novo headache due to N95 mask. Donning of N95 respirator-mask resulted in significant increase in MFV (4.4 ± 10.4 cm/s, p < 0.001) and decrease in PI (0.13 ± 0.12; p < 0.001) while ET-CO2 increased by 3.1 ± 1.2 mmHg (p < 0.001). TCD monitoring in 24 (16%) participants donning PAPR and N95 respirator mask together showed normalization of PI, accompanied by normalization of ET-CO2 values within 5-min. Combined use of N95 respirator-mask and PAPR was more comfortable as compared to N95 respirator-mask alone.

Conclusion

Use of N95 respirator-mask results in significant alterations in cerebral hemodynamics. However, these effects are mitigated by the use of additional PAPR. We recommend the use of PAPR together with the N95 mask for healthcare-workers doing longer duties in the hospital wards.

•

Most N95 mask users develop de novo headache or worsening of pre-existing headache.

•

End-tidal carbon dioxide levels increase while donning N95 respirator mask.

•

Significant cerebral hemodynamic changes occur during donning of N95 mask.

•

Powered air-purifying respirator restores N95 mask induced cerebral hemodynamic changes.

Variability Predictors of Vasospasm in Subarachnoid Hemorrhage: A Feasibility Study

BACKGROUND

Mean cerebral blood flow velocity (mean-CBFV) obtained from Transcranial Doppler (TCD) poorly predicts cerebral vasospasm in patients with aneurysmal subarachnoid hemorrhage (aSAH). Variability descriptors of mean-CBFV obtained during extended TCD recordings may improve this prediction. We assessed the feasibility of generating reliable linear and non-linear descriptors of mean-CBFV variability using extended recordings in aSAH patients and in healthy controls. We also explored which of those metrics might have the ability to discriminate between aSAH patients and healthy controls, and among patients who would go on to develop vasospasm and those who would not.

METHODS

Bilateral mean-CBFV, blood pressure, and heart rate were continuously recorded for 40 minutes in aSAH patients (n = 8) within the first 5 days after ictus, in age-matched healthy controls (n = 8) and in additional young controls (n = 8). We obtained linear [standard deviation, coefficient of variations, and the very-low (0.003-0.040 Hz), low (0.040-0.150 Hz), and high-frequency (0.15-0.4 Hz) power spectra] and non-linear (Fractality, deterministic Chaos analyses) variability metrics.

RESULTS

We successfully obtained TCD recordings from patients and healthy controls and calculated the desired metrics of mean-CBFV variability. Differences were appreciable between aSAH patients and healthy controls, as well as between aSAH patients who later developed vasospasm and those who did not.

CONCLUSIONS

A 40-minute TCD recording provides reliable variability metrics in aSAH patients and healthy controls. Future studies are required to determine if mean-CBFV variability metrics remain stable over time, and whether they may serve to identify patients who are at greatest risk of developing cerebral vasospasm after aSAH.

Steady-state cerebral autoregulation in older adults with amnestic mild cognitive impairment: linear mixed model analysis

We examined whether the efficacy of steady-state cerebral autoregulation (CA) is reduced in older adults with amnestic mild cognitive impairment (aMCI), a prodromal stage of clinical Alzheimer disease (AD). Forty-two patients with aMCI and 24 cognitively normal older adults (NC) of similar age, sex, and education underwent stepwise decreases and increases in mean arterial pressure (MAP) induced by intravenous infusion of sodium nitroprusside and phenylephrine, respectively. Changes in cerebral blood flow (CBF) were measured repeatedly in the internal carotid and vertebral artery. Linear mixed modeling, including random effects of both individual intercept and regression slope, was used to quantify the MAP-CBF relationship accounting for nonindependent, repeated CBF measures. Changes in end-tidal CO₂ (EtCO₂) associated with changes in MAP were also included in the model to account for their effects on CBF. Marginal mean values of MAP were reduced by 13-14 mmHg during sodium nitroprusside and increased by 20-24 mmHg during phenylephrine infusion in both groups with similar doses of drug infusion. A steeper slope of changes in CBF in response to changes in MAP was observed in aMCI relative to NC, indicating reduced efficacy of CA (MAP × Group, P = 0.040). These findings suggest that cerebrovascular dysfunction may occur early in the development of AD.NEW & NOTEWORTHY Cerebral autoregulation is a fundamental regulatory mechanism to protect brain perfusion against changes in blood pressure that, if impaired, may contribute to the development of Alzheimer's disease. Using a linear mixed model, we demonstrated that the efficacy of cerebral autoregulation, assessed during stepwise changes in arterial pressure, was reduced in individuals with amnestic mild cognitive impairment, a prodromal stage of Alzheimer's disease. These findings support the hypothesis that cerebrovascular dysfunction may be an important underlying pathophysiological mechanism for the development of clinical Alzheimer's disease.

Lower body negative pressure protects brain perfusion in aviation gravitational stress induced by push-pull manoeuvre

KEY POINTS

Rapid alterations of gravitational stress during high-performance aircraft push-pull manoeuvres induce dramatic shifts in volume and pressure within the circulation system, which may result in loss of consciousness due to the rapid and significant reduction in cerebral perfusion. There are still no specific and effective countermeasures so far. We found that lower body negative pressure (LBNP), applied prior to and during -Gz and released at the subsequent transition to +Gz, could effectively counteract gravitational haemodynamic stress induced by a simulated push-pull manoeuvre and improve cerebral diastolic perfusion in human subjects. We developed a LBNP strategy that effectively protects cerebral perfusion at rapid -Gz to +Gz transitions via improving cerebral blood flow and blood pressure during push-pull manoeuvres and highlight the importance of the timing of the intervention. Our findings provide a systemic link of integrated responses between the peripheral and cerebral haemodynamic changes during push-pull manoeuvres.

ABSTRACT

The acute negative (-Gz) to positive (+Gz) gravity stress during high-performance aircraft push-pull manoeuvres dramatically reduces transient cerebral perfusion, which may lead to loss of vision or even consciousness. The aim of this study was to explore a specific and effective counteractive strategy. Twenty-three healthy young male volunteers (age 21 ± 1 year) were subjected to tilting-simulated push-pull manoeuvres. Lower body negative pressure (LBNP) of -40 mmHg was applied prior to and during -Gz stress (-0.50 or -0.87 Gz) and released at the subsequent transition to +1.00 Gz stress. Beat-to-beat cerebral and systemic haemodynamics were continuously recorded during the simulated push-pull manoeuvre in LBNP bouts and corresponding control bouts. During the rapid gravitational transition from -Gz to +Gz, the mean cerebral blood flow velocity decreased significantly in control bouts, while it increased in LBNP bouts (control vs. LBNP bouts, -6.6 ± 4.6 vs. 5.1 ± 6.8 cm s^-1 for -0.50 Gz, and -7.4 ± 4.8 vs. 3.4 ± 4.6 cm s^-1 for -0.87 Gz, P < 0.01), which was attributed mainly to the elevation of diastolic flow. The LBNP bouts showed much smaller reduction of mean arterial blood pressure at the brain level than control bouts (control bouts vs. LBNP bouts, -38 ± 12 vs. -23 ± 10 mmHg for -0.50 to +1.00 Gz, and -62 ± 16 vs. -43 ± 11 mmHg for -0.87 to +1.00 Gz, P < 0.01). LBNP applied at -Gz and released at subsequent +Gz had biphasic counteractive effects against the gravitational responses to the push-pull manoeuvre. These data demonstrate that this LBNP strategy could effectively protect cerebral perfusion with dominant improvement of diastolic flow during push-pull manoeuvres.

The effect of age on cerebral blood flow responses during repeated and sustained stand to sit transitions

INTRODUCTION

Aging is associated with impaired cerebrovascular blood flow and function, attributed to reduced vasodilatory capacity of the cerebrovascular network. Older adults may also have an impaired relationship between changes in blood pressure and cerebral blood flow; however, previous reports conflict. This study aimed to compare the blood pressure and cerebral blood flow responses to both repeated and sustained stand-to-sit transitions in young and older adults, and to assess the relationship with cerebrovascular reactivity.

METHODS

In 20 young (age: 24 ± 4 years) and 20 older (age: 71 ± 7 years) adults we compared middle cerebral artery flow velocity (MCAv), end-tidal partial pressure of carbon dioxide (PET CO2 ), and blood pressure (mean arterial blood pressure [MAP]) during repeated stand-to-sit (10 s standing and 10 s sitting) and sustained stand-to-sit (3 min standing followed by 2 min sitting) transitions. Cerebrovascular reactivity to changes in carbon dioxide levels was assessed using a repeated breath-hold test.

RESULTS

The % change in MCAv per % change in MAP (%∆MCAv/%∆MAP) was higher in the older adults than in the young adults during repeated stand-to-sit transitions. During the sustained protocol the %∆MCAv/%∆MAP response was similar in both age groups. A high %∆MCAv/%∆MAP response during the repeated stand-to-sit protocol was associated with low cerebrovascular reactivity to CO2 (r = -.39; p < .01), which was significantly lower in the older adults.

CONCLUSION

These findings suggest that the higher %∆MCAv/%∆MAP during repeated stand-sit transitions was associated with impaired cerebrovascular reactivity. Impairments in endothelial function and vascular stiffness with age may contribute to the altered transient cerebral pressure-flow responses in older adults.

Benign Regulation of the Astrocytic Phospholipase A2-Arachidonic Acid Pathway: The Underlying Mechanism of the Beneficial Effects of Manual Acupuncture on CBF

Background

The astrocytic phospholipase A₂ (PLA₂)-arachidonic acid (AA) pathway is crucial in understanding the reduction of cerebral blood flow (CBF) prior to cognitive deterioration. In complementary and alternative medicine, manual acupuncture (MA) is used as one of the most important therapies for Alzheimer’s disease (AD). The beneficial effects of MA on CBF were reported in our previous study. However, the underlying molecular mechanism remains largely elusive.

Objective

To investigate the effect of MA on the astrocytic PLA₂-AA pathway in SAMP8 mice hippocampi.

Methods

SAMP8 mice were divided into the SAMP8 control (Pc) group, the SAMP8 MA (Pm) group and the SAMP8 donepezil (Pd) group. SAMR1 mice were used as the SAMRl control (Rc) group. Mice in the Pd group were treated with donepezil hydrochloride at 0.65 μg/g. In the Pm group, MA was applied at Baihui (GV20) and Yintang (GV29) for 20 min. The above treatments were administered once a day for 26 consecutive days. The Morris water maze was applied to assess spatial learning and memory. Immunofluorescence staining, western blot and liquid chromatography-tandem mass spectrometry were used to investigate the expression of related proteins and measure the contents of the metabolic intermediates of the PLA₂-AA pathway.

Results

Compared with that in the Rc group, the escape latency in the Pc group significantly increased (p < 0.01); whereas, the platform crossover number and percentage of time and swimming distance in the platform quadrant decreased (p < 0.01). The hippocampal expression of PLA₂, cyclooxygenase-1, cytochrome P450 proteins 2C23 and the levels of AA, prostaglandin E₂ and epoxyeicosatrienoic acids of the Pc group was drastically higher than that in the Rc group (p < 0.01). These changes were reversed by MA and donepezil (p < 0.01 or p < 0.05).

Conclusion

MA can effectively improve the learning and memory abilities of SAMP8 mice and has a negative regulatory effect on the PLA₂-AA pathway. We propose that the increase of the arterial tone, which is induced by the inhibition of vasodilatory pathway, may be a reason for the beneficial effect of MA on CBF.

Single mean arterial blood pressure drops during stroke thrombectomy under general anaesthesia are associated with poor outcome

Background

We examined the influence of periprocedural blood pressure (BP), especially critical BP drops, on 3-month functional outcome in stroke patients undergoing mechanical thrombectomy (MT) under general anaesthesia (GA).

Methods

We screened all patients with anterior circulation large vessel occlusion receiving MT under GA at our centre from January 2011 to June 2016 and selected those who had continuous invasive periinterventional BP monitoring. Clinical and radiological data were prospectively collected as part of an ongoing cohort study, monitoring data were extracted from electronic anaesthesia records. We used uni- and multivariable regression to investigate the association of BP values with unfavourable outcome, defined as modified Rankin Scale scores 3–6 3 months post-stroke.

Results

115 patients were included in this study (mean age 65.3 ± 13.0 years, 55.7% male). Periinterventional systolic, diastolic, and mean arterial BP (MAP) values averaged across MT had no effect on outcome. However, single BP drops were related to unfavourable outcome, with absolute MAP drops showing the highest association compared to both systolic and relative BP drops (with reference to pre-interventional values). The BP value with the strongest association with unfavourable outcome was identified as an MAP ever < 60 mmHg (p = 0.01) with a pronounced effect in patients with poor collaterals. An MAP < 60 mmHg remained independently associated with poor functional outcome in multivariable analysis (p < 0.01).

Conclusions

For patients undergoing MT under GA, single MAP drops < 60 mmHg are independently related to unfavourable 3-month outcome. Therefore, every effort should be made to prevent periinterventional hypotensive episodes, especially below this threshold.

Electronic supplementary material

The online version of this article (10.1007/s00415-020-09701-x) contains supplementary material, which is available to authorized users.

Comparative Analysis of Simultaneous Transcranial Doppler and Perfusion Computed Tomography for Cerebral Perfusion Evaluation in Patients with Traumatic Brain Injury

The aim was to investigate the feasibility of simultaneous comparison of cerebral circulation in major vessels and microvasculature in patients suffering traumatic brain injury (TBI) with or without intracranial hematomas (IH).

METHODS

170 patients were divided into two groups: Group 1 - diffuse TBI (75 patients); and Group 2 - TBI with IH (95 patients: 18 epidural, 65 subdural and 12 multiple). Perfusion computed tomography (PCT) for assessment of volumetric cerebral blood flow (CBF) was done 2-15 days after admission to hospital. Simultaneous assessment of cerebral blood flow velocity (CBFV) in both middle cerebral arteries was done by transcranial Doppler.

RESULTS

In patients with diffuse TBI, CBF had statistically valid correlations with CBFV (r = 0.28, p = 0.0149 on the left side; r = 0.382, p = 0.00075 on the right side). In patients with TBI and IH, the analysis did not reveal any reliable correlations between the CBFV and CBF velocity in the temporal lobes, either on the side of the removed IH or on the opposite side.

CONCLUSION

The greatest linear correlation was noted in patients with diffuse TBI without the development of a coarse shift of the midline structures and dislocation syndrome. This correlation decreases with the increase in injury severity and development of secondary complications in the acute period, which probably reflects impairment of the coupling of oxygen consumption by brain tissue and cerebral microcirculation.

Ultrasound vector projectile imaging for detection of altered carotid bifurcation hemodynamics during reductions in cardiac output

PURPOSE

Complex blood flow is commonly observed in the carotid bifurcation, although the factors that regulate these patterns beyond arterial geometry are unknown. The emergence of high-frame-rate ultrasound vector flow imaging allows for noninvasive, time-resolved analysis of complex hemodynamic behavior in humans, and it can potentially help researchers understand which physiological stressors can alter carotid bifurcation hemodynamics in vivo. Here, we seek to pursue the first use of vector projectile imaging (VPI), a dynamic form of vector flow imaging, to analyze the regulation of carotid bifurcation hemodynamics during experimental reductions in cardiac output induced via a physiological stressor called lower body negative pressure (LBNP).

METHODS

Seven healthy adults (age: 27 ± 4 yr, 4 men) underwent LBNP at -45 mmHg to simulate a postural hemodynamic response in a controlled environment. Using a research-grade, high-frame-rate ultrasound platform, vector flow estimation in each subject's right carotid bifurcation was performed through a multi-angle plane wave imaging (two transmission angles of 10° and -10°) formulation, and VPI cineloops were generated at a frame rate of 750 fps. Vector concentration was quantified by the resultant blood velocity vector angles within a region of interest; lower concentration indicated greater flow dispersion. Discrete concentration values during peak and late systole were compared across different segments of the carotid artery bifurcation before, and during, LBNP.

RESULTS

Vector projectile imaging revealed that external and internal carotid arteries exhibited regional hemodynamic changes during LBNP, which acted to reduce both the subject's cardiac output (Δ - 1.2 ± 0.5 L/min, -19%; P < 0.01) and peak carotid blood velocity (Δ - 6.30 ± 8.27 cm/s, -7%; P = 0.05). In these carotid artery branches, the vector concentration time trace before and during LBNP were observed to be different. The impact of LBNP on flow complexity in the two carotid artery branches showed variations between subjects.

CONCLUSIONS

Using VPI, intuitive visualization of complex hemodynamic changes can be obtained in healthy humans subjected to LBNP. This imaging tool has potential for further applications in vascular physiology to identify and quantify complex hemodynamic features in humans during different physiological stressor tests that regulate hemodynamics.

The effect of steady-state CO2 on regional brain blood flow responses to increases in blood pressure via the cold pressor test

The pressure-passive cerebrovasculature is affected by alterations in cerebral perfusion pressure (CPP) and arterial blood gases (e.g., pressure of arterial [Pa]CO₂), where acute changes in either stimulus can influence cerebral blood flow (CBF). The effect of superimposed increases in CPP at different levels of steady-state PaCO₂ on regional CBF regulation is unclear. In 17 healthy participants, we simultaneously recorded continuous heart rate (electrocardiogram), blood pressure (finometer), pressure of end-tidal CO₂ (P_ETCO₂; gas analyzer), and middle (MCA) and posterior (PCA) cerebral artery blood velocity (CBV; transcranial Doppler ultrasound). Three separate CPTs were administered by passive immersion of both feet into 0-1 °C of ice water for 3-min under three randomized and coached steady-state P_ETCO₂ conditions: normocapnia (room air), hypocapnia (-10 Torr; hyperventilation) and hypercapnia (+9 Torr; 5% inspired CO₂;). CBV responses were calculated as the absolute difference (∆) between baseline and mean MCAv and PCAv during the 3-min CPT. Both the ∆MCAv and ∆PCAv responses to the CPT were larger under hypercapnic conditions. The absolute ∆MCAv response was larger than the ∆PCAv during the CPT across all three CO₂ trials. Cerebrovascular CO₂ reactivity (CVR) was larger in the MCA than PCA in both CPT and baseline conditions, but there were no differences in CVR between CPT and baseline conditions. Our data indicate that (a) increases in CO₂ increases the CBV responses to a CPT, (b) the anterior cerebrovasculature is more responsive to a CPT-induced increases in MAP, and (c) although unchanged during a CPT, CVR is larger in the anterior cerebral circulation.

Cerebral Blood Flow, Oxygen Delivery, and Pulsatility Responses to Oxygen Inhalation at High Altitude: Highlanders vs. Lowlanders

Objective: To determine whether the acute cerebral hemodynamic responses to oxygen inhalation are impacted by race or acclimation to high altitude.

Methods: Three groups of young healthy males, who were Tibetans (highlanders, n = 15) with lifelong exposure to high altitude, and Han Chinese (lowlanders) with five-year (Han-5 yr, n = 15) and three-day (Han-3 d, n = 16) exposures, participated in the study at an altitude of 3658 m. Cerebral blood flow velocity (CBFV) was recorded for three minutes prior to and during pure oxygen inhalation (2 L/min), respectively, using a transcranial color-coded duplex (TCCD) sonography at the middle cerebral artery (MCA). The blood draw and simultaneous monitoring of blood pressure (BP), heart rate (HR), and finger arterial oxygen saturation (SaO₂) were also performed.

Results: Values are Mean ± SEM. The three groups had similar demographic characteristics and HR responses, with the group differences (P < 0.05) found in hemoglobin concentration (16.9 ± 0.9, 18.4 ± 1.3, and 15.5 ± 1.0 gm/dL), baseline BPs and HR as expected. Both the Tibetans and Han-5yr groups presented blunted BP responses to O₂-inhalation when compared to the Han-3d group; more interestingly, the Tibetans showed significantly reduced responses compared with Han-5yr and Han-3d in CBFV, cerebral oxygen delivery (COD), and pulsatility index (PI) as assessed by Δ%CBFV/ΔSaO₂ (-1.50 ± 0.25 vs. -2.24 ± 0.25 and -2.23 ± 0.27, P = 0.049 and 0.048), Δ%COD/ΔSaO₂ (-0.52 ± 0.27 vs. -1.33 ± 0.26 and -1.38 ± 0.28, P = 0.044 and 0.031), and Δ%PI (7 ± 2 vs. 16 ± 3 and 16 ± 3 %, P = 0.036 and 0.023), respectively.

Conclusion: These findings provide evidence on the Tibetans trait of a distinct cerebral hemodynamic regulatory pattern to keep more stable cerebral blood flow (CBF), oxygen delivery, and pulsatility in response to oxygen inhalation as compared with Han Chinese, which is likely due to a genetic adaptation to altitude.

Slow sinusoidal tilt movements demonstrate the contribution to orthostatic tolerance of cerebrospinal fluid movement to and from the spinal dural space

Standing up elicits a host of cardiovascular changes which all affect the cerebral circulation. Lowered mean arterial blood pressure (ABP) at brain level, change in the cerebral venous outflow path, lowered end-tidal PCO2 (PET CO2 ), and intracranial pressure (ICP) modify cerebral blood flow (CBF). The question we undertook to answer is whether gravity-induced blood pressure (BP) changes are compensated in CBF with the same dynamics as are spontaneous or induced ABP changes in a stable position. Twenty-two healthy subjects (18/4 m/f, 40 ± 8 years) were subjected to 30° and 70° head-up tilt (HUT) and sinusoidal tilts (SinTilt, 0°↨60° around 30° at 2.5-10 tilts/min). Additionally, at those three tilt levels, they performed paced breathing at 6-15 breaths/min to induce larger than spontaneous cardiovascular oscillations. We measured continuous finger BP and cerebral blood flow velocity (CBFv) in the middle cerebral artery by transcranial Doppler to compute transfer functions (TFs) from ABP- to CBFv oscillations. SinTilt induces the largest ABP oscillations at brain level with CBFv gains strikingly lower than for paced breathing or spontaneous variations. This would imply better autoregulation for dynamic gravitational changes. We demonstrate in a mathematical model that this difference is explained by ICP changes due to movement of cerebrospinal fluid (CSF) into and out of the spinal dural sack. Dynamic cerebrovascular autoregulation seems insensitive to how BP oscillations originate if the effect of ICP is factored in. CSF-movement in-and-out of the spinal dural space contributes importantly to orthostatic tolerance by its effect on cerebral perfusion pressure.

Sodium nitroprusside dilates cerebral vessels and enhances internal carotid artery flow in young men

KEY POINTS

Sodium nitroprusside lowers blood pressure by vasodilatation but is reported to reduce cerebral blood flow. In healthy young men sodium nitroprusside reduced blood pressure, total peripheral resistance, and arterial CO₂ tension and yet cerebral blood flow was maintained, with an increase in internal carotid artery blood flow and cerebrovascular conductance. Sodium nitroprusside induces both systemic and cerebral vasodilatation affecting internal carotid artery more than vertebral artery flow.

ABSTRACT

Cerebral autoregulation maintains cerebral blood flow (CBF) despite marked changes in mean arterial pressure (MAP). Sodium nitroprusside (SNP) reduces blood pressure by vasodilatation but is reported to lower CBF, probably by a reduction in its perfusion pressure. We evaluated the influence of SNP on CBF and aimed for a 20% and then 40% reduction in MAP, while keeping MAP ≥ 50 mmHg, to challenge cerebral autoregulation. In 19 healthy men (age 24 ± 4 years; mean ± SD) duplex ultrasound determined right internal carotid (ICA) and vertebral artery (VA) blood flow. The SNP reduced MAP (from 83 ± 8 to 69 ± 8 and 58 ± 4 mmHg; both P < 0.0001), total peripheral resistance, and arterial CO₂ tension (P aC O2; 41 ± 3 vs. 39 ± 3 and 37 ± 4 mmHg; both P < 0.01). Yet ICA flow increased with the moderate reduction in MAP but returned to the baseline value with the large reduction in MAP (336 ± 66 vs. 365 ± 69; P = 0.013 and 349 ± 82 ml min^-1 ; n.s.), while VA flow (114 ± 34 vs. 112 ± 38 and 110 ± 42 ml min^-1 ; both n.s.) and CBF ((ICA + VA flow) × 2; 899 ± 135 vs. 962 ± 127 and 918 ± 197 ml min^-1 ; both n.s.) were maintained with increased cerebrovascular conductance. In conclusion, CBF is maintained during SNP-induced reduction in MAP despite reduced P aC O2 and the results indicate that SNP dilates cerebral vessels and increases ICA flow.

Cerebral blood flow changes during tilt table testing in healthy volunteers, as assessed by Doppler imaging of the carotid and vertebral arteries

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Extracranial cerebral artery Doppler imaging show CBF changes during tilt testing.

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Total CBF during tilt testing decreases 6% in healthy volunteers.

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Flow decrease of internal carotid and vertebral arteries during tilting is similar.

Objectives

Using different techniques, reduction of cerebral blood flow (CBF) during orthostatic stress were demonstrated. One study reported flow reduction of the right internal carotid (ICA) and vertebral (VA) artery during orthostatic stress by Doppler imaging, with different effects on the 2 vessels. Global CBF changes, using this technique, have not been reported. Therefore, flow of the ICA, VA and global CBF were measured during head-up tilt testing.

Methods

33 healthy volunteers underwent tilt testing. At three time points (supine, half way and at the end of the test) Doppler imaging of the ICA and VA was performed, as well as PetCO2 measurements.

Results

Global CBF was significantly reduced by 4.5 ± 2.8% halfway the test and by 6.0 ± 3.4% at the end. All 4 artery flows were significantly reduced during the tilt, without differences between them. Despite small changes in PetCO2 there was a significant relation between de CBF decrease and PetCO2 decrease (p < 0.05).

Conclusions

Orthostatic stress in HV results in a small but significant reduction of CBF by a homogenous reduction in the four cerebral vessels and is modulated by PetCO2 changes.

Significance

CBF changes can be measured during tilt testing using Doppler VA and ICA imaging.

Lack of linear correlation between dynamic and steady-state cerebral autoregulation

Key points

For correct application and interpretation of cerebral autoregulation (CA) measurements in research and in clinical care, it is essential to understand differences and similarities between dynamic and steady‐state CA.

The present study found no correlation between dynamic and steady‐state CA indices in healthy older adults.

There was variability between individuals in all (steady‐state and dynamic) autoregulatory indices, ranging from low (almost absent) to highly efficient CA in this healthy population.

These findings challenge the assumption that assessment of a single CA parameter or a single set of parameters can be generalized to overall CA functioning. Therefore, depending on specific research purposes, the choice for either steady‐state or dynamic measures or both should be weighed carefully.

Abstract

The present study aimed to investigate the relationship between dynamic (dCA) and steady‐state cerebral autoregulation (sCA). In 28 healthy older adults, sCA was quantified by a linear regression slope of proportionate (%) changes in cerebrovascular resistance (CVR) in response to proportionate (%) changes in mean blood pressure (BP) induced by stepwise sodium nitroprusside (SNP) and phenylephrine (PhE) infusion. Cerebral blood flow (CBF) was measured at the internal carotid artery (ICA) and vertebral artery (VA) and CBF velocity at the middle cerebral artery (MCA). With CVR = BP/CBF, Slope‐CVR_ICA, Slope‐CVR_VA and Slope‐CVRi_MCA were derived. dCA was assessed (i) in supine rest, analysed with transfer function analysis (gain and phase) and autoregulatory index (ARI) fit from spontaneous oscillations (ARI_Baseline), and (ii) with transient changes in BP using a bolus injection of SNP (ARI_SNP) and PhE (ARI_PhE). Comparison of sCA and dCA parameters (using Pearson's r for continuous and Spearman's ρ for ordinal parameters) demonstrated a lack of linear correlations between sCA and dCA measures. However, comparisons of parameters within dCA and within sCA were correlated. For sCA slope‐CVR_VA with Slope‐CVRi_MCA (r = 0.45, P < 0.03); for dCA ARI_SNP with ARI_PhE (ρ = 0.50, P = 0.03), ARI_Baseline (ρ = 0.57, P = 0.03) and Phase_LF (ρ = 0.48, P = 0.03); and for Gain_VLF with Gain_LF (r = 0.51, P = 0.01). By contrast to the commonly held assumption based on an earlier study, there were no linear correlations between sCA and dCA. As an additional observation, there was strong inter‐individual variability, both in dCA and sCA, in this healthy group of elderly, in a range from low to high CA efficiency.

For correct application and interpretation of cerebral autoregulation (CA) measurements in research and in clinical care, it is essential to understand differences and similarities between dynamic and steady‐state CA.

The present study found no correlation between dynamic and steady‐state CA indices in healthy older adults.

There was variability between individuals in all (steady‐state and dynamic) autoregulatory indices, ranging from low (almost absent) to highly efficient CA in this healthy population.

These findings challenge the assumption that assessment of a single CA parameter or a single set of parameters can be generalized to overall CA functioning. Therefore, depending on specific research purposes, the choice for either steady‐state or dynamic measures or both should be weighed carefully.

Dynamic Cerebral Autoregulation Assessment Using Extracranial Internal Carotid Artery Doppler Ultrasonography

Transcranial Doppler ultrasonography of the middle cerebral artery (MCA) is frequently used to assess dynamic cerebral autoregulation (dCA); however, this is difficult in patients with poor temporal bone windows. In the study described here, we investigated the agreement and sensitivity of dCA indices determined from the extracranial internal carotid artery (ICA) and those determined from the MCA. Measurements for 32 stroke patients and 59 controls were analyzed. Measurement of the mean flow correlation index (Mx) and transfer function analysis based on spontaneous blood pressure fluctuation were simultaneously performed for the extracranial ICA and MCA. The mean values of Mx and phase shift did not significantly differ between the ICA and MCA (mean difference: Mx = 0.01; phase shift of very low frequency [VLF] = 0.7°, low frequency [LF] = 3.3° and high frequency = 4.5°), but the gains in VLF and LF in the ICA were significantly lower than those in the MCA (mean difference: gain of VLF = -0.13, gain of LF = -0.10). The intra-class correlation coefficient between the dCA indices of the ICA and MCA was favorable in Mx (0.76) and the phase shift of VLF (0.72). The area under the receiver operating characteristic curve for stroke diagnosis did not differ among the dCA indices. We conclude that dCA assessed from the ICA is as effective as that from the MCA, but the results are not interchangeable.

Extra- and intracranial blood flow regulation during the cold pressor test: influence of age

We determined how the extra- and intracranial circulations respond to generalized sympathetic activation evoked by a cold pressor test (CPT) and whether this is affected by healthy aging. Ten young [23 ± 2 yr (means ± SD)] and nine older (66 ± 3 yr) individuals performed a 3-min CPT by immersing the left foot into 0.8 ± 0.3°C water. Common carotid artery (CCA) and internal carotid artery (ICA) diameter, velocity, and flow were simultaneously measured (duplex ultrasound) along with middle cerebral artery and posterior cerebral artery mean blood velocity (MCAv_mean and PCAv_mean) and cardiorespiratory variables. The increases in heart rate (~6 beats/min) and mean arterial blood pressure (~14 mmHg) were similar in young and older groups during the CPT (P < 0.01 vs. baseline). In the young group, the CPT elicited an ~5% increase in CCA diameter (P < 0.01 vs. baseline) and a tendency for an increase in CCA flow (~12%, P = 0.08); in contrast, both diameter and flow remained unchanged in the older group. Although ICA diameter was not changed during the CPT in either group, ICA flow increased (~8%, P = 0.02) during the first minute of the CPT in both groups. Whereas the CPT elicited an increase in MCAv_mean and PCAv_mean in the young group (by ~20 and ~10%, respectively, P < 0.01 vs. baseline), these intracranial velocities were unchanged in the older group. Collectively, during the CPT, these findings suggest a differential mechanism(s) of regulation between the ICA compared with the CCA in young individuals and a blunting of the CCA and intracranial responses in older individuals.NEW & NOTEWORTHY Sympathetic activation evoked by a cold pressor test elicits heterogeneous extra- and intracranial blood vessel responses in young individuals that may serve an important protective role. The extra- and intracranial responses to the cold pressor test are blunted in older individuals.

Impaired cerebral autoregulation: measurement and application to stroke

Cerebral autoregulation (CA) is a protective mechanism that maintains cerebral blood flow at a relatively constant level despite fluctuations of cerebral perfusion pressure or arterial blood pressure. It is a universal physiological mechanism that may involve myogenic, neural control as well as metabolic regulations of cerebral vasculature in response to changes in pressure or cerebral blood flow. Traditionally, CA has been represented by a sigmoid curve with a wide plateau between about 50 mm Hg and 170 mm Hg of steady-state changes in mean arterial pressure, defined as static CA. With the advent of transcranial Doppler, measurement of cerebral blood flow in response to transient changes in arterial pressure has been used to assess dynamic CA. However, a gold standard for measuring CA is not currently available. Stroke has been the leading cause of long-term adult disability throughout the world. A better understanding of CA and its response to pathological derangements can help assess the severity of stroke, guide management decisions, assess response to interventions and provide prognostic information. The objective of this review is to provide a comprehensive insight about physiology of autoregulation, measurement methodologies and clinical applications in stroke to help build a consensus for what should be included in an internationally agreed protocol for CA testing and monitoring, and to promote its translation into clinical bedside practice for stroke management.

New Wavelet Neurovascular Bundle for Bedside Evaluation of Cerebral Autoregulation and Neurovascular Coupling in Newborns with Hypoxic-Ischemic Encephalopathy

Neonatal encephalopathy (NE) resulting from birth asphyxia constitutes a major global public health burden for millions of infants every year, and despite therapeutic hypothermia, half of these neonates have poor neurological outcomes. As new neuroprotective interventions are being studied in clinical trials, there is a critical need to establish physiological surrogate markers of therapeutic efficacy, to guide patient selection and/or to modify the therapeutic intervention. The challenge in the field of neonatal brain injury has been the difficulty of clinically discerning NE severity within the short therapeutic window after birth or of analyzing the dynamic aspects of the cerebral circulation in sick NE newborns. To address this roadblock, we have recently developed a new "wavelet neurovascular bundle" analytical system that can measure cerebral autoregulation (CA) and neurovascular coupling (NVC) at multiple time scales under dynamic, nonstationary clinical conditions. This wavelet analysis may allow noninvasive quantification at the bedside of (1) CA (combining metrics of blood pressure and cerebral near-infrared spectroscopy, NIRS) and (2) NVC (combining metrics obtained from NIRS and EEG) in newborns with encephalopathy without mathematical assumptions of linear and stationary systems. In this concept paper, we present case examples of NE using the proposed physiological wavelet metrics of CA and NVC. The new approach, once validated in large NE studies, has the potential to optimize the selection of candidates for therapeutic decision-making, and the prediction of neurocognitive outcomes.

Oscillatory lower body negative pressure impairs working memory task-related functional hyperemia in healthy volunteers

Neurovascular coupling (NVC) describes the link between an increase in task-related neural activity and increased cerebral blood flow denoted "functional hyperemia." We previously showed induced cerebral blood flow oscillations suppressed functional hyperemia; conversely functional hyperemia also suppressed cerebral blood flow oscillations. We used lower body negative pressure (OLBNP) oscillations to force oscillations in middle cerebral artery cerebral blood flow velocity (CBFv). Here, we used N-back testing, an intellectual memory challenge as a neural activation task, to test the hypothesis that OLBNP-induced oscillatory cerebral blood flow can reduce functional hyperemia and NVC produced by a working memory task and can interfere with working memory. We used OLBNP (-30 mmHg) at 0.03, 0.05, and 0.10 Hz and measured spectral power of CBFv at all frequencies. Neither OLBNP nor N-back, alone or combined, affected hemodynamic parameters. 2-Back power and OLBNP individually were compared with 2-back power during OLBNP. 2-Back alone produced a narrow band increase in oscillatory arterial pressure (OAP) and oscillatory cerebral blood flow power centered at 0.0083 Hz. Functional hyperemia in response to 2-back was reduced to near baseline and 2-back memory performance was decreased by 0.03-, 0.05-, and 0.10-Hz OLBNP. OLBNP alone produced increased oscillatory power at frequencies of oscillation not suppressed by added 2-back. However, 2-back preceding OLBNP suppressed OLBNP power. OLBNP-driven oscillatory CBFv blunts NVC and memory performance, while memory task reciprocally interfered with forced CBFv oscillations. This shows that induced cerebral blood flow oscillations suppress functional hyperemia and functional hyperemia suppresses cerebral blood flow oscillations.NEW & NOTEWORTHY We show that induced cerebral blood flow oscillations suppress functional hyperemia produced by a working memory task as well as memory task performance. We conclude that oscillatory cerebral blood flow produces causal reductions of memory task neurovascular coupling and memory task performance. Reductions of functional hyperemia are constrained by autoregulation.

Effects of race and sex on cerebral hemodynamics, oxygen delivery and blood flow distribution in response to high altitude

To assess racial, sexual, and regional differences in cerebral hemodynamic response to high altitude (HA, 3658 m). We performed cross-sectional comparisons on total cerebral blood flow (TCBF = sum of bilateral internal carotid and vertebral arterial blood flows = Q_ICA + Q_VA), total cerebrovascular resistance (TCVR), total cerebral oxygen delivery (TCOD) and Q_VA/TCBF (%), among six groups of young healthy subjects: Tibetans (2-year staying) and Han (Han Chinese) at sea level, Han (2-day, 1-year and 5-year) and Tibetans at HA. Bilateral ICA and VA diameters and flow velocities were derived from duplex ultrasonography; and simultaneous measurements of arterial pressure, oxygen saturation, and hemoglobin concentration were conducted. Neither acute (2-day) nor chronic (>1 year) responses showed sex differences in Han, except that women showed lower TCOD compared with men. Tibetans and Han exhibited different chronic responses (percentage alteration relative to the sea-level counterpart value) in TCBF (−17% vs. 0%), TCVR (22% vs. 12%), TCOD (0% vs. 10%) and Q_VA/TCBF (0% vs. 2.4%, absolute increase), with lower resting TCOD found in SL- and HA-Tibetans. Our findings indicate racial but not sex differences in cerebral hemodynamic adaptations to HA, with Tibetans (but not Han) demonstrating an altitude-related change of CBF distribution.

Correlation Between the Reduction in Lenticulostriate Arteries Caused by Hypertension and Changes in Brain Metabolism Detected With MRI

OBJECTIVE

Hypertension can alter the vascular structure, mechanics, and function of small arteries and arterioles. It remains unknown whether microvascular changes are associated with brain metabolism. The purpose of this study was to analyze the correlation between the reduction in small arteries and changes in brain metabolism in patients with hypertension.

SUBJECTS AND METHODS

The study population comprised 50 patients with hypertension and 50 volunteers without hypertension. The two groups underwent 3-T 3D time-of-flight MR angiography, and the numbers of lenticulostriate arteries (LSAs) were determined for both groups. Single-voxel proton MR spectroscopic data on the basal ganglia regions were also acquired. The ratios of N-acetylaspartate to creatine (NAA/Cr), myo-inositol to creatine (Mi/Cr), and choline to creatine (Cho/Cr) were measured. Statistical analysis was performed to evaluate the differences between the two groups with respect to metabolite ratios.

RESULTS

The average total number of LSA stems on both sides in patients with hypertension was 5.12 ± 0.98 compared with 6.10 ± 0.95 in volunteers without hypertension (p < 0.0001). The NAA/Cr ratio decreased according to a reduction in the number of LSAs in the hypertension group, which was significantly reduced when the number of LSAs was 3 or fewer.

CONCLUSION

Hypertension can lead to a statistically significant reduction in NAA/Cr ratio in the basal ganglia regions when the number of LSAs decreases to a certain extent. Reduced numbers of LSAs correlated with brain metabolism changes caused by hypertension, which can provide important insights for understanding the pathophysiologic mechanism of hypertension and may be valuable in evaluating this disease.

Individual variability of cerebral autoregulation, posterior cerebral circulation and white matter hyperintensity

KEY POINTS

Cerebral autoregulation (CA) is a key mechanism to protect brain perfusion in the face of changes in arterial blood pressure, but little is known about individual variability of CA and its relationship to the presence of brain white matter hyperintensity (WMH) in older adults, a type of white matter lesion related to cerebral small vessel disease (SVD). This study demonstrated the presence of large individual variability of CA in healthy older adults during vasoactive drug-induced changes in arterial pressure assessed at the internal carotid and vertebral arteries. We also observed, unexpectedly, that it was the 'over-' rather than the 'less-reactive' CA measured at the vertebral artery that was associated with WMH severity. These findings challenge the traditional concept of CA and suggest that the presence of cerebral SVD, manifested as WMH, is associated with posterior brain hypoperfusion during acute increase in arterial pressure.

ABSTRACT

This study measured the individual variability of static cerebral autoregulation (CA) and determined its associations with brain white matter hyperintensity (WMH) in older adults. Twenty-seven healthy older adults (13 females, 66 ± 6 years) underwent assessment of CA during steady-state changes in mean arterial pressure (MAP) induced by intravenous infusion of sodium nitroprusside (SNP) and phenylephrine. Cerebral blood flow (CBF) was measured using colour-coded duplex ultrasonography at the internal carotid (ICA) and vertebral arteries (VA). CA was quantified by a linear regression slope (CA slope) between percentage changes in cerebrovascular resistance (CVR = MAP/CBF) and MAP relative to baseline values. Periventricular and deep WMH volumes were measured with T2-weighted magnetic resonance imaging. MAP was reduced by -11 ± 7% during SNP, and increased by 21 ± 8% during phenylephrine infusion. CA demonstrated large individual variability with the CA slopes ranging from 0.37 to 2.20 at the ICA and from 0.17 to 3.18 at the VA; no differences in CA were found between the ICA and VA. CA slopes measured at the VA had positive correlations with the total and periventricular WMH volume (r = 0.55 and 0.59, P < 0.01). Collectively, these findings demonstrated the presence of large individual variability of CA in older adults, and that, when measured in the posterior cerebral circulation, it is the higher rather than lower CA reactivity that is associated with WMH severity.

Oscillatory lower body negative pressure impairs task related functional hyperemia in healthy volunteers

Neurovascular coupling refers to the link between an increase in neural activity in response to a task and an increase in cerebral blood flow denoted "functional hyperemia." Recent work on postural tachycardia syndrome indicated that increased oscillatory cerebral blood flow velocity (CBFv) was associated with reduced functional hyperemia. We hypothesized that a reduction in functional hyperemia could be causally produced in healthy volunteers by using oscillations in lower body negative pressure (OLBNP) to force oscillations in CBFv. CBFv was measured by transcranial Doppler ultrasound of the left middle cerebral artery. We used passive arm flexion applied during eight periodic 60-s flexion/60-s relaxation epochs to produce 120-s periodic changes in functional hyperemia (at 0.0083 Hz). We used -30 mmHg of OLBNP at 0.03, 0.05, and 0.10 Hz, the range for cerebral autoregulation, and measured spectral power of CBFv at all frequencies. Arm flexion power performed without OLBNP was compared with arm flexion power during OLBNP. OLBNP power performed in isolation was compared with power during OLBNP plus arm flexion. Cerebral flow velocity oscillations at 0.05 Hz reduced and at 0.10 Hz eliminated functional hyperemia, while 0.03 Hz did not reach significance. In contrast, arm flexion reduced OLBNP-induced oscillatory power at all frequencies. The interactions between OLBNP-driven CBFv oscillations and arm flexion-driven CBFv oscillations are reciprocal. Thus induced cerebral blood flow oscillations suppress functional hyperemia, and functional hyperemia suppresses cerebral blood flow oscillations. We conclude that oscillatory cerebral blood flow produces a causal reduction of functional hyperemia.

Feasibility and Safety of Using External Counterpulsation to Augment Cerebral Blood Flow in Acute Ischemic Stroke-The Counterpulsation to Upgrade Forward Flow in Stroke (CUFFS) Trial

BACKGROUND

External counterpulsation (ECP) increases perfusion to a variety of organs and may be helpful for acute stroke.

METHODS

We conducted a single-blinded, prospective, randomized controlled feasibility and safety trial of ECP for acute middle cerebral artery (MCA) ischemic stroke. Twenty-three patients presenting within 48 hours of symptom onset were randomized into one of two groups. One group was treated with ECP for 1 hour at a pressure of up to 300 mmHg ("full pressure"). During the procedure, we also determined the highest possible pressure that would augment MCA mean flow velocity (MFV) by 15%. The other group was treated with ECP at 75 mmHg ("sham pressure"). Transcranial Doppler MCA flow velocities and National Institutes of Health Stroke Scale (NIHSS) scores of both groups were checked before, during, and after ECP. Outcomes were assessed at 30 days after randomization.

RESULTS

Although the procedures were feasible to implement, there was a frequent inability to augment MFV by 15% despite maximal pressures in full-pressure patients. In sham-pressure patients, however, MFV frequently increased as shown by increases in peak systolic velocity and end diastolic velocity. In both groups, starting ECP was often associated with contemporaneous improvements in NIHSS stroke scores. There were no between-group differences in NIHSS, modified Rankin Scale Scores, and Barthel Indices, and no device or treatment-related serious adverse events, deaths, intracerebral hemorrhages, or episodes of acute neuro-worsening.

CONCLUSIONS

ECP was safe and feasible to use in patients with acute ischemic stroke. It was associated with unexpected effects on flow velocity, and contemporaneous improvements in NIHSS score regardless of pressure used, with a possibility that even very low ECP pressures had an effect. Further study is warranted.

Heterogeneous patterns of vasoreactivity in the middle cerebral and internal carotid arteries

This study compared changes in cross-sectional area (CSA) and flow (Q) between the middle cerebral artery (MCA) and the internal carotid artery (ICA) at baseline and during 5 min of hypercapnia (HC; 6% CO2) and hypocapnia (HO; hyperventilation) and quantified how these changes contribute to estimates of cerebrovascular reactivity (CVR). Measures of MCA CSA were made using 3T magnetic resonance imaging. On a separate day, MCA flow velocity was measured with transcranial Doppler ultrasound and ICA diameters and flow velocity were measured with duplex ultrasound. Fourteen subjects (23 ± 3 yr, 7 females) participated, providing data for 11 subjects during HC and 9 subjects during HO. An increase in MCA CSA ( P < 0.05) was observed within the first minute of HC. During HO, the decrease in MCA CSA ( P < 0.05) was delayed until minute 4. No changes were observed in ICA CSA during HC or HO. The relative changes in QICA and QMCA were similar during HC and HO. Therefore, the MCA, but not ICA, dilates and constricts during 5 min of HC and HO, respectively. The consequent impact on QMCA significantly affects estimates of CVR, and reactivity cannot be attributed solely to changes in smaller arterioles.

Prior head-down tilt does not impair the cerebrovascular response to head-up tilt

The hypothesis that cerebrovascular autoregulation was not impaired during head-up tilt (HUT) that followed brief exposures to varying degrees of prior head-down tilt (HDT) was tested in 10 healthy young men and women. Cerebral mean flow velocity (MFV) and cardiovascular responses were measured in transitions to a 60-s period of 75° HUT that followed supine rest (control) or 15 s HDT at -10°, -25°, and -55°. During HDT, heart rate (HR) was reduced for -25° and -55°, and cardiac output was lower at -55° HDT. MFV increased during -10° HDT, but not in the other conditions even though blood pressure at the middle cerebral artery (BPMCA) increased. On the transition to HUT, HR increased only for -55° condition, but stroke volume and cardiac output transiently increased for -25° and -55°. Total peripheral resistance index decreased in proportion to the magnitude of HDT and recovered over the first 20 s of HUT. MFV was significantly less in all HDT conditions compared with the control in the first 5-s period of HUT, but it recovered quickly. An autoregulation correction index derived from MFV recovery relative to BPMCA decline revealed a delay in the first 5 s for prior HDT compared with control but then a rapid increase to briefly exceed control after -55° HDT. This study showed that cerebrovascular autoregulation is modified by but not impaired by brief HDT prior to HUT and that cerebral MFV recovered quickly and more rapidly than arterial blood pressure to protect against cerebral hypoperfusion and potential syncope.

Oscillatory cerebral blood flow is associated with impaired neurocognition and functional hyperemia in postural tachycardia syndrome during graded tilt

We hypothesize that upright cognitive impairment in patients with postural tachycardia syndrome (POTS) is caused by reduced cerebral blood flow (CBF). The CBF velocity (CBF(v)) measured by transcranial Doppler ultrasound decreased excessively during 70° tilt in a minority of patients with intermittent hyperpnea/hypocapnia. Incremental tilt showed no difference in mean CBF(v). But N-back memory tasking indicated progressive compromised memory, reduced functional hyperemia, and reduced neurovascular coupling. Orthostasis caused slow oscillations in CBF(v) linked to oscillations in arterial pressure in patients with POTS. We also hypothesize that oscillatory CBF(v) degrades neurovascular coupling. We performed 2-back testing when subjects were in supine position and during incremental tilts to 15°, 30°, 45°, and 60° in 11 patients with POTS and 9 controls. Oscillatory arterial pressure, oscillatory CBF(v), and neurovascular coupling were similar in supine position. The oscillatory arterial pressure increased by 31%, 45%, 67%, and 93% in patients with POTS during tilt and remained unchanged in the controls. Oscillatory CBF(v) increased by 61%, 82%, 161%, and 264% in patients with POTS during tilt and remained unchanged in the controls. Functional hyperemia decreased from 4.1% to 3.0%, 1.1%, 0.2%, and to 0.04% in patients with POTS, but it was unchanged at 4% in the controls. Percent correct N-back responses decreased from 78% to 33% in patients with POTS, whereas they remained at 89% in the controls. In patients with POTS, oscillatory CBF(v) was linearly correlated with functional hyperemia (r(2)=0.76). Increased oscillatory CBF is associated with reduced neurovascular coupling and diminished cognitive performance in patients with POTS.

Impact of hypocapnia and cerebral perfusion on orthostatic tolerance

We examined two novel hypotheses: (1) that orthostatic tolerance (OT) would be prolonged when hyperventilatory-induced hypocapnia (and hence cerebral hypoperfusion) was prevented; and (2) that pharmacological reductions in cerebral blood flow (CBF) at baseline would lower the 'CBF reserve', and ultimately reduce OT. In study 1 (n = 24; aged 25 ± 4 years) participants underwent progressive lower-body negative pressure (LBNP) until pre-syncope; end-tidal carbon dioxide (P ET , CO 2) was clamped at baseline levels (isocapnic trial) or uncontrolled. In study 2 (n = 10; aged 25 ± 4 years), CBF was pharmacologically reduced by administration of indomethacin (INDO; 1.2 mg kg(-1)) or unaltered (placebo) followed by LBNP to pre-syncope. Beat-by-beat measurements of middle cerebral artery blood flow velocity (MCAv; transcranial Doppler), heart rate (ECG), blood pressure (BP; Finometer) and end-tidal gases were obtained continuously. In a subset of subjects' arterial-to-jugular venous differences were obtained to examine the independent impact of hypocapnia or cerebral hypoperfusion (following INDO) on cerebral oxygen delivery and extraction. In study 1, during the isocapnic trial, P ET , CO 2 was successfully clamped at baseline levels at pre-syncope (38.3 ± 2.7 vs. 38.5 ± 2.5 mmHg respectively; P = 0.50). In the uncontrolled trial, P ET , CO 2 at pre-syncope was reduced by 10.9 ± 3.9 mmHg (P ≤ 0.001). Compared to the isocapnic trial, the decline in mean MCAv was 15 ± 4 cm s(-1) (35%; P ≤ 0.001) greater in the uncontrolled trial, yet the time to pre-syncope was comparable between trials (544 ± 130 vs. 572 ± 180 s; P = 0.30). In study 2, compared to placebo, INDO reduced resting MCAv by 19 ± 4 cm s(-1) (31%; P ≤ 0.001), but time to pre-syncope remained similar between trials (placebo: 1123 ± 138 s vs. INDO: 1175 ± 212 s; P = 0.53). The brain extracted more oxygen in face of hypocapnia (34% to 53%) or cerebral hypoperfusion (34% to 57%) to compensate for reductions in delivery. In summary, cerebral hypoperfusion either at rest or induced by hypocapnia at pre-syncope does not impact OT, probably due to a compensatory increase in oxygen extraction.

Cerebral blood flow velocity underestimates cerebral blood flow during modest hypercapnia and hypocapnia

To establish the accuracy of transcranial Doppler ultrasound (TCD) measures of middle cerebral artery (MCA) cerebral blood flow velocity (CBFV) as a surrogate of cerebral blood flow (CBF) during hypercapnia (HC) and hypocapnia (HO), we examined whether the cross-sectional area (CSA) of the MCA changed during HC or HO and whether TCD-based estimates of CBFV were equivalent to estimates from phase contrast (PC) magnetic resonance imaging. MCA CSA was measured from 3T magnetic resonance images during baseline, HO (hyperventilation at 30 breaths/min), and HC (6% carbon dioxide). PC and TCD measures of CBFV were measured during these protocols on separate days. CSA and TCD CBFV were used to calculate CBF. During HC, CSA increased from 5.6 ± 0.8 to 6.5 ± 1.0 mm2 ( P < 0.001, n = 13), while end-tidal carbon dioxide partial pressure (PetCO2) increased from 37 ± 3 to 46 ± 5 Torr ( P < 0.001). During HO, CSA decreased from 5.8 ± 0.9 to 5.3 ± 0.9 mm2 ( P < 0.001, n = 15), while PetCO2 decreased from 36 ± 4 to 23 ± 3 Torr ( P < 0.001). CBFVs during baseline, HO, and HC were compared between PC and TCD, and the intraclass correlation coefficient was 0.83 ( P < 0.001). The relative increase from baseline was 18 ± 8% greater ( P < 0.001) for CBF than TCD CBFV during HC, and the relative decrease of CBF during HO was 7 ± 4% greater than the change in TCD CBFV ( P < 0.001). These findings challenge the assumption that the CSA of the MCA does not change over modest changes in PetCO2.

Fundamental relationships between blood pressure and cerebral blood flow in humans

Cerebral blood flow responses to transient blood pressure challenges are frequently attributed to cerebral autoregulation (CA), yet accumulating evidence indicates vascular properties like compliance are also influential. We hypothesized that middle cerebral blood velocity (MCAv) dynamics during or following a transient blood pressure perturbation can be accurately explained by the windkessel mechanism. Eighteen volunteers underwent blood pressure manipulations, including bilateral thigh-cuff deflation and sit-to-stand maneuvers under normocapnic and hypercapnic (5% CO2) conditions. Pressure-flow recordings were analyzed using a windkessel analysis approach that partitions the frequency-dependent resistance and compliance contributions to MCAv dynamics. The windkessel was typically able to explain more than 50% of the MCAv variance, as indicated by R2 values for both the flow recovery and postrecovery phase. The most consistent predictors of MCAv dynamics under the control condition were the windkessel capacitive gain and high-frequency resistive gain. However, there were significant interindividual variations in the composition of windkessel predictors. Hypercapnia consistently reduced the capacitive gain and enhanced the low-frequency (0.04–0.20 Hz) resistive gain for both thigh-cuff deflation and sit-to-stand trials. These findings indicate that 1) MCAv dynamics during acute transient hypotension challenges are dominated by cerebrovascular windkessel properties independent of CA; 2) there is significant heterogeneity in windkessel properties between individuals; and 3) hemodynamic effects of hypercapnia during transient blood pressure challenges primarily reflect changes in windkessel properties rather than pure CA impairment.

Blood tracer kinetics in the arterial tree

Evaluation of blood supply of different organs relies on labeling blood with a suitable tracer. The tracer kinetics is linear: Tracer concentration at an observation site is a linear response to an input somewhere upstream the arterial flow. The corresponding impulse response functions are currently treated empirically without incorporating the relation to the vascular morphology of an organ. In this work we address this relation for the first time. We demonstrate that the form of the response function in the entire arterial tree is reduced to that of individual vessel segments under approximation of good blood mixing at vessel bifurcations. The resulting expression simplifies significantly when the geometric scaling of the vascular tree is taken into account. This suggests a new way to access the vascular morphology in vivo using experimentally determined response functions. However, it is an ill-posed inverse problem as demonstrated by an example using measured arterial spin labeling in large brain arteries. We further analyze transport in individual vessel segments and demonstrate that experimentally accessible tracer concentration in vessel segments depends on the measurement principle. Explicit expressions for the response functions are obtained for the major middle part of the arterial tree in which the blood flow in individual vessel segments can be treated as laminar. When applied to the analysis of regional cerebral blood flow measurements for which the necessary arterial input is evaluated in the carotid arteries, present theory predicts about 20% underestimation, which is in agreement with recent experimental data.

Intraoperative blood pressure and cerebral perfusion: strategies to clarify hemodynamic goals

Blood pressure can vary considerably during anesthesia. If blood pressure falls outside the limits of cerebrovascular autoregulation, children can become at risk of cerebral ischemic or hyperemic injury. However, the blood pressure limits of autoregulation are unclear in infants and children, and these limits can shift after brain injury. This article will review autoregulation, considerations for the hemodynamic management of children with brain injuries, and research on autoregulation monitoring techniques.