Determinants of human cerebral pressure-flow velocity relationships: new insights from vascular modelling and Ca²⁺ channel blockade

The effects of habitual resistance exercise training on cerebrovascular responses to lower body dynamic resistance exercise: A cross-sectional study

Dynamic resistance exercise (RE) produces sinusoidal fluctuations in blood pressure with simultaneous fluctuations in middle cerebral artery blood velocity (MCAv). Some evidence indicates that RE may alter cerebrovascular function. This study aimed to examine the effects of habitual RE training on the within-RE cerebrovascular responses. RE-trained (n = 15, Female = 4) and healthy untrained individuals (n = 15, Female = 12) completed four sets of 10 paced repetitions (15 repetitions per minute) of unilateral leg extension exercise at 60% of predicted 1 repetition maximum. Beat-to-beat blood pressure, MCAv and end-tidal carbon dioxide were measured throughout. Zenith, nadir and zenith-to-nadir difference in mean arterial blood pressure (MAP) and mean MCAv (MCAvmean) for each repetition were averaged across each set. Two-way ANOVA was used to analyse dependent variables (training × sets), Bonferroni corrected t-tests were used for post hoc pairwise comparisons. Group age (26 ± 7 trained vs. 25 ± 6 years untrained, P = 0.683) and weight (78 ± 15 vs. 71 ± 15 kg, P = 0.683) were not different. During exercise average MAP was greater for the RE-trained group in sets 2, 3 and 4 (e.g., set 4: 101 ± 11 vs. 92 ± 7 mmHg for RE trained and untrained, respectively, post hoc tests all P = < 0.012). Zenith MAP and zenith-to-nadir MAP difference demonstrated a training effect (P < 0.039). Average MCAvmean and MCAvmean zenith-to-nadir difference was not different between groups (interaction effect P = 0.166 and P = 0.459, respectively). Despite RE-trained individuals demonstrating greater fluctuations in MAP during RE compared to untrained, there were no differences in MCAvmean. Regular RE may lead to vascular adaptations that stabilise MCAv during RE.

Rapid changes in cerebrovascular compliance during vasovagal syncope

PURPOSE

The compensatory mechanisms supporting cerebral perfusion throughout head-up tilt (HUT) in patients with vasovagal syncope (VVS) remain unclear. We tested the hypothesis that increased cerebrovascular compliance (Ci) and decreased cerebrovascular resistance (CVR) support cerebral blood velocity (CBV) during pre-syncope in VVS.

METHODS

Finger arterial blood pressure (ABP) and right middle cerebral artery blood velocity (CBV) were recorded in 15 individuals diagnosed with VVS (n = 11 female, mean age: 40 ± 16 years, mean body mass index: 24.9 ± 4.0 kg/m2) at supine rest and during HUT (80 degree angle). Individual ABP and CBV waveforms during VVS were input into a modified Windkessel model to calculate Ci and ohmic CVR. Gosling's pulsatility index (Pi; pulse amplitude/mean CBV) was calculated.

RESULTS

Diastolic ABP, systolic ABP, mean ABP (72 ± 11 to 51 ± 12 mmHg), and CVR decreased progressively during presyncope (all P ≤ 0.04). As expected, systolic CBV was sustained (all P ≥ 0.29) while diastolic and mean CBV (51 ± 13 to 38 ± 13 mmHg) fell during presyncope (all P ≤ 0.04). Both Ci and Pi increased during presyncope (128 ± 97 and 60 ± 41%, respectively; all P ≤ 0.049) and were positively correlated (R2 = 0.79, P < 0.01). Increased Ci contributed to changes in mean CBV (P < 0.01) but decreased CVR did not (P = 0.28).

CONCLUSIONS

These data provide evidence that Ci increases during presyncope in patients with VVS and is likely involved in the maintenance of systolic CBV during a fall in diastolic CBV. However, this regulation is not sufficient to preserve CBV in the presence of such extreme and progressive reductions in ABP.

A systematic review, meta-analysis, and meta-regression amalgamating the driven approaches used to quantify dynamic cerebral autoregulation

Numerous driven techniques have been utilized to assess dynamic cerebral autoregulation (dCA) in healthy and clinical populations. The current review aimed to amalgamate this literature and provide recommendations to create greater standardization for future research. The PubMed database was searched with inclusion criteria consisting of original research articles using driven dCA assessments in humans. Risk of bias were completed using Scottish Intercollegiate Guidelines Network and Methodological Index for Non-Randomized Studies. Meta-analyses were conducted for coherence, phase, and gain metrics at 0.05 and 0.10 Hz using deep-breathing, oscillatory lower body negative pressure (OLBNP), sit-to-stand maneuvers, and squat-stand maneuvers. A total of 113 studies were included, with 40 of these incorporating clinical populations. A total of 4126 participants were identified, with younger adults (18-40 years) being the most studied population. The most common techniques were squat-stands (n = 43), deep-breathing (n = 25), OLBNP (n = 20), and sit-to-stands (n = 16). Pooled coherence point estimates were: OLBNP 0.70 (95%CI:0.59-0.82), sit-to-stands 0.87 (95%CI:0.79-0.95), and squat-stands 0.98 (95%CI:0.98-0.99) at 0.05 Hz; and deep-breathing 0.90 (95%CI:0.81-0.99); OLBNP 0.67 (95%CI:0.44-0.90); and squat-stands 0.99 (95%CI:0.99-0.99) at 0.10 Hz. This review summarizes clinical findings, discusses the pros/cons of the 11 unique driven techniques included, and provides recommendations for future investigations into the unique physiological intricacies of dCA.

Quantification of dynamic cerebral autoregulation: welcome to the jungle!

PURPOSE

Patients with dysautonomia often experience symptoms such as dizziness, syncope, blurred vision and brain fog. Dynamic cerebral autoregulation, or the ability of the cerebrovasculature to react to transient changes in arterial blood pressure, could be associated with these symptoms.

METHODS

In this narrative review, we go beyond the classical view of cerebral autoregulation to discuss dynamic cerebral autoregulation, focusing on recent advances pitfalls and future directions.

RESULTS

Following some historical background, this narrative review provides a brief overview of the concept of cerebral autoregulation, with a focus on the quantification of dynamic cerebral autoregulation. We then discuss the main protocols and analytical approaches to assess dynamic cerebral autoregulation, including recent advances and important issues which need to be tackled.

CONCLUSION

The researcher or clinician new to this field needs an adequate comprehension of the toolbox they have to adequately assess, and interpret, the complex relationship between arterial blood pressure and cerebral blood flow in healthy individuals and clinical populations, including patients with autonomic disorders.

Cerebrovascular and cardiovascular responses to the Valsalva manoeuvre during hyperthermia

BACKGROUND

During hyperthermia, the perturbations in mean arterial blood pressure (MAP) produced by the Valsalva manoeuvre (VM) are more severe. However, whether these more severe VM-induced changes in MAP are translated to the cerebral circulation during hyperthermia is unclear.

METHODS

Healthy participants (n = 12, 1 female, mean ± SD: age 24 ± 3 years) completed a 30 mmHg (mouth pressure) VM for 15 s whilst supine during normothermia and mild hyperthermia. Hyperthermia was induced passively using a liquid conditioning garment with core temperature measured via ingested temperature sensor. Middle cerebral artery blood velocity (MCAv) and MAP were recorded continuously during and post-VM. Tieck's autoregulatory index was calculated from the VM responses, with pulsatility index, an index of pulse velocity (pulse time) and mean MCAv (MCAvmean ) also calculated.

RESULTS

Passive heating significantly raised core temperature from baseline (37.9 ± 0.2 vs. 37.1 ± 0.1°C at rest, p < 0.01). MAP during phases I through III of the VM was lower during hyperthermia (interaction effect p < 0.01). Although an interaction effect was observed for MCAvmean (p = 0.02), post-hoc differences indicated only phase IIa was lower during hyperthermia (55 ± 12 vs. 49.3 ± 8 cm s- 1 for normothermia and hyperthermia, respectively, p = 0.03). Pulsatility index was increased 1-min post-VM in both conditions (0.71 ± 0.11 vs. 0.76 ± 0.11 for pre- and post-VM during normothermia, respectively, p = 0.02, and 0.86 ± 0.11 vs. 0.99 ± 0.09 for hyperthermia p < 0.01), although for pulse time only main effects of time (p < 0.01), and condition (p < 0.01) were apparent.

CONCLUSION

These data indicate that the cerebrovascular response to the VM is largely unchanged by mild hyperthermia.

Regulation of cerebrovascular compliance compared with forearm vascular compliance in humans: a pharmacological study

Increasing evidence indicates that cerebrovascular compliance contributes to the dynamic regulation of cerebral blood flow but the mechanisms regulating cerebrovascular compliance in humans are unknown. This retrospective study investigated the impact of neural, endothelial, and myogenic mechanisms on the regulation of vascular compliance in the cerebral vascular bed compared with the forearm vascular bed. An index of vascular compliance (C_i) was assessed using a Windkessel model applied to blood pressure waveforms (finger photoplethysmography) and corresponding middle cerebral artery blood velocity or brachial artery blood velocity waveforms (Doppler ultrasound). Data were analyzed during a 5-min baseline period (10 waveforms) under control conditions and during distinct sympathetic blockade (experiment 1, phentolamine; 10 adults), cholinergic blockade (experiment 2, glycopyrrolate; 9 adults), and myogenic blockade (experiment 3, nicardipine; 14 adults). In experiment 1, phentolamine increased C_i similarly in the cerebral vascular bed (131 ± 135%) and forearm vascular bed (93 ± 75%; P = 0.45). In experiment 2, glycopyrrolate increased cerebrovascular C_i (72 ± 61%) and forearm vascular C_i (74 ± 64%) to a similar extent (P = 0.88). In experiment 3, nicardipine increased C_i but to a greater extent in the cerebral vascular bed (88 ± 88%) than forearm vascular bed (20 ± 45%; P = 0.01). Therefore, adrenergic, cholinergic, and myogenic mechanisms contribute to the regulation of cerebrovascular and forearm vascular compliance. However, myogenic mechanisms appear to exert more specific control over vascular compliance in the brain relative to the forearm.NEW & NOTEWORTHY Vascular compliance represents an important determinant in the dynamics and regulation of blood flow through a vascular bed. However, the mechanisms that regulate vascular compliance remain poorly understood. This study examined the impact of neural, endothelial, and myogenic mechanisms on cerebrovascular compliance compared with forearm vascular compliance. Distinct pharmacological blockade of α-adrenergic, endothelial muscarinic, and myogenic inputs altered cerebrovascular and forearm vascular compliance. These results further our understanding of vascular control and blood flow regulation in the brain.

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.

The potential therapeutic benefits of low frequency haemodynamic oscillations

Haemodynamic oscillations occurring at frequencies below the rate of respiration have been observed experimentally for more than a century. Much of the research regarding these oscillations, observed in arterial pressure and blood flow, has focused on mechanisms of generation and methods of quantification. However, examination of the physiological role of these oscillations has been limited. Multiple studies have demonstrated that oscillations in arterial pressure and blood flow are associated with the protection in tissue oxygenation or functional capillary density during conditions of reduced tissue perfusion. There is also evidence that oscillatory blood flow can improve clearance of interstitial fluid, with a growing number of studies demonstrating a role for oscillatory blood flow to aid in clearance of debris from the brain. The therapeutic potential of these haemodynamic oscillations is an important new area of research which may have beneficial impact in treating conditions such as stroke, cardiac arrest, blood loss injuries, sepsis, or even Alzheimer's disease and vascular dementia.

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.

Integrative cerebral blood flow regulation in ischemic stroke

Optimizing cerebral perfusion is key to rescuing salvageable ischemic brain tissue. Despite being an important determinant of cerebral perfusion, there are no effective guidelines for blood pressure (BP) management in acute stroke. The control of cerebral blood flow (CBF) involves a myriad of complex pathways which are largely unaccounted for in stroke management. Due to its unique anatomy and physiology, the cerebrovascular circulation is often treated as a stand-alone system rather than an integral component of the cardiovascular system. In order to optimize the strategies for BP management in acute ischemic stroke, a critical reappraisal of the mechanisms involved in CBF control is needed. In this review, we highlight the important role of collateral circulation and re-examine the pathophysiology of CBF control, namely the determinants of cerebral perfusion pressure gradient and resistance, in the context of stroke. Finally, we summarize the state of our knowledge regarding cardiovascular and cerebrovascular interaction and explore some potential avenues for future research in ischemic stroke.

Effect of drug interventions on cerebral hemodynamics in ischemic stroke patients

The ischemic penumbra is sensitive to alterations in cerebral perfusion. A myriad of drugs are used in acute ischemic stroke (AIS) management, yet their impact on cerebral hemodynamics is poorly understood. As part of the Cerebral Autoregulation Network led INFOMATAS project (Identifying New Targets for Management and Therapy in Acute Stroke), this paper reviews some of the most common drugs a patient with AIS will come across and their potential influence on cerebral hemodynamics with a particular focus being on cerebral autoregulation (CA). We first discuss how compounds that promote clot lysis and prevent clot formation could potentially impact cerebral hemodynamics, before focusing on how the different classes of antihypertensive drugs can influence cerebral hemodynamics. We discuss the different properties of each drug and their potential impact on cerebral perfusion and CA. With emerging interest in CA status of AIS patients, either during or soon after treatment when timely reperfusion and salvageable tissue is at its most critical, the properties of these pharmacological agents may be relevant for modelling cerebral perfusion accuracy and for setting individualised treatment strategies.

The Acute Cardiorespiratory and Cerebrovascular Response to Resistance Exercise

Resistance exercise (RE) is a popular modality for the general population and athletes alike, due to the numerous benefits of regular participation. The acute response to dynamic RE is characterised by temporary and bidirectional physiological extremes, not typically seen in continuous aerobic exercise (e.g. cycling) and headlined by phasic perturbations in blood pressure that challenge cerebral blood flow (CBF) regulation. Cerebral autoregulation has been heavily scrutinised over the last decade with new data challenging the effectiveness of this intrinsic flow regulating mechanism, particularly to abrupt changes in blood pressure over the course of seconds (i.e. dynamic cerebral autoregulation), like those observed during RE. Acutely, RE can challenge CBF regulation, resulting in adverse responses (e.g. syncope). Compared with aerobic exercise, RE is relatively understudied, particularly high-intensity dynamic RE with a concurrent Valsalva manoeuvre (VM). However, the VM alone challenges CBF regulation and generates additional complexity when trying to dissociate the mechanisms underpinning the circulatory response to RE. Given the disparate circulatory response between aerobic and RE, primarily the blood pressure profiles, regulation of CBF is ostensibly different. In this review, we summarise current literature and highlight the acute physiological responses to RE, with a focus on the cerebral circulation.

Cerebral blood flow, cerebrovascular reactivity and their influence on ventilatory sensitivity

NEW FINDINGS

What is the topic of this review? Cerebrovascular reactivity to CO₂ , which is a principal factor in determining ventilatory responses to CO₂ through the role reactivity plays in determining cerebral extra- and intracellular pH. What advances does it highlight? Recent animal evidence suggests central chemoreceptor vasculature may demonstrate regionally heterogeneous cerebrovascular reactivity to CO₂ , potentially as a protective mechanism against excessive CO₂ washout from the central chemoreceptors, thereby allowing ventilation to reflect the systemic acid-base balance needs (respiratory changes in P aC O 2 ) rather than solely the cerebral needs. Ventilation per se does not influence cerebrovascular reactivity independent of changes in P aC O 2 .

ABSTRACT

Alveolar ventilation and cerebral blood flow are both predominantly regulated by arterial blood gases, especially arterial P C O 2 , and so are intricately entwined. In this review, the fundamental mechanisms underlying cerebrovascular reactivity and central chemoreceptor control of breathing are covered. We discuss the interaction of cerebral blood flow and its reactivity with the control of ventilation and ventilatory responsiveness to changes in P C O 2 , as well as the lack of influence of ventilation itself on cerebrovascular reactivity. We briefly summarize the effects of arterial hypoxaemia on the relationship between ventilatory and cerebrovascular response to both P C O 2 and P O 2 . We then highlight key methodological considerations regarding the interaction of reactivity and ventilatory sensitivity, including the following: regional heterogeneity of cerebrovascular reactivity; a pharmacological approach for the reduction of cerebral blood flow; reactivity assessment techniques; the influence of mean arterial blood pressure; and sex-related differences. Finally, we discuss ventilatory and cerebrovascular control in the context of high altitude and congestive heart failure. Future research directions and pertinent questions of interest are highlighted throughout.

Hypoperfusion assessed by pressure reactivity index is associated with delayed cerebral ischemia after subarachnoid hemorrhage: an observational study

Background

Dysfunction of cerebral autoregulation is one of the pathophysiological mechanisms that causes delayed cerebral ischemia (DCI) after subarachnoid hemorrhage (SAH). Pressure reactivity index (PRx) have been confirmed to reflect the level of cerebral autoregulation and used to derive optimal cerebral perfusion pressure (CPPopt). The goal of this study is to explore the associations between autoregulation, CPPopt, PRx, and DCI.

Methods

Continuous intracranial pressure (ICP), arterial blood pressure (ABP), and cerebral perfusion pressure (CPP) signals acquired from 61 aSAH patients were retrospectively analyzed. PRx was calculated and collected by Pneumatic computer system. The CPP at the lowest PRx was determined as the CPPopt. The duration of a hypoperfusion event (dHP) was defined as the cumulative time that the PRx was > 0.3 and the CPP was <CPPopt. The duration of CPP more than 10 mmHg below CPPopt (ΔCPPopt < − 10 mmHg) was also used to assess hypoperfusion. The percent of the time of hypoperfusion by dHP and ΔCPPopt < − 10 mmHg (%dHP and %ΔCPPopt) were compared between DCI group and control group, utilizing univariate and multivariable logistic regression. It was the clinical prognosis at 3 months after hemorrhage that was assessed with the modified Rankin Scale, and logistic regression and ROC analysis were used for predictive power for unfavorable outcomes (mRs 3–5).

Results

Data from 52 patients were included in the final analysis of 61 patients. The mean %dHP in DCI was 29.23% and 10.66% in control. The mean %ΔCPPopt < − 10 mmHg was 22.28%, and 5.90% in control. The %dHP (p < 0.001) and the %ΔCPPopt < − 10mmHg (p < 0.001) was significantly longer in the DCI group. In multivariate logistic regression model, %ΔCPPopt <− 10 mmHg (p < 0.001) and %dHP (p < 0.001) were independent risk factor for predicting DCI, and %ΔCPPopt <− 10 mmHg (p = 0.010) and %dHP (p = 0.026) were independent risk factor for predicting unfavorable outcomes.

Conclusions

The increase of duration of hypoperfusion events and duration of CPP below CPPopt over 10 mmHg, evaluated as time of lowered CPP, is highly indicative of DCI and unfavorable outcomes.

Microcirculatory model predicts blood flow and autoregulation range in the human retina: in vivo investigation with laser speckle flowgraphy

In this study, we mathematically predict retinal vascular resistance (RVR) and retinal blood flow (RBF), we test predictions using laser speckle flowgraphy (LSFG), we estimate the range of vascular autoregulation, and we examine the relationship of RBF with the retinal nerve fiber layer (RNFL) and ganglion cell complex (GCC). Fundus, optical coherence tomography (OCT), and OCT-angiography images, systolic/diastolic blood pressure (SBP/DBP), and intraocular pressure (IOP) measurements were obtained from 36 human subjects. We modeled two circulation markers (RVR and RBF) and estimated individualized lower/higher autoregulation limits (LARL/HARL), using retinal vessel calibers, fractal dimension, perfusion pressure, and population-based hematocrit values. Quantitative LSFG waveforms were extracted from vessels of the same eyes, before and during IOP elevation. LSFG metrics explained most variance in RVR (R² = 0.77/P = 6.9·10^-9) and RBF (R² = 0.65/P = 1.0·10^-6), suggesting that the markers strongly reflect blood flow physiology. Higher RBF was associated with thicker RNFL (P = 4.0·10^-4) and GCC (P = 0.003), thus also verifying agreement with structural measurements. LARL was at SBP/DBP of 105/65 mmHg for the average subject without arterial hypertension and at 115/75 mmHg for the average hypertensive subject. Moreover, during IOP elevation, changes in RBF were more pronounced than changes in RVR. These observations physiologically imply that healthy subjects are already close to LARL, thus prone to hypoperfusion. In conclusion, we modeled two clinical markers and described a novel method to predict individualized autoregulation limits. These findings could improve understanding of retinal perfusion and pave the way for personalized intervention decisions, when treating patients with coexisting ophthalmic and cardiovascular pathologies.NEW & NOTEWORTHY We describe and test a new approach to quantify retinal blood flow, based on standard clinical examinations and imaging techniques, linked together with a physiological model. We use these findings to generate individualized estimates of the autoregulation range. We provide evidence that healthy subjects are closer to the lower autoregulation limit than thought before. This suggests that some retinas are less prepared to withstand hypoperfusion, even after small intraocular pressure rises or blood pressure drops.

Rapid changes in vascular compliance contribute to cerebrovascular adjustments during transient reductions in blood pressure in young, healthy adults

Characterization of dynamic cerebral autoregulation has focused primarily on adjustments in cerebrovascular resistance in response to blood pressure (BP) alterations. However, the role of vascular compliance in dynamic autoregulatory processes remains elusive. The present study examined changes in cerebrovascular compliance and resistance during standing-induced transient BP reductions in nine young, healthy adults (3 women). Brachial artery BP (Finometer) and middle cerebral artery blood velocity (BV; Multigon) waveforms were collected. Beginning 20 beats before standing and continuing 40 beats after standing, individual BP and BV waveforms of every second heartbeat were extracted and input into a four-element modified Windkessel model to calculate indexes of cerebrovascular resistance (R_i) and compliance (C_i). Standing elicited a transient reduction in mean BP of 20 ± 9 mmHg. In all participants, a large increase in C_i (165 ± 84%; P < 0.001 vs. seated baseline) occurred 2 ± 2 beats following standing. Reductions in R_i occurred 11 ± 3 beats after standing (C_i vs. R_i delay: P < 0.001). The increase in C_i contributed to maintained systolic BV before the decrease in R_i. The present results demonstrate rapid, large but transient increases in C_i that precede reductions in R_i, in response to standing-induced reductions in BP. Therefore, C_i represents a discreet component of cerebrovascular responses during acute decreases in BP and, consequently, dynamic autoregulation.NEW & NOTEWORTHY Historically, dynamic cerebral autoregulation has been characterized by adjustments in cerebrovascular resistance following systematic changes in blood pressure. However, with the use of Windkessel modeling approaches, this study revealed rapid and large increases in cerebrovascular compliance that preceded reductions in cerebrovascular resistance following standing-induced blood pressure reductions. Importantly, the rapid cerebrovascular compliance response contributed to preservation of systolic blood velocity during the transient hypotensive phase. These results broaden our understanding of dynamic cerebral autoregulation.

Incomplete recovery of cerebral blood flow dynamics in sufficiently treated high blood pressure

Objective:

Whether cerebrovascular regulation is different in patients with controlled high blood pressure (HBP) with and without small vessel disease (SVD).

Methods:

Sixty-seven healthy controls (mean age ± SD, 45 ± 16 years; 30 women, 37 men) and 40 patients (mean age, 64 ± 13 years; 14 women, 26 men) with HBP and different stages of SVD, underwent simultaneous recordings of the spontaneous fluctuations of BP, blood flow velocity (CBFV) in both middle cerebral arteries (MCA), and of end-tidal CO₂ (ETCO₂). Coherence and transfer function gain and phase between BP and CBFV were assessed in the frequency ranges of VLF (0.02–0.07 Hz), low frequency (0.07–0.15), and high frequency (>0.15). BP SD indicated BP variability (BPV).

Results:

In controls (BP, 86 ± 13 mmHg; ETCO₂, 39 ± 4 mmHg; BPV, 15 ± 6 mmHg), gain, phase and coherence were not age-dependent in simple or a multiple regression models. BPV correlated significantly in both MCAs with gain in low frequency and high frequency, and with phase in VLF and high frequency. In patients (BP, 91 ± 16 mmHg, ETCO₂, 39 ± 4 mmHg, BPV 18 ± 5 mmHg), only gain showed some differences between different SVD groups. Comparing all patients with 25 controls of similar age and sex, patients exhibited significantly (P < 0.05–P < 0.005): increased coherence and gain in VLF, decreased phase in VLF and low frequency, correlations between BPV with phase in low frequency (left) and with gain in VLF (left) and in high frequency (left and right).

Conclusion:

Phase seems an age independent autoregulatory index. In controlled HBP, CBF regulation is degraded at longlasting CBF changes; BPV effects lose their physiological bilateral distribution.

Responses of cerebral blood velocity and tissue oxygenation to low-frequency oscillations during simulated haemorrhagic stress in humans

NEW FINDINGS

What is the central question of this study? Do low-frequency oscillations in arterial pressure and cerebral blood velocity protect cerebral blood velocity and oxygenation during central hypovolaemia? What is the main finding and its importance? Low-frequency oscillations in arterial pressure and cerebral blood velocity attenuate reductions in cerebral oxygen saturation but do not protect absolute cerebral blood velocity during central hypovolaemia. This finding indicates the potential importance of haemodynamic oscillations in maintaining cerebral oxygenation and therefore viability of tissues during challenges to cerebral blood flow and oxygen delivery.

ABSTRACT

Tolerance to both real and simulated haemorrhage varies between individuals. Exaggerated low-frequency (∼0.1 Hz) oscillations in mean arterial pressure and brain blood flow [indexed via middle cerebral artery velocity (MCAv)] have been associated with improved tolerance to reduced central blood volume. The mechanism for this association has not been explored. We hypothesized that inducing low-frequency oscillations in arterial pressure and cerebral blood velocity would attenuate reductions in cerebral blood velocity and oxygenation during simulated haemorrhage. Fourteen subjects (11 men and three women) were exposed to oscillatory (0.1 and 0.05 Hz) and non-oscillatory (0 Hz) lower-body negative pressure profiles with an average chamber pressure of -60 mmHg (randomized and counterbalanced order). Measurements included arterial pressure and stroke volume via finger photoplethysmography, MCAv via transcranial Doppler ultrasound, and cerebral oxygenation of the frontal lobe via near-infrared spectroscopy. Tolerance was higher during the two oscillatory profiles compared with the 0 Hz profile (0.05 Hz, P = 0.04; 0.1 Hz, P = 0.09), accompanied by attenuated reductions in stroke volume (P < 0.001) and cerebral oxygenation of the frontal lobe (P ≤ 0.02). No differences were observed between profiles for reductions in mean arterial pressure (P = 0.17) and MCAv (P = 0.30). In partial support of our hypothesis, cerebral oxygenation, but not cerebral blood velocity, was protected during the oscillatory profiles. Interestingly, more subjects tolerated the oscillatory profiles compared with the static 0 Hz profile, despite similar arterial pressure responses. These findings emphasize the potential importance of haemodynamic oscillations in maintaining perfusion and oxygenation of cerebral tissues during haemorrhagic stress.

Spinal Cord Disruption Is Associated with a Loss of Cushing-Like Blood Pressure Interactions

The capacity of the cerebrovasculature to buffer changes in blood pressure (BP) likely plays an important role in the prevention of stroke, which is three- to fourfold more common after spinal cord injury (SCI). Although the directional relationship between BP and cerebral blood flow (CBF) has traditionally been thought to travel solely from BP to CBF, a Cushing-like mechanism functioning in the inverse direction, in which changes in CBF influence BP, has recently been revealed using Granger causality analysis. Although both CBF buffering of BP and the Cushing-like mechanism are influenced by the sympathetic nervous system, we do not understand the impact of disruption of descending sympathetic pathways within the spinal cord, caused by cervical SCI on these regulatory systems. We hypothesized that people with cervical SCI would have greater BP to CBF transmission, as well as a reduced Cushing-like mechanism. The directional relationships between mean arterial BP (MAP; Finometer^® PRO) and middle cerebral artery blood velocity (MCAv; transcranial Doppler) were assessed at rest in 14 cervical SCI subjects and 16 uninjured individuals using Granger causality analysis, while also accounting for end-tidal CO₂ tension. Those with SCI exhibited 66% increased forward MAP→MCAv information transmission as compared with the uninjured group (p = 0.0003), indicating reduced cerebrovascular buffering of BP, and did not have a predominant backward Cushing-like MCAv→MAP phenotype. These results indicate that both forward and backward communication between BP and CBF are influenced by SCI, which may be associated with impaired cerebrovascular BP buffering after SCI as well as widespread BP instability.

Effects of anesthesia on cerebral blood flow, metabolism, and neuroprotection

Administration of anesthetic agents fundamentally shifts the responsibility for maintenance of homeostasis from the patient and their intrinsic physiological regulatory mechanisms to the anesthesiologist. Continuous delivery of oxygen and nutrients to the brain is necessary to prevent irreversible injury and arises from a complex series of regulatory mechanisms that ensure uninterrupted cerebral blood flow. Our understanding of these regulatory mechanisms and the effects of anesthetics on them has been driven by the tireless work of pioneers in the field. It is of paramount importance that the anesthesiologist shares this understanding. Herein, we will review the physiological determinants of cerebral blood flow and how delivery of anesthesia impacts these processes.

System dynamics of active and passive postural changes: Insights from principal dynamic modes analysis of baroreflex loop

The baroreflex being a key modulator of cardiovascular control ensures adequate blood pressure regulation under orthostatic stress which otherwise may cause severe hypotension. Contrary to conventional baroreflex sensitivity indices derived across a-priori traditional frequency bands, the present study is aimed at proposing new indices for the assessment of baroreflex drive which follows active (supine to stand-up) and passive (supine to head-up tilt) postural changes. To achieve this, a novel system identification approach of principal dynamic modes (PDM) was utilized to extract data-adaptive frequency components of closed-loop interactions between beat-to-beat interval and systolic blood pressure recorded from 10 healthy humans. We observed that the gain of low-pass global PDM of cardiac arm (:feedback reflex loop, mediated by pressure sensors to adjust heart rate in response to arterial blood pressure), and 0.2 Hz global PDM of mechanical arm (:feed-forward pathways, originating changes in arterial blood pressure in response to heart rate variations) may function as potential markers to distinguish active and passive orthostatic tests in healthy subjects.

Wavelet decomposition analysis is a clinically relevant strategy to evaluate cerebrovascular buffering of blood pressure after spinal cord injury

The capacity of the cerebrovasculature to buffer changes in blood pressure (BP) is crucial to prevent stroke, the incidence of which is three- to fourfold elevated after spinal cord injury (SCI). Disruption of descending sympathetic pathways within the spinal cord due to cervical SCI may result in impaired cerebrovascular buffering. Only linear analyses of cerebrovascular buffering of BP, such as transfer function, have been used in SCI research. This approach does not account for inherent nonlinearity and nonstationarity components of cerebrovascular regulation, often depends on perturbations of BP to increase the statistical power, and does not account for the influence of arterial CO₂ tension. Here, we used a nonlinear and nonstationary analysis approach termed wavelet decomposition analysis (WDA), which recently identified novel sympathetic influences on cerebrovascular buffering of BP occurring in the ultra-low-frequency range (ULF; 0.02-0.03Hz). WDA does not require BP perturbations and can account for influences of CO₂ tension. Supine resting beat-by-beat BP (Finometer), middle cerebral artery blood velocity (transcranial Doppler), and end-tidal CO₂ tension were recorded in cervical SCI ( n = 14) and uninjured ( n = 16) individuals. WDA revealed that cerebral blood flow more closely follows changes in BP in the ULF range ( P = 0.0021, Cohen's d = 0.89), which may be interpreted as an impairment in cerebrovascular buffering of BP. This persisted after accounting for CO₂. Transfer function metrics were not different in the ULF range, but phase was reduced at 0.07-0.2 Hz ( P = 0.03, Cohen's d = 0.31). Sympathetically mediated cerebrovascular buffering of BP is impaired after SCI, and WDA is a powerful strategy for evaluating cerebrovascular buffering in clinical populations.

Chronic kidney disease and poor outcomes in ischemic stroke: is impaired cerebral autoregulation the missing link?

BACKGROUND

Chronic kidney disease increases stroke incidence and severity but the mechanisms behind this cerebro-renal interaction are mostly unexplored. Since both vascular beds share similar features, microvascular dysfunction could be the possible missing link. Therefore, we examined the relationship between renal function and cerebral autoregulation in the early hours post ischemia and its impact on outcome.

METHODS

We enrolled 46 ischemic strokes (middle cerebral artery). Dynamic cerebral autoregulation was assessed by transfer function (coherence, phase and gain) of spontaneous blood pressure oscillations to blood flow velocity within 6 h from symptom-onset. Estimated glomerular filtration rate (eGFR) was calculated. Hemorrhagic transformation (HT) and white matter lesions (WML) were collected from computed tomography performed at presentation and 24 h. Outcome was evaluated with modified Rankin Scale at 3 months.

RESULTS

High gain (less effective autoregulation) was correlated with lower eGFR irrespective of infarct side (p < 0.05). Both lower eGFR and higher gain correlated with WML grade (p < 0.05). Lower eGFR and increased gain, alone and in combination, progressively reduced the odds of a good functional outcome [ipsilateral OR = 4.39 (CI95% 3.15-25.6), p = 0.019; contralateral OR = 8.15 (CI95% 4.15-15.6), p = 0.002] and increased risk of HT [ipsilateral OR = 3.48 (CI95% 0.60-24.0), p = 0.132; contralateral OR = 6.43 (CI95% 1.40-32.1), p = 0.034].

CONCLUSIONS

Lower renal function correlates with less effective dynamic cerebral autoregulation in acute ischemic stroke, both predicting a bad outcome. The evaluation of serum biomarkers of renal dysfunction could have interest in the future for assessing cerebral microvascular risk and relationship with stroke complications.

Compromised Cerebrovascular Regulation and Cerebral Oxygenation in Pulmonary Arterial Hypertension

BACKGROUND

Functional cerebrovascular regulatory mechanisms are important for maintaining constant cerebral blood flow and oxygen supply in heathy individuals and are altered in heart failure. We aim to examine whether pulmonary arterial hypertension (PAH) is associated with abnormal cerebrovascular regulation and lower cerebral oxygenation and their physiological and clinical consequences.

METHODS AND RESULTS

Resting mean flow velocity in the middle cerebral artery mean flow velocity in the middle cerebral artery (MCAvmean); transcranial Doppler), cerebral pressure-flow relationship (assessed at rest and during squat-stand maneuvers; analyzed using transfer function analysis), cerebrovascular reactivity to CO2, and central chemoreflex were assessed in 11 patients with PAH and 11 matched healthy controls. Both groups also completed an incremental ramp exercise protocol until exhaustion, during which MCAvmean, mean arterial pressure, cardiac output (photoplethysmography), end-tidal partial pressure of CO2, and cerebral oxygenation (near-infrared spectroscopy) were measured. Patients were characterized by a significant decrease in resting MCAvmean (P<0.01) and higher transfer function gain at rest and during squat-stand maneuvers (both P<0.05). Cerebrovascular reactivity to CO2 was reduced (P=0.03), whereas central chemoreceptor sensitivity was increased in PAH (P<0.01), the latter correlating with increased resting ventilation (R2=0.47; P<0.05) and the exercise ventilation/CO2 production slope (V˙E/V˙CO2 slope; R2=0.62; P<0.05) during exercise for patients. Exercise-induced increases in MCAvmean were limited in PAH (P<0.05). Reduced MCAvmean contributed to impaired cerebral oxygen delivery and oxygenation (both P<0.05), the latter correlating with exercise capacity in patients with PAH (R2=0.52; P=0.01).

CONCLUSIONS

These findings provide comprehensive evidence for physiologically and clinically relevant impairments in cerebral hemodynamic regulation and oxygenation in PAH.

Increased risk of critical CBF levels in SAH patients with actual CPP below calculated optimal CPP

Background

Cerebral pressure autoregulation can be quantified with the pressure reactivity index (PRx), based on the correlation between blood pressure and intracranial pressure. Using PRx optimal cerebral perfusion pressure (CPPopt) can be calculated, i.e., the level of CPP where autoregulation functions best. The relation between cerebral blood flow (CBF) and CPPopt has not been examined. The objective was to assess to which extent CPPopt can be calculated in SAH patients and to investigate CPPopt in relation to CBF.

Methods

Retrospective study of prospectively collected data. CBF was measured bedside with Xenon-enhanced CT (Xe-CT). The difference between actual CPP and CPPopt was calculated (CPP∆). Correlations between CPP∆ and CBF parameters were calculated with Spearman’s rank order correlation coefficient (rho). Separate calculations were done using all patients (day 0–14 after onset) as well as in two subgroups (day 0–3 and day 4–14).

Results

Eighty-two patients with 145 Xe-CT scans were studied. Automated calculation of CPPopt was possible in adjunct to 60% of the Xe-CT scans. Actual CPP < CPPopt was associated with higher numbers of low-flow regions (CBF <10 ml/100 g/min) in both the early phase (day 0–3, n = 39, Spearman’s rho = −0.38, p = 0.02) and late acute phase of the disease (day 4–14, n = 35, Spearman’s rho = −0.39, p = 0.02). CPP level per se was not associated with CBF.

Conclusions

Calculation of CPPopt is possible in a majority of patients with severe SAH. Actual CPP below CPPopt is associated with low CBF.

Compartmental and Data-Based Modeling of Cerebral Hemodynamics: Nonlinear Analysis

OBJECTIVE

As an extension to our study comparing a putative compartmental and data-based model of linear dynamic cerebral autoregulation (CA) and CO2-vasomotor reactivity (VR), we study the CA-VR process in a nonlinear context.

METHODS

We use the concept of principal dynamic modes (PDM) in order to obtain a compact and more easily interpretable input-output model. This in silico study permits the use of input data with a dynamic range large enough to simulate the classic homeostatic CA and VR curves using a putative structural model of the regulatory control of the cerebral circulation. The PDM model obtained using theoretical and experimental data are compared.

RESULTS

It was found that the PDM model was able to reflect accurately both the simulated static CA and VR curves in the associated nonlinear functions (ANFs). Similar to experimental observations, the PDM model essentially separates the pressure-flow relationship into a linear component with fast dynamics and nonlinear components with slow dynamics. In addition, we found good qualitative agreement between the PDMs representing the dynamic theoretical and experimental CO2-flow relationship.

CONCLUSION

Under the modeling assumption and in light of other experimental findings, we hypothesize that PDMs obtained from experimental data correspond with passive fluid dynamical and active regulatory mechanisms.

SIGNIFICANCE

Both hypothesis-based and data-based modeling approaches can be combined to offer some insight into the physiological basis of PDM model obtained from human experimental data. The PDM modeling approach potentially offers a practical way to quantify the status of specific regulatory mechanisms in the CA-VR process.

Evidence for hysteresis in the cerebral pressure-flow relationship in healthy men

The cerebrovasculature is more efficient at compensating for pharmacologically induced transient hypertension versus transient hypotension. Whether this phenomenon exists during nonpharmacologically induced hypertension and hypotension is currently unknown. We compared the percent change in mean velocity in the middle cerebral artery (MCAvmean) per percent change in mean arterial pressure (MAP) (%ΔMCAVmean/%ΔMAP) during transient hypertension and hypotension induced during squat-stand maneuvers performed at 0.05 Hz (20-s cycles) and 0.10 Hz (10-s cycles) in 58 male volunteers. %ΔMCAvmean/%ΔMAP was attenuated by 25% ( P = 0.03, 0.05 Hz) and 47% ( P < 0.0001, 0.10 Hz) during transient hypertension versus hypotension. Thus, these findings indicate that the brain in healthy men is better adapted to compensate for physiologically relevant transient hypertension than hypotension.

NEW & NOTEWORTHY The novel finding of this study is that the change in middle cerebral artery mean flow velocity is attenuated during hypertension compared with hypotension physiologically induced by oscillations in blood pressure in men. These results support that the human brain is more effective at compensating for transient hypertension than hypotension.

Effects of a Dietary Beetroot Juice Treatment on Systemic and Cerebral Haemodynamics- A Pilot Study

INTRODUCTION

Beetroot Juice (BJ) contains dietary nitrates that increase the blood Nitric Oxide (NO) level, decrease Blood Pressure (BP), increase athletic performance and improve cognitive functions but the mechanism remains unclear. Ultrasonographic measurement of middle cerebral artery blood flow velocity with computation of Cerebral Augmentation Index (CAIx) is a measure of the reflected flow signal, modulated by changes in cerebrovascular resistance and compliance.

AIM

This pilot study tests the hypothesis that ingestion of an amount of BJ sufficient to raise the blood NO level two-to three-fold, decreases Transcranial Doppler (TCD) measured CAIx.

MATERIALS AND METHODS

Ten healthy young-adult African-American women were studied at two levels of submaximal exercise, 40% and 80% of their predetermined peak oxygen consumptions. The subjects ingested nitrate-free orange juice (OJ, control) and an isocaloric BJ beverage (1.5 mg/mL nitrate, 220 Cal), on different days, 1-2 weeks apart.

RESULTS

The BJ treatment increased blood NO and decreased systolic BP at rest and at the two levels of exercise. The BJ treatment decreased CAIx only at the two levels of exercise (from 79 ± 2% to 62 ± 2% and from 80 ± 2% to 60 ± 3%, p<0.05). Exercise increased TCD-measured resistance and pulsatility indices (RIx, PIx) without changing AIx. The BJ treatment had no effect on RIx and PIx.

CONCLUSION

These findings suggest that decreased CAIx associated with aerobic exercise reflects the change in cerebral haemodynamics resulting from dietary nitrate supplementation. Future studies should determine whether the BJ-induced decrement in CAIx is correlated with an improvement in brain function.

Cerebral blood flow and autoregulation: current measurement techniques and prospects for noninvasive optical methods

Cerebral blood flow (CBF) and cerebral autoregulation (CA) are critically important to maintain proper brain perfusion and supply the brain with the necessary oxygen and energy substrates. Adequate brain perfusion is required to support normal brain function, to achieve successful aging, and to navigate acute and chronic medical conditions. We review the general principles of CBF measurements and the current techniques to measure CBF based on direct intravascular measurements, nuclear medicine, X-ray imaging, magnetic resonance imaging, ultrasound techniques, thermal diffusion, and optical methods. We also review techniques for arterial blood pressure measurements as well as theoretical and experimental methods for the assessment of CA, including recent approaches based on optical techniques. The assessment of cerebral perfusion in the clinical practice is also presented. The comprehensive description of principles, methods, and clinical requirements of CBF and CA measurements highlights the potentially important role that noninvasive optical methods can play in the assessment of neurovascular health. In fact, optical techniques have the ability to provide a noninvasive, quantitative, and continuous monitor of CBF and autoregulation.

Assessment of the Brain's Macro- and Micro-Circulatory Blood Flow Responses to CO2 via Transfer Function Analysis

OBJECTIVES

At present, there is no standard bedside method for assessing cerebral autoregulation (CA) with high temporal resolution. We combined the two methods most commonly used for this purpose, transcranial Doppler sonography (TCD, macro-circulation level), and near-infrared spectroscopy (NIRS, micro-circulation level), in an attempt to identify the most promising approach.

METHODS

In eight healthy subjects (5 women; mean age, 38 ± 10 years), CA disturbance was achieved by adding carbon dioxide (CO2) to the breathing air. We simultaneously recorded end-tidal CO2 (ETCO2), blood pressure (BP; non-invasively at the fingertip), and cerebral blood flow velocity (CBFV) in both middle cerebral arteries using TCD and determined oxygenated and deoxygenated hemoglobin levels using NIRS. For the analysis, we used transfer function calculations in the low-frequency band (0.07-0.15 Hz) to compare BP-CBFV, BP-oxygenated hemoglobin (OxHb), BP-tissue oxygenation index (TOI), CBFV-OxHb, and CBFV-TOI.

RESULTS

ETCO2 increased from 37 ± 2 to 44 ± 3 mmHg. The CO2-induced CBFV increase significantly correlated with the OxHb increase (R (2) = 0.526, p < 0.001). Compared with baseline, the mean CO2 administration phase shift (in radians) significantly increased (p < 0.005) from -0.67 ± 0.20 to -0.51 ± 0.25 in the BP-CBFV system, and decreased from 1.21 ± 0.81 to -0.05 ± 0.91 in the CBFV-OxHb system, and from 0.94 ± 1.22 to -0.24 ± 1.0 in the CBFV-TOI system; no change was observed for BP-OxHb (0.38 ± 1.17 to 0.41 ± 1.42). Gain changed significantly only in the BP-CBFV system. The correlation between the ETCO2 change and phase change was higher in the CBFV-OxHb system [r = -0.60; 95% confidence interval (CI): -0.16, -0.84; p < 0.01] than in the BP-CBFV system (r = 0.52; 95% CI: 0.03, 0.08; p < 0.05).

CONCLUSION

The transfer function characterizes the blood flow transition from macro- to micro-circulation by time delay only. The CBFV-OxHb system response with a broader phase shift distribution offers the prospect of a more detailed grading of CA responses. Whether this is of clinical relevance needs further studies in different patient populations.

External Counterpulsation Reduces Beat-to-Beat Blood Pressure Variability When Augmenting Blood Pressure and Cerebral Blood Flow in Ischemic Stroke

BACKGROUND AND PURPOSE

External counterpulsation (ECP) is a noninvasive method used to enhance cerebral perfusion by elevating the blood pressure in ischemic stroke. However, the response of the beat-to-beat blood pressure variability (BPV) in ischemic stroke patients during ECP remains unknown.

METHODS

We enrolled recent ischemic stroke patients and healthy controls. Changes in the blood flow velocities in bilateral middle cerebral arteries and the continuous beat-to-beat blood pressure before, during, and after ECP were monitored. Power spectral analysis revealed that the BPV included oscillations at very low frequency (VLF; <0.04 Hz), low frequency (LF; 0.04-0.15 Hz), and high frequency (HF; 0.15-0.40 Hz), and the total power spectral density (TP; <0.40 Hz) and LF/HF ratio were calculated.

RESULTS

We found that ECP significantly increased the systolic and diastolic blood pressures in both stroke patients and controls. ECP decreased markedly the systolic and diastolic BPVs at VLF and LF and the TP, and the diastolic BPV at HF when compared with baseline. The decreases in diastolic and systolic BPV reached 37.56% and 23.20%, respectively, at VLF, 21.15% and 12.19% at LF, 8.76% and 16.59% at HF, and 31.92% and 23.62% for the total TP in stroke patients, which did not differ from those in healthy controls. The change in flow velocity on the contralateral side was positively correlated with the total TP systolic BPV change induced by ECP (r=0.312, p=0.035).

CONCLUSIONS

ECP reduces the beat-to-beat BPV when increasing the blood pressure and cerebral blood flow velocity in ischemic stroke patients. ECP might be able to improve the clinical outcome by decreasing the beat-to-beat BPV in stroke patients, and this should be explored further in future studies.

Relationship between blood pressure and cerebral blood flow during supine cycling: influence of aging

The cerebral pressure-flow relationship can be quantified as a high-pass filter, where slow oscillations are buffered (<0.20 Hz) and faster oscillations are passed through relatively unimpeded. During moderate intensity exercise, previous studies have reported paradoxical transfer function analysis (TFA) findings (altered phase or intact gain). This study aimed to determine whether these previous findings accurately represent this relationship. Both younger (20-30 yr; n = 10) and older (62-72 yr; n = 9) adults were examined. To enhance the signal-to-noise ratio, large oscillations in blood pressure (via oscillatory lower body negative pressure; OLBNP) were induced during steady-state moderate intensity supine exercise (∼45-50% of heart rate reserve). Beat-to-beat blood pressure, cerebral blood velocity, and end-tidal Pco2 were monitored. Very low frequency (0.02-0.07 Hz) and low frequency (0.07-0.20 Hz) range spontaneous data were quantified. Driven OLBNP point estimates were sampled at 0.05 and 0.10 Hz. The OLBNP maneuvers augmented coherence to >0.97 at 0.05 Hz and >0.98 at 0.10 Hz in both age groups. The OLBNP protocol conclusively revealed the cerebrovascular system functions as a high-pass filter during exercise throughout aging. It was also discovered that the older adults had elevations (+71%) in normalized gain (+0.46 ± 0.36%/%: 0.05 Hz) and reductions (-34%) in phase (-0.24 ± 0.22 radian: 0.10 Hz). There were also age-related phase differences between resting and exercise conditions. It is speculated that these age-related changes in the TFA metrics are mediated by alterations in vasoactive factors, sympathetic tone, or the mechanical buffering of the compliance vessels.

Non-Linear Characterisation of Cerebral Pressure-Flow Dynamics in Humans

Cerebral metabolism is critically dependent on the regulation of cerebral blood flow (CBF), so it would be expected that vascular mechanisms that play a critical role in CBF regulation would be tightly conserved across individuals. However, the relationships between blood pressure (BP) and cerebral blood velocity fluctuations exhibit inter-individual variations consistent with heterogeneity in the integrity of CBF regulating systems. Here we sought to determine the nature and consistency of dynamic cerebral autoregulation (dCA) during the application of oscillatory lower body negative pressure (OLBNP). In 18 volunteers we recorded BP and middle cerebral artery blood flow velocity (MCAv) and examined the relationships between BP and MCAv fluctuations during 0.03, 0.05 and 0.07Hz OLBNP. dCA was characterised using project pursuit regression (PPR) and locally weighted scatterplot smoother (LOWESS) plots. Additionally, we proposed a piecewise regression method to statistically determine the presence of a dCA curve, which was defined as the presence of a restricted autoregulatory plateau shouldered by pressure-passive regions. Results show that LOWESS has similar explanatory power to that of PPR. However, we observed heterogeneous patterns of dynamic BP-MCAv relations with few individuals demonstrating clear evidence of a dCA central plateau. Thus, although BP explains a significant proportion of variance, dCA does not manifest as any single characteristic BP-MCAv function.

Methodological comparison of active- and passive-driven oscillations in blood pressure; implications for the assessment of cerebral pressure-flow relationships

We examined the between-day reproducibility of active (squat-stand maneuvers)- and passive [oscillatory lower-body negative pressure (OLBNP) maneuvers]-driven oscillations in blood pressure. These relationships were examined in both younger (n = 10; 25 ± 3 yr) and older (n = 9; 66 ± 4 yr) adults. Each testing protocol incorporated rest (5 min), followed by driven maneuvers at 0.05 (5 min) and 0.10 (5 min) Hz to increase blood-pressure variability and improve assessment of the pressure-flow dynamics using linear transfer function analysis. Beat-to-beat blood pressure, middle cerebral artery velocity, and end-tidal partial pressure of CO2 were monitored. The pressure-flow relationship was quantified in the very low (0.02-0.07 Hz) and low (0.07-0.20 Hz) frequencies (LF; spontaneous data) and at 0.05 and 0.10 Hz (driven maneuvers point estimates). Although there were no between-age differences, very few spontaneous and OLBNP transfer function metrics met the criteria for acceptable reproducibility, as reflected in a between-day, within-subject coefficient of variation (CoV) of <20%. Combined CoV data consist of LF coherence (15.1 ± 12.2%), LF gain (15.1 ± 12.2%), and LF normalized gain (18.5 ± 10.9%); OLBNP data consist of 0.05 (12.1 ± 15.%) and 0.10 (4.7 ± 7.8%) Hz coherence. In contrast, the squat-stand maneuvers revealed that all metrics (coherence: 0.6 ± 0.5 and 0.3 ± 0.5%; gain: 17.4 ± 12.3 and 12.7 ± 11.0%; normalized gain: 16.7 ± 10.9 and 15.7 ± 11.0%; and phase: 11.6 ± 10.2 and 17.3 ± 10.8%) at 0.05 and 0.10 Hz, respectively, were considered biologically acceptable for reproducibility. These findings have important implications for the reliable assessment and interpretation of cerebral pressure-flow dynamics in humans.

Coupling between arterial pressure, cerebral blood velocity, and cerebral tissue oxygenation with spontaneous and forced oscillations

We tested the hypothesis that transmission of arterial pressure to brain tissue oxygenation is low under conditions of arterial pressure instability. Two experimental models of hemodynamic instability were used in healthy human volunteers; (1) oscillatory lower body negative pressure (OLBNP) (N = 8; 5 male, 3 female), and; (2) maximal LBNP to presyncope (N = 21; 13 male, 8 female). Mean arterial pressure (MAP), middle cerebral artery velocity (MCAv), and cerebral tissue oxygen saturation (ScO2) were measured non-invasively. For the OLBNP protocol, between 0 and -60 mmHg negative pressure was applied for 20 cycles at 0.05 Hz, then 20 cycles at 0.1 Hz. For the maximal LBNP protocol, progressive 5 min stages of chamber decompression were applied until the onset of presyncope. Spectral power of MAP, mean MCAv, and ScO2 were calculated within the VLF (0.04-0.07 Hz), and LF (0.07-0.2 Hz) ranges, and cross-spectral coherence was calculated for MAP-mean MCAv, MAP-ScO2, and mean MCAv-ScO2 at baseline, during each OLBNP protocol, and at the level prior to pre-syncope during maximal LBNP (sub-max). The key findings are (1) both 0.1 Hz OLBNP and sub-max LBNP elicited increases in LF power for MAP, mean MCAv, and ScO2 (p ≤ 0.08); (2) 0.05 Hz OLBNP increased VLF power in MAP and ScO2 only (p ≤ 0.06); (3) coherence between MAP-mean MCAv was consistently higher (≥0.71) compared with MAP-ScO2, and mean MCAv-ScO2 (≤0.43) during both OLBNP protocols, and sub-max LBNP (p ≤ 0.04). These data indicate high linearity between pressure and cerebral blood flow variations, but reduced linearity between cerebral tissue oxygenation and both arterial pressure and cerebral blood flow. Measuring arterial pressure variability may not always provide adequate information about the downstream effects on cerebral tissue oxygenation, the key end-point of interest for neuronal viability.

Impact of intraoperative hypotension and blood pressure fluctuations on early postoperative delirium after non-cardiac surgery

INTRODUCTION

Postoperative delirium is common in older patients. Despite its prognostic significance, the pathophysiology is incompletely understood. Although many risk factors have been identified, no reversible factors, particularly ones potentially modifiable by anaesthetic management, have been identified. The goal of this prospective cohort study was to investigate whether intraoperative hypotension was associated with postoperative delirium in older patients undergoing major non-cardiac surgery.

METHODS

Study subjects were patients >65 years of age, undergoing major non-cardiac surgery, who were enrolled in an ongoing prospective observational study of the pathophysiology of postoperative delirium. Intraoperative blood pressure was measured and predefined criteria were used to define hypotension. Delirium was measured by the Confusion Assessment Method on the first two postoperative days. Data were analysed using t-tests, two-sample proportion tests and ordered logistic regression multivariable models, including correction for multiple comparisons.

RESULTS

Data from 594 patients with a mean age of 73.6 years (sd 6.2) were studied. Of these 178 (30%) developed delirium on day 1 and 176 (30%) on day 2. Patients developing delirium were older, more often female, had lower preoperative cognitive scores, and underwent longer operations. Relative hypotension (decreases by 20, 30, or 40%) or absolute hypotension [mean arterial pressure (MAP)<50 mm Hg] were not significantly associated with postoperative delirium, nor was the duration of hypotension (MAP<50 mm Hg). Conversely, intraoperative blood pressure variance was significantly associated with postoperative delirium.

DISCUSSION

These results showed that increased blood pressure fluctuation, not absolute or relative hypotension, was predictive of postoperative delirium.

Assessing cerebral autoregulation via oscillatory lower body negative pressure and projection pursuit regression

The process by which cerebral perfusion is maintained constant over a wide range of systemic pressures is known as "cerebral autoregulation." Effective dampening of flow against pressure changes occurs over periods as short as ~15 sec and becomes progressively greater over longer time periods. Thus, slower changes in blood pressure are effectively blunted and faster changes or fluctuations pass through to cerebral blood flow relatively unaffected. The primary difficulty in characterizing the frequency dependence of cerebral autoregulation is the lack of prominent spontaneous fluctuations in arterial pressure around the frequencies of interest (less than ~0.07 Hz or ~15 sec). Oscillatory lower body negative pressure (OLBNP) can be employed to generate oscillations in central venous return that result in arterial pressure fluctuations at the frequency of OLBNP. Moreover, Projection Pursuit Regression (PPR) provides a nonparametric method to characterize nonlinear relations inherent in the system without a priori assumptions and reveals the characteristic non-linearity of cerebral autoregulation. OLBNP generates larger fluctuations in arterial pressure as the frequency of negative pressure oscillations become slower; however, fluctuations in cerebral blood flow become progressively lesser. Hence, the PPR shows an increasingly more prominent autoregulatory region at OLBNP frequencies of 0.05 Hz and below (20 sec cycles). The goal of this approach it to allow laboratory-based determination of the characteristic nonlinear relationship between pressure and cerebral flow and could provide unique insight to integrated cerebrovascular control as well as to physiological alterations underlying impaired cerebral autoregulation (e.g., after traumatic brain injury, stroke, etc.).

Selective alpha adrenergic antagonist reduces severity of transient hypertension during sexual stimulation after spinal cord injury

On a daily basis, the majority of those with high-level spinal cord injury have autonomic dysreflexia, which describes a life-threatening episode of transient extreme hypertension (i.e., as high as 300 mm Hg) as many as 90% of people living with this condition. Unfortunately, ejaculation is a major initiating factor for autonomic dysreflexia, which discourages sexual activity. In order to obtain a sperm specimen, or for initial assessment of fertility, penile vibrostimulation is clinically performed. Nifedipine, a selective calcium channel blocker, is the most commonly prescribed pharmaceutical for a priori management of autonomic dysreflexia secondary to ejaculation or other causes; however, it is limited because of its potential exacerbation of low resting pressure, which also affects this population. The present study examined the effect of a short-acting selective α1 antagonist (prazosin) on autonomic dysreflexia severity using a randomized placebo trial during medically supervised penile vibrostimulation in six males with cervical spinal cord injury. Beat-by-beat blood pressure and heart rate were recorded throughout penile vibrostimulation during placebo and prazosin-treated days. The increase in systolic blood pressure was mitigated during vibrostimulation in subjects administered prazosin as compared with those administered placebo (+140±19 mm Hg vs. +96±14 mmHg; p<0.05). On average, the peak in systolic blood pressure was 46 mm Hg lower during penile vibrostimulation when patients were administered prazosin (p<0.05), whereas resting blood pressure was not affected. Prazosin appears to be effective at reducing the severity of autonomic dysreflexia during sexual stimulation in patients with spinal cord injury, without exacerbating resting hypotension in high-level spinal cord injury.

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.

Cerebrovascular regulation, exercise, and mild traumatic brain injury

A substantial number of people who sustain a mild traumatic brain injury report persistent symptoms. Most common among these symptoms are headache, dizziness, and cognitive difficulties. One possible contributor to sustained symptoms may be compromised cerebrovascular regulation. In addition to injury-related cerebrovascular dysfunction, it is possible that prolonged rest after mild traumatic brain injury leads to deconditioning that may induce physiologic changes in cerebral blood flow control that contributes to persistent symptoms in some people. There is some evidence that exercise training may reduce symptoms perhaps because it engages an array of cerebrovascular regulatory mechanisms. Unfortunately, there is very little work on the degree of impairment in cerebrovascular control that may exist in patients with mild traumatic brain injury, and there are no published studies on the subacute phase of recovery from this injury. This review aims to integrate the current knowledge of cerebrovascular mechanisms that might underlie persistent symptoms and seeks to synthesize these data in the context of exploring aerobic exercise as a feasible intervention to treat the underlying pathophysiology.

Influence of cerebrovascular resistance on the dynamic relationship between blood pressure and cerebral blood flow in humans

We examined the hypothesis that changes in the cerebrovascular resistance index (CVRi), independent of blood pressure (BP), will influence the dynamic relationship between BP and cerebral blood flow in humans. We altered CVRi with (via controlled hyperventilation) and without [via indomethacin (INDO, 1.2 mg/kg)] changes in PaCO2. Sixteen subjects (12 men, 27 ± 7 yr) were tested on two occasions (INDO and hypocapnia) separated by >48 h. Each test incorporated seated rest (5 min), followed by squat-stand maneuvers to increase BP variability and improve assessment of the pressure-flow dynamics using linear transfer function analysis (TFA). Beat-to-beat BP, middle cerebral artery velocity (MCAv), posterior cerebral artery velocity (PCAv), and end-tidal Pco2 were monitored. Dynamic pressure-flow relations were quantified using TFA between BP and MCAv/PCAv in the very low and low frequencies through the driven squat-stand maneuvers at 0.05 and 0.10 Hz. MCAv and PCAv reductions by INDO and hypocapnia were well matched, and CVRi was comparably elevated (P < 0.001). During the squat-stand maneuvers (0.05 and 0.10 Hz), the point estimates of absolute gain were universally reduced, and phase was increased under both conditions. In addition to an absence of regional differences, our findings indicate that alterations in CVRi independent of PaCO2 can alter cerebral pressure-flow dynamics. These findings are consistent with the concept of CVRi being a key factor that should be considered in the correct interpretation of cerebral pressure-flow dynamics as indexed using TFA metrics.

Arterial pressure and cerebral blood flow variability: friend or foe? A review

Variability in arterial pressure and cerebral blood flow has traditionally been interpreted as a marker of cardiovascular decompensation, and has been associated with negative clinical outcomes across varying time scales, from impending orthostatic syncope to an increased risk of stroke. Emerging evidence, however, suggests that increased hemodynamic variability may, in fact, be protective in the face of acute challenges to perfusion, including significant central hypovolemia and hypotension (including hemorrhage), and during cardiac bypass surgery. This review presents the dichotomous views on the role of hemodynamic variability on clinical outcome, including the physiological mechanisms underlying these patterns, and the potential impact of increased and decreased variability on cerebral perfusion and oxygenation. We suggest that reconciliation of these two apparently discrepant views may lie in the time scale of hemodynamic variability; short time scale variability appears to be cerebroprotective, while mid to longer term fluctuations are associated with primary and secondary end-organ dysfunction.

Integrative regulation of human brain blood flow

Herein, we review mechanisms regulating cerebral blood flow (CBF), with specific focus on humans. We revisit important concepts from the older literature and describe the interaction of various mechanisms of cerebrovascular control. We amalgamate this broad scope of information into a brief review, rather than detailing any one mechanism or area of research. The relationship between regulatory mechanisms is emphasized, but the following three broad categories of control are explicated: (1) the effect of blood gases and neuronal metabolism on CBF; (2) buffering of CBF with changes in blood pressure, termed cerebral autoregulation; and (3) the role of the autonomic nervous system in CBF regulation. With respect to these control mechanisms, we provide evidence against several canonized paradigms of CBF control. Specifically, we corroborate the following four key theses: (1) that cerebral autoregulation does not maintain constant perfusion through a mean arterial pressure range of 60-150 mmHg; (2) that there is important stimulatory synergism and regulatory interdependence of arterial blood gases and blood pressure on CBF regulation; (3) that cerebral autoregulation and cerebrovascular sensitivity to changes in arterial blood gases are not modulated solely at the pial arterioles; and (4) that neurogenic control of the cerebral vasculature is an important player in autoregulatory function and, crucially, acts to buffer surges in perfusion pressure. Finally, we summarize the state of our knowledge with respect to these areas, outline important gaps in the literature and suggest avenues for future research.