The sit-to-stand technique for the measurement of dynamic cerebral autoregulation

Intensive antihypertensive treatment does not lower cerebral blood flow or cause orthostatic hypotension in frail older adults

This study aimed to examine the effects of intensive antihypertensive treatment (AHT), i.e., systolic blood pressure target ≤ 140 mmHg, on cerebral blood flow, cerebral autoregulation, and orthostatic hypotension, in a representative population of frail older adults. Fourteen frail hypertensive patients (six females; age 80.3 ± 5.2 years; Clinical Frailty Scale 4-7; unattended SBP ≥ 150 mmHg) underwent measurements before and after a median 7-week AHT targeting SBP ≤ 140 mmHg. Transcranial Doppler measurements of middle cerebral artery velocity (MCAv), reflecting changes in cerebral blood flow (CBF), were combined with finger plethysmography recordings of continuous BP. Transfer function analysis assessed cerebral autoregulation (CA). ANCOVA analysed AHT-induced changes in CBF and CA and evaluated non-inferiority of the relative change in CBF (margin: -10%; covariates: pre-AHT values and AHT-induced relative mean BP change). McNemar-tests analysed whether the prevalence of OH and initial OH, assessed by sit/supine-to-stand challenges, increased with AHT. Unattended mean arterial pressure decreased by 15 mmHg following AHT. Ten (71%) participants had good quality TCD assessments. Non-inferiority was confirmed for the relative change in MCAv (95%CI: -2.7, 30.4). CA remained normal following AHT (P > 0.05), and the prevalence of OH and initial OH did not increase (P ≥ 0.655). We found that AHT in frail, older patients does not reduce CBF, impair autoregulation, or increase (initial) OH prevalence. These observations may open doors for more intensive AHT targets upon individualized evaluation and monitoring of hypertensive frail patients.Clinical Trial Registration: This study is registered at ClinicalTrials.gov (NCT05529147; September 1, 2022) and EudraCT (2022-001283-10; June 28, 2022).

Time-domain methods for quantifying dynamic cerebral blood flow autoregulation: Review and recommendations. A white paper from the Cerebrovascular Research Network (CARNet)

Cerebral Autoregulation (CA) is an important physiological mechanism stabilizing cerebral blood flow (CBF) in response to changes in cerebral perfusion pressure (CPP). By maintaining an adequate, relatively constant supply of blood flow, CA plays a critical role in brain function. Quantifying CA under different physiological and pathological states is crucial for understanding its implications. This knowledge may serve as a foundation for informed clinical decision-making, particularly in cases where CA may become impaired. The quantification of CA functionality typically involves constructing models that capture the relationship between CPP (or arterial blood pressure) and experimental measures of CBF. Besides describing normal CA function, these models provide a means to detect possible deviations from the latter. In this context, a recent white paper from the Cerebrovascular Research Network focused on Transfer Function Analysis (TFA), which obtains frequency domain estimates of dynamic CA. In the present paper, we consider the use of time-domain techniques as an alternative approach. Due to their increased flexibility, time-domain methods enable the mitigation of measurement/physiological noise and the incorporation of nonlinearities and time variations in CA dynamics. Here, we provide practical recommendations and guidelines to support researchers and clinicians in effectively utilizing these techniques to study CA.

Lower dynamic cerebral autoregulation following acute bout of low-volume high-intensity interval exercise in chronic stroke compared to healthy adults

Fluctuating arterial blood pressure during high-intensity interval exercise (HIIE) may challenge dynamic cerebral autoregulation (dCA), specifically after stroke after an injury to the cerebrovasculature. We hypothesized that dCA would be attenuated at rest and during a sit-to-stand transition immediately after and 30 min after HIIE in individuals poststroke compared with age- and sex-matched control subjects (CON). HIIE switched every minute between 70% and 10% estimated maximal watts for 10 min. Mean arterial pressure (MAP) and middle cerebral artery blood velocity (MCAv) were recorded. dCA was quantified during spontaneous fluctuations in MAP and MCAv via transfer function analysis. For sit-to-stand, time delay before an increase in cerebrovascular conductance index (CVCi = MCAv/MAP), rate of regulation, and % change in MCAv and MAP were measured. Twenty-two individuals poststroke (age 60 ± 12 yr, 31 ± 16 mo) and twenty-four CON (age 60 ± 13 yr) completed the study. Very low frequency (VLF) gain (P = 0.02, η2 = 0.18) and normalized gain (P = 0.01, η2 = 0.43) had a group × time interaction, with CON improving after HIIE whereas individuals poststroke did not. Individuals poststroke had lower VLF phase (P = 0.03, η2 = 0.22) after HIIE compared with CON. We found no differences in the sit-to-stand measurement of dCA. Our study showed lower dCA during spontaneous fluctuations in MCAv and MAP following HIIE in individuals poststroke compared with CON, whereas the sit-to-stand response was maintained.NEW & NOTEWORTHY This study provides novel insights into poststroke dynamic cerebral autoregulation (dCA) following an acute bout of high-intensity interval exercise (HIIE). In people after stroke, dCA appears attenuated during spontaneous fluctuations in mean arterial pressure (MAP) and middle cerebral artery blood velocity (MCAv) following HIIE. However, the dCA response during a single sit-to-stand transition after HIIE showed no significant difference from controls. These findings suggest that HIIE may temporarily challenge dCA after exercise in individuals with stroke.

Cerebral Oxygenation Responses to Standing in Young Patients with Vasovagal Syncope

Vasovagal syncope (VVS) is common in young adults and is attributed to cerebral hypoperfusion. However, during active stand (AS) testing, only peripheral and not cerebral hemodynamic responses are measured. We sought to determine whether cerebral oxygenation responses to an AS test were altered in young VVS patients when compared to the young healthy controls. A sample of young healthy adults and consecutive VVS patients attending a Falls and Syncope unit was recruited. Continuous beat-to-beat blood pressure (BP), heart rate, near-infrared spectroscopy (NIRS)-derived tissue saturation index (TSI), and changes in concentration of oxygenated/deoxygenated Δ[O₂Hb]/Δ[HHb] hemoglobin were measured. BP and NIRS-derived features included nadir, peak, overshoot, trough, recovery rate, normalized recovery rate, and steady-state. Multivariate linear regression was used to adjust for confounders and BP. In total, 13 controls and 27 VVS patients were recruited. While no significant differences were observed in the TSI and Δ[O₂Hb], there was a significantly smaller Δ[HHb] peak-to-trough and faster Δ[HHb] recovery rate in VVS patients, independent of BP. A higher BP steady-state was observed in patients but did not remain significant after multiple comparison correction. Young VVS patients demonstrated a similar cerebral circulatory response with signs of altered peripheral circulation with respect to the controls, potentially due to a hyper-reactive autonomic nervous system. This study sets the grounds for future investigations to understand the role of cerebral regulation during standing in VVS.

Force sensor reduced measurement error compared with verbal command during sit-to-stand assessment of cerebral autoregulation

Current methods estimate the time delay (TD) before the onset of dynamic cerebral autoregulation (dCA) from verbal command to stand. A force sensor used during a sit-to-stand dCA measure provides an objective moment an individual stands (arise-and-off, AO). We hypothesized that the detection of AO would improve the accuracy of TD compared with estimation. We measured middle cerebral artery blood velocity (MCAv) and mean arterial pressure (MAP) for 60 s sitting followed by 2-min standing, three times separated by 20 min. TD was calculated as the time from: (1) verbal command and (2) AO, until an increase in cerebrovascular conductance index (CVCi = MCAv/MAP). Sixty-five participants were enrolled: young adults (n = 25), older adults (n = 20), and individuals post-stroke (n = 20). The TD calculated from AO (x ¯ $$ \overline{x} $$  = 2.98 ± 1.64 s) was shorter than TD estimated from verbal command (x ¯ $$ \overline{x} $$  = 3.35 ± 1.72 s, η2  = 0.49, p < 0.001), improving measurement error by ~17%. TD measurement error was not related to age or stroke. Therefore, the force sensor provided an objective method to improve the calculation of TD compared with current methods. Our data support using a force sensor during sit-to-stand dCA measures in adults across the lifespan and post-stroke.

Cerebral autoregulation across the menstrual cycle in eumenorrheic women

There is emerging evidence that ovarian hormones play a significant role in the lower stroke incidence observed in pre‐menopausal women compared with men. However, the role of ovarian hormones in cerebrovascular regulation remains to be elucidated. We examined the blood pressure‐cerebral blood flow relationship (cerebral autoregulation) across the menstrual cycle in eumenorrheic women (n = 12; mean ± SD: age, 31 ± 7 years). Participants completed sit‐to‐stand and Valsalva maneuvers (VM, mouth pressure of 40 mmHg for 15 s) during the early follicular (EF), late follicular (LF), and mid‐luteal (ML) menstrual cycle phases, confirmed by serum measurement of progesterone and 17β‐estradiol. Middle cerebral artery blood velocity (MCAv), arterial blood pressure and partial pressure of end‐tidal carbon dioxide were measured. Cerebral autoregulation was assessed by transfer function analysis during spontaneous blood pressure oscillations, rate of regulation (RoR) during sit‐to‐stand maneuvers, and Tieck’s autoregulatory index during VM phases II and IV (AI‐II and AI‐IV, respectively). Resting mean MCAv (MCAv_mean), blood pressure, and cerebral autoregulation were unchanged across the menstrual cycle (all p > 0.12). RoR tended to be different (EF, 0.25 ± 0.06; LF; 0.19 ± 0.04; ML, 0.18 ± 0.12 sec⁻¹; p = 0.07) and demonstrated a negative relationship with 17β‐estradiol (R² = 0.26, p = 0.02). No changes in AI‐II (EF, 1.95 ± 1.20; LF, 1.67 ± 0.77 and ML, 1.20 ± 0.55) or AI‐IV (EF, 1.35 ± 0.21; LF, 1.27 ± 0.26 and ML, 1.20 ± 0.2) were observed (p = 0.25 and 0.37, respectively). Although, a significant interaction effect (p = 0.02) was observed for the VM MCAv_mean response. These data indicate that the menstrual cycle has limited impact on cerebrovascular autoregulation, but individual differences should be considered.

Novel application of a force sensor during sit-to-stands to measure dynamic cerebral autoregulation onset

Current sit-to-stand methods measuring dynamic cerebral autoregulation (dCA) do not capture the precise onset of the time delay (TD) response. Reduced sit-to-stand reactions in older adults and individuals post-stroke could inadvertently introduce variability, error, and imprecise timing. We applied a force sensor during a sit-to-stand task to more accurately determine how TD before the onset of dCA may be altered. Middle cerebral artery blood velocity (MCAv) and mean arterial pressure (MAP) were measured during two sit-to-stands separated by 15 min. Recordings started with participants sitting on a force-sensitive resistor for 60 s, then asked to stand for 2 min. Upon standing, the force sensor voltage immediately dropped and marked the exact moment of arise-and-off (AO). Time from AO until an increase in cerebrovascular conductance (CVC = MCAv/MAP) was calculated as TD. We tested the sensor in four healthy young adults, two older adults, and two individuals post-stroke. Healthy young adults stood quickly and the force sensor detected a small change in TD compared to classically estimated AO, from verbal command to stand. When compared to the estimated AO, older adults had a delayed measured AO and TD decreased up to ~53% while individuals post-stroke had an early AO and TD increased up to ~14%. The stance time during the sit-to-stand has the potential to influence the TD before the onset of dCA metric. As observed in the older adults and participants with stroke, this response may drastically vary and influence TD.

Machine Learning Models and Statistical Complexity to Analyze the Effects of Posture on Cerebral Hemodynamics

The mechanism of cerebral blood flow autoregulation can be of great importance in diagnosing and controlling a diversity of cerebrovascular pathologies such as vascular dementia, brain injury, and neurodegenerative diseases. To assess it, there are several methods that use changing postures, such as sit-stand or squat-stand maneuvers. However, the evaluation of the dynamic cerebral blood flow autoregulation (dCA) in these postures has not been adequately studied using more complex models, such as non-linear ones. Moreover, dCA can be considered part of a more complex mechanism called cerebral hemodynamics, where others (CO2 reactivity and neurovascular-coupling) that affect cerebral blood flow (BF) are included. In this work, we analyzed postural influences using non-linear machine learning models of dCA and studied characteristics of cerebral hemodynamics under statistical complexity using eighteen young adult subjects, aged 27 ± 6.29 years, who took the systemic or arterial blood pressure (BP) and cerebral blood flow velocity (BFV) for five minutes in three different postures: stand, sit, and lay. With models of a Support Vector Machine (SVM) through time, we used an AutoRegulatory Index (ARI) to compare the dCA in different postures. Using wavelet entropy, we estimated the statistical complexity of BFV for three postures. Repeated measures ANOVA showed that only the complexity of lay-sit had significant differences.

Prenatal exercise and cardiovascular health (PEACH) study: impact of acute and chronic exercise on cerebrovascular hemodynamics and dynamic cerebral autoregulation

We performed a randomised controlled trial measuring dynamic cerebral autoregulation (dCA) using a sit-to-stand maneuver before (SS1) and following (SS2) an acute exercise test at 16-20 weeks gestation (trimester 2; TM2) and then again at 34-37 weeks gestation (third trimester; TM3). Following the first assessment, women were randomised into exercise training or control (standard care) groups; women in the exercise training group were prescribed moderate intensity aerobic exercise for 25-40 minutes on 3-4 days per week for 14±1weeks. Resting seated mean blood velocity in the middle cerebral artery (MCAv_mean) was lower in TM3 compared to TM2 but not impacted by exercise training intervention. dCA was not impacted by gestational age, or exercise training during SS1. During SS2, dCA was altered such that there were greater absolute and relative decreases in mean arterial blood pressure (MAP) and MCAv_mean, but this was not impacted by the intervention. There was also no difference in the relationship between the decrease in MCAv_mean compared to the decrease in MAP (%/%), or the onset of the regulatory response with respect to acute exercise, gestational age, or intervention; however, rate of regulation was faster in women in the exercise group following acute exercise (interaction effect, p=0.048). These data highlight the resilience of the cerebral circulation in that dCA was well maintained or improved in healthy pregnant women between TM2 and TM3. However, future work addressing the impact of acute and chronic exercise on dCA in women who are at risk for cardiovascular complications during pregnancy is needed.

Diagnostic criteria for initial orthostatic hypotension: a narrative review

Abnormalities in orthostatic blood pressure changes upon active standing are associated with morbidity, mortality, and reduced quality of life. However, over the last decade, several population-based cohort studies have reported a remarkably high prevalence (between 25 and 70%) of initial orthostatic hypotension (IOH) among elderly individuals. This has raised the question as to whether the orthostatic blood pressure patterns in these community-dwelling elderly should truly be considered as pathological. If not, redefining of the systolic cutoff values for IOH (i.e., a value ≥ 40 mmHg in systolic blood pressure in the first 15 s after standing up) might be necessary to differ between normal aging and true pathology. Therefore, in this narrative review, we provide a critical analysis of the current reference values for the changes in systolic BP in the first 60 s after standing up and discuss how these values should be applied to large population studies. We will address factors that influence the magnitude of the systolic blood pressure changes following active standing and the importance of standardization of the stand-up test, which is a prerequisite for quantitative, between-subject comparisons of the postural hemodynamic response.

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.

Supine Hypertension Is Associated With an Impaired Cerebral Oxygenation Response to Orthostasis: Finding From The Irish Longitudinal Study on Ageing

[Figure: see text].

Dynamic cerebral autoregulation and cerebrovascular carbon dioxide reactivity in middle and posterior cerebral arteries in young endurance-trained women

The integrated responses regulating cerebral blood flow are understudied in women, particularly in relation to potential regional differences. In this study, we compared dynamic cerebral autoregulation (dCA) and cerebrovascular reactivity to carbon dioxide (CVRco₂) in the middle (MCA) and posterior cerebral arteries (PCA) in 11 young endurance-trained women (age, 25 ± 4 yr; maximal oxygen uptake, 48.1 ± 4.1 mL·kg^-1·min^-1). dCA was characterized using a multimodal approach including a sit-to-stand and a transfer function analysis (TFA) of forced blood pressure oscillations (repeated squat-stands executed at 0.05 Hz and 0.10 Hz). The hyperoxic rebreathing test was utilized to characterize CVRco₂. Upon standing, the percent reduction in blood velocity per percent reduction in mean arterial pressure during initial orthostatic stress (0-15 s after sit-to-stand), the onset of the regulatory response, and the rate of regulation did not differ between MCA and PCA (all P > 0.05). There was an ANOVA effect of anatomical location for TFA gain (P < 0.001) and a frequency effect for TFA phase (P < 0.001). However, normalized gain was not different between arteries (P = 0.18). Absolute CVRco₂ was not different between MCA and PCA (1.55 ± 0.81 vs. 1.30 ± 0.49 cm·s^-1/Torr, P = 0.26). Relative CVRco₂ was 39% lower in the MCA (2.16 ± 1.02 vs. 3.00 ± 1.09%/Torr, P < 0.01). These findings indicate that the cerebral pressure-flow relationship appears to be similar between the MCA and the PCA in young endurance-trained women. The absence of regional differences in absolute CVRco₂ could be women specific, although a direct comparison with a group of men will be necessary to address that issue.NEW & NOTEWORTHY Herein, we describe responses from two major mechanisms regulating cerebral blood flow with a special attention on regional differences in young endurance-trained women. The novel findings are that dynamic cerebral autoregulation and absolute cerebrovascular reactivity to carbon dioxide appear similar between the middle and posterior cerebral arteries of these young women.

Effect of menopause on cerebral artery blood flow velocity and cerebrovascular reactivity: Systematic review and meta-analysis

BACKGROUND

Menopause and its associated decline in oestrogen is linked to chronic conditions like cardiovascular disease and osteoporosis, which may be difficult to disentangle from the effects of ageing. Further, post-menopausal women are at increased risk of cerebrovascular disease, linked to declines in cerebral blood flow (CBF) and cerebrovascular reactivity (CVR), yet the direct understanding of the impact of the menopause on cerebrovascular function is unclear. The aim of this systematic review and meta-analysis was to examine the literature investigating CBF and CVR in pre- compared with post-menopausal women METHODS: Five databases were searched for studies assessing CBF or CVR in pre- and post-menopausal women. Meta-analysis examined the effect of menopausal status on middle cerebral artery velocity (MCAv), and GRADE-assessed evidence certainty RESULTS: Nine studies (n=504) included cerebrovascular outcomes. Six studies (n=239) reported negligible differences in MCAv between pre- and post-menopausal women [2.11cm/s (95% CI: -8.94 to 4.73, p=0.54)], but with a "low" certainty of evidence. MCAv was lower in post-menopausal women in two studies, when MCAv was adjusted for blood pressure. CVR was lower in post- compared with pre-menopausal women in two of three studies, but high-quality evidence is lacking. Across outcomes, study methodology and reporting criteria for menopause were inconsistent CONCLUSIONS: MCAv was similar in post- compared with pre-menopausal women. Methodological differences in characterising menopause and inconsistent reporting of cerebrovascular outcomes make comparisons difficult. Comprehensive assessments of cerebrovascular function of the intra- and extracranial arteries to determine the physiological implications of menopause on CBF with healthy ageing is warranted.

Two-Tiered Response of Cardiorespiratory-Cerebrovascular Network to Orthostatic Challenge

Dynamic interdependencies within and between physiological systems and subsystems are key for homeostatic mechanisms to establish an optimal state of the organism. These interactions mediate regulatory responses elicited by various perturbations, such as the high-pressure baroreflex and cerebral autoregulation, alleviating the impact of orthostatic stress on cerebral hemodynamics and oxygenation. The aim of this study was to evaluate the responsiveness of the cardiorespiratory-cerebrovascular networks by capturing linear and nonlinear interdependencies to postural changes. Ten young healthy adults participated in our study. Non-invasive measurements of arterial blood pressure (from that cardiac cycle durations were derived), breath-to-breath interval, cerebral blood flow velocity (BFV, recorded by transcranial Doppler sonography), and cerebral hemodynamics (HbT, total hemoglobin content monitored by near-infrared spectroscopy) were performed for 30-min in resting state, followed by a 1-min stand-up and a 1-min sit-down period. During preprocessing, noise was filtered and the contribution of arterial blood pressure was regressed from BFV and HbT signals. Cardiorespiratory-cerebrovascular networks were reconstructed by computing pair-wise Pearson-correlation or mutual information between the resampled signals to capture their linear and/or nonlinear interdependencies, respectively. The interdependencies between cardiac, respiratory, and cerebrovascular dynamics showed a marked weakening after standing up persisting throughout the sit-down period, which could mainly be attributed to strikingly attenuated nonlinear coupling. To summarize, we found that postural changes induced topological changes in the cardiorespiratory-cerebrovascular network. The dissolution of nonlinear networks suggests that the complexity of key homeostatic mechanisms maintaining cerebral hemodynamics and oxygenation is indeed sensitive to physiological perturbations such as orthostatic stress.

Systemic and cerebral circulatory adjustment within the first 60 s after active standing: An integrative physiological view

Transient cardiovascular and cerebrovascular responses within the first minute of active standing provide the means to assess autonomic, cardiovascular and cerebrovascular regulation using a real-world everyday stimulus. Traditionally, these responses have been used to detect autonomic dysfunction, and to identify the hemodynamic correlates of patient symptoms and attributable causes of (pre)syncope and falls. This review addresses the physiology of systemic and cerebrovascular adjustment within the first 60 s after active standing. Mechanical factors induced by standing up cause a temporal mismatch between cardiac output and vascular conductance which leads to an initial blood pressure drops with a nadir around 10 s. The arterial baroreflex counteracts these initial blood pressure drops, but needs 2-3 s to be initiated with a maximal effect occurring at 10 s after standing while, in parallel, cerebral autoregulation buffers these changes within 10 s to maintain adequate cerebral perfusion. Interestingly, both the magnitude of the initial drop and these compensatory mechanisms are thought to be quite well-preserved in healthy aging. It is hoped that the present review serves as a reference for future pathophysiological investigations and epidemiological studies. Further experimental research is needed to unravel the causal mechanisms underlying the emergence of symptoms and relationship with aging and adverse outcomes in variants of orthostatic hypotension.

Diffuse optical assessment of cerebral-autoregulation in older adults stratified by cerebrovascular risk

Diagnosis of cerebrovascular disease (CVD) at early stages is essential for preventing sequential complications. CVD is often associated with abnormal cerebral microvasculature, which may impact cerebral-autoregulation (CA). A novel hybrid near-infrared diffuse optical instrument and a finger plethysmograph were used to simultaneously detect low-frequency oscillations (LFOs) of cerebral blood flow (CBF), oxy-hemoglobin concentration ([HbO₂ ]), deoxy-hemoglobin concentration ([Hb]) and mean arterial pressure (MAP) in older adults before, during and after 70° head-up-tilting (HUT). The participants with valid data were divided based on Framingham risk score (FRS, 1-30 points) into low-risk (FRS ≤15, n = 13) and high-risk (FRS >15, n = 11) groups for developing CVD. The LFO gains were determined by transfer function analyses with MAP as the input, and CBF, [HbO₂ ] and [Hb] as the outputs (CA ∝ 1/Gain). At resting-baseline, LFO gains in the high-risk group were relatively lower compared to the low-risk group. The lower baseline gains in the high-risk group may attribute to compensatory mechanisms to maintain stronger steady-state CAs. However, HUT resulted in smaller gain reductions in the high-risk group compared to the low-risk group, suggesting weaker dynamic CAs. LFO gains are potentially valuable biomarkers for early detection of CVD based on associations with CAs.

Does depth of squat-stand maneuver affect estimates of dynamic cerebral autoregulation?

Repeated squat-stand maneuvers (SSM) are an effective way of measuring dynamic cerebral autoregulation (dCA), but the depth of SSM required to improve dCA estimations has never been studied. We compared beat-to-beat cerebral hemodynamic parameters between maximal depth SSM (SSMD ) and a shallower alternative (SSMS ) in two age groups (younger [20-34 years] vs. older [50-71 years]) at a frequency of 0.05 Hz. Cerebral blood flow velocity, continuous blood pressure (BP) and end-tidal CO2 (EtCO2 ) were measured using transcranial Doppler ultrasound, the Finometer device, and capnography, respectively. Coherence (at 0.05 Hz) was significantly higher in both SSM recordings compared to spontaneous BP oscillations at baseline standing (BS ). Median (IQR) autoregulation index (ARI) was reduced during SSMD (4.46 [4.03-5.22], p < .01) compared to SSMS (5.96 [5.40-6.69]) and BS (6.03 [5.20-6.49], p < .01) with similar relative differences also observed for phase (at 0.05 Hz). End-tidal CO2 was increased in SSMD (38.3 ± 3.7 mmHg, p < .01) compared to both SSMS (36.6 ± 3.6 mmHg) and BS (35.5 ± 3.2 mmHg). The older group demonstrated significantly lower ARI and phase estimates during SSM and found SSMS more effortful than SSMD . In conclusion, both SSMD and SSMS are effective at estimating dCA, and dCA appears to be less efficient during maximal depth SSM compared to baseline rest or a shallower alternative.

Acute reduction in cerebral blood velocity on supine-to-stand transition increases postural instability in young adults

We tested the hypothesis that transient deficits in cerebral blood flow are associated with postural sway. In 19 young, healthy adults, we examined the association between the drop in cerebral blood flow during supine-to-stand transitions, indexed by transcranial Doppler ultrasound [middle cerebral artery blood velocity at diastole (MCAdv)] and near-infrared spectroscopy [tissue saturation index (TSI)] and the center of pressure displacement while standing. Participants performed transitions under three conditions aimed at progressively increasing the drop in MCAdv, in a randomized order: 1) a control transition (Con); 2) a transition that coincided with deflation of bilateral thigh cuffs; and 3) a transition that coincided with both thigh-cuff deflation and 90 s of prior hyperventilation (HTC). The deficit in diastolic blood velocity (MCAdv deficit) was quantified as the difference between MCAdv and its preceding baseline value, summed over 10 s, beginning at the MCAdv nadir. Compared with Con, HTC led to greater drops in MCAdv (P = 0.003) and TSI (P < 0.001) at nadir. The MCAdv deficit was positively associated with the center of pressure displacement vector-average using repeated-measures correlation (repeated-measures correlation coefficient = 0.56, P < 0.001). An a posteriori analysis identified a sub-group of participants that showed an exaggerated increase in MCAdv deficit and greater postural instability in both the anterior-posterior (P = 0.002) and medial-lateral (P = 0.021) directions in response to the interventions. These findings support the theory that individuals who experience greater initial cerebral hypoperfusion on standing may be at a greater risk for falls.NEW & NOTEWORTHY Dizziness and risk for falls after standing might link directly to reduced delivery of oxygen to the brain. By introducing challenges that increased the drop in brain blood flow in healthy young adults, we have shown for the first time a direct link to greater postural instability. These results point to a need to measure cerebral blood flow and/or oxygenation after postural transitions in populations, such as older adults, to assist in fall risk assessment.

Inflight leg cuff test does not identify the risk for orthostatic hypotension after long-duration spaceflight

Landing day symptoms from orthostatic hypotension after prolonged spaceflight can be debilitating, but severity of these symptoms can be unpredictable and highly individual. We tested the hypothesis that an impaired baroreflex response to an inflight leg cuff test could predict orthostatic intolerance on return to Earth. Eight male astronauts (44 ± 7 years of age (mean ± SD); mean mission length: 167 ± 12 days) participated in a standardized supine-to-sit-to-stand test (5 min-30s-3 min) pre- and postflight, and a 3 min thigh cuff occlusion test pre- and inflight with continuous monitoring of heart rate and arterial blood pressure. The arterial baroreflex was not changed inflight as shown by similar reductions in mean arterial pressure (MAP) response to leg cuff deflation (preflight -19 ± 2 mmHg vs. inflight -18 ± 5 mmHg). With the sit/stand test, the nadir of MAP was lower postflight (-17 ± 9 mmHg) than preflight (-11 ± 6 mmHg, p < 0.05). A greater increase in heart rate (25 ± 7; 16 ± 3 bpm) and decrease in stroke volume (-24 ± 11; -6 ± 4 mL) occurred with sit/stand postflight than leg cuffs inflight (p < 0.001). Inflight testing was influenced by elevated cardiac output resulting in a smaller drop in total peripheral resistance. Two of eight subjects exhibited orthostatic hypotension during the postflight stand test; their responses were not predicted by the inflight leg cuff deflation test. These results suggest that the baroreflex response examined by inflight leg cuff deflation was not a reliable indicator of postflight stand responses.

Exploring Cerebrovascular Function in Osteoarthritis: "Heads-up"

Individuals with osteoarthritis (OA) are at greater risk of cardiovascular and cerebrovascular incidents; yet, cerebrovascular control remains uncharacterized. Our primary outcome was to acquire cerebrovascular control metrics in patients with OA and compare measures to healthy control adults (CTL) without OA or cardiovascular complications. Our primary covariate was a 10-year risk factor for cardiovascular and stroke incidents, and secondary covariates were other cardiovascular disease risk factors (i.e., body mass index, carotid intima media thickness, and brachial flow-mediated dilation). Our secondary outcomes were to assess anatomical and functional changes that may be related to cerebrovascular reactivity were also acquired such as white matter lesion volume and brief cognitive assessments. In 25 adults (n = 13 CTL, n = 12 OA), under hypercapnia, magnetic resonance imaging (3T) was used to acquire a "Global Cerebrovascular Reactivity" index across the larger intracranial cerebral arteries and white matter lesions, and transcranial Doppler was used for both middle cerebral artery hemodynamic responses to hypercapnia and to assess autoregulation via a sit-to-stand task. Compared to CTL, OA had lower "Global Cerebrovascular Reactivity" index responses to hypercapnia, autoregulatory responses, and greater white matter lesions (P < 0.05). These differences persisted after covarying for the outlined primary and secondary covariates. Patients with OA, in the absence of known cardiovascular disease, can exhibit pre-clinical and impaired (compared to CTL) peripheral and cerebrovascular control metrics.

Sex differences in cerebral autoregulation are unaffected by menstrual cycle phase in young, healthy women

Sex is known to affect the prevalence of conditions such as stroke. However, effects of sex on cerebral blood flow regulation are still not well understood. Critical to this understanding is how fluctuations in hormones across the menstrual cycle affect cerebral autoregulation. We measured autoregulation in the early follicular, late follicular, and midluteal phases during spontaneous and induced blood pressure oscillations in 26 young, healthy individuals (13 women and 13 men, age: 26 ± 4 yr). Men participated three times, ~1–3 wk apart. Beat-by-beat blood pressure, heart rate, end-tidal CO2, and transcranial Doppler ultrasonography of the middle (MCA) and anterior (ACA) cerebral arteries were obtained. We did not find a difference in cerebral autoregulation across the menstrual cycle in women but found significantly improved autoregulation in the MCA and ACA of women compared with men. Women demonstrated significantly lower MCA gain (0.97 ± 0.13 vs. 1.17 ± 0.14%/mmHg, P = 0.001), higher MCA phase (46.1 ± 12.6 vs. 35.8 ± 7.9°, P = 0.019), and higher ACA phase (40.5 ± 10.8 vs 31.5 ± 8.5°, P = 0.040) during repeated squat-to-stand maneuvers. Women also had lower MCA gain (1.50 ± 0.11 vs. 1.72 ± 0.30%/mmHg, P = 0.029) during spontaneous fluctuations in pressure while standing and less of a decrease in MCA flow velocity (−18.7 ± 2.7 vs. −23.2 ± 6.0%, P = 0.014) during sit-to-stand maneuvers. Our results suggest that young women have improved cerebral autoregulation compared with young men regardless of menstrual cycle phase and that autoregulation is relatively robust to acute fluctuations in female sex hormones.

NEW & NOTEWORTHY This is the first study to investigate thoroughly the effects of menstrual cycle phase and sex differences in cerebral autoregulation in young, healthy individuals. Cerebral autoregulation was unaffected by menstrual cycle phase during both repeated squat-to-stand and sit-to-stand maneuvers. However, women demonstrated significantly improved cerebral autoregulation in the middle and anterior cerebral arteries, suggesting women were able to maintain cerebral blood flow during changes in blood pressure more efficiently than men.

Dynamic cerebral autoregulation is attenuated in young fit women

Young women exhibit higher prevalence of orthostatic hypotension with presyncopal symptoms compared to men. These symptoms could be influenced by an attenuated ability of the cerebrovasculature to respond to rapid blood pressure (BP) changes [dynamic cerebral autoregulation (dCA)]. The influence of sex on dCA remains unclear. dCA in 11 fit women (25 ± 2 years) and 11 age-matched men (24 ± 1 years) was compared using a multimodal approach including a sit-to-stand (STS) and forced BP oscillations (repeated squat-stand performed at 0.05 and 0.10 Hz). Prevalence of initial orthostatic hypotension (IOH; decrease in systolic ≥ 40 mmHg and/or diastolic BP ≥ 20 mmHg) during the first 15 sec of STS was determined as a functional outcome. In women, the decrease in mean middle cerebral artery blood velocity (MCAvmean ) following the STS was greater (-20 ± 8 vs. -11 ± 7 cm sec-1 ; P = 0.018) and the onset of the regulatory change (time lapse between the beginning of the STS and the increase in the conductance index (MCAvmean /mean arterial pressure) was delayed (P = 0.007). Transfer function analysis gain during 0.05 Hz squat-stand was ~48% higher in women (6.4 ± 1.3 vs. 3.8 ± 2.3 cm sec-1 mmHg-1 ; P = 0.017). Prevalence of IOH was comparable between groups (women: 4/9 vs. men: 5/9, P = 0.637). These results indicate the cerebrovasculature of fit women has an attenuated ability to react to rapid changes in BP in the face of preserved orthostasis, which could be related to higher resting cerebral blood flow allowing women to better face transient hypotension.

Dynamic cerebral autoregulation is impaired in Veterans with Gulf War Illness: A case-control study

Neurological dysfunction has been reported in Gulf War Illness (GWI), including abnormal cerebral blood flow (CBF) responses to physostigmine challenge. However, it is unclear whether the CBF response to normal physiological challenges and regulation is similarly dysfunctional. The goal of the present study was to evaluate the CBF velocity response to orthostatic stress (i.e., sit-to-stand maneuver) and increased fractional concentration of carbon dioxide. 23 cases of GWI (GWI+) and 9 controls (GWI) volunteered for this study. Primary variables of interest included an index of dynamic autoregulation and cerebrovascular reactivity. Dynamic autoregulation was significantly lower in GWI+ than GWI- both for autoregulatory index (2.99±1.5 vs 4.50±1.5, p = 0.017). In addition, we observed greater decreases in CBF velocity both at the nadir after standing (-18.5±6.0 vs -9.8±4.9%, p = 0.001) and during steady state standing (-5.7±7.1 vs -1.8±3.2%, p = 0.042). In contrast, cerebrovascular reactivity was not different between groups. In our sample of Veterans with GWI, dynamic autoregulation was impaired and consistent with greater cerebral hypoperfusion when standing. This reduced CBF may contribute to cognitive difficulties in these Veterans when upright.

How many squat-stand manoeuvres to assess dynamic cerebral autoregulation?

Purpose

Squat–stand manoeuvres (SSMs) have been used to induce blood pressure (BP) changes for the reliable assessment of dynamic cerebral autoregulation. However, they are physically demanding and thus multiple manoeuvres can be challenging for older subjects. This study aimed to determine the minimum number of SSMs required to obtain satisfactory coherence, thus minimising the subjects’ workload.

Method

20 subjects performed SSMs at a frequency of 0.05 Hz. End-tidal CO₂, cerebral blood flow velocity, heart rate, continuous BP and the depth of the squat were measured. 11 subjects returned for a repeat visit. The time points at which subjects had performed 3, 6, 9, 12 and 15 SSMs were determined. Transfer function analysis was performed on files altered to the required length to obtain estimates of coherence and the autoregulation index (ARI).

Results

After three SSMs, coherence (0.05 Hz) was 0.93 ± 0.05, and peaked at 0.95 ± 0.02 after 12 manoeuvres. ARI decreased consecutively with more manoeuvres. ARI was comparable across the two visits (p = 0.92), but coherence was significantly enhanced during the second visit (p < 0.01). The intra-subject coefficients of variation (CoV) for ARI remained comparable as the number of manoeuvres varied.

Conclusions

This analysis can aid those designing SSM protocols, especially where participants are unable to tolerate a standard 5-min protocol or when a shorter protocol is needed to accommodate additional tests. We emphasise that fewer manoeuvres should only be used in exceptional circumstances, and where possible a full set of manoeuvres should be performed. Furthermore, these results need replicating at 0.10 Hz to ensure their applicability to different protocols.

Dynamic Regulation of Cerebral Blood Flow in Patients With Alzheimer Disease

Cerebral autoregulation and baroreflex sensitivity are key mechanisms that maintain cerebral blood flow. This study assessed whether these control mechanisms are affected in patients with dementia and mild cognitive impairment due to Alzheimer disease, as this would increase the risks of antihypertensive treatment. We studied 53 patients with dementia (73.1 years [95% confidence interval (CI), 71.4–74.8]), 37 patients with mild cognitive impairment (69.2 years [95% CI, 66.4–72.0]), and 47 controls (69.4 years [95% CI, 68.3–70.5]). Beat-to-beat blood pressure (photoplethysmography), heart rate, and cerebral blood flow velocity (transcranial Doppler) were measured during 5-minute rest (sitting) and 5 minutes of orthostatic challenges, using repeated sit-to-stand maneuvers. Cerebral autoregulation was assessed using transfer function analysis and the autoregulatory index. Baroreflex sensitivity was estimated with transfer function analysis and by calculating the heart rate response to blood pressure changes during the orthostatic challenges. Dementia patients had the lowest cerebral blood flow velocity ( P =0.004). During rest, neither transfer function analysis nor the autoregulatory index indicated impairments in cerebral autoregulation. During the orthostatic challenges, higher autoregulatory index ( P =0.011) and lower transfer function gain ( P =0.017), indicating better cerebral autoregulation, were found in dementia (4.56 arb. unit [95% CI, 4.14–4.97]; 0.59 cm/s per mm Hg [95% CI, 0.51–0.66]) and mild cognitive impairment (4.59 arb. unit [95% CI, 4.04–5.13]; 0.51 cm/s per mm Hg [95% CI, 0.44–0.59]) compared with controls (3.71 arb. unit [95% CI, 3.35–4.07]; 0.67 cm/s per mm Hg [95% CI, 0.59–0.74]). Baroreflex sensitivity measures did not differ between groups. In conclusion, the key mechanisms to control blood pressure and cerebral blood flow are not reduced in 2 stages of Alzheimer disease compared with controls, both in rest and during orthostatic changes that reflect daily life challenges.

Diminished dynamic cerebral autoregulatory capacity with forced oscillations in mean arterial pressure with elevated cardiorespiratory fitness

The effect that cardiorespiratory fitness has on the dynamic cerebral autoregulatory capacity during changes in mean arterial pressure (MAP) remains equivocal. Using a multiple‐metrics approach, challenging MAP across the spectrum of physiological extremes (i.e., spontaneous through forced MAP oscillations), we characterized dynamic cerebral autoregulatory capacity in 19 male endurance athletes and eight controls via three methods: (1) onset of regulation (i.e., time delay before an increase in middle cerebral artery (MCA) conductance [MCA blood velocity (MCAv)/MAP] and rate of regulation, after transient hypotension induced by sit‐to‐stand, and transfer function analysis (TFA) of MAP and MCAv responses during (2) spontaneous and (3) forced oscillations (5‐min of squat‐stand maneuvers performed at 0.05 and 0.10 Hz). Reductions in MAP and mean MCAv (MCAV_mean) during initial orthostatic stress (0‐30 sec after sit‐to‐stand) and the prevalence of orthostatic hypotension were also determined. Onset of regulation was delayed after sit‐to‐stand in athletes (3.1 ± 1.7 vs. 1.5 ± 1.0 sec; P = 0.03), but rate of regulation was not different between groups (0.24 ± 0.05 vs. 0.21 ± 0.09 sec⁻¹; P = 0.82). While both groups had comparable TFA metrics during spontaneous oscillations, athletes had higher TFA gain during 0.10 Hz squat‐stand versus recreational controls (P = 0.01). Reductions in MAP (P = 0.15) and MCAV_mean (P = 0.11) during orthostatic stress and the prevalence of initial orthostatic hypotension (P = 0.65) were comparable between groups. These results indicate an intact ability of the cerebral vasculature to react to spontaneous oscillations but an attenuated capability to counter rapid and large changes in MAP in individuals with elevated cardiorespiratory fitness.

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.

Autoregulation in the Neuro ICU

PURPOSE OF REVIEW

The purpose of this review is to briefly describe the concept of cerebral autoregulation, to detail several bedside techniques for measuring and assessing autoregulation, and to outline the impact of impaired autoregulation on clinical and functional outcomes in acute brain injury. Furthermore, we will review several autoregulation studies in select forms of acute brain injuries, discuss the potential for its use in patient management in the ICU, and suggest further avenues for research.

RECENT FINDINGS

Cerebral autoregulation plays a critical role in regulating cerebral blood flow, and impaired autoregulation has been associated with worse functional and clinical outcomes in various acute brain injuries. There exists a multitude of methods to assess the autoregulatory state in patients using both invasive and non-invasive modalities. Continuous monitoring of patients in the ICU has yielded autoregulatory-derived optimal perfusion pressures that may prevent secondary injury and improve outcomes. Measuring autoregulation continuously at the bedside is now a feasible option for clinicians working in the ICU, although there exists a great need to standardize autoregulatory measurement. While the clinical benefits await prospective and randomized trials, autoregulation-derived parameters show enormous potential for creating an optimal physiological environment for the injured brain.

Non-linear models for the detection of impaired cerebral blood flow autoregulation

The ability to discriminate between normal and impaired dynamic cerebral autoregulation (CA), based on measurements of spontaneous fluctuations in arterial blood pressure (BP) and cerebral blood flow (CBF), has considerable clinical relevance. We studied 45 normal subjects at rest and under hypercapnia induced by breathing a mixture of carbon dioxide and air. Non-linear models with BP as input and CBF velocity (CBFV) as output, were implemented with support vector machines (SVM) using separate recordings for learning and validation. Dynamic SVM implementations used either moving average or autoregressive structures. The efficiency of dynamic CA was estimated from the model's derived CBFV response to a step change in BP as an autoregulation index for both linear and non-linear models. Non-linear models with recurrences (autoregressive) showed the best results, with CA indexes of 5.9 ± 1.5 in normocapnia, and 2.5 ± 1.2 for hypercapnia with an area under the receiver-operator curve of 0.955. The high performance achieved by non-linear SVM models to detect deterioration of dynamic CA should encourage further assessment of its applicability to clinical conditions where CA might be impaired.

Comparing Models of Spontaneous Variations, Maneuvers and Indexes to Assess Dynamic Cerebral Autoregulation

OBJECTIVE

We analyzed the performance of linear and nonlinear models to assess dynamic cerebral autoregulation (dCA) from spontaneous variations in healthy subjects and compared it with the use of two known maneuvers to abruptly change arterial blood pressure (BP): thigh cuffs and sit-to-stand.

MATERIALS AND METHODS

Cerebral blood flow velocity and BP were measured simultaneously at rest and while the maneuvers were performed in 20 healthy subjects. To analyze the spontaneous variations, we implemented two types of models using support vector machine (SVM): linear and nonlinear finite impulse response models. The classic autoregulation index (ARI) and the more recently proposed model-free ARI (mfARI) were used as measures of dCA. An ANOVA analysis was applied to compare the different methods and the coefficient of variation was calculated to evaluate their variability.

RESULTS

There are differences between indexes, but not between models and maneuvers. The mfARI index with the sit-to-stand maneuver shows the least variability.

CONCLUSIONS

Support vector machine modeling of spontaneous variation with the mfARI index could be used for the assessment of dCA as an alternative to maneuvers to introduce large BP fluctuations.

Cerebrovascular Hemodynamics in Women

Sex and gender, as biological and social factors, significantly influence health outcomes. Among the biological factors, sex differences in vascular physiology may be one specific mechanism contributing to the observed differences in clinical presentation, response to treatment, and clinical outcomes in several vascular disorders. This review focuses on the cerebrovascular bed and summarizes the existing literature on sex differences in cerebrovascular hemodynamics to highlight the knowledge deficit that exists in this domain. The available evidence is used to generate mechanistically plausible and testable hypotheses to underscore the unmet need in understanding sex-specific mechanisms as targets for more effective therapeutic and preventive strategies.

Increased blood pressure variability upon standing up improves reproducibility of cerebral autoregulation indices

Dynamic cerebral autoregulation, that is the transient response of cerebral blood flow to changes in arterial blood pressure, is currently assessed using a variety of different time series methods and data collection protocols. In the continuing absence of a gold standard for the study of cerebral autoregulation it is unclear to what extent does the assessment depend on the choice of a computational method and protocol. We use continuous measurements of blood pressure and cerebral blood flow velocity in the middle cerebral artery from the cohorts of 18 normotensive subjects performing sit-to-stand manoeuvre. We estimate cerebral autoregulation using a wide variety of black-box approaches (including the following six autoregulation indices ARI, Mx, Sx, Dx, FIR and ARX) and compare them in the context of reproducibility and variability. For all autoregulation indices, considered here, the intra-class correlation was greater during the standing protocol, however, it was significantly greater (Fisher's Z-test) for Mx (p < 0.03), Sx (p < 0.003) and Dx (p < 0.03). In the specific case of the sit-to-stand manoeuvre, measurements taken immediately after standing up greatly improve the reproducibility of the autoregulation coefficients. This is generally coupled with an increase of the within-group spread of the estimates.

Elevated Soluble Vascular Cell Adhesion Molecule-1 Is Associated With Cerebrovascular Resistance and Cognitive Function

BACKGROUND

Elevated plasma soluble vascular cell adhesion molecule-1 (sVCAM-1) is a presumed marker of endothelial dysfunction, both in the brain and systemic circulation. Impairments in memory and cognition have been associated with cardiovascular diseases, but little is known about their relationships to abnormal cerebral endothelial function.

METHODS

We studied the cross-sectional association between sVCAM-1 and markers of cerebrovascular hemodynamics and cognitive function in 680 community-dwelling participants in the MOBILIZE Boston Study, aged 65 years and older. Cognitive function was assessed using the Hopkins Verbal Learning Memory Test and Trail Making Tests (TMTs) A and B. Global cognitive impairment was defined as Mini-Mental State Examination (MMSE) score less than 24. sVCAM-1 was measured by ELISA assay. Beat-to-beat blood flow velocity (BFV) and cerebrovascular resistance (CVR = mean arterial pressure / BFV) in the middle cerebral artery were assessed at rest by transcranial Doppler ultrasound.

RESULTS

sVCAM-1 concentrations were higher among participants with an MMSE score <24 versus ≥24 (1,201±417 vs 1,122±494ng/mL). In regression models adjusted for sociodemographic characteristics and health conditions, increasing levels of sVCAM-1 were linearly associated with higher resting CVR (p = .006) and lower performance on the Hopkins Verbal Learning Memory (immediate recall and delayed recall) and adjusted TMT B tests (p < .05). Higher levels of sVCAM-1 were also associated with global cognitive impairment on the MMSE (odds ratio = 3.9; 95% confidence interval: 1.4-10.9; p = .011).

CONCLUSIONS

In this cohort of elderly participants, we observed a cross-sectional association between elevated sVCAM-1 levels and both cognitive impairment and increased cerebrovascular resistance. Longitudinal studies are needed to determine whether elevated sVCAM-1 is a cause or consequence of cerebrovascular damage.

Poststroke alterations in heart rate variability during orthostatic challenge

Older adults following recovery from ischemic stroke have a higher incidence of orthostatic hypotension, syncope, and fall risk, which may be related to impaired autonomic responses limiting the ability to maintain cerebral blood flow. Thus, we investigated cerebrovascular and cardiovascular regulation in 23 adults ≥55 years of age, 10 diagnosed with ischemic stroke, and 13 age-matched healthy controls when sitting at rest and upon standing to compare differences of autonomic variables at ∼7 months (218 ± 41 days) poststroke.Arterial blood pressure via finger plethysmography, muscle-pump baroreflex via electromyography, heart rate variability via 3-lead ECG, and cerebral blood flow velocity via transcranial Doppler were analyzed while sitting for 5 minutes and then during quiet standing for 5 minutes.From the seated to standing position, the stroke group had significantly greater decline in the low frequency component of heart rate variability (164 [79] vs 25 [162] ms; P = 0.043). All other cardiovascular parameters and assessments of autonomic function were not significantly different between the two groups.Our findings support the hypothesis of continued autonomic dysfunction after recovery from ischemic stroke, with potential attenuation of the cardiovascular response to standing. However, further investigation is required to determine the mechanisms underlying the increased risk of orthostatic hypotension, syncope, and falls poststroke.

Ultrasound and dynamic functional imaging in vascular cognitive impairment and Alzheimer's disease

BACKGROUND

The vascular contributions to neurodegeneration and neuroinflammation may be assessed by magnetic resonance imaging (MRI) and ultrasonography (US). This review summarises the methodology for these widely available, safe and relatively low cost tools and analyses recent work highlighting their potential utility as biomarkers for differentiating subtypes of cognitive impairment and dementia, tracking disease progression and evaluating response to treatment in various neurocognitive disorders.

METHODS

At the 9th International Congress on Vascular Dementia (Ljubljana, Slovenia, October 2015) a writing group of experts was formed to review the evidence on the utility of US and arterial spin labelling (ASL) as neurophysiological markers of normal ageing, vascular cognitive impairment (VCI) and Alzheimer's disease (AD). Original articles, systematic literature reviews, guidelines and expert opinions published until September 2016 were critically analysed to summarise existing evidence, indicate gaps in current knowledge and, when appropriate, suggest standards of use for the most widely used US and ASL applications.

RESULTS

Cerebral hypoperfusion has been linked to cognitive decline either as a risk or an aggravating factor. Hypoperfusion as a consequence of microangiopathy, macroangiopathy or cardiac dysfunction can promote or accelerate neurodegeneration, blood-brain barrier disruption and neuroinflammation. US can evaluate the cerebrovascular tree for pathological structure and functional changes contributing to cerebral hypoperfusion. Microvascular pathology and hypoperfusion at the level of capillaries and small arterioles can also be assessed by ASL, an MRI signal. Despite increasing evidence supporting the utility of these methods in detection of microvascular pathology, cerebral hypoperfusion, neurovascular unit dysfunction and, most importantly, disease progression, incomplete standardisation and missing validated cut-off values limit their use in daily routine.

CONCLUSIONS

US and ASL are promising tools with excellent temporal resolution, which will have a significant impact on our understanding of the vascular contributions to VCI and AD and may also be relevant for assessing future prevention and therapeutic strategies for these conditions. Our work provides recommendations regarding the use of non-invasive imaging techniques to investigate the functional consequences of vascular burden in dementia.

Traumatic brain injury-induced autoregulatory dysfunction and spreading depression-related neurovascular uncoupling: Pathomechanisms, perspectives, and therapeutic implications

Traumatic brain injury (TBI) is a major health problem worldwide. In addition to its high mortality (35-40%), survivors are left with cognitive, behavioral, and communicative disabilities. While little can be done to reverse initial primary brain damage caused by trauma, the secondary injury of cerebral tissue due to cerebromicrovascular alterations and dysregulation of cerebral blood flow (CBF) is potentially preventable. This review focuses on functional, cellular, and molecular changes of autoregulatory function of CBF (with special focus on cerebrovascular myogenic response) that occur in cerebral circulation after TBI and explores the links between autoregulatory dysfunction, impaired myogenic response, microvascular impairment, and the development of secondary brain damage. We further provide a synthesized translational view of molecular and cellular mechanisms involved in cortical spreading depolarization-related neurovascular dysfunction, which could be targeted for the prevention or amelioration of TBI-induced secondary brain damage.

Further understanding of cerebral autoregulation at the bedside: possible implications for future therapy

Cerebral autoregulation reflects the ability of the brain to keep the cerebral blood flow (CBF) relatively constant despite changes in cerebral perfusion pressure. It is an intrinsic neuroprotective physiological phenomenon often suggested as part of pathophysiological pathways in brain research. However, despite increasing knowledge of this phenomenon for over 50 years, harnessing cerebral autoregulation as a basis for therapy remains an elegant concept rather than a practical reality. This raises the question is it useful to measure at the bedside or is it merely a scientific curiosity that is too complex and has little pragmatic relevance. In this article, we attempt to answer this question by demonstrating how cerebral autoregulation assessment can have prognostic value, indicate pathological states, and potentially even influence therapy with the use of the 'optimal cerebral perfusion pressure' paradigm. Evidence from the literature is combined with bedside clinical examples to address the following fundamental questions about cerebral autoregulation: What is it? How do we measure it? Why is it important? Can we use it as a basis for therapy?

Circulating vascular cell adhesion molecule-1 is associated with cerebral blood flow dysregulation, mobility impairment, and falls in older adults

Soluble vascular cell adhesion molecule-1 (sVCAM-1) is associated with hypertension, vascular inflammation, and systemic endothelial dysfunction. We evaluated whether elevated plasma sVCAM-1 is associated with impaired cerebrovascular function and mobility impairments in elderly people. We studied the cross-sectional relationships between plasma sVCAM-1 level, gait speed, and cerebrovascular hemodynamics, and its longitudinal relationship with falls in 680 community-dwelling participants aged ≥65 years in the Maintenance of Balance, Independent Living, Intellect, and Zest in the Elderly (MOBILIZE) Boston Study. Falls were recorded prospectively for 1 year on daily calendars. sVCAM-1 was measured by ELISA assay and beat-to-beat blood flow velocity in the middle cerebral artery during rest and in response to changes in end-tidal CO2 was measured by transcranial Doppler ultrasound. sVCAM-1 concentration was 1094±340 ng/mL in normotensives, 1195±438 ng/mL in controlled hypertensives, and 1250±445 ng/mL in uncontrolled hypertensives (P=0.008). The mean resting blood flow velocity and cerebral vasomotor range were, respectively, 41.0±10.3 cm/s and 1.3±0.4 cm/s per millimeter of mercury. Elevated sVCAM-1 levels indicative of endothelial dysfunction were associated with reduced resting blood flow velocity (P=0.017) and cerebral vasomotor range (P=0.0048). Elevated sVCAM-1 levels were associated with slower gait speed (<0.8 m/s; odds ratio, 3.01; 95% confidence interval, 1.56-5.83; P=0.0011) and an increased odds of injurious falls (odds ratio, 2.4; 95% confidence interval, 1.4-4.2; P=0.0028). An elevated sVCAM-1 level may be a marker of cerebral blood flow dysregulation because of endothelial damage from hypertension. It may also signal the presence of cerebral microvascular disease and its clinical consequences, including slow gait speed and falls.

Reexamining the Effect of Antihypertensive Medications on Falls in Old Age

Conflicting data on the relationship between antihypertensive medications and falls in elderly people may lead to inappropriate undertreatment of hypertension in an effort to prevent falls. We aimed to clarify the relationships between the chronic use of different classes of antihypertensive medications and different types of falls, to determine the effect of medication dose, and to assess whether the risk of falls is associated with differences in cerebral blood flow. We assessed demographics, clinical characteristics, and chronic antihypertensive medication use in 598 community-dwelling people with hypertension, aged 70 to 97 years, then followed them prospectively for self-reported falls using monthly calendar postcards and telephone interviews. Antihypertensive medication use was not associated with an increased risk of falls. Participants reporting use of angiotensin-converting enzyme inhibitors had a significantly decreased 1-year risk of injurious falls (odds ratio, 0.62; 95% confidence interval, 0.39-0.96), whereas those using calcium channel blockers had a decreased risk of all falls (odds ratio, 0.62; 95% confidence interval, 0.42-0.91) and indoor falls (odds ratio, 0.57; 95% confidence interval, 0.36-0.91), compared with participants not taking these drugs. Larger doses of these classes were associated with a lower fall risk. Participants taking calcium channel blockers had higher cerebral blood flow than those not taking these medications. In relatively healthy community-dwelling elderly people, high doses of antihypertensive agents are not associated with an increased risk of falls.

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.

A Novel Approach for Classification of Chinese Herbal Medicines Using Diffusion Maps

The quality of Chinese herbal medicines (CHMs) and the authenticity of drugs are directly related to the effect of treatment. This paper presents a method based on diffusion maps for the identification of pungent CHMs. An electronic nose (E-nose) was employed to collect the smell print of different groups of pungent CHMs with different kinds and different harvesting time. First, the collected odor data was mapped to a high-dimensional space, then using the diffusion maps algorithm, the low-dimensional manifold features were extended to the high-dimensional odor data. Finally, the linear discriminant analysis (LDA) algorithm was employed to implement a linear classifier. The results show that, the combination of Diffusion maps and LDA algorithm can well distinguish four different kinds of pungent CHMs and three different harvesting time of the same kind, provides a new way for the classification and identification of CHMs.

The repeated sit-to-stand maneuver is a superior method for cardiac baroreflex assessment: a comparison with the modified Oxford method and Valsalva maneuver

Baroreflex assessment has diagnostic and prognostic utility in the clinical and research environments, and there is a need for a reliable, simple, noninvasive method of assessment. The repeated sit-to-stand method induces oscillatory changes in blood pressure (BP) at a desired frequency and is suitable for assessing dynamic baroreflex sensitivity (BRS). However, little is known about the reliability of this method and its ability to discern fundamental properties of the baroreflex. In this study we sought to: 1) evaluate the reliability of the sit-to-stand method for assessing BRS and compare its performance against two established methods (Oxford method and Valsalva maneuver), and 2) examine whether the frequency of the sit-to-stand method influences hysteresis. Sixteen healthy participants underwent three trials of each method. For the sit-to-stand method, which was performed at 0.1 and 0.05 Hz, BRS was quantified as an integrated response (BRSINT) and in response to falling and rising BP (BRSDOWN and BRSUP, respectively). Test retest reliability was assessed using the intraclass correlation coefficient (ICC). Irrespective of frequency, the ICC for BRSINT during the sit-to-stand method was ≥0.88. The ICC for a rising BP evoked by phenylephrine (PEGAIN) in the Oxford method was 0.78 and ≤0.5 for the remaining measures. During the sit-to-stand method, hysteresis was apparent in all participants at 0.1 Hz but was absent at 0.05 Hz. These findings indicate the sit-to-stand method is a statistically reliable BRS assessment tool and suitable for the examination of baroreflex hysteresis. Using this approach we showed that baroreflex hysteresis is a frequency-dependent phenomenon.

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.

A new model-free index of dynamic cerebral blood flow autoregulation

The classic dynamic autoregulatory index (ARI), proposed by Aaslid and Tiecks, is one of the most widely used methods to assess the efficiency of dynamic cerebral autoregulation. Although this index is often used in clinical research and is also included in some commercial equipment, it exhibits considerable intra-subject variability, and has the tendency to produce false positive results in clinical applications. An alternative index of dynamic cerebral autoregulation is proposed, which overcomes most of the limitations of the classic method and also has the advantage of being model-free. This new index uses two parameters that are obtained directly from the response signal of the cerebral blood flow velocity to a transient decrease in arterial blood pressure provoked by the sudden release of bilateral thigh cuffs, and a third parameter measuring the difference in slope of this response and the change in arterial blood pressure achieved. With the values of these parameters, a corresponding classic autoregulatory index value could be calculated by using a linear regression model built from theoretical curves generated with the Aaslid-Tiecks model. In 16 healthy subjects who underwent repeated thigh-cuff manoeuvres, the model-free approach exhibited significantly lower intra-subject variability, as measured by the unbiased coefficient of variation, than the classic autoregulatory index (p = 0.032) and the Rate of Return (p<0.001), another measure of cerebral autoregulation used for this type of systemic pressure stimulus, from 39.23%±41.91% and 55.31%±31.27%, respectively, to 15.98%±7.75%.

Ambient fine particulate matter alters cerebral hemodynamics in the elderly

BACKGROUND AND PURPOSE

Short-term elevations in fine particulate matter air pollution (PM2.5) are associated with increased risk of acute cerebrovascular events. Evidence from the peripheral circulation suggests that vascular dysfunction may be a central mechanism. However, the effects of PM2.5 on cerebrovascular function and hemodynamics are unknown.

METHODS

We used transcranial Doppler ultrasound to measure beat-to-beat blood flow velocity in the middle cerebral artery at rest and in response to changes in end-tidal CO2 (cerebral vasoreactivity) and arterial blood pressure (cerebral autoregulation) in 482 participants from the Maintenance of Balance, Independent Living, Intellect, and Zest in the Elderly (MOBILIZE) of Boston study. We used linear mixed effects models with random subject intercepts to evaluate the association between cerebrovascular hemodynamic parameters and mean PM2.5 levels 1 to 28 days earlier adjusting for age, race, medical history, meteorologic covariates, day of week, temporal trends, and season.

RESULTS

An interquartile range increase (3.0 µg/m(3)) in mean PM2.5 levels during the previous 28 days was associated with an 8.6% (95% confidence interval, 3.7%-13.8%; P<0.001) higher cerebral vascular resistance and a 7.5% (95% confidence interval, 4.2%-10.6%; P<0.001) lower blood flow velocity at rest. Measures of cerebral vasoreactivity and autoregulation were not associated with PM2.5 levels.

CONCLUSIONS

In this cohort of community-dwelling seniors, exposure to PM2.5 was associated with higher resting cerebrovascular resistance and lower cerebral blood flow velocity. If replicated, these findings suggest that alterations in cerebrovascular hemodynamics may underlie the increased risk of particle-related acute cerebrovascular events.

Advances in transcranial Doppler clinical applications

IMPORTANCE OF THE FIELD

Diagnostic neurosonology techniques including transcranial Doppler (TCD), transcranial color Doppler imaging (TCDI) and power motion-mode (PMD) TCD provide information about various aspects of cerebrovascular status such as microemboli detection, dynamic autoregulation and long-duration real-time monitoring of flow characteristics. Although most of the information provided cannot be obtained by any other imaging methodology, and is critical in clinical decision-making in the care of various neurovascular diseases, these modalities are widely underutilized. Increasing the familiarity to neurosonological techniques is of crucial importance.

AREAS COVERED IN THIS REVIEW

After briefly reviewing TCD, TCDI and PMD techniques, classical features are summarized and recent developments in the clinical neurosonology applications with specific interest in the neurovascular disorders.

WHAT THE READER WILL GAIN

Practical perspectives of ultrasound evaluation of intracranial arterial status in various neurovascular diseases including sickle cell vasculopathy and vasospasm are reviewed in detail. Pearls on the neurosonological monitoring of acute ischemic stroke and increased intracranial pressure increase is provided. Standards of cerebral microembolism detection, right to left shunts diagnosis and cerebral autoregulation assessment are discussed methodologically. Future perspectives of therapeutic neurosonology including sonothrombolysis, microbubble-ultrasound-mediated gene and drug delivery into the brain, and alteration of the brain-blood barrier permeability are summarized.

TAKE HOME MESSAGE

Suitable with future medicine, neurosonology brings imaging to the bedside, which enables the treating physician to monitor a given intervention in real time. A non-invasive neurosonology-guided treatment of various diseases could be possible in the near future. The first and foremost step in gaining mastery in this very fruitful field is beginning to use it.

Blood pressure regulation IX: cerebral autoregulation under blood pressure challenges

Cerebral autoregulation (CA) is integral to the delicate process of maintaining stable cerebral perfusion and brain tissue oxygenation against changes in arterial blood pressure. The last four decades has seen dramatic advances in understanding CA physiology, and the role that CA might play in the causation and progression of disease processes that affect the cerebral circulation such as stroke. However, the translation of these basic scientific advances into clinical practice has been limited by the maintenance of old constructs and because there are persistent gaps in our understanding of how this vital vascular mechanism should be quantified. In this review, we re-evaluate relevant studies that challenge established paradigms about how the cerebral perfusion pressure and blood flow are related. In the context of blood pressure being a major haemodynamic challenge to the cerebral circulation, we conclude that: (1) the physiological properties of CA remain inconclusive, (2) many extant methods for CA characterisation are based on simplistic assumptions that can give rise to misleading interpretations, and (3) robust evaluation of CA requires thorough consideration not only of active vasomotor function, but also the unique properties of the intracranial environment.