Cerebral autoregulation: an overview of current concepts and methodology with special focus on the elderly

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.

Cerebral haemodynamics and intracranial pressure during haemorrhagic shock and resuscitation with total endovascular balloon occlusion of the aorta in an animal model

PURPOSE

To assess changes of cerebral haemodynamic and intracranial pressure (ICP) in animals, with or without elevated ICP, during controlled haemorrhagic shock and resuscitation with Total REBOA (tREBOA).

METHOD

In 22 anaesthetized and normoventilated pigs, after placement of catheters for monitoring invasive proximal blood pressure (pMAP), ICP, and vital parameters, and 60 min stabilisation phase, a controlled haemorrhagic shock (HS), was conducted. In 11 pigs (EICPG), an elevated ICP of 25-30 mmHg at the end HS was achieved by simulating an epidural mass. In 11 pigs (NICPG), the ICP was normal. tREBOA was then applied for 120 min. The changes of pMAP and ICP were followed, and cerebral perfusion pressure (CPP) calculated. The integrity of the autoregulation was estimated using a calculated Modified-Long Pressure Reactivity Index (mL-PRx).

RESULTS

After stabilisation, hemodynamics and physiological parameters were similar and normal in both groups. At the end of the HS, ICP was 16 mmHg in NICPG vs. 32 in EICPG (p = 0.0010). CPP was 30 mmHg in NICPG vs. 6 mmHg in EICPG (p = 0.0254). After aorta occlusion CPP increased immediately in both groups reaching after 15 min up to104 mmHg in NICPG vs. 126 mmHg in EICPG. Cerebrovascular reactivity seems to be altered during bleeding and occlusion phases in both groups with positive mL-PRx. The alteration was more pronounced in EICPG, but reversible in both groups.

CONCLUSION

tREBOA is lifesaving by restoration the cerebral circulation defined as CPP in animals with HS with normal or elevated ICP. Despite the observation of short episodes of cerebral autoregulation impairment during the occlusion, mainly in EICPG, tREBOA seems to be an effective tool for improving cerebral perfusion in HS that extends the crucial early window sometimes known as the "golden hour" for resuscitation even after a traumatic brain injury.

Haemosync: A synchronisation algorithm for multimodal haemodynamic signals

BACKGROUND

Synchronous acquisition of haemodynamic signals is crucial for their multimodal analysis, such as dynamic cerebral autoregulation (DCA) analysis of arterial blood pressure (ABP) and transcranial Doppler (TCD)-derived cerebral blood velocity (CBv). Several technical problems can, however, lead to (varying) time-shifts between the different signals. These can be difficult to recognise and can strongly influence the multimodal analysis results.

METHODS

We have developed a multistep, cross-correlation-based time-shift detection and synchronisation algorithm for multimodal pulsatile haemodynamic signals. We have developed the algorithm using ABP and CBv measurements from a dataset that contained combinations of several time-shifts. We validated the algorithm on an external dataset with time-shifts. We additionally quantitatively validated the algorithm's performance on a dataset with artificially added time-shifts, consisting of sample clock differences ranging from -0.2 to 0.2 s/min and sudden time-shifts between -4 and 4 s. The influence of superimposed noise and variation in waveform morphology on the time-shift estimation was quantified, and their influence on DCA-indices was determined.

RESULTS

The instantaneous median absolute error (MedAE) between the artificially added time-shifts and the estimated time-shifts was 12 ms (median, IQR 12-12, range 11-14 ms) for drifts between -0.1 and 0.1 s/min and sudden time-shifts between -4 and 4 s. For drifts above 0.1 s/min, MedAE was higher (median 753, IQR 19 - 766, range 13 - 772 ms). When a certainty threshold was included (peak cross-correlation > 0.9), MedAE for all drifts-shift combinations decreased to 12 ms, with smaller variability (IQR 12 - 13, range 8 - 22 ms, p < 0.001). The time-shift estimation is robust to noise, as the MedAE was similar for superimposed white noise with variance equal to the signal variance. After time-shift correction, DCA-indices were similar to the original, non-time-shifted signals. Phase shift differed by 0.17° (median, IQR 0.13-0.2°, range 0.0038-1.1°) and 0.54° (median, IQR 0.23-1.7°, range 0.0088-5.6°) for the very low frequency and low frequency ranges, respectively.

DISCUSSION

This algorithm allows visually interpretable detection and accurate correction of time-shifts between pulsatile haemodynamic signals (ABP and CBv).

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.

Dynamic cerebral autoregulation is governed by two time constants: Arterial transit time and feedback time constant

Dynamic cerebral autoregulation (dCA) is the mechanism that describes how the brain maintains cerebral blood flow approximately constant in response to short-term changes in arterial blood pressure. This is known to be impaired in many different pathological conditions, including ischaemic and haemorrhagic stroke, dementia and traumatic brain injury. Many different approaches have thus been used both to analyse and to quantify this mechanism in a range of healthy and diseased subjects, including data-driven models (in both the time and the frequency domain) and biophysical models. However, despite the substantial body of work on both biophysical models and data-driven models of dCA, there remains little work that links the two together. One of the reasons for this is proposed to be the discrepancies between the time constants that govern dCA in models and in experimental data. In this study, the processes that govern dCA are examined and it is proposed that the application of biophysical models remains limited due to a lack of understanding about the physical processes that are being modelled, partly due to the specific model formulation that has been most widely used (the equivalent electrical circuit). Based on the analysis presented here, it is proposed that the two most important time constants are arterial transit time and feedback time constant. It is therefore time to revisit equivalent electrical circuit models of dCA and to develop a more physiologically realistic alternative, one that can more easily be related to experimental data. KEY POINTS: Dynamic cerebral autoregulation is governed by two time constants. The first time constant is the arterial transit time, rather than the traditional 'RC' time constant widely used in previous models. This arterial transit time is approximately 1 s in the brain. The second time constant is the feedback time constant, which is less accurately known, although it is somewhat larger than the arterial transit time. The equivalent electrical circuit model of dynamic cerebral autoregulation should be replaced with a more physiologically representative model.

Individual Patient Data Meta-Analysis of Dynamic Cerebral Autoregulation and Functional Outcome After Ischemic Stroke

BACKGROUND

The relationship between dynamic cerebral autoregulation (dCA) and functional outcome after acute ischemic stroke (AIS) is unclear. Previous studies are limited by small sample sizes and heterogeneity.

METHODS

We performed a 1-stage individual patient data meta-analysis to investigate associations between dCA and functional outcome after AIS. Participating centers were identified through a systematic search of the literature and direct invitation. We included centers with dCA data within 1 year of AIS in adults aged over 18 years, excluding intracerebral or subarachnoid hemorrhage. Data were obtained on phase, gain, coherence, and autoregulation index derived from transfer function analysis at low-frequency and very low-frequency bands. Cerebral blood velocity, arterial pressure, end-tidal carbon dioxide, heart rate, stroke severity and sub-type, and comorbidities were collected where available. Data were grouped into 4 time points after AIS: <24 hours, 24 to 72 hours, 4 to 7 days, and >3 months. The modified Rankin Scale assessed functional outcome at 3 months. Modified Rankin Scale was analyzed as both dichotomized (0 to 2 versus 3 to 6) and ordinal (modified Rankin Scale scores, 0-6) outcomes. Univariable and multivariable analyses were conducted to identify significant relationships between dCA parameters, comorbidities, and outcomes, for each time point using generalized linear (dichotomized outcome), or cumulative link (ordinal outcome) mixed models. The participating center was modeled as a random intercept to generate odds ratios with 95% CIs.

RESULTS

The sample included 384 individuals (35% women) from 7 centers, aged 66.3±13.7 years, with predominantly nonlacunar stroke (n=348, 69%). In the affected hemisphere, higher phase at very low-frequency predicted better outcome (dichotomized modified Rankin Scale) at <24 (crude odds ratios, 2.17 [95% CI, 1.47-3.19]; P<0.001) hours, 24-72 (crude odds ratios, 1.95 [95% CI, 1.21-3.13]; P=0.006) hours, and phase at low-frequency predicted outcome at 3 (crude odds ratios, 3.03 [95% CI, 1.10-8.33]; P=0.032) months. These results remained after covariate adjustment.

CONCLUSIONS

Greater transfer function analysis-derived phase was associated with improved functional outcome at 3 months after AIS. dCA parameters in the early phase of AIS may help to predict functional outcome.

Compromised Dynamic Cerebral Autoregulation in Patients with Restless Legs Syndrome

Background

Restless legs syndrome (RLS) is a prevalent sensorimotor nervous system disorder in patients accompanied with insomnia, blood pressure fluctuation, and sympathetic dysfunction. These symptoms may disrupt cerebral hemodynamics. Dynamic cerebral autoregulation (dCA) describes the temporary response of cerebrovascular system to abrupt fluctuations in blood pressure, which keep cerebral blood flow stable and serve as a marker of cerebrovascular system ability.

Objective

This research aimed to assess dCA in RLS patients.

Methods

In this study, RLS patients were recruited and subsequently classified into four groups (mild, moderate, severe, and very severe) based on the International RLS Rating Scale (IRLS). Healthy controls matched for age and sex were enrolled. All participants were evaluated dCA by assessing phase difference (PD). A portion of patients with RLS was reassessed for dCA after one month of medication therapy (pramipexole [0.125 mg/day] and gabapentin [300 mg/day]).

Results

There were altogether 120 patients with RLS and 30 controls completed the polysomnography and dCA assessment. PD was lower in the moderate, severe, and very severe RLS groups than that in the controls and mild RLS groups. Periodic limb movement index (PLMI), arousal index, and IRLS all showed a linear correlation with PD in RLS patients. Additionally, PD increased in RLS patients after therapy.

Conclusion

The dCA was compromised in moderate, severe, and very severe RLS patients and was negatively correlated with the IRLS, arousal index, and PLMI. After 1 month of therapy, dCA improved in RLS patients.

Myths and methodologies: Assessment of dynamic cerebral autoregulation by the mean flow index

The mean flow index-usually referred to as Mx-has been used for assessing dynamic cerebral autoregulation (dCA) for almost 30 years. However, concerns have arisen regarding methodological consistency, construct and criterion validity, and test-retest reliability. Methodological nuances, such as choice of input (cerebral perfusion pressure, invasive or non-invasive arterial pressure), pre-processing approach and artefact handling, significantly influence mean flow index values, and previous studies correlating mean flow index with other established dCA metrics are confounded by inherent methodological flaws like heteroscedasticity, while the mean flow index also fails to discriminate individuals with presumed intact versus impaired dCA (discriminatory validity), and its prognostic performance (predictive validity) across various conditions remains inconsistent. The test-retest reliability, both within and between days, is generally poor. At present, no single approach for data collection or pre-processing has proven superior for obtaining the mean flow index, and caution is advised in the further use of mean flow index-based measures for assessing dCA, as current evidence does not support their clinical application.

The effect of relative hypotension on 30-day mortality in older people receiving emergency care

Research has observed increased mortality among older people attending the emergency department (ED) who had systolic pressure > 7 mmHg lower than baseline primary care values. This study aimed to (1) assess feasibility of identifying this 'relative hypotension' using readily available ED data, (2) externally validate the 7 mmHg threshold, and (3) refine a threshold for clinically important relative hypotension. A single-centre retrospective cohort study linked year 2019 data for ED attendances by people aged over 64 to hospital discharge vital signs within the previous 18 months. Frailty and comorbidity scores were calculated. Previous discharge ('baseline') vital signs were subtracted from initial ED values to give individuals' relative change. Cox regression analysis compared relative hypotension > 7 mmHg with mean time to mortality censored at 30 days. The relative hypotension threshold was refined using a fully adjusted risk tool formed of logistic regression models. Receiver operating characteristics were compared to NEWS2 models with and without incorporation of relative systolic. 5136 (16%) of 32,548 ED attendances were linkable with recent discharge vital signs. Relative hypotension > 7 mmHg was associated with increased 30-day mortality (HR 1.98; 95% CI 1.66-2.35). The adjusted risk tool (AUC: 0.69; sensitivity: 0.61; specificity: 0.68) estimated each 1 mmHg relative hypotension to increase 30-day mortality by 2% (OR 1.02; 95% CI 1.02-1.02). 30-day mortality prediction was marginally better with NEWS2 (AUC: 0.73; sensitivity: 0.59; specificity: 0.78) and NEWS2 + relative systolic (AUC: 0.74; sensitivity: 0.63; specificity: 0.75). Comparison of ED vital signs with recent discharge observations was feasible for 16% individuals. The association of relative hypotension > 7 mmHg with 30-day mortality was externally validated. Indeed, any relative hypotension appeared to increase risk, but model characteristics were poor. These findings are limited to the context of older people with recent hospital admissions.

The effect of hypercapnia on the directional sensitivity of dynamic cerebral autoregulation and the influence of age and sex

The cerebral circulation responds differently to increases in mean arterial pressure (MAP), compared to reductions in MAP. We tested the hypothesis that this directional sensitivity is reduced by hypercapnia. Retrospective analysis of 104 healthy subjects (46 male (44%), age range 19-74 years), with five minute recordings of middle cerebral blood velocity (MCAv, transcranial Doppler), non-invasive MAP (Finometer) and end-tidal CO2 (capnography) at rest, during both poikilocapnia and hypercapnia (5% CO2 breathing in air) produced MCAv step responses allowing estimation of the classical Autoregulation Index (ARIORIG), and corresponding values for both positive (ARI+D) and negative (ARI-D) changes in MAP. Hypercapnia led to marked reductions in ARIORIG, ARI+D and ARI-D (p < 0.0001, all cases). Females had a lower value of ARIORIG compared to males (p = 0.030) at poikilocapnia (4.44 ± 1.74 vs 4.74 ± 1.48) and hypercapnia (2.44 ± 1.93 vs 3.33 ± 1.61). The strength of directional sensitivity (ARI+D-ARI-D) was not influenced by hypercapnia (p = 0.46), sex (p = 0.76) or age (p = 0.61). During poikilocapnia, ARI+D decreased with age in females (p = 0.027), but not in males. Directional sensitivity was not affected by hypercapnia, suggesting that its origins are more likely to be inherent to the mechanics of vascular smooth muscle than to myogenic pathways.

Comparable dynamic cerebral autoregulation and neurovascular coupling of the posterior cerebral artery between healthy men and women

AIMS

Most studies focus on dynamic cerebral autoregulation (dCA) in the middle cerebral artery (MCA), and few studies investigated neurovascular coupling (NVC) and dCA in the posterior cerebral artery (PCA). We investigated NVC and dCA of the PCA in healthy volunteers to identify sex differences.

METHODS

Thirty men and 30 age-matched women completed dCA and NCV assessments. The cerebral blood flow velocity (CBFV) and mean arterial pressure were evaluated using transcranial Doppler ultrasound and a servo-controlled plethysmograph, respectively. The dCA parameters were analyzed using transfer function analysis. The NCV was evaluated by eyes-open and eyes-closed (24 s each) periodically based on voice prompts. The eyes-open visual stimulation comprised silent reading of Beijing-related tourist information.

RESULTS

The PCA gain was lower than that of the MCA in all frequency ranges (all p < 0.05). Phase was consistent across the cerebrovascular territories. The cerebrovascular conductance index (CVCi) and mean CBFV (MV) of the PCA were significantly higher during the eyes-open than eyes-closed period (CVCi: 0.50 ± 0.12 vs. 0.38 ± 0.10; MV: 42.89 ± 8.49 vs. 32.98 ± 7.25, both p < 0.001). The PCA dCA and NVC were similar between the sexes.

CONCLUSION

We assessed two major mechanisms that maintain cerebral hemodynamic stability in healthy men and women. The visual stimulation-evoked CBFV of the PCA was significantly increased compared to that during rest, confirming the activation of NVC. Men and women have similar functions in PCA dCA and NCV.

Were Frailty Identification Criteria Created Equal? A Comparative Case Study on Continuous Non-Invasively Collected Neurocardiovascular Signals during an Active Standing Test in the Irish Longitudinal Study on Ageing (TILDA)

BACKGROUND

In this observational study, we compared continuous physiological signals during an active standing test in adults aged 50 years and over, characterised as frail by three different criteria, using data from The Irish Longitudinal Study on Ageing (TILDA).

METHODS

This study utilised data from TILDA, an ongoing landmark prospective cohort study of community-dwelling adults aged 50 years or older in Ireland. The initial sampling strategy in TILDA was based on random geodirectory sampling. Four independent groups were identified: those characterised as frail only by one of the frailty tools used (the physical Frailty Phenotype (FP), the 32-item Frailty Index (FI), or the Clinical Frailty Scale (CFS) classification tree), and a fourth group where participants were not characterised as frail by any of these tools. Continuous non-invasive physiological signals were collected during an active standing test, including systolic (sBP) and diastolic (dBP) blood pressure, as well as heart rate (HR), using digital artery photoplethysmography. Additionally, the frontal lobe cerebral oxygenation (Oxy), deoxygenation (Deoxy), and tissue saturation index (TSI) were also non-invasively measured using near-infrared spectroscopy (NIRS). The signals were visualised across frailty groups and statistically compared using one-dimensional statistical parametric mapping (SPM).

RESULTS

A total of 1124 participants (mean age of 63.5 years; 50.2% women) were included: 23 were characterised as frail only by the FP, 97 by the FI, 38 by the CFS, and 966 by none of these criteria. The SPM analyses revealed that only the group characterised as frail by the FI had significantly different signals (p < 0.001) compared to the non-frail group. Specifically, they exhibited an attenuated gain in HR between 10 and 15 s post-stand and larger deficits in sBP and dBP between 15 and 20 s post-stand.

CONCLUSIONS

The FI proved to be more adept at capturing distinct physiological responses to standing, likely due to its direct inclusion of cardiovascular morbidities in its definition. Significant differences were observed in the dynamics of cardiovascular signals among the frail populations identified by different frailty criteria, suggesting that caution should be taken when employing frailty identification tools on physiological signals, particularly the neurocardiovascular signals in an active standing test.

Compromised dynamic cerebral autoregulation is a hemodynamic marker for predicting poor prognosis even with good recanalization after endovascular thrombectomy

PURPOSE

In patients undergoing endovascular thrombectomy (EVT) with acute ischemic stroke (AIS), dynamic cerebral autoregulation (dCA) may minimize neurological injury from blood pressure fluctuations. This study set out to investigate the function of dCA in predicting clinical outcomes following EVT.

METHODS

43 AIS of the middle cerebral or internal carotid artery patients underwent with EVT, and 43 healthy individuals (controls) were enrolled in this case control research. The dCA was evaluated using transcranial Doppler 12 h and five days after EVT. The transfer function analysis was used to derive the dCA parameters, such as phase, gain, and coherence. The modified Rankin scale (mRS) at 3 months after EVT was used to assess the clinical outcomes. Thefavorable outcome group was defined with mRS ≤2 and the unfavorable outcome group was defined with mRS score of 3-6. Logistic regression analysis was performed to determine the risk factors of clinical outcomes.

RESULTS

A significant impairment in dCA was observed on the ipsilateral side after EVT, particularly in patients with unfavorable outcomes. After 5 days, the ipsilateral phase was associated with poor functional outcomes (adjusted odds ratio [OR] = 0.911, 95% confidence interval [CI]: 0.854-0.972; P = 0.005) and the area under the curve (AUC) (AUC, 0.878, [95% CI: 0.756-1.000] P < 0.001) (optimal cutoff, 35.0°). Phase change was an independent predictor of clinical outcomes from 12 h to 5 days after EVT (adjusted OR = 1.061, 95% CI: 1.016-1.109, P = 0.008).

CONCLUSIONS

dCA is impaired in patients with AIS after EVT. Change in dCA could be an independent factor related to the clinical outcomes.

Mean Arterial Pressure and Cerebral Hemodynamics Across The Lifespan: A Cross-Sectional Study From Human Connectome Project-Aging

BACKGROUND

Cerebral perfusion is directly affected by systemic blood pressure, which has been shown to be negatively correlated with cerebral blood flow (CBF). The impact of aging on these effects is not fully understood.

PURPOSE

To determine whether the relationship between mean arterial pressure (MAP) and cerebral hemodynamics persists throughout the lifespan.

STUDY TYPE

Retrospective, cross-sectional study.

POPULATION

Six hundred and sixty-nine participants from the Human Connectome Project-Aging ranging between 36 and 100+ years and without a major neurological disorder.

FIELD STRENGTH/SEQUENCE

Imaging data was acquired at 3.0 Tesla using a 32-channel head coil. CBF and arterial transit time (ATT) were measured by multi-delay pseudo-continuous arterial spin labeling.

ASSESSMENT

The relationships between cerebral hemodynamic parameters and MAP were evaluated globally in gray and white matter and regionally using surface-based analysis in the whole group, separately within different age groups (young: <60 years; younger-old: 60-79 years; oldest-old: ≥80 years).

STATISTICAL TESTS

Chi-squared, Kruskal-Wallis, ANOVA, Spearman rank correlation and linear regression models. The general linear model setup in FreeSurfer was used for surface-based analyses. P < 0.05 was considered significant.

RESULTS

Globally, there was a significant negative correlation between MAP and CBF in both gray (ρ = -0.275) and white matter (ρ = -0.117). This association was most prominent in the younger-old [gray matter CBF (β = -0.271); white matter CBF (β = -0.241)]. In surface-based analyses, CBF exhibited a widespread significant negative association with MAP throughout the brain, whereas a limited number of regions showed significant prolongation in ATT with higher MAP. The associations between regional CBF and MAP in the younger-old showed a different topographic pattern in comparison to young subjects.

DATA CONCLUSION

These observations further emphasize the importance of cardiovascular health in mid-to-late adulthood for healthy brain aging. The differences in the topographic pattern with aging indicate a spatially heterogeneous relationship between high blood pressure and CBF.

LEVEL OF EVIDENCE

3 TECHNICAL EFFICACY STAGE: 3.

A systematic review on the assessment of cerebral autoregulation in patients with Large Vessel Occlusion

As the majority of large vessel occlusion (LVO) patients are not treated with revascularization therapies or efficiently revascularized, complementary management strategies are needed. In this article we explore the importance of cerebral autoregulation (CA) assessment in the prediction and/or modification of infarct growth and hemorrhagic transformation. In patients with LVO, these are important factors that affect prognosis. A systematic search of the PubMed, EMBASE databases and a targeted Google search was conducted, resulting in the inclusion of 34 relevant articles. There is an agreement that CA is impaired in patients with LVO; several factors have been identified such as time course, revascularization status, laterality, disease subtype and location, some of which may be potentially modifiable and affect outcomes. The personalized CA assessment of these patients suggests potential for better understanding of the inter-individual variability. Further research is needed for the development of more accurate, noninvasive techniques for continuous monitoring and personalized thresholds for CA.

Neurovascular dysfunction in glaucoma

Retinal ganglion cells, the neurons that die in glaucoma, are endowed with a high metabolism requiring optimal provision of oxygen and nutrients to sustain their activity. The timely regulation of blood flow is, therefore, essential to supply firing neurons in active areas with the oxygen and glucose they need for energy. Many glaucoma patients suffer from vascular deficits including reduced blood flow, impaired autoregulation, neurovascular coupling dysfunction, and blood-retina/brain-barrier breakdown. These processes are tightly regulated by a community of cells known as the neurovascular unit comprising neurons, endothelial cells, pericytes, Müller cells, astrocytes, and microglia. In this review, the neurovascular unit takes center stage as we examine the ability of its members to regulate neurovascular interactions and how their function might be altered during glaucomatous stress. Pericytes receive special attention based on recent data demonstrating their key role in the regulation of neurovascular coupling in physiological and pathological conditions. Of particular interest is the discovery and characterization of tunneling nanotubes, thin actin-based conduits that connect distal pericytes, which play essential roles in the complex spatial and temporal distribution of blood within the retinal capillary network. We discuss cellular and molecular mechanisms of neurovascular interactions and their pathophysiological implications, while highlighting opportunities to develop strategies for vascular protection and regeneration to improve functional outcomes in glaucoma.

Additional predictors of the lower limit of cerebral autoregulation during cardiac surgery

OBJECTIVES

The lower limit of autoregulation (LLA) of cerebral blood flow was previously shown to vary directly with the Ambulatory Arterial Stiffness Index (AASI) redefined as 1-regression slope of DBP-versus-SBP readings invasively measured from the radial artery before the bypass. We aimed expanding the predictive capacity of the LLA with AASI by combining it with additional predictors and provide new indications whether mean arterial pressure (MAP) is above/below the LLA.

DESIGN AND METHOD

In 181 patients undergoing cardiac surgery, mean (SD) age 71 (8) years), we identified from the demographic, preoperative and intraoperative characteristics independent and statistically significant 'single predictors' of the LLA (including AASI). This was achieved using multivariate linear regression with a backward-elimination technique. The single predictors combined with 1-AASI generated new multiplicative and additive composite predictors of the LLA. Indicators for the MAP-to-LLA difference (DIF) were determined using DIF-versus-predictor plots. The odds ratio (OR) for the DIF sign (Outcome = 1 for DIF≤0) and predictor-minus-median sign (Exposure = 1 for Predictor  ≤ Median) were calculated using logistic regression.

RESULTS

BMI, 1-AASI and systolic coefficient of variation were identified single predictors that correlated similarly with the LLA ( r  = -0.26 to -0.27, P  < 0.001). The multiplicative and additive composite predictors displayed higher correlation with LLA ( r  = -0.41 and r  = -0.43, respectively, P  < 0.001) and improved LLA estimation. The adjusted OR for the composite predictors was nearly twice that of the single predictors.

CONCLUSION

The novel composite predictors may enhance the LLA estimation and the ability to maintain MAP in the cerebral autoregulatory range during cardiac surgery.

Age and sex affect the association of systolic blood pressure with clinical outcomes in thrombolysed stroke patient: a secondary analysis of the INTRECIS study

Background and purpose

Blood pressure is associated with outcomes in acute ischemic stroke (AIS) patients receiving intravenous alteplase. The study aimed to explore the effect of sex and age on their association.

Methods

Based on a prospective cohort, we retrospectively enrolled consecutive AIS patients who received intravenous alteplase and had complete blood pressure data, including baseline systolic blood pressure (SBP 01), SBP at 1 h (SBP 02), and SBP at 24 h (SBP 03) after alteplase. Maximum SBP (SBP max), minimum SBP (SBP min), and mean SBP (SBP mean) were calculated. Poor outcome was defined as having a modified Rankin Scale (mRS) score of 2-6 at 90 days. We explored the effect of age and sex on the association of different SBP indicators with the 3-month outcomes.

Results

A total of 1,593 eligible patients were included in the present study. All SBP indicators were found to be higher in patients with poor vs. good outcomes. Multivariate logistic regression analysis showed that all SBP indicators except baseline SBP were associated with poor outcomes with good prediction powers (AUC, 0.762-0.766). More SBP indicators (SBP 02, SBP 03, SBP min, and SBP mean) were associated with poor outcomes in women vs. men, while all SBP indicators after alteplase were associated with poor outcomes in patients aged ≥ 60 years, but none was seen in patients aged < 60 years. Furthermore, all SBP indicators after alteplase were associated with poor outcomes in women aged ≥ 60 years, while only SBP 03 in men aged < 60 years.

Conclusion

Among Chinese stroke patients treated with intravenous alteplase, SBP after alteplase was associated with clinical outcomes, which were affected by age and sex.

The neurovascular unit in healthy and injured spinal cord

The neurovascular unit (NVU) reflects the close temporal and spatial link between neurons and blood vessels. However, the understanding of the NVU in the spinal cord is far from clear and largely based on generalized knowledge obtained from the brain. Herein, we review the present knowledge of the NVU and highlight candidate approaches to investigate the NVU, particularly focusing on the spinal cord. Several unique features maintain the highly regulated microenvironment in the NVU. Autoregulation and neurovascular coupling ensure regional blood flow meets the metabolic demand according to the blood supply or local neural activation. The blood-central nervous system barrier partitions the circulating blood from neural parenchyma and facilitates the selective exchange of substances. Furthermore, we discuss spinal cord injury (SCI) as a common injury from the perspective of NVU dysfunction. Hopefully, this review will help expand the understanding of the NVU in the spinal cord and inspire new insights into SCI.

Dynamic cerebral autoregulation in postpartum individuals with and without preeclampsia

BACKGROUND

Changes in dynamic cerebral autoregulation (DCA) may contribute to postpartum maternal cerebrovascular complications after preeclampsia. We hypothesized that DCA is impaired in the first week postpartum after diagnosis of preeclampsia with severe features (PSF), compared with normotensive postpartum individuals and healthy non-pregnant female volunteers.

METHODS

We measured DCA within seven days after delivery in individuals with and without PSF, using transcranial Doppler and continuous arterial blood pressure monitoring with finger plethysmography. Historical data from 28 healthy female non-pregnant volunteers, collected using the same methods, were used for comparison. We used generalized harmonic wavelets to estimate autoregulation parameters (phase shift and gain) in very low frequency and low frequency bands, with lower phase shift and higher gain indicating impaired DCA function. We compared DCA parameters between the three groups using the Kruskal Wallis test.

RESULTS

A total of 69 postpartum participants contributed data, of whom 49 had preeclampsia with severe features. Median phase shifts in both postpartum groups were higher compared with historical controls across all frequency ranges (p = 0.001), indicating faster autoregulatory response. Gain was higher in both postpartum groups than in historical controls across all frequency ranges (p = 0.04), indicating impaired dampening effect.

CONCLUSION

We found that postpartum individuals, regardless of preeclampsia diagnosis, had higher phase shifts and higher gain than healthy non-pregnant/postpartum female volunteers. Our results suggest hyperdynamic DCA with impaired dampening effect in the first week postpartum, regardless of preeclampsia diagnosis.

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

Background

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

Methods

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

Results

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

Conclusion

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

Quantification of Cerebral Vascular Autoregulation Immediately Following Resuscitation from Cardiac Arrest

Cerebral vascular autoregulation is impaired following resuscitation from cardiac arrest (CA), and its quantification may allow assessing CA-induced brain injury. However, hyperemia occurring immediately post-resuscitation limits the application of most metrics that quantify autoregulation. Therefore, to characterize autoregulation during this critical period, we developed three novel metrics based on how the cerebrovascular resistance (CVR) covaries with changes in cerebral perfusion pressure (CPP): (i) θCVR, which quantifies the CVR vs CPP gradient, (ii) a CVR-based transfer function analysis, and (iii) CVRx, the correlation coefficient between CPP and CVR. We tested these metrics in a model of asphyxia induced CA and resuscitation using seven adult male Wistar rats. Mean arterial pressure (MAP) and cortical blood flow recorded for 30 min post-resuscitation via arterial cannulation and laser speckle contrast imaging, were used as surrogates of CPP and cerebral blood flow (CBF), while CVR was computed as the CPP/CBF ratio. Using our metrics, we found that the status of cerebral vascular autoregulation altered substantially during hyperemia, with changes spread throughout the 0-0.05 Hz frequency band. Our metrics push the boundary of how soon autoregulation can be assessed, and if validated against outcome markers, may help develop a reliable metric of brain injury post-resuscitation.

Candidate neuroinflammatory markers of cerebral autoregulation dysfunction in human acute brain injury

The loss of cerebral autoregulation (CA) is a common and detrimental secondary injury mechanism following acute brain injury and has been associated with worse morbidity and mortality. However patient outcomes have not as yet been conclusively proven to have improved as a result of CA-directed therapy. While CA monitoring has been used to modify CPP targets, this approach cannot work if the impairment of CA is not simply related to CPP but involves other underlying mechanisms and triggers, which at present are largely unknown. Neuroinflammation, particularly inflammation affecting the cerebral vasculature, is an important cascade that occurs following acute injury. We hypothesise that disturbances to the cerebral vasculature can affect the regulation of CBF, and hence the vascular inflammatory pathways could be a putative mechanism that causes CA dysfunction. This review provides a brief overview of CA, and its impairment following brain injury. We discuss candidate vascular and endothelial markers and what is known about their link to disturbance of the CBF and autoregulation. We focus on human traumatic brain injury (TBI) and subarachnoid haemorrhage (SAH), with supporting evidence from animal work and applicability to wider neurologic diseases.

A decade of aging in healthy older adults: longitudinal findings on cerebrovascular and cognitive health

Research suggests an association between cerebrovascular health and cognitive decline, but previous work is limited by its cross-sectional nature or short (< 1-2 years) follow-up. Our aim was to examine, across 10 years of follow-up in healthy older adults, changes in cerebrovascular health and their relationship with subjective memory complaints as an early marker of cognitive decline. Between 2008 and 2010, twenty-eight healthy older adults (69 ± 4 years) underwent baseline blood pressure and transcranial Doppler measurements to assess middle cerebral artery blood velocity (MCAv), cerebrovascular resistance index (CVRi), and measures of cerebral autoregulation (CA). After 9-12 years of follow-up, these measurements were repeated, and presence of memory complaints was evaluated. Linear mixed-model analyses explored effects of aging on cerebrovascular parameters and whether memory complaints were associated with cerebrovascular changes. Across a median follow-up of 10.9 years, no changes in MCAv, CVRi, or CA were found. At baseline, these parameters were not different between subjects with (n = 15) versus without (n = 13) memory complaints. During follow-up, subjects with memory complaints showed larger decreases in MCAv (- 10% versus + 9%, P = 0.041) and increases in CVRi (+ 26% versus - 9%, P = 0.029) compared to other peers without memory complaints, but no distinct changes in CA parameters (P > 0.05). Although a decade of aging does not lead to deterioration in cerebral blood flow or autoregulation, our findings suggest that reductions in cerebral blood flow and increases in cerebrovascular resistance are associated with early subjective cognitive decline.

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.

Hypertension, antihypertensive drugs, and age at onset of Huntington's disease

BACKGROUND

Associations between blood pressure (BP) with age at onset of Huntington's disease (HD) have reported inconsistent findings. We used Mendelian randomization (MR) to assess effects of BP and lowering systolic BP (SBP) via the genes encoding targets of antihypertensive drugs on age at onset of HD.

METHODS

Genetic variants from genome-wide association studies(GWAS) of BP traits and BP-lowering variants in genes encoding antihypertensive drugs targets were extracted. Summary statistics for age at onset of HD were retrieved from the GWAS meta-analysis of HD residual age at onset from the GEM-HD Consortium included 9064 HD patients of European ancestry (4417 males and 4,647 females). MR estimates were calculated using the inverse variance weighted method, supplemented by MR-Egger, weighted median, and MR-PRESSO methods.

RESULTS

Genetically predicted SBP or diastolic BP increase was associated with a later age at onset of HD. However, after SBP/DBP was present as a covariate using multivariable MR method, no significant causal association was suggested. A 10-mm Hg reduction in SBP through variants in genes encoding targets of calcium channel blockers (CCB) was associated with an earlier age at onset of HD (β=-0.220 years, 95% CI =-0.337 to -0.102, P = 2.42 × 10^- 4). We did not find a causal association between angiotensin converting enzyme inhibitors and β-blockers with the earlier HD onset. No heterogeneity and horizontal pleiotropy were identified.

CONCLUSIONS

This MR analysis provided evidence that genetically determined SBP lowering through antihypertensive drugs might be associated with an earlier age at onset of HD. The results may have a potential impact on management of hypertension in the pre-motor-manifest HD population.

Dynamic cerebral autoregulation in Alzheimer's disease and mild cognitive impairment: A systematic review

Dynamic cerebral autoregulation (dCA) is a key mechanism that regulates cerebral blood flow (CBF) in response to transient changes in blood pressure (BP). Impairment of dCA could increase vulnerability to hypertensive vascular damage, but also to BP lowering effects of antihypertensive treatment. The literature remains conflicted on whether dCA is altered in Alzheimer's disease (AD) and mild cognitive impairment (MCI). We summarized available data on dCA in AD and MCI, by searching PubMed, Embase, PsycINFO and Web of Science databases (inception-January 2022). Eight studies (total n = 443) were included in the qualitative synthesis of which seven were eligible for meta-analysis. All studies used Transcranial Doppler (TCD) ultrasonography and transfer function analysis or the autoregulatory index to assess dCA during spontaneous or induced BP fluctuations. Meta-analysis indicated no significant difference between AD, MCI and healthy controls in dCA parameters for spontaneous fluctuations. For induced fluctuations, the available data were limited, but indicative of at least preserved and possibly better autoregulatory functioning in AD and MCI compared to controls. In summary, current evidence does not suggest poorer dCA efficiency in AD or MCI. Further work is needed to investigate dCA in dementia with induced fluctuations controlling for changes in end-tidal carbon dioxide.

Ischemic brain edema: Emerging cellular mechanisms and therapeutic approaches

Brain edema is one of the most devastating consequences of ischemic stroke. Malignant cerebral edema is the main reason accounting for the high mortality rate of large hemispheric strokes. Despite decades of tremendous efforts to elucidate mechanisms underlying the formation of ischemic brain edema and search for therapeutic targets, current treatments for ischemic brain edema remain largely symptom-relieving rather than aiming to stop the formation and progression of edema. Recent preclinical research reveals novel cellular mechanisms underlying edema formation after brain ischemia and reperfusion. Advancement in neuroimaging techniques also offers opportunities for early diagnosis and prediction of malignant brain edema in stroke patients to rapidly adopt life-saving surgical interventions. As reperfusion therapies become increasingly used in clinical practice, understanding how therapeutic reperfusion influences the formation of cerebral edema after ischemic stroke is critical for decision-making and post-reperfusion management. In this review, we summarize these research advances in the past decade on the cellular mechanisms, and evaluation, prediction, and intervention of ischemic brain edema in clinical settings, aiming to provide insight into future preclinical and clinical research on the diagnosis and treatment of brain edema after stroke.

Dynamic cerebral autoregulation measured by diffuse correlation spectroscopy

Dynamic cerebral autoregulation (dCA) can be derived from spontaneous oscillations in arterial blood pressure (ABP) and cerebral blood flow (CBF). Transcranial Doppler (TCD) measures CBF-velocity and is commonly used to assess dCA. Diffuse correlation spectroscopy (DCS) is a promising optical technique for non-invasive CBF monitoring, so here we aimed to validate DCS as a tool for quantifying dCA. In 33 healthy adults and 17 acute ischemic stroke patients, resting-state hemodynamic were monitored simultaneously with high-speed (20 Hz) DCS and TCD. dCA parameters were calcaulated by a transfer function analysis using a Fourier decomposition of ABP and CBF (or CBF-velocity). Strong correlation was found between DCS and TCD measured gain (magnitude of regulation) in healthy volunteers (r = 0.73, p < 0.001) and stroke patients (r = 0.76, p = 0.003). DCS-gain retained strong test-retest reliability in both groups (ICC 0.87 and 0.82, respectively). DCS and TCD-derived phase (latency of regulation) did not significantly correlate in healthy volunteers (r = 0.12, p = 0.50) but moderately correlated in stroke patients (r = 0.65, p = 0.006). DCS-derived phase was reproducible in both groups (ICC 0.88 and 0.90, respectively). High-frequency DCS is a promising non-invasive bedside technique that can be leveraged to quantify dCA from resting-state data, but the discrepancy between TCD and DCS-derived phase requires further investigation.

Neurobiological Mechanisms of Cognitive Decline Correlated with Brain Aging

Cognitive decline has emerged as one of the greatest health threats of old age. Meanwhile, aging is the primary risk factor for Alzheimer's disease (AD) and other prevalent neurodegenerative disorders. Developing therapeutic interventions for such conditions demands a greater understanding of the processes underlying normal and pathological brain aging. Despite playing an important role in the pathogenesis and incidence of disease, brain aging has not been well understood at a molecular level. Recent advances in the biology of aging in model organisms, together with molecular- and systems-level studies of the brain, are beginning to shed light on these mechanisms and their potential roles in cognitive decline. This chapter seeks to integrate the knowledge about the neurological mechanisms of age-related cognitive changes that underlie aging.

Transfer function analysis of dynamic cerebral autoregulation: A CARNet white paper 2022 update

Cerebral autoregulation (CA) refers to the control of cerebral tissue blood flow (CBF) in response to changes in perfusion pressure. Due to the challenges of measuring intracranial pressure, CA is often described as the relationship between mean arterial pressure (MAP) and CBF. Dynamic CA (dCA) can be assessed using multiple techniques, with transfer function analysis (TFA) being the most common. A 2016 white paper by members of an international Cerebrovascular Research Network (CARNet) that is focused on CA strove to improve TFA standardization by way of introducing data acquisition, analysis, and reporting guidelines. Since then, additional evidence has allowed for the improvement and refinement of the original recommendations, as well as for the inclusion of new guidelines to reflect recent advances in the field. This second edition of the white paper contains more robust, evidence-based recommendations, which have been expanded to address current streams of inquiry, including optimizing MAP variability, acquiring CBF estimates from alternative methods, estimating alternative dCA metrics, and incorporating dCA quantification into clinical trials. Implementation of these new and revised recommendations is important to improve the reliability and reproducibility of dCA studies, and to facilitate inter-institutional collaboration and the comparison of results between studies.

Incidence of Alzheimer's Disease in Men with Late-Life Hypertension Is Ameliorated by FOXO3 Longevity Genotype

BACKGROUND

It is well established that mid-life hypertension increases risk of dementia, whereas the association of late-life hypertension with dementia is unclear.

OBJECTIVE

To determine whether FOXO3 longevity-associated genotype influences the association between late-life hypertension and incident dementia.

METHODS

Subjects were 2,688 American men of Japanese ancestry (baseline age: 77.0±4.1 years, range 71-93 years) from the Kuakini Honolulu Heart Program. Status was known for FOXO3 rs2802292 genotype, hypertension, and diagnosis of incident dementia to 2012. Association of FOXO3 genotype with late-life hypertension and incident dementia, vascular dementia (VaD) and Alzheimer's disease (AD) was assessed using Cox proportional hazards models.

RESULTS

During 21 years of follow-up, 725 men were diagnosed with all-cause dementia, 513 with AD, and 104 with VaD. A multivariable Cox model, adjusting for age, education, APOEɛ4, and cardiovascular risk factors, showed late-life hypertension increased VaD risk only (HR = 1.71, 95% CI = 1.08-2.71, p = 0.022). We found no significant protective effect of FOXO3 longevity genotype on any type of dementia at the population level. However, in a full Cox model adjusting for age, education, APOEɛ4, and other cardiovascular risk factors, there was a significant interaction effect of late-life hypertension and FOXO3 longevity genotype on incident AD (β= -0.52, p = 0.0061). In men with FOXO3 rs2802292 longevity genotype (TG/GG), late-life hypertension showed protection against AD (HR = 0.72; 95% CI = 0.55-0.95, p = 0.021). The non-longevity genotype (TT) (HR = 1.16; 95% CI = 0.90-1.51, p = 0.25) had no protective effect.

CONCLUSION

This longitudinal study found late-life hypertension was associated with lower incident AD in subjects with FOXO3 genotype.

Risk of perioperative stroke and cerebral autoregulation monitoring: a systematic review

BACKGROUND

Perioperative stroke, delirium, and cognitive impairment could be related to management and to variations in blood pressure control, cerebral hypoperfusion and raised blood volume. Cerebral autoregulation (CAR) is a mechanism to maintain cerebral perfusion through the control of the vascular tone and hemodynamic reactions in the circulation.

OBJECTIVE

The present systematic review addresses the relationship between impaired CAR and perioperative stroke by evaluating the rate of neurological complications after surgery in studies in which perioperative CAR was tested or monitored.

METHODS

We included randomized clinical trials and prospective observational studies. All studies had adjusted the relative risk, hazard ratio or 95% confidence interval (95%CI) values. These estimation effects were tested using random-effects models. Heterogeneity among the selected studies was assessed using the Higgins and Thompson I² statistics.

RESULTS

The Web of Science, PubMed and EMBASE electronic databases were searched to retrieve articles. A total of 4,476 studies published between 1983 and 2019 were analyzed, but only 5 qualified for the data extraction and were included in the final analysis. The combined study cohort comprised 941 patients who underwent CAR monitoring during surgical procedures. All studies provided information about perioperative stroke, which equated to 16% (158 of 941) of the overall patient population.

CONCLUSION

The present meta-analysis showed evidence of the impact of CAR impairment in the risk of perioperative stroke. On the pooled analysis, blood fluctuations or other brain insults large enough to compromise CAR were associated with the outcome of stroke (odds ratio [OR]: 2.26; 95%CI: 1.54-2.98; p < 0.0001).

Relationships between common carotid artery blood flow and anesthesia, pneumoperitoneum, and head-down tilt position: a linear mixed-effect analysis

This study investigated the effects of pneumoperitoneum and the head-down tilt position on common carotid artery (CCA) blood flow in surgical patients.

METHODS

This prospective observational study included 20 patients who underwent gynecological surgery. CCA blood flow was measured using Doppler ultrasound at four-time points: awake in the supine position [T1], 3 min after anesthesia induction in the supine position [T2], 3 min after pneumoperitoneum in the supine position [T3], and 3 min after pneumoperitoneum in the head-down tilt position [T4]. Hemodynamic and respiratory parameters were also recorded at each time point. Linear mixed-effect analyses were performed to compare CCA blood flow across the time points and assess its relationship with hemodynamic parameters.

RESULTS

Compared with T1, CCA blood flow decreased significantly at T2 (345.4 [288.0-392.9] vs. 293.1 [253.0-342.6], P = 0.048). CCA blood flow were also significantly lower at T3 and T4 compared with T1 (345.4 [288.0-392.9] vs. 283.6 [258.8-307.6] and 287.1 [242.1-321.4], P = 0.005 and 0.016, respectively). CCA blood flow at T3 and T4 did not significantly differ from that at T2. Changes in CCA blood flow were significantly associated with changes in cardiac index and stroke volume index (P = 0.011 and 0.024, respectively).

CONCLUSION

CCA blood flow was significantly decreased by anesthesia induction. Inducing pneumoperitoneum, with or without the head-down tilt position, did not further decrease CCA blood flow if the cardiac index remained unchanged. The cardiac index and stroke volume index were significantly associated with CCA blood flow.

CLINICAL TRIAL REGISTRATION

Clinicaltrials.gov (NCT04233177, January 18, 2020).

Effects of Hypertension on Alzheimer's Disease and Related Disorders

PURPOSE OF REVIEW

To review the pathophysiology of hypertension in Alzheimer's disease and related dementias and explore the current landscape of clinical trials involving treatment of hypertension to improve cognition.

RECENT FINDINGS

Hypertension is increasingly recognized as a contributor to cognitive impairment. Clinical trials that explore blood pressure reductions with cognitive outcomes have been promising. Various antihypertensives have been evaluated in clinical trials, with growing interest in those agents that impact the renin-angiotensin-aldosterone system due to its own association with cognitive impairment. No antihypertensive agent has been found to be superior to others in reducing cognitive impairment risk or conferring neuroprotective benefits. In this review, the pathophysiology of and clinical trial data involving hypertension and dementia will be explored. Hypertension is a significant risk factor for the development of neurodegenerative dementias, and clinical trials have been overall favorable in improving cognition by reductions in blood pressure using antihypertensive agents.

Investigation of Cerebral Autoregulation Using Time-Frequency Transformations

The authors carried out the study of the state of systemic and cerebral hemodynamics in normal conditions and in various neurosurgical pathologies using modern signal processing methods. The results characterize the condition for the mechanisms of cerebral circulation Institute of Computer Science and Control, Higher School of Cyber-Physical Systems and Control regulation, which allows for finding a solution to fundamental and specific clinical problems for the effective treatment of patients with various pathologies. The proposed method is based on the continuous wavelet transform of systemic arterial pressure and blood flow velocity signals in the middle cerebral artery recorded by non-invasive methods of photoplethysmography and transcranial doppler ultrasonography. The study of these signals in real-time in the frequency range of Mayer waves makes it possible to determine the cerebral autoregulation state in certain diseases before and after surgical interventions. The proposed method uses a cross-wavelet spectrum, which helps obtain wavelet coherence and a phase shift between the wavelet coefficients of systemic arterial pressure signals and blood flow velocity in the Mayer wave range. The obtained results enable comparing the proposed method with that based on the short-time Fourier transform. The comparison showed that the proposed method has higher sensitivity to changes in cerebral autoregulation and better localization of changes in time and frequency.

Dynamic effects of cholinergic blockade upon cerebral blood flow autoregulation in healthy adults

Background: Cerebral flow autoregulation (CFA) is a homeostatic mechanism critical for survival. The autonomic nervous system (ANS) plays a key role in maintaining proper CFA function. More quantitative studies of how the ANS influences CFA are desirable. Objective: To discover and quantify the dynamic effects of cholinergic blockade upon CFA in response to changes of arterial blood pressure and blood CO2 tension in healthy adults. Methods: We analyzed time-series data of spontaneous beat-to-beat mean arterial blood pressure (ABP) and cerebral blood flow velocity in the middle cerebral arteries (CFV), as well as breath-to-breath end-tidal CO2 (CO2), collected in 9 adults before and after cholinergic blockade, in order to obtain subject-specific predictive input-output models of the dynamic effects of changes in ABP and CO2 (inputs) upon CFV (output). These models are defined in convolutional form using "kernel" functions (or, equivalently, Transfer Functions in the frequency domain) that are estimated via the robust method of Laguerre expansions. Results: Cholinergic blockade caused statistically significant changes in the obtained kernel estimates (and the corresponding Transfer Functions) that define the linear dynamics of the ABP-to-CFV and CO2-to-CFV causal relations. The kernel changes due to cholinergic blockade reflect the effects of the cholinergic mechanism and exhibited, in the frequency domain, resonant peaks at 0.22 Hz and 0.06 Hz for the ABP-to-CFV and CO2-to-CFV dynamics, respectively. Conclusion: Quantitative estimates of the dynamics of the cholinergic component in CFA are found as average changes of the ABP-to-CFV and CO2-to-CFV kernels, and corresponding Transfer Functions, before and after cholinergic blockade.

Longitudinal Changes in Global Cerebral Blood Flow in Cognitively Normal Older Adults: A Phase-Contrast MRI Study

BACKGROUND

Characterization of blood supply changes in older individuals is important in understanding brain aging and diseases. However, prior studies largely focused on cross-sectional design, thus change in cerebral blood flow (CBF) could not be assessed on an individual level.

PURPOSE

To evaluate longitudinal short-term changes in global CBF in cognitively normal older adults.

STUDY TYPE

Prospective, longitudinal, and cohort.

POPULATION

One-hundred twenty-seven cognitive-normal participants (mean age 69 ± 7 years, 47 males) underwent serial MRI with an average follow-up time of 2.1 years.

FIELD STRENGTH/SEQUENCE

3 T phase-contrast (PC), three-dimensional magnetization-prepared-rapid-acquisition-of-gradient-echo (MPRAGE) and fluid-attenuated inversion recovery (FLAIR) MRI.

ASSESSMENT

Total CBF was measured with PC MRI allowing assessment of quantitative flow in four major feeding arteries by a trained radiologist with >3 years' experience (O.K.). Brain volume was obtained from MPRAGE MRI and measured by T1-MultiAtlas MRICloud tool. The ratio between total CBF and brain volume yielded global CBF in mL/100 g/min. White matter hyperintensity (WMH) was measured automatically using a Bayesian probability approach on FLAIR.

STATISTICAL TESTS

Linear mixed effect model was used to simultaneously assess cross-sectional age-differences and longitudinal age-changes in CBF. Spearman rank correlation was used to evaluate the relationship between CBF change and WMH progression. A P-value of <0.05 (two-tailed) was considered significant.

RESULTS

Global CBF decreased with age at a longitudinal rate of -0.56 mL/100 g/min/year (95% confidence interval [CI]: -1.09, -0.03), compared to a cross-sectional rate of -0.26 mL/100 g/min/year (95% CI: -0.41, -0.11). Changes in CBF were significantly associated with progression of WMH (Spearman rank correlation r = -0.25), as those participants who had a more rapid CBF reduction had greater increases in WMH volumes and the relationship remained significant when adjusting for baseline vascular risk scores. Additionally, age-related changes in whole-brain volume were found to be -0.151%/year (95% CI: -0.186, -0.116).

DATA CONCLUSION

These findings suggest that brain aging in older adults is accompanied by a rapid longitudinal reduction in CBF, the rate of which is associated with white matter damage.

LEVEL OF EVIDENCE

1 TECHNICAL EFFICACY STAGE: 2.

Diagnostic and prognostic performance of Mxa and transfer function analysis-based dynamic cerebral autoregulation metrics

Dynamic cerebral autoregulation is often assessed by continuously recorded arterial blood pressure (ABP) and transcranial Doppler-derived mean cerebral blood flow velocity followed by analysis in the time and frequency domain, respectively. Sequential correlation (in the time domain, yielding e.g., the measure mean flow index, Mxa) and transfer function analysis (TFA) (in the frequency domain, yielding, e.g., normalised and non-normalised gain as well as phase in the low frequency domain) are commonly used approaches. This study investigated the diagnostic and prognostic performance of these metrics. We included recordings from 48 healthy volunteers, 19 patients with sepsis, 36 with traumatic brain injury (TBI), and 14 patients admitted to a neurorehabilitation unit. The diagnostic (between healthy volunteers and patients) and prognostic performance (to predict death or poor functional outcome) of Mxa and the TFA measures were assessed by area under the receiver-operating characteristic (AUROC) curves. AUROC curves generally indicated that the measures were 'no better than chance' (AUROC ∼0.5) both for distinguishing between healthy volunteers and patient groups, and for predicting outcomes in our cohort. No metric emerged as superior for distinguishing between healthy volunteers and different patient groups, for assessing the effect of interventions, or for predicting mortality or functional outcome.

Impaired dynamic cerebral autoregulation: A potential mechanism of orthostatic hypotension and dementia in Parkinson’s disease

Background

Orthostatic hypotension (OH) and cognitive impairment are common non-motor symptoms of Parkinson’s disease (PD). This study aimed to investigate whether impaired dynamic cerebral autoregulation (dCA) is associated with OH and Parkinson’s disease dementia (PDD), and analyze the related risk factors in patients with PDD.

Materials and methods

We enrolled 89 patients with PD and 20 age- and sex-matched healthy controls (HCs). Cognition and different cognitive domains were assessed by the Montreal Cognitive Assessment scale. Non-invasive continuous beat-to-beat blood pressure and cerebral blood flow velocity were assessed using a servo-controlled finger plethysmograph and transcranial Doppler, respectively. dCA was examined using supine and orthostatic changes with transfer function analysis to derive the autoregulatory parameters of phase, gain, and coherence. Logistic regression analysis was performed to determine the risk factors for PDD.

Results

We found that 21 (23.6%) patients with PD had OH. These patients showed worse cognitive performance in specific cognitive tasks, such as language and orientation. The patients with OH also had poorer dCA; the very low frequency (VLF) phase in two different postures was lower than that in patients without OH as well as HCs (both P < 0.05). And the normalized gain in the VLF and low frequency (LF) in standing position was higher in PD patients with and without OH than in HCs. PDD patients also had significantly higher LF normalized gain when standing than patients without dementia (P = 0.015), indicating impaired dCA. LF normalized gain in standing (odds ratio: 3.756, 95% confidence interval: 1.241–11.367) and education were significantly associated with PDD.

Conclusion

Diminished dCA may represent a potential mechanism for OH and cognitive impairment and low educational level might be a significant factor contributing to the increased risk of PDD.

Cerebral Autoregulation Assessment Using the Near Infrared Spectroscopy ‘NIRS-Only’ High Frequency Methodology in Critically Ill Patients: A Prospective Cross-Sectional Study

Impairments in cerebral autoregulation (CA) are related to poor clinical outcome. Near infrared spectroscopy (NIRS) is a non-invasive technique applied to estimate CA. Our general purpose was to study the clinical feasibility of a previously published ‘NIRS-only’ CA methodology in a critically ill intensive care unit (ICU) population and determine its relationship with clinical outcome. Bilateral NIRS measurements were performed for 1–2 h. Data segments of ten-minutes were used to calculate transfer function analyses (TFA) CA estimates between high frequency oxyhemoglobin (oxyHb) and deoxyhemoglobin (deoxyHb) signals. The phase shift was corrected for serial time shifts. Criteria were defined to select TFA phase plot segments (segments) with ‘high-pass filter’ characteristics. In 54 patients, 490 out of 729 segments were automatically selected (67%). In 34 primary neurology patients the median (q1–q3) low frequency (LF) phase shift was higher in 19 survivors compared to 15 non-survivors (13° (6.3–35) versus 0.83° (−2.8–13), p = 0.0167). CA estimation using the NIRS-only methodology seems feasible in an ICU population using segment selection for more robust and consistent CA estimations. The ‘NIRS-only’ methodology needs further validation, but has the advantage of being non-invasive without the need for arterial blood pressure monitoring.

The Spatiotemporal Dynamics of Cerebral Autoregulation in Functional Magnetic Resonance Imaging

The thigh-cuff release (TCR) maneuver is a physiological challenge that is widely used to assess dynamic cerebral autoregulation (dCA). It is often applied in conjunction with Transcranial Doppler ultrasound (TCD), which provides temporal information of the global flow response in the brain. This established method can only yield very limited insights into the regional variability of dCA, whereas functional MRI (fMRI) has the ability to reveal the spatial distribution of flow responses in the brain with high spatial resolution. The aim of this study was to use whole-brain blood-oxygenation-level-dependent (BOLD) fMRI to characterize the spatiotemporal dynamics of the flow response to the TCR challenge, and thus pave the way toward mapping dCA in the brain. We used a data driven approach to derive a novel basis set that was then used to provide a voxel-wise estimate of the TCR associated haemodynamic response function (HRF TCR ). We found that the HRF TCR evolves with a specific spatiotemporal pattern, with gray and white matter showing an asynchronous response, which likely reflects the anatomical structure of cerebral blood supply. Thus, we propose that TCR challenge fMRI is a promising method for mapping spatial variability in dCA, which will likely prove to be clinically advantageous.

Blood flow estimation via numerical integration of temporal autocorrelation function in diffuse correlation spectroscopy

BACKGROUND AND OBJECTIVE

Diffuse correlation spectroscopy (DCS) is an optical technique widely used to monitor blood flow. Recently, efforts have been made to derive new signal processing methods to minimize the systems used and shorten the signal processing time. Herein, we propose alternative approaches to obtain blood flow information via DCS by numerically integrating the temporal autocorrelation curves.

METHODS

We use the following methods: the inverse of K² (IK2)-based on the framework of diffuse speckle contrast analysis-and the inverse of the numerical integration of squared g₁ (INISg1) which, based on the normalized electric field autocorrelation curve, is more simplified than IK2. In addition, g₁ thresholding is introduced to further reduce computational time and make the suggested methods comparable to the conventional nonlinear fitting approach. To validate the feasibility of the suggested methods, studies using simulation, liquid phantom, and in vivo settings were performed. In the meantime, the suggested methods were implemented and tested on three types of Arduino (Arduino Due, Arduino Nano 33 BLE Sense, and Portenta H7) to demonstrate the possibility of miniaturizing the DCS systems using microcotrollers for signal processing.

RESULTS

The simulation and experimental results confirm that both IK2 and INISg1 are sufficiently relevant to capture the changes in blood flow information. More interestingly, when g₁ thresholding was applied, our results showed that INISg1 outperformed IK2. It was further confirmed that INISg1 with g₁ thresholding implemented on a PC and Portenta H7, an advanced Arduino board, performed faster than did the deep learning-based, state-of-the-art processing method.

CONCLUSION

Our findings strongly indicate that INISg1 with g₁ thresholding could be an alternative approach to derive relative blood flow information via DCS, which may contribute to the simplification of DCS methodologies.

Vascular pathology and pathogenesis of cognitive impairment and dementia in older adults

It is well recognized that brains of older people often harbor cerebrovascular disease pathology including vessel disease and vascular-related tissue injuries and that this is associated with vascular cognitive impairment and contributes to dementia. Here we review vascular pathologies, cognitive impairment, and dementia. We highlight the importance of mixed co-morbid AD/non-AD neurodegenerative and vascular pathology that has been collected in multiple clinical pathologic studies, especially in community-based studies. We also provide an update of vascular pathologies from the Rush Memory and Aging Project and Religious Orders Study cohorts with special emphasis on the differences across age in persons with and without dementia. Finally, we discuss neuropathological perspectives on the interpretation of clinical-pathological studies and emerging data in community-based studies.

Identifying Cognitive Impairment in Elderly Using Coupling Functions Between Cerebral Oxyhemoglobin and Arterial Blood Pressure

Background

This study aimed to assess brain oxygenation status and cerebral autoregulation function in subjects with cognitive dysfunction.

Methods

The Montreal Cognitive Assessment (MoCA) was applied to divide the subjects into three groups: cognitive impairment (Group CI, 72.50 ± 10.93 y), mild cognitive impairment (Group MCI, 72.02 ± 9.90 y), and normal cognition (Group NC, 70.72 ± 7.66 y). Near-infrared spectroscopy technology and a non-invasive blood pressure device were used to simultaneously measure changes in cerebral tissue oxygenation signals in the bilateral prefrontal lobes (LPFC/RPFC) and arterial blood pressure (ABP) signals from subjects in the resting state (15 min). The coupling between ABP and cerebral oxyhemoglobin concentrations (Δ [O2Hb]) was calculated in very-low-frequency (VLF, 0.02-0.07 Hz) and low-frequency (LF, 0.07-0.2 Hz) bands based on the dynamical Bayesian inference approach. Pearson correlation analyses were used to study the relationships between MoCA scores, tissue oxygenation index, and strength of coupling function.

Results

In the interval VLF, Group CI (p = 0.001) and Group MCI (p = 0.013) exhibited significantly higher coupling strength from ABP to Δ [O2Hb] in the LPFC than Group NC. In the interval LF, coupling strength from ABP to Δ [O2Hb] in the LPFC was significantly higher in Group CI than in Group NC (p = 0.001). Pearson correlation results showed that MoCA scores had a significant positive correlation with the tissue oxygenation index and a significant negative correlation with the coupling strength from ABP to Δ [O2Hb].

Conclusion

The significantly increased coupling strength may be evidence of impaired cerebral autoregulation function in subjects with cognitive dysfunction. The Pearson correlation results suggest that indicators of brain oxygenation status and cerebral autoregulation function can reflect cognitive function. This study provides insights into the mechanisms underlying the pathophysiology of cognitive impairment and provides objective indicators for screening cognitive impairment in the elderly population.

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.

Cerebrovascular function and its association with systemic artery function and stiffness in older adults with and without mild cognitive impairment

PURPOSE

Our aim was to compare cerebrovascular and systemic vascular function between older adults with and without mild cognitive impairment (MCI), and to determine which measures of vascular function best predict the presence of MCI.

METHODS

In 41 adults with MCI and 33 adults without MCI (control) we compared middle cerebral artery velocity (MCAv) and cerebrovascular pulsatility index (PI) at rest, cerebrovascular reactivity to CO₂, and responsiveness to changes in blood pressure (%∆MCAv/%∆MAP). Systemic vascular function was assessed by flow-mediated dilation (FMD) and stiffness by pulse wave velocity (PWV).

RESULTS

Cerebrovascular PI was higher in MCI compared with control (mean ± SD: 1.17 ± 0.27 vs. 1.04 ± 0.21), and MCI exhibited a lower %∆MCAv/%∆MAP (1.26 ± 0.44 vs. 1.50 ± 0.55%). Absolute (p = 0.76) and relative cerebrovascular reactivity to CO₂ (p = 0.34) was similar between MCI and control. When age was included as a covariate the significant difference in cerebral PI between groups was lost. PWV was higher (13.2 ± 2.2 vs. 11.3 ± 2.5 m s^-1) and FMD% (4.41 ± 1.70 vs. 5.43 ± 2.15%) was lower in MCI compared with control. FMD% was positively associated with PI across the cohort. Logistic regression analysis indicated that FMD and PWV significantly discriminated between MCI and controls, independent of age, whereas the inclusion of cerebrovascular measures did not improve the predictive accuracy of the model.

CONCLUSION

These findings raise the possibility that early changes in systemic vascular stiffness and endothelial function may contribute to altered cerebrovascular haemodynamics and impaired cognitive function, and present potential targets for prevention and treatment strategies in people with MCI.

Cerebral Augmentation Effect Induced by External Counterpulsation Is Not Related to Impaired Dynamic Cerebral Autoregulation in Ischemic Stroke

Background and Purpose

Dynamic cerebral autoregulation is impaired after ischemic stroke. External counterpulsation (ECP) augments the cerebral blood flow of patients with ischemic stroke by elevation of blood pressure (BP). We aimed to investigate if cerebral augmentation effects during ECP were associated with impaired dynamic cerebral autoregulation in patients after acute ischemic stroke.

Methods

Forty patients with unilateral ischemic stroke and large artery atherosclerosis in the anterior circulation territory within 7 days from symptom onset and eighteen healthy controls were recruited. We monitored changes in mean flow velocity over both middle cerebral arteries (MCA) by transcranial Doppler (TCD) before, during, and immediately after ECP. Cerebral augmentation index was MCA mean flow velocity increase in percentage during ECP compared with baseline to evaluate the augmentation effects of ECP. Spontaneous arterial BP and cerebral blood flow velocity in both bilateral MCAs were recorded using a servo-controlled plethysmograph and TCD, respectively. Transfer function analysis was used to derive the autoregulatory parameters, including phase difference (PD), and gain.

Results

The cerebral augmentation index in patients with stroke was significantly higher on both the ipsilateral and contralateral sides than that in controls, while the PD in patients with stroke was significantly lower on both sides than those in controls (all P < 0.05). The cerebral augmentation index did not correlate with PD and gain on either the ipsilateral or contralateral side of patients with stroke or in controls (all P > 0.05). The cerebral augmentation index of patients with stroke was significantly related to mean BP change on the ipsilateral side (R² = 0.108, P = 0.038).

Conclusion

The degree of ECP-induced cerebral augmentation effects as measured by the cerebral augmentation index did not correlate with the magnitude of impaired dynamic cerebral autoregulation.

Effect of Antihypertensive Treatment on Cerebral Blood Flow in Older Adults: a Systematic Review and Meta-Analysis

BACKGROUND

In older age, the benefits of antihypertensive treatment (AHT) become less evident, with greater associated risk. Of particular concern is compromising cerebral blood flow (CBF), especially in those with cognitive impairment.

METHODS

We created a synthesis of the published evidence by searching multiple electronic databases from 1970 to May 2021. Included studies had participants with mean age ≥50 years, hypertension or cognitive impairment, and assessed CBF before and after initiating AHT. Two authors independently determined eligibility and extracted data. Study quality was assessed using The Risk of Bias in Nonrandomized Studies of Interventions tool. We summarized study characteristics (qualitative synthesis) and performed random-effects meta-analyses (quantitative synthesis).

RESULTS

Thirty-two studies (total n=1306) were included, of which 23 were eligible for meta-analysis. In line with the qualitative synthesis, the meta-analysis indicated no effect of AHT initiation on CBF (standardized mean difference, 0.08 [95% CI, -0.07 to 0.22]; P=0.31, I2=42%). This was consistent across subgroups of acute versus chronic AHT, drug class, study design, and CBF measurement. Subgroups by age demonstrated an increase in CBF after AHT in those aged >70 years (standardized mean difference, 4.15 [95% CI, 0.16-8.15]; P=0.04, I2=42%), but not in those aged 50 to 65 and 65 to 70 years (standardized mean difference, 0.18 [95% CI,-2.02 to 2.38]; P=0.87, I2=49%; standardized mean difference, 1.22 [95% CI, -0.45 to 2.88]; P=0.15, I2=68%). Overall, risk of bias was moderate-to-high and quality of evidence (Grading of Recommendations Assessment, Development and Evaluation) was very low, reflecting the observational nature of the data.

CONCLUSIONS

Accepting the observed limitations, current evidence does not suggest a harmful effect of AHT on CBF. Concerns over CBF should not preclude treatment of hypertension.

Cerebrovascular reactivity deficits in cognitively unimpaired older adults: vasodilatory versus vasoconstrictive responses

Cerebrovascular reactivity (CVR) deficits may index vulnerability to vascular brain injury and cognitive impairment, but findings on age-related changes in CVR have been mixed, and no studies to date have directly compared age-related changes in CVR to hypercapnia versus hypocapnia. The present study compared CVR in 31 cognitively unimpaired older adults (ages 55-87) and 30 healthy younger adults (ages 18-28). Breath control tasks induced CVR to hypocapnia (0.1 Hz paced breathing) and hypercapnia (15s breath holds) during pseudo-continuous arterial spin labeling MRI. Relative to younger adults, cognitively unimpaired older adults displayed lower levels of global CVR under both hypocapnia and hypercapnia. In region-of-interest analyses, older adults exhibited attenuated CVR to hypocapnia in select frontal and temporal regions, and lower CVR to hypercapnia in all cortical, limbic, and subcortical regions examined, relative to younger adults. Results indicate age-related deficits in CVR are detectible even in cognitively unimpaired older adults and are disproportionately related to vasodilatory (hypercapnia) responses relative to vasoconstrictive (hypocapnia) responses. Findings may offer means for early detection of cerebrovascular dysfunction.