Dynamics of middle cerebral artery blood flow velocity during moderate-intensity exercise

Ergodic speckle contrast optical coherence tomography velocimetry of rapid blood flow

Visualizing a 3D blood flow velocity field through noninvasive imaging is crucial for analyzing hemodynamic mechanisms in areas prone to disorders. However, traditional correlation-based optical coherence tomography (OCT) velocimetry techniques have a maximum measurable flow velocity depending on the A-line rate. We presented the ergodic speckle contrast OCT (ESCOCT) to break the bottleneck in measuring the rapid blood flow velocity. It achieved a measurement of blood flow velocity ranging from 9.5 to 280 mm/s using a 100 kHz swept-source (SS) OCT based on 100 A-repeats scanning mode. Addressing the non-ergodic problem of temporal OCT signals by integrating more consecutive A-scans, ESCOCT can enable the estimation for lower velocity flows by increasing A-repeats. ESCOCT provided a wide dynamic range with no upper limit on measuring blood flow velocity with an adequate signal-to-noise ratio and improved the sensitivity and accuracy of the hemodynamic assessment.

Feasibility of a Recumbent Stepper for Short-Interval, Low-Volume High-Intensity Interval Exercise in Stroke

BACKGROUND AND PURPOSE

Studies investigating high-intensity interval exercise (HIIE) in stroke typically emphasize treadmill training. However, a literature review suggested that seated devices such as a recumbent stepper or cycle offer a promising alternative for HIIE since exercise can be prescribed using peak power output (PPO). Therefore, this would give health care professionals the ability to monitor and adapt power output for the target heart rate range. The purpose of this secondary analysis was to examine the feasibility of prescribing short-interval, low-volume HIIE using PPO in chronic stroke.

METHODS

We used several methods to test feasibility: (1)Acceptability: Measured by the percentage of participants who completed the entire HIIE protocol; (2) Implementation was assessed by the number of reported cardiac or serious adverse events during submaximal exercise testing and HIIE and the average percentage of participants reaching vigorous intensity, defined by the American College of Sports Medicine as at least 77% of age-predicted maximal heart rate (HR max ).

RESULTS

Data were available for 28 participants who were 32.2 (17.2) months post-stroke and 61.4 (11.9) years of age. Twenty-eight participants completed HIIE per protocol. No cardiac or serious adverse events occurred during the submaximal exercise test or during HIIE. The rapid switching between HIIE and recovery showed no evidence of blood pressure reaching unsafe thresholds. Average intensity during HIIE reached 76.8% HR max , which is slightly below the target of 77.0%.

DISCUSSION AND CONCLUSIONS

A single bout of short-interval, low-volume HIIE, prescribed using PPO, was feasible in chronic stroke.

Video Abstract : Available for more insights from the authors (Supplemental Digital Content, Video, available at: http://links.lww.com/JNPT/A474 ).

Lower middle cerebral artery blood velocity during low-volume high-intensity interval exercise in chronic stroke

High-intensity interval training (HIIE) may present unique challenges to the cerebrovascular system in individuals post-stroke. We hypothesized lower middle cerebral artery blood velocity (MCAv) in individuals post-stroke: 1) during 10 minutes of HIIE, 2) immediately following HIIE, and 3) 30 minutes after HIIE, compared to age- and sex-matched controls (CON). We used a recumbent stepper submaximal exercise test to determine workloads for high-intensity and active recovery. Our low volume HIIE protocol consisted of 1-minute intervals for 10 minutes. During HIIE, we measured MCAv, mean arterial pressure (MAP), heart rate (HR), and end tidal carbon dioxide (PETCO2). We assessed carotid-femoral pulse wave velocity as a measure of arterial stiffness. Fifty participants completed the study (25 post-stroke, 76% ischemic, 32% moderate disability). Individuals post-stroke had lower MCAv during HIIE compared to CON (p = 0.03), which remained 30 minutes after HIIE. Individuals post-stroke had greater arterial stiffness (p = 0.01) which was moderately associated with a smaller MCAv responsiveness during HIIE (r = -0.44). No differences were found for MAP, HR, and PETCO2. This study suggests individuals post-stroke had a lower MCAv during HIIE compared to their peers, which remained during recovery up to 30 minutes. Arterial stiffness may contribute to the lower cerebrovascular responsiveness post-stroke.

Effect of electrical muscle stimulation on cerebrovascular function and cognitive performance

It is known that electrical muscle stimulation (EMS) can enhance physical function, but its impact on cognition and cerebral hemodynamics is not well understood. Thus, the purpose of this study was to investigate the effects of one EMS session on cerebrovascular function and cognitive performance. The 17 recruited young healthy participants undertook a 25-min session of EMS and a resting control session (Ctrl group) in a random order. Cerebral blood flow velocity (CBFv) in the middle and posterior cerebral arteries (right MCAv and left PCAv, respectively), cerebral oxygenation, cardiac output, and heart rate were measured throughout the sessions, whereas cognitive function was assessed before and after each experimental condition. MCAv, cardiac output, heart rate, and cerebral oxygenation were increased throughout the EMS session, whereas PCAv remained unchanged. In addition, EMS led to improved scores at the Rey auditory verbal learning test-part B and congruent Stroop task versus Ctrl. The present study demonstrates that a single session of EMS may improve cognitive performance and concomitantly increase CBFv and cerebral oxygenation. Therefore, EMS appears to be a valuable surrogate for voluntary exercise and could therefore be advantageously used in populations with severe physical limitations who would not be able to perform physical exercise otherwise.NEW & NOTEWORTHY This study is the first to demonstrate that one session of EMS applied to the quadriceps increases cerebral blood flow velocity and cerebral oxygenation, which are pivotal factors for brain health. Thus, EMS has the potential to be used as an interesting option in rehabilitation to increase cerebral perfusion and defend if not improve cognitive function sustainably for people with severe physical limitations who would not be able to perform physical exercise voluntarily.

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

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

Acute effects of high-intensity exercise on brain mechanical properties and cognitive function

Previous studies have shown that engagement in even a single session of exercise can improve cognitive performance in the short term. However, the underlying physiological mechanisms contributing to this effect are still being studied. Recently, with improvements to advanced quantitative neuroimaging techniques, brain tissue mechanical properties can be sensitively and noninvasively measured with magnetic resonance elastography (MRE) and regional brain mechanical properties have been shown to reflect individual cognitive performance. Here we assess brain mechanical properties before and immediately after engagement in a high-intensity interval training (HIIT) regimen, as well as one-hour post-exercise. We find that immediately after exercise, subjects in the HIIT group had an average global brain stiffness decrease of 4.2% (p < 0.001), and an average brain damping ratio increase of 3.1% (p = 0.002). In contrast, control participants who did not engage in exercise showed no significant change over time in either stiffness or damping ratio. Changes in brain mechanical properties with exercise appeared to be regionally dependent, with the hippocampus decreasing in stiffness by 10.4%. We also found that one-hour after exercise, brain mechanical properties returned to initial baseline values. The magnitude of changes to brain mechanical properties also correlated with improvements in reaction time on executive control tasks (Eriksen Flanker and Stroop) with exercise. Understanding the neural changes that arise in response to exercise may inform potential mechanisms behind improvements to cognitive performance with acute exercise.

Impaired cerebrovascular CO2 reactivity at high altitude in prematurely born adults

Premature birth impairs cardiac and ventilatory responses to both hypoxia and hypercapnia, but little is known about cerebrovascular responses. Both at sea level and after 2 days at high altitude (3375 m), 16 young preterm-born (gestational age, 29 ± 1 weeks) and 15 age-matched term-born (40 ± 0 weeks) adults were exposed to two consecutive 4 min bouts of hyperoxic hypercapnic conditions (3% CO2 -97% O2 ; 6% CO2 -94% O2 ), followed by two periods of voluntary hyperventilation-induced hypocapnia. We measured middle cerebral artery blood velocity, end-tidal CO2 , pulmonary ventilation, beat-by-beat mean arterial pressure and arterialized capillary blood gases. Baseline middle cerebral artery blood velocity increased at high altitude compared with sea level in term-born (+24 ± 39%, P = 0.036), but not in preterm-born (-4 ± 27%, P = 0.278) adults. The end-tidal CO2 , pulmonary ventilation and mean arterial pressure were similar between groups at sea level and high altitude. Hypocapnic cerebrovascular reactivity was higher at high altitude compared with sea level in term-born adults (+173 ± 326%, P = 0.026) but not in preterm-born adults (-21 ± 107%, P = 0.572). Hypercapnic reactivity was altered at altitude only in preterm-born adults (+125 ± 144%, P < 0.001). Collectively, at high altitude, term-born participants showed higher hypocapnic (P = 0.012) and lower hypercapnic (P = 0.020) CO2 reactivity compared with their preterm-born peers. In conclusion, exposure to high altitude revealed different cerebrovascular responses in preterm- compared with term-born adults, despite similar ventilatory responses. These findings suggest a blunted cerebrovascular response at high altitude in preterm-born adults, which might predispose these individuals to an increased risk of high-altitude illnesses. KEY POINTS: Cerebral haemodynamics and cerebrovascular reactivity in normoxia are known to be similar between term-born and prematurely born adults. In contrast, acute exposure to high altitude unveiled different cerebrovascular responses to hypoxia, hypercapnia and hypocapnia. In particular, cerebral vasodilatation was impaired in prematurely born adults, leading to an exaggerated cerebral vasoconstriction. Cardiovascular and ventilatory responses to both hypo- and hypercapnia at sea level and at high altitude were similar between control subjects and prematurely born adults. Other mechanisms might therefore underlie the observed blunted cerebral vasodilatory responses in preterm-born adults at high altitude.

Middle cerebral artery blood velocity and cognitive function after high- and moderate-intensity aerobic exercise sessions

This crossover study explored the acute effect of a session of high-intensity interval training (HIIT) or moderate-intensity continuous training (MICT) on middle cerebral artery (MCA) variables such as cerebral blood velocity, pulsatility index (PI) and resistivity index (RI) through transcranial Doppler (TCD), and cognitive function (CF - verbal fluency and Digit Span) in healthy young adults. Participants (26 healthy young adults, 13 women, 24 ± 3 years) underwent two different randomized exercise sessions: (1) MICT (60 % heart rate reserve, HRR) and (2) HIIT (80 % HRR). MCA velocity, PI, RI, CF, and serum lactate were measured immediately before and after the sessions. HIIT demonstrated improved executive function/semantic fluency (20 %, p = 0.019), while both MICT and HIIT increased lactate (625 %, HIIT, p < 0.001, and 238 %, MICT, p < 0.001). Other assessments remained stable, except for reduced PI (p = 0.029) and RI (p = 0.023) after MICT, with no significant difference (pre-post for HIIT-MICT). Notably, cognition improvement correlated with lactate increase in HIIT (ρ = 0.436; p < 0.001). Executive function/semantic fluency increased after HIIT relative to MICT. The findings show that there are no systematic out-of-normal changes in the cerebrovascular circulation of clinically healthy adults undergoing HIIT and MICT.

Influence of sex on the reliability of cerebral blood velocity regulation during lower body negative pressure and supine cycling with considerations of the menstrual cycle

Objective.To evaluate sex differences in the reliability of absolute and relative cerebral blood velocity (CBv) during concurrent supine cycling with lower body negative pressure (LBNP).Approach. A total of 19 participants (11 females; aged 20-33 years) completed five testing sessions, occurring on 7 d intervals. Visit 1 was a maximal-ramp-cycle test to ascertain peak CBv wattage. During visits 2-5, supine cycling protocol occurred at individualized peak CBv wattages with progressive decreases in LBNP from 0 to -20, -40, -60, -70, and -80 Torr. Menstrual cycle day was self-reported via the Rhinessa Women's Questionnaire. Transcranial Doppler ultrasound insonated bilateral middle cerebral artery velocity (MCAv). Two-way ANOVA assessed potential day- and sex-differences at each LBNP stage. Reliability was determined using intraclass correlation coefficients (ICC) and coefficient of variation (CoV).Main results. For all physiological measures, no main-effects were present for day- or interaction-terms (p> 0.067; negligible-to-small effect sizes), while sex differences were noted for MCAv, blood pressure, and heart rate (p< 0.046). Across visits, males and females displayed excellent and good-to-excellent levels of reliability for MCAv metrics, respectively (ICC range: 0.745-0.999; CoV range: 0.33%-9.90%).Significance. During the current investigation, both relative and absolute CBv demonstrated high reliability in both male and female participants during a supine LBNP cycling protocol. An exploratory analysis revealed increased variance was found in female participants dependent on contraceptive use. Despite this, results indicate future LBNP studies may include females at any menstrual cycle stage.

Exercise breaks prevent attenuation in cerebrovascular function following an acute bout of uninterrupted sitting in healthy children

The purpose of this study was to examine the effect of an acute bout of prolonged sitting with and without exercise breaks on cerebrovascular function in 7- to 13-year-old children. Forty-two children and adolescents were recruited to a crossover trial, with 15 girls (mean age 10.1 ± 2.5 years) and 16 boys (mean age 10.5 ± 1.3 years) completing the two trial conditions: SIT, uninterrupted sitting for 3 h and CYCLE, 3 h of sitting interrupted hourly with a 10-min moderate intensity exercise break. Cerebrovascular function was measured Pre and Post SIT and CYCLE from blood flow (Q ̇ ${\dot{Q}}$ ), diameter, and shear rate of the internal carotid artery (ICA) at rest and in response to CO2 . Blood velocity in the middle (MCA) and posterior (PCA) cerebral arteries was assessed at rest, during a neurovascular coupling task (NVC) and in response to CO2 . We demonstrate that SIT but not CYCLE reduced ICA cerebrovascular reactivity to CO2 (%Δ ICAQ ̇ ${\dot{Q}}$ /Δ end-tidal CO2 : SIT: Pre 5.0 ± 2.4%/mmHg to Post 3.3 ± 2.8%/mmHg vs. CYCLE: Pre 4.4 ± 2.3%/mmHg to Post 5.3 ± 3.4%/mmHg, P = 0.05) and slowed the MCA blood velocity onset response time to hypercapnia (SIT: Pre 57.2 ± 32.6 s to Post 76.6 ± 55.2 s, vs. CYCLE: Pre 64.1 ± 40.4 s to Post 52.3 ± 28.8 s, P = 0.05). There were no changes in NVC. Importantly, breaking up prolonged sitting with hourly exercise breaks prevented the reductions in cerebrovascular reactivity to CO2 and the slowed intracranial blood velocity onset response time to hypercapnia apparent with uninterrupted sitting in children. NEW FINDINGS: What is the central question of this study? What are the effects of interrupting prolonged sitting on cerebrovascular function in children? What is the main finding and its importance? Prolonged sitting results in declines in cerebrovascular reactivity, a valuable index of cerebrovascular health. Breaking up prolonged sitting with hourly 10 min exercise breaks prevented these changes. These initial findings suggest excessive sedentary behaviour does impact cerebrovascular function in childhood, but taking exercise breaks prevents declines.

The Association of Tacrolimus Formulation on Cerebral Blood Flow and Cognitive Function

Calcineurin inhibitors are inherent vasoconstrictors. Cerebral vasoconstriction can reduce cerebral blood flow (CBF), and negatively impact cerebrovascular response (CVR) to exercise, and cognitive function. The once-daily extended-release (LCP) tacrolimus has fewer side effects than the immediate-release (IR) tacrolimus. The role of calcineurin inhibitors on CBF and the impact of specific formulations of tacrolimus on CBF, CVR, and cognitive function are unknown. In this pilot study, we evaluated whether changing from IR tacrolimus to LCP tacrolimus modulates CBF, CVR, or cognitive function in kidney transplant (KT) recipients.

Methods

We randomized (2:1) 30 stable KT recipients on IR tacrolimus to intervention (switch to LCP tacrolimus) and control (continue IR tacrolimus) arms. We measured CBF, CVR, and cognitive function at baseline and at 12 wk. We used ANCOVA to evaluate changes in outcome variables, with baseline values and study arm as covariates. We used descriptive statistics with mean changes in outcome variables to compare the 2 groups.

Results

Participants were 51 ± 13 y old. There was no difference in plasma tacrolimus levels at baseline and at 12 wk in the 2 arms. The changes in CBF, resting middle cerebral artery velocity, CVR, and cognitive function were more favorable in the intervention arm than in the control group.

Conclusions

Changing IR tacrolimus to LCP tacrolimus may improve CBF, cerebrovascular dynamics, and cognitive function in KT recipients. Larger studies are needed to confirm these results.

Agreement between left and right middle cerebral artery blood velocity responses to incremental and constant work-rate exercise in healthy males and females

Objective.To quantify the agreement between left and right middle cerebral artery blood velocity (MCAv) responses to incremental and constant work-rate exercise in adults.ApproachSeventeen healthy adults (23.8 ± 2.4 years, 9 females) completed a ramp incremental test to exhaustion on a cycle ergometer, three 6-minute transitions at a moderate-intensity, and three at a heavy-intensity, all on separate days. Bilateral MCAv was measured throughout using transcranial Doppler ultrasonography, with left and right MCAv data analysed separately. Data were analysed at baseline, gas exchange threshold, respiratory compensation point and exhaustion during ramp incremental exercise. MCAv responses to constant work-rate exercise were analysed using a mono-exponential model, to determine time- and amplitude-based kinetic response parameters.Main ResultsLeft and right MCAv responses to incremental and constant work-rate exercise were significantly, strongly and positively correlated (r≥ 0.61,P< 0.01). Coefficient of variation (left versus right) ranged from 7.3%-20.7%, 6.4%-26.2% and 5.9%-22.5% for ramp, moderate and heavy-intensity exercise, respectively. The relative change in MCAv from baseline was higher in the right compared to left MCAv during ramp, moderate and heavy-intensity exercise (allP< 0.05), but the effect sizes were small (d≤ 0.4). Small mean left-right differences were present during ramp incremental exercise at all time-points (<6 cm s-1; <4%), and for all kinetic parameters during moderate and heavy-intensity exercise (<3 cm s-1, <3%, <4 s).SignificanceThese findings demonstrate similarities between left and right MCAv responses to incremental and constant-work rate exercise in adults on a group-level, but also highlight individual variation in the agreement between left and right MCAv exercise responses.

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.

Effect of Acute Cardiovascular Exercise on Cerebral Blood Flow: A Systematic Review

A single bout of cardiovascular exercise can have a cascade of physiological effects, including increased blood flow to the brain. This effect has been documented across multiple modalities, yet studies have reported mixed findings. Here, we systematically review evidence for the acute effect of cardiovascular exercise on cerebral blood flow across a range of neuroimaging techniques and exercise characteristics. Based on 52 studies and a combined sample size of 1,174 individuals, our results indicate that the acute effect of cardiovascular exercise on cerebral blood flow generally follows an inverted U-shaped relationship, whereby blood flow increases early on but eventually decreases as exercise continues. However, we also find that this effect is not uniform across studies, instead varying across a number of key variables including exercise characteristics, brain regions, and neuroimaging modalities. As the most comprehensive synthesis on the topic to date, this systematic review sheds light on the determinants of exercise-induced change in cerebral blood flow, a necessary step toward personalized interventions targeting brain health across a range of populations.

Continuous non-invasive estimates of cerebral blood flow using electrocardiography signals: a feasibility study

This paper describes a potential method to detect changes in cerebral blood flow (CBF) using electrocardiography (ECG) signals, measured across scalp electrodes with reference to the same signal across the chest—a metric we term the Electrocardiography Brain Perfusion index (EBPi). We investigated the feasibility of EBPi to monitor CBF changes in response to specific tasks. Twenty healthy volunteers wore a head-mounted device to monitor EBPi and electroencephalography (EEG) during tasks known to alter CBF. Transcranial Doppler (TCD) ultrasound measurements provided ground-truth estimates of CBF. Statistical analyses were applied to EBPi, TCD right middle cerebral artery blood flow velocity (rMCAv) and EEG relative Alpha (rAlpha) data to detect significant task-induced changes and correlations. Breath-holding and aerobic exercise induced highly significant increases in EBPi and TCD rMCAv (p < 0.01). Verbal fluency also increased both measures, however the increase was only significant for EBPi (p < 0.05). Hyperventilation induced a highly significant decrease in TCD rMCAv (p < 0.01) but EBPi was unchanged. Combining all tasks, EBPi exhibited a highly significant, weak positive correlation with TCD rMCAv (r = 0.27, p < 0.01) and the Pearson coefficient between EBPi and rAlpha was r = − 0.09 (p = 0.05). EBPi appears to be responsive to dynamic changes in CBF and, can enable practical, continuous monitoring. CBF is a key parameter of brain health and function but is not easily measured in a practical, continuous, non-invasive fashion. EBPi may have important clinical implications in this context for stroke monitoring and management. Additional studies are required to support this claim.

Proceedings from the Albert Charitable Trust Inaugural Workshop on 'Understanding the Acute Effects of Exercise on the Brain'

An inaugural workshop supported by "The Leo and Anne Albert Charitable Trust," was held October 4-7, 2019 in Scottsdale, Arizona, to focus on the effects of exercise on the brain and to discuss how physical activity may prevent or delay the onset of aging-related neurodegenerative conditions. The Scientific Program Committee (led by Dr. Jeff Burns) assembled translational, clinical, and basic scientists who research various aspects of the effects of exercise on the body and brain, with the overall goal of gaining a better understanding as to how to delay or prevent neurodegenerative diseases. In particular, research topics included the links between cardiorespiratory fitness, the cerebrovasculature, energy metabolism, peripheral organs, and cognitive function, which are all highly relevant to understanding the effects of acute and chronic exercise on the brain. The Albert Trust workshop participants addressed these and related topics, as well as how other lifestyle interventions, such as diet, affect age-related cognitive decline associated with Alzheimer's and other neurodegenerative diseases. This report provides a synopsis of the presentations and discussions by the participants, and a delineation of the next steps towards advancing our understanding of the effects of exercise on the aging brain.

Capillary communication: the role of capillaries in sensing the tissue environment, coordinating the microvascular, and controlling blood flow

Historically, capillaries have been viewed as the microvascular site for flux of nutrients to cells and removal of waste products. Capillaries are the most numerous blood vessel segment within the tissue, whose vascular wall consists of only a single layer of endothelial cells and are situated within microns of each cell of the tissue, all of which optimizes capillaries for the exchange of nutrients between the blood compartment and the interstitial space of tissues. There is, however, a growing body of evidence to support that capillaries play an important role in sensing the tissue environment, coordinating microvascular network responses, and controlling blood flow. Much of our growing understanding of capillaries stems from work in skeletal muscle and more recent work in the brain, where capillaries can be stimulated by products released from cells of the tissue during increased activity and are able to communicate with upstream and downstream vascular segments, enabling capillaries to sense the activity levels of the tissue and send signals to the microvascular network to coordinate the blood flow response. This review will focus on the emerging role that capillaries play in communication between cells of the tissue and the vascular network required to direct blood flow to active cells in skeletal muscle and the brain. We will also highlight the emerging central role that disruptions in capillary communication may play in blood flow dysregulation, pathophysiology, and disease.

Dynamic onset response of the internal carotid artery to hypercapnia is blunted in children compared with adults

Intracranial blood velocity reactivity to a steady‐state hypercapnic stimulus has been shown to be similar in children and adults, but the onset response to hypercapnia is slower in the child. Given the vasodilatory effect of hypercapnia on the cerebrovasculature, assessment of vessel diameter, and blood flow are vital to fully elucidate whether the temporal hypercapnic response differs in children versus adults. Assessment of internal carotid artery (ICA) vessel diameter (ICAd), blood velocity (ICAv), volumetric blood flow (Q_ICA), and shear rate (ICA_SR) in response to a 4 min hypercapnic challenge was completed in children (n = 14, 8 girls; 9.8 ± 0.7 years) and adults (n = 17, 7 females; 24.7 ± 1.8 years). The dynamic onset responses of partial pressure of end‐tidal CO₂ (P_ETCO₂), Q_ICA, ICAv, and ICA_SR to hypercapnia were modeled, and mean response time (MRT) was computed. Following 4 min of hypercapnia, ICA reactivity and ICAd were comparable between the groups. Despite a similar MRT in P_ETCO₂ in children and adults, children had slower Q_ICA (children 108 ± 60 s vs. adults 66 ± 37 s; p = 0.023), ICAv (children 120 ± 52 s vs. adults 52 ± 31 s; p = 0.001), and ICA_SR (children 90 ± 27 s vs. adults 47 ± 36 s; p = 0.001) MRTs compared with adults. This is the first study to show slower hypercapnic hyperemic kinetic responses of the ICA in children. The mechanisms determining these differences and the need to consider the duration of hypercapnic exposure when assessing CVR in children should be considered in future studies.

Dementia risk and dynamic response to exercise: A non-randomized clinical trial

Background

Physical exercise may support brain health and cognition over the course of typical aging. The goal of this nonrandomized clinical trial was to examine the effect of an acute bout of aerobic exercise on brain blood flow and blood neurotrophic factors associated with exercise response and brain function in older adults with and without possession of the Apolipoprotein epsilon 4 (APOE4) allele, a genetic risk factor for developing Alzheimer’s. We hypothesized that older adult APOE4 carriers would have lower cerebral blood flow regulation and would demonstrate blunted neurotrophic response to exercise compared to noncarriers.

Methods

Sixty-two older adults (73±5 years old, 41 female [67%]) consented to this prospectively enrolling clinical trial, utilizing a single arm, single visit, experimental design, with post-hoc assessment of difference in outcomes based on APOE4 carriership. All participants completed a single 15-minute bout of moderate-intensity aerobic exercise. The primary outcome measure was change in cortical gray matter cerebral blood flow in cortical gray matter measured by magnetic resonance imaging (MRI) arterial spin labeling (ASL), defined as the total perfusion (area under the curve, AUC) following exercise. Secondary outcomes were changes in blood neurotrophin concentrations of insulin-like growth factor-1 (IGF-1), vascular endothelial growth factor (VEGF), and brain derived neurotrophic factor (BDNF).

Results

Genotyping failed in one individual (n = 23 APOE4 carriers and n = 38 APOE4 non-carriers) and two participants could not complete primary outcome testing. Cerebral blood flow AUC increased immediately following exercise, regardless of APOE4 carrier status. In an exploratory regional analyses, we found that cerebral blood flow increased in hippocampal brain regions, while showing no change in cerebellum across both groups. Among high inter-individual variability, there were no significant changes in any of the 3 neurotrophic factors for either group immediately following exercise.

Conclusions

Our findings show that both APOE4 carriers and non-carriers show similar effects of exercise-induced increases in cerebral blood flow and neurotrophic response to acute aerobic exercise. Our results provide further evidence that acute exercise-induced increases in cerebral blood flow may be regional specific, and that exercise-induced neurotrophin release may show a differential effect in the aging cardiovascular system. Results from this study provide an initial characterization of the acute brain blood flow and neurotrophin responses to a bout of exercise in older adults with and without this known risk allele for cardiovascular disease and Alzheimer’s disease.

Trial registration

Dementia Risk and Dynamic Response to Exercise (DYNAMIC); Identifier: NCT04009629.

The effect of exercise intensity and cardiorespiratory fitness on the kinetic response of middle cerebral artery blood velocity during exercise in adults

The aim of this study was to compare the kinetic response of middle cerebral artery blood velocity (MCAv) to moderate and heavy-intensity cycling in adults, and explore the relationship between maximal oxygen uptake (V̇O_2max) and MCAv kinetics. Seventeen healthy adults (23.8±2.4 years, 9 females) completed a ramp incremental test to exhaustion on a cycle ergometer to determine V̇O_2max and the gas exchange threshold (GET). Across six separate visits, participants completed three 6-minute transitions at a moderate-intensity (90% GET) and three at a heavy-intensity (40% of the difference between GET and V̇O_2max). Bilateral MCAv was measured using transcranial Doppler ultrasonography and analysed using a mono-exponential model with a time delay. The time constant (τ) of the MCAv response was not different between moderate- and heavy-intensity cycling (25±10 vs. 26±8 s, P=0.82), as was the time delay (29±11 vs. 29±10 s, P=0.95). The amplitude of the exponential increase in MCAv from baseline was greater during heavy (23.9±10.0 cm^.s^-1, 34.1±14.4%) compared to moderate (12.7±4.4 cm^.s^-1, 18.7±7.5%) intensity cycling (P<0.01). Following the exponential increase, a greater fall in MCAv was observed during heavy compared to moderate-intensity exercise (9.5±6.9 vs 2.8±3.8 cm^.s^-1, P<0.01). MCAv after 6 minutes of exercise remained elevated during heavy compared to moderate-intensity exercise (85.2±9.6 vs. 79.3±7.7cm^.s^-1, P≤0.01). V̇O_2max was not correlated with MCAv τ or amplitude (r=0.11-0.26, P>0.05). These data suggest that the intensity of constant-work rate exercise influences the amplitude, but not time-based, response parameters of MCAv in healthy adults, and found no relationship between cardiorespiratory fitness and MCAv kinetics.

Influence of cardiac output response to the onset of exercise on cerebral blood flow

PURPOSE

Change in cardiac output (Q) contributes to cerebral blood flow (CBF) regulation at rest and even during steady-state exercise. At the onset of cycling exercise, Q increases acutely and largely via muscle pump. The purpose of the present study was to examine whether onset exercise-induced a large increase in Q contributes to CBF regulation at the onset of exercise.

METHODS

In 20 young healthy participants (10 males and 10 females), Q, mean arterial pressure (MAP), and mean blood velocities of middle and posterior cerebral arteries (MCA Vm and PCA Vm) were continuously measured during light cycling exercise for 3 min.

RESULTS

At the onset of exercise, Q increased acutely to the peak (P < 0.001), while the CBF peak responses were not significantly higher than the values during the steady-state exercise (MCA Vm and PCA Vm; P = 0.183 and P = 0.101, respectively). The change in Q was correlated with that of MCA Vm or PCA Vm from resting baseline to the steady-state exercise (r = 0.404, P < 0.001 and r = 0.393, P < 0.001, respectively). However, the change in Q was not correlated with that of MCA Vm or PCA Vm at the onset of exercise (P = 0.853 and P = 0.893, respectively). Any sex differences in the onset response of peripheral and cerebral hemodynamics to exercise were not observed.

CONCLUSION

These findings suggest that the acute change in Q does not contribute to CBF regulation at the onset of exercise for protecting cerebral vasculature against a large and acute elevation in Q at the onset of exercise.

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

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

Sex Differences in Resilience and Resistance to Brain Pathology and Dysfunction Moderated by Cerebrovascular Response to Exercise and Genetic Risk for Alzheimer's Disease

Sex as a biological variable appears to contribute to the multifactorial etiology of Alzheimer's disease. We tested sex-based interactions between cerebrovascular function and APOE4 genotype on resistance and resilience to brain pathology and cognitive executive dysfunction in cognitively-normal older adults. Female APOE4 carriers had higher amyloid-β deposition yet achieved similar cognitive performance to males and female noncarriers. Further, female APOE4 carriers with robust cerebrovascular responses to exercise possessed lower amyloid-β. These results suggest a unique cognitive resilience and identify cerebrovascular function as a key mechanism for resistance to age-related brain pathology in females with high genetic vulnerability to Alzheimer's disease.

Cerebrovascular response to an acute bout of low-volume high-intensity interval exercise and recovery in young healthy adults

High-intensity interval exercise (HIIT) is performed widely. However, there is a gap in knowledge regarding the acute cerebrovascular response to low-volume HIIT. Our objective was to characterize the middle cerebral artery blood velocity (MCAv) response during an acute bout of low-volume HIIT in young healthy adults. We hypothesized that MCAv would decrease below the baseline (BL), 1) during HIIT, 2) immediately following HIIT, and 3) 30 min after HIIT. As a secondary objective, we investigated sex differences in the MCAv response during HIIT. Twenty-four young healthy adults completed HIIT [12 males, age = 25 (SD = 2)]. HIIT included 10 min of 1-min high intensity (∼70% estimated maximal Watts) and active recovery (10% estimated maximal Watts) intervals on a recumbent stepper. MCAv, mean arterial pressure (MAP), heart rate (HR), and end-tidal carbon dioxide ([Formula: see text]) were recorded at BL, during HIIT, immediately following HIIT, and 30 min after HIIT. Contrary to our hypothesis, MCAv remained above BL during HIIT. MCAv peaked at minute 3 then decreased concomitantly with [Formula: see text]. MCAv was lower than BL immediately following HIIT (P < 0.001). Thirty minutes after HIIT, MCAv returned to BL (P = 0.47). Compared with men, women had a higher MCAv at BL (P = 0.001), during HIIT (P = 0.009), immediately following HIIT (P = 0.004), and 30 min after HIIT (P = 0.001). MCAv did not decrease below BL during low-volume HIIT. However, MCAv decreased below BL immediately following HIIT and returned to resting values 30 min after HIIT. MCAv also differed between sexes.NEW & NOTEWORTHY We are the first, to our knowledge, to characterize the cerebrovascular and hemodynamic response to low-volume high-intensity interval exercise (HIIT, 1-min intervals) in young healthy adults. Middle cerebral artery blood velocity (MCAv) decreased during the HIIT bout and rebounded during active recovery. Women demonstrated a significantly higher resting MCAv than men and the difference remained during HIIT. Here, we report a novel protocol and characterized the MCAv response during an acute bout of low-volume HIIT.

The within- and between-day reliability of cerebrovascular reactivity using traditional and novel analytical approaches

NEW FINDINGS

What is the central question of the study? What is the reliability of middle cerebral artery velocity cerebrovascular reactivity (CVR) when using traditional and novel outcomes, as measured by transcranial Doppler? What is the main finding and its importance? Traditional CVR approaches presented acceptable reproducibility but should be expressed as an absolute CVR. Large within- and between-individual differences in the middle cerebral artery velocity response profile support using a dynamic peak, rather than a set time point, for the most reliable interpretation. The study highlights the utility of novel kinetic CVR outcomes, but due to increased variability in time-based metrics, this analysis requires larger sample sizes than traditional methods.

ABSTRACT

Cerebrovascular reactivity (CVR) of middle cerebral artery velocity (MCAv) to CO₂ is a common method to assess cerebrovascular function. Yet, the approaches used to calculate CVR outcomes vary. The aim of this study was to explore the within- and between-day reliability of traditional CVR outcomes. The second aim was to explore the reliability of novel kinetic-based analyses. Healthy adults (n = 10, 22.3 ± 3.4 years) completed assessments of CVR over 4 min using a fixed fraction of inspired CO₂ (6%). This was repeated across four separate visits (between-day), and on one visit measures were repeated 2.5 h later (within-day). No mean biases were present between assessments for traditional CVR metrics, expressed as absolute (cm/s/mmHg) or relative (%/mmHg) outcomes (minute 3, minute 4, peak 1 s, peak 30 s) (between-day: P > 0.14, η_p ² < 0.20; within-day: P > 0.22, d > 0.27). Absolute, rather than relative, CVR yielded the most reproducible parameters (coefficient of variation: 8.1-13.2% vs. 14-83%, respectively). There were significant differences between CVR outcomes (P < 0.001, η_p ² > 0.89) dependent on the time point used to determine CVR, as a steady state MCAv response was rarely observed. Furthermore, the MCAv response was not reproducible within an individual (κ = 0.15, P = 0.09). No mean differences were present for novel kinetic outcomes (amplitude, time-delay, time constant) (between-day: P > 0.05, d < 0.33; within-day: P > 0.38, d < 0.25). The results support the need for standardisation and indicate CVR should be defined as a dynamic peak, rather than a set time point for increased reliability. For novel kinetic outcomes variability was greater (CV: 8.7-120.9%) due to the nature of time-based metrics.

A methodology for an acute exercise clinical trial called dementia risk and dynamic response to exercise

Exercise likely has numerous benefits for brain and cognition. However, those benefits and their causes remain imprecisely defined. If the brain does benefit from exercise it does so primarily through cumulative brief, "acute" exposures over a lifetime. The Dementia Risk and Dynamic Response to Exercise (DYNAMIC) clinical trial seeks to characterize the acute exercise response in cerebral perfusion, and circulating neurotrophic factors in older adults with and without the apolipoprotein e4 genotype (APOE4), the strongest genetic predictor of sporadic, late onset Alzheimer's disease. DYNAMIC will enroll 60 older adults into a single moderate intensity bout of exercise intervention, measuring pre- and post-exercise cerebral blood flow (CBF) using arterial spin labeling, and neurotrophic factors. We expect that APOE4 carriers will have poor CBF regulation, i.e. slower return to baseline perfusion after exercise, and will demonstrate blunted neurotrophic response to exercise, with concentrations of neurotrophic factors positively correlating with CBF regulation. Preliminary findings on 7 older adults and 9 younger adults demonstrate that the experimental method can capture CBF and neurotrophic response over a time course. This methodology will provide important insight into acute exercise response and potential directions for clinical trial outcomes.ClinicalTrials.gov NCT04009629, Registered 05/07/2019.

High-but not moderate-intensity exercise acutely attenuates hypercapnia-induced vasodilation of the internal carotid artery in young men

PURPOSE

Exercise-induced increases in shear rate (SR) across different exercise intensities may differentially affect hypercapnia-induced vasodilation of the internal carotid artery (ICA), a potential index of cerebrovascular function. We aimed to elucidate the effects of exercise intensity on ICA SR during exercise and post-exercise hypercapnia-induced vasodilation of the ICA in young men.

METHODS

Twelve healthy men completed 30 min of cycling at moderate [MIE; 65 ± 5% of age-predicted maximal heart rate (HR_max)] and high (HIE; 85 ± 5% HR_max) intensities. Hypercapnia-induced vasodilation was induced by 3 min of hypercapnia (target end-tidal partial pressure of CO₂ + 10 mmHg) and was assessed at pre-exercise, 5 min and 60 min after exercise. Doppler ultrasound was used to measure ICA diameter and blood velocity during exercise and hypercapnia tests.

RESULTS

SR was not altered during either exercise (interaction and main effects of time; both P > 0.05). ICA conductance decreased during HIE from resting values (5.1 ± 1.3 to 3.2 ± 1.0 mL·min^-1·mmHg^-1; P < 0.01) but not during MIE (5.0 ± 1.3 to 4.0 ± 0.8 mL·min^-1·mmHg^-1; P = 0.11). Consequently, hypercapnia-induced vasodilation declined immediately after HIE (6.9 ± 1.7% to 4.0 ± 1.4%; P < 0.01), but not after MIE (7.2 ± 2.1% to 7.3 ± 1.8%; P > 0.05). Sixty minutes after exercise, hypercapnia-induced vasodilation returned to baseline values in both trials (MIE 8.0 ± 3.1%; HIE 6.4 ± 2.9%; both P > 0.05).

CONCLUSION

The present study showed blunted hypercapnia-induced vasodilation of the ICA immediately after high-intensity exercise, but not a moderate-intensity exercise in young men. Given that the acute response is partly linked to the adaptive response in the peripheral endothelial function, the effects of aerobic training on cerebrovascular health may vary depending on exercise intensity.

Contrasting Measures of Cerebrovascular Reactivity Between MRI and Doppler: A Cross-Sectional Study of Younger and Older Healthy Individuals

Cerebrovascular reactivity (CVR) is used as an outcome measure of brain health. Traditionally, lower CVR is associated with ageing, poor fitness and brain-related conditions (e.g. stroke, dementia). Indeed, CVR is suggested as a biomarker for disease risk. However, recent findings report conflicting associations between ageing or fitness and CVR measures. Inconsistent findings may relate to different neuroimaging modalities used, which include transcranial Doppler (TCD) and blood-oxygen-level-dependant (BOLD) contrast magnetic resonance imaging (MRI). We assessed the relationship between CVR metrics derived from two common imaging modalities, TCD and BOLD MRI, within the same individuals and with expected significant differences (i.e., younger vs. older) to maximise the expected spread in measures. We conducted two serial studies using TCD- and MRI-derived measures of CVR (via inspired 5% CO2 in air). Study 1 compared 20 younger (24 ± 7 years) with 15 older (66 ± 7 years) participants, Study 2 compared 10 younger (22 ± 2 years) with 10 older (72 ± 4 years) participants. Combining the main measures across studies, no significant correlation (r = 0.15, p = 0.36) was observed between individual participant TCD- and BOLD-CVR measures. Further, these measures showed differential effects between age groups; with TCD-CVR higher in the older compared to younger group (4 ± 1 vs. 3 ± 1 %MCAv/mmHg P ET CO2; p < 0.05, Hedges' g = 0.75), whereas BOLD-CVR showed no difference (p = 0.104, Hedges' g = 0.38). In Study 2 additional measures were obtained to understand the origin of the discrepancy: phase contrast angiography (PCA) MRI of the middle cerebral artery, showed a significantly lower blood flow (but not velocity) CVR response in older compared with younger participants (p > 0.05, Hedges' g = 1.08). The PCA CVR metrics did not significantly correlate with the BOLD- or TCD-CVR measures. The differing CVR observations between imaging modalities were despite expected, correlated (r = 0.62-0.82), age-related differences in resting CBF measures across modalities. Taken together, findings across both studies show no clear relationship between TCD- and BOLD-CVR measures. We hypothesize that CVR differences between imaging modalities are in part due to the aspects of the vascular tree that are assessed (TCD:arteries; BOLD:venules/veins). Further work is needed to understand the between-modality CVR response differences, but caution is needed when comparing CVR metrics derived from different imaging modalities.

Pilot Study to Characterize Middle Cerebral Artery Dynamic Response to an Acute Bout of Moderate Intensity Exercise at 3- and 6-Months Poststroke

Background The primary aim of this study was to characterize the middle cerebral artery blood velocity (MCAv) dynamic response to an acute bout of exercise in humans at 3- and 6-months poststroke. As a secondary objective, we grouped individuals according to the MCAv dynamic response to the exercise bout as responder or nonresponder. We tested whether physical activity, aerobic fitness, and exercise mean arterial blood pressure differed between groups. Methods and Results Transcranial Doppler ultrasound measured MCAv during a 90-second baseline followed by a 6-minute moderate intensity exercise bout. Heart rate, mean arterial blood pressure, and end-tidal CO2 were additional variables of interest. The MCAv dynamic response variables included the following: baseline, time delay, amplitude, and time constant. Linear mixed model revealed no significant differences in our selected outcomes between 3- and 6-months poststroke. Individuals characterized as responders demonstrated a faster time delay, higher amplitude, and reported higher levels of physical activity and aerobic fitness when compared with the nonresponders. No between-group differences were identified for baseline, time constant, or exercise mean arterial blood pressure. In the nonresponders, we observed an immediate rise in MCAv following exercise onset followed by an immediate decline to near baseline values, while the responders showed an exponential rise until steady state was reached. Conclusions The MCAv dynamic response profile has the potential to provide valuable information during an acute exercise bout following stroke. Individuals with a greater MCAv response to the exercise stimulus reported statin use and regular participation in exercise.

Chronic hyperglycemia before acute ischemic stroke impairs the bilateral cerebrovascular response to exercise during the subacute recovery period

BACKGROUND AND PURPOSE

Chronic hyperglycemia contributes to cerebrovascular dysfunction by damaging blood vessels. Poor glucose control has been tied to impairments in cerebral blood flow, which may be particularly detrimental for people recovering from major cerebrovascular events such as acute ischemic stroke. In this secondary analysis, we explore for the first time the connection between chronic hyperglycemia before acute stroke and the cerebrovascular response (CVR) to exercise 3 and 6 month into the subacute recovery period.

METHODS

We recorded middle cerebral artery velocity (MCAv) using transcranial Doppler ultrasound bilaterally at rest and during moderate-intensity exercise in stroke patients at 3 (n = 19) and 6 (n = 12) months post-stroke. We calculated CVR as the difference between MCAv during steady-state exercise and resting MCAv. We obtained hemoglobin A1c levels (HbA1c; a measure of blood glucose over the prior 3 months) from the electronic medical record (EMR) and divided participants by HbA1c greater or less than 7%.

RESULTS

Participants with high HbA1c (>7%) at the time of acute stroke had significantly lower CVR to exercise for both the stroke-affected (p = .009) and non-affected (p = .007) hemispheres at 3 months post-stroke. These differences remained significant at 6 months post-stroke (stroke-affected, p = .008; non-affected, p = .016).

CONCLUSIONS

Patients with chronic hyperglycemia before acute ischemic stroke demonstrated impaired cerebrovascular function during exercise months into the subacute recovery period. These findings highlight the importance of maintaining tight glucose control to reduce morbidity and improve recovery post-stroke and could have implications for understanding cerebrovascular pathophysiology.

Cerebrovascular assessments to help understand brain-related changes associated with aerobic exercise after stroke

Evidence suggests exercise is "good medicine" after stroke, yet consensus is lacking on the time to initiate, type, exertion level, and duration per session. It remains a challenge to identify outcome measures for stroke-exercise trials that are sufficiently sensitive to intervention parameters. Cerebrovascular assessments, namely cerebral blood flow and intracranial pulsatility, are herein discussed as examples of quantitative brain-specific measures that may be useful to monitor exercise-related brain changes and help to guide stroke rehabilitation interventions. Novelty: Cerebral blood flow and arterial stiffness are potential vascular targets for stroke exercise trials.

Cerebrovascular responses to graded exercise in young healthy males and females

Although systemic sex-specific differences in cardiovascular responses to exercise are well established, the comparison of sex-specific cerebrovascular responses to exercise has gone under-investigated especially, during high intensity exercise. Therefore, our purpose was to compare cerebrovascular responses in males and females throughout a graded exercise test (GXT). Twenty-six participants (13 Females and 13 Males, 24 ± 4 yrs.) completed a GXT on a recumbent cycle ergometer consisting of 3-min stages. Each sex completed 50W, 75W, 100W stages. Thereafter, power output increased 30W/stage for females and 40W/stage for males until participants were unable to maintain 60-80 RPM. The final stage completed by the participant was considered maximum workload(Wmax ). Respiratory gases (End-tidal CO2 , EtCO2 ), middle cerebral artery blood velocity (MCAv), heart rate (HR), non-invasive mean arterial pressure (MAP), cardiac output (CO), and stroke volume (SV) were continuously recorded on a breath-by-breath or beat-by-beat basis. Cerebral perfusion pressure, CPP = MAP (0. 7,355 distance from heart-level to doppler probe) and cerebral vascular conductance index, CVCi = MCAv/CPP 100mmHg were calculated. The change from baseline (Δ) in MCAv was similar between the sexes during the GXT (p = .091, ωp2  = 0.05). However, ΔCPP (p < .001, ωp2  = 0.25) was greater in males at intensities ≥ 80% Wmax and ΔCVCi (p = .005, ωp2  = 0.15) was greater in females at 100% Wmax . Δ End-tidal CO2 (ΔEtCO2 ) was not different between the sexes during exercise (p = .606, ωp2  = -0.03). These data suggest there are sex-specific differences in cerebrovascular control, and these differences may only be identifiable at high and severe intensity exercise.

Measurement and Changes in Cerebral Oxygenation and Blood Flow at Rest and During Exercise in Normotensive and Hypertensive Individuals

PURPOSE OF REVIEW

Summarize the methods used for measurement of cerebral blood flow and oxygenation; describe the effects of hypertension on cerebral blood flow and oxygenation.

RECENT FINDINGS

Information regarding the effects of hypertension on cerebrovascular circulation during exercise is very limited, despite a plethora of methods to help with its assessment. In normotensive individuals performing incremental exercise testing, total blood flow to the brain increases. In contrast, the few studies performed in hypertensive patients suggest a smaller increase in cerebral blood flow, despite higher blood pressure levels. Endothelial dysfunction and increased vasoconstrictor concentration, as well as large vessel atherosclerosis and decreased small vessel number, have been proposed as the underlying mechanisms. Hypertension may adversely impact oxygen and blood delivery to the brain, both at rest and during exercise. Future studies should utilize the newer, noninvasive techniques to better characterize the interplay between the brain and exercise in hypertension.

The Effect of Stroke on Middle Cerebral Artery Blood Flow Velocity Dynamics During Exercise

BACKGROUND AND PURPOSE

Previous work demonstrates that older adults have a lower response in the middle cerebral artery velocity (MCAv) to an acute bout of moderate-intensity exercise when compared with young adults. However, no information exists regarding MCAv response to exercise after stroke. We tested whether MCAv response to an acute bout of moderate-intensity exercise differed between participants 3 months after stroke and an age- and sex-matched control group of older adults (CON). A secondary objective was to compare MCAv response between the stroke- and non-stroke-affected MCAv.

METHODS

Using transcranial Doppler ultrasound, we recorded MCAv during a 90-second baseline (BL) followed by a 6-minute moderate-intensity exercise bout using a recumbent stepper. Heart rate (HR), end-tidal CO2 (PETCO2), and beat-to-beat mean arterial blood pressure (MAP) were additional variables of interest. The MCAv response measures included BL, peak response amplitude (Amp), time delay (TD), and time constant (τ).

RESULTS

The Amp was significantly lower in the stroke-affected MCAv compared with CON (P < 0.01) and in the nonaffected MCAv compared with CON (P = 0.03). No between-group differences were found between TD and τ. No significant differences were found during exercise for PETCO2 and MAP while HR was lower in participants with stroke (P < 0.01). Within the group of participants with stroke, no differences were found between the stroke-affected and non-stroke-affected sides for any measures.

DISCUSSION AND CONCLUSIONS

Resolution of the dynamic response profile has the potential to increase our understanding of the cerebrovascular control mechanisms and test cerebrovascular response to physical therapy-driven interventions such as exercise.Video Abstract available for more insights from the authors (see the Video, Supplemental Digital Content 1, available at: http://links.lww.com/JNPT/A284).

The influence of age and sex on cerebrovascular reactivity and ventilatory response to hypercapnia in children and adults

NEW FINDINGS

What is the central question of this study? In this study, we investigated intracranial cerebrovascular and ventilatory reactivity to 6% CO₂ in children and adults and explored dynamic ventilatory and cerebrovascular onset responses. What is the main finding and its importance? We showed that cerebrovascular reactivity was similar in children and adults, but the intracranial blood velocity onset response was markedly attenuated in children. Sex differences were apparent, with greater increases in intracranial blood velocity in females and lower ventilatory reactivity in adult females. Our study confirms the importance of investigating dynamic onset responses when assessing the influence of development on cerebrovascular regulation.

ABSTRACT

The purpose of this study was to compare the integrated intracranial cerebrovascular reactivity (CVR) and hypercapnic ventilatory response between children and adults and to explore the dynamic response of the middle cerebral artery mean velocity (MCA_V ). Children (n = 20; 9.9 ± 0.7 years of age) and adults (n = 21; 24.4 ± 2.0 years of age) completed assessment of CVR over 240 s using a fixed fraction of inspired CO₂ (0.06). Baseline MCA_V was higher in the adult females compared with the males (P ≤ 0.05). The MCA_V was greater in female children compared with male children (P ≤ 0.05) and in female adults compared with male adults (P ≤ 0.05) with hypercapnia. Relative CVR was similar in children and adults (3.71 ± 1.06 versus 4.12 ± 1.32% mmHg^-1 ; P = 0.098), with absolute CVR being higher in adult females than males (3.27 ± 0.86 versus 2.53 ± 0.70 cm s^-1  mmHg^-1 ; P ≤ 0.001). Likewise, the hypercapnic ventilatory response did not differ between the children and adults (1.89 ± 1.00 versus 1.77 ± 1.34 l min^-1  mmHg^-1 ; P = 0.597), but was lower in adult females than males (1.815 ± 0.37 versus 2.33 ± 1.66 l min^-1  mmHg^-1 ; P ≤ 0.05). The heart rate response to hypercapnia was greater in children than in adults (P = 0.001). A monoexponential regression model was used to characterize the dynamic onset, consisting of a delay term, amplitude and time constant (τ). The results revealed that MCA_V τ was faster in adults than in children (34 ± 18 versus 74 ± 28 s; P = 0.001). Our study provides new insight into the impact of age and sex on CVR and the dynamic response of the MCA_V to hypercapnia.

Cardiovascular disease risk is associated with middle cerebral artery blood flow velocity in older adults

PURPOSE

The aim of this study was to evaluate the relationship of cardiovascular disease (CVD) on middle cerebral blood flow velocity (MCAv) at rest and during exercise. A secondary aim was to explore the relationship between MCAv and 1) the presence of white matter lesions and 2) cognitive function.

METHODS

We recruited individuals who were cognitively normal older adults. CVD risk was assessed by the Pooled Cohort atherosclerotic cardiovascular disease (ASCVD) risk score. Transcranial Doppler ultrasound measured middle cerebral artery at rest and during a bout of moderate intensity exercise. We quantified white matter lesions from MRI and cognitive function outcomes included executive function, language, processing speed, and attention.

RESULTS

Seventy-two participants 70.1 ± 4.7 years of age completed the study protocol. ASCVD risk score was significantly associated with resting and exercise MCAv (p<0.01) but not associated with white matter lesions (p>0.468). We observed a significant association between resting and exercise MCAv and language processing (p=0.010) but not other cognitive domains.

CONCLUSION

In cognitively normal older adults, higher ASCVD risk score was associated with blunted resting and exercise MCAv and with lower language processing performance. These results highlight the need for CVD risk management to maintain optimal brain health.

Cerebrovascular Pulsatility During Rest and Exercise Reflects Hemodynamic Impairment in Stroke and Cerebral Small Vessel Disease

Although aerobic exercise is recommended as a core component of stroke rehabilitation, knowledge of acute cerebrovascular responses in patients is limited. This study tested the hypothesis that older adults with chronic stroke or cerebral small vessel disease (SVD) exhibit a greater increase in pulsatile hemodynamics during exercise compared with young and age-matched healthy adults. Middle cerebral artery blood flow velocity was acquired during 20 min of moderate intensity cycling in 51 participants from four groups (young, old, SVD and stroke). During rest, only the stroke group had a higher pulsatility index (PI) compared with the young group (1.02 ± 0.17 vs 0.83 ± 0.13; p = 0.038). During exercise, however, the SVD group exhibited a larger increase in PI (68 ± 20% relative to rest) than the young (47 ± 19%), old (45 ± 17%) and stroke (40 ± 25%) groups (p < 0.05, for each). The stress of aerobic exercise may reveal arterial dysfunction associated with latent and overt cerebrovascular disease.

Middle cerebral artery velocity dynamic response profile during exercise is attenuated following multiple ischemic strokes: a case report

Blood flow regulation is impaired in people with stroke. However, the time course of change in middle cerebral artery velocity (MCAv) following repeated stroke at rest and during exercise remains unknown. In this case study, we provide novel characterization of the dynamic kinetic MCAv response profile to moderate-intensity exercise before and after repeated ischemic MCA stroke. The initial stroke occurred in the left MCA. At 3 months poststroke, left MCAv amplitude (Amp) was ~50% lower than the right. At the 6-month follow-up visit, MCAv Amp declined in both MCA with the left MCAv Amp ~50% lower than the right MCAv Amp. Following a second right MCA stroke, we report further decline in Amp for the left MCA. At the 3- and 6-month visit following the second stroke, the left MCAv Amp declined further (~10%). The right MCAv Amp dramatically decreased by 81.3% when compared to the initial study visit. The MCAv kinetic analysis revealed a marked impairment in the cerebrovascular response to exercise following stroke. We discuss potential pathophysiological mechanisms contributing to poststroke cerebrovascular dysfunction and the need to test therapeutic interventions (such as exercise) that might attenuate cerebrovascular decline in people following stroke.

Swimming-related effects on cerebrovascular and cognitive function

Both acute and regular exercise influence vascular and cognitive function. Upright aquatic exercise increases mean middle cerebral artery blood velocity (MCAvmean ) and has been suggested as favorable for cerebrovascular adaptations. However, MCAvmean has not been reported during swimming. Thus, we examined the cerebrovascular and cognitive effects of swimming. Ten land-based athletes (22 ± 5 years) and eight swimmers (19 ± 1 years) completed three cognitive tasks and four conditions that were used to independently and collectively delineate the swimming-related factors (i.e., posture, immersion, CO2 retention [end-tidal CO2 ; PETCO2 ], and motor involvement). Measurements of MCAvmean and PETCO2 were taken throughout each condition. Prone posture increased MCAvmean by 11% (P < 0.01 vs. upright land). Water immersion independently increased MCAvmean when upright (12%; P < 0.01) but not prone (P = 0.76). The consequent rise in PETCO2 during head-out, breast-stroke swimming (50% heart rate range) independently increased MCAvmean by 14% (P < 0.01), while the motor involvement of swimming per se did not significantly change MCAvmean (P = 0.32). While accounting for sex, swimmers had ~17% lower MCAvmean during all rest conditions (P ≤ 0.05). However, in a subset of participants, both groups had similar internal carotid artery diameters (P = 0.99) and velocities (P = 0.97). Water immersion per se did not alter cognition (P ≥ 0.15), but 20 min of moderate-intensity swimming improved visuomotor performance by 4% (P = 0.03), regardless of athlete group (P = 0.12). In conclusion, breast-stroke swimming increased MCAvmean mostly due to postural and PETCO2 effects, with minimal contributions from water immersion or motor activity. Lastly, swimming improved cognitive functioning acutely, regardless of athlete group. Future research should explore the chronic effects of swimming on cerebrovascular function and cognition, particularly in aging.

Intracranial Vascular Responses to High-Intensity Interval Exercise and Moderate-Intensity Steady-State Exercise in Children

PURPOSE

To understand the extent different types of acute exercise influence cerebral blood flow during and following exercise in children.

METHODS

Eight children (7-11 y; 4 girls) completed 2 conditions: high-intensity interval exercise (HIIE; 6 × 1-min sprints at 90% watt maximum) and moderate-intensity steady-state exercise (MISS; 15 min at 44% watt maximum). Blood velocity in the middle cerebral artery (MCAV) and heart rate were assessed continuously. The partial pressure of end-tidal carbon dioxide and mean arterial pressure were assessed at baseline and following exercise.

RESULTS

Percentage of maximum heart rate during HIIE was 82% (4%), compared with 69% (4%) during MISS. MCAV was increased above baseline in MISS after 75 seconds (5.8% [3.9%], P × .004) but was unchanged during HIIE. MCAV was reduced below baseline (-10.7% [4.1%], P × .004) during the sixth sprint of HIIE. In both conditions, MCAV remained below baseline postexercise, but returned to baseline values 30-minute postexercise (P < .001). A postexercise increase in mean arterial pressure was apparent following HIIE and MISS, and persisted 30-minute postexercise. Partial pressure of end-tidal carbon dioxide declined post HIIE (-3.4 mm Hg, P < .05), but not following MISS.

CONCLUSION

These preliminary findings show HIIE and MISS elicit differing intracranial vascular responses; however, research is needed to elucidate the implications and underlying regulatory mechanisms of these responses.

Exercise intensity and middle cerebral artery dynamics in humans

Despite its necessity for understanding healthy brain aging, the influence of exercise intensity on cerebrovascular kinetics is currently unknown. We, therefore characterized middle cerebral artery blood flow velocity (MCAv) kinetics associated with two exercise intensities: low and moderate. We hypothesized that increasing exercise intensity would increase the MCAv amplitude response (Amp) and that age and estimated fitness (V̇O2max) would be related to Amp. Baseline (BL) values were collected for 90-seconds followed by a 6-minute exercise bout. Heart rate, end-tidal CO2, mean arterial pressure and MCAv were recorded throughout. MCAv kinetics were described by Amp, time delay (TD) and time constant (τ). Sixty-four adults completed the study. Amp was greater during moderate compared to low exercise intensity (p < 0.001) while no difference was observed in either TD (p = 0.65) or τ (p = 0.47). Amp was negatively associated with age (p < 0.01) and positively correlated with estimated V̇O2max (p < 0.01). Although Amp declines with age, maintaining higher V̇O2max may benefit the cerebrovascular response to exercise.

Blunted cerebrovascular response is associated with elevated beta-amyloid

The goal of this study was to explore the association of beta-amyloid accumulation and cerebrovascular response (CVR) in cognitively normal older adults. Beta-amyloid accumulation was characterized with [18F] Florbetapir positron emission tomography scans. CVR was calculated as middle cerebral artery blood flow velocity change from rest to moderate intensity exercise. We found that individuals with elevated beta-amyloid aggregation had a blunted CVR ( n = 25, age 70.1 ± 4.8; 3.3 ± 3.7 cm/s) compared to non-elevated individuals ( n = 45, age 72.0 ± 4.9; 7.2 ± 5.0 cm/s, p < 0.001). Further, greater beta-amyloid burden was linearly associated with less CVR across all participants (b = -11.7, p < 0.001). Greater CVR and less beta-amyloid burden were associated with processing speed ( p < 0.05). This study is the first to show that CVR from rest to exercise is blunted across increased global beta-amyloid burden.

Effect of healthy aging and sex on middle cerebral artery blood velocity dynamics during moderate-intensity exercise

Blood velocity measured in the middle cerebral artery (MCA_V) increases with finite kinetics during moderate-intensity exercise, and the amplitude and dynamics of the response provide invaluable insights into the controlling mechanisms. The MCA_V response after exercise onset is well fit to an exponential model in young individuals but remains to be characterized in their older counterparts. The responsiveness of vasomotor control degrades with advancing age, especially in skeletal muscle. We tested the hypothesis that older subjects would evince a slower and reduced MCA_V response to exercise. Twenty-nine healthy young (25 ± 1 yr old) and older (69 ± 1 yr old) adults each performed a rapid transition from rest to moderate-intensity exercise on a recumbent stepper. Resting MCA_V was lower in older than young subjects (47 ± 2 vs. 64 ± 3 cm/s, P < 0.001), and amplitude from rest to steady-state exercise was lower in older than young subjects (12 ± 2 vs. 18 ± 3 cm/s, P = 0.04), even after subjects were matched for work rate. As hypothesized, the time constant was significantly longer (slower) in the older than young subjects (51 ± 10 vs. 31 ± 4 s, P = 0.03), driven primarily by older women. Neither age-related differences in fitness, end-tidal CO₂, nor blood pressure could account for this effect. Thus, MCA_V kinetic analyses revealed a marked impairment in the cerebrovascular response to exercise in older individuals. Kinetic analysis offers a novel approach to evaluate the efficacy of therapeutic interventions for improving cerebrovascular function in elderly and patient populations. NEW & NOTEWORTHY Understanding the dynamic cerebrovascular response to exercise has provided insights into sex-related cerebrovascular control mechanisms throughout the aging process. We report novel differences in the kinetics response of cerebrovascular blood velocity after the onset of moderate-intensity exercise. The exponential increase in brain blood flow from rest to exercise revealed that 1) the kinetics profile of the older group was blunted compared with their young counterparts and 2) the older women demonstrated a slowed response.