The effect of age and sex on intracranial and extracranial cerebrovascular function is poorly understood. We investigated the relationships between age, sex and cerebrovascular reactivity (CVR) to hypercapnia in 73 healthy adults (18-80 years, N=39 female). CVR to hypercapnia was assessed in the middle cerebral artery (MCA) using transcranial Doppler ultrasound and at the internal carotid artery (ICA) using duplex ultrasound. MCA CVR was characterised by peak MCA velocity (MCAv) response per mmHg increase in end-tidal CO2, and by using a mono-exponential model to characterize the kinetics (time-constant) of the MCAv response. ICA reactivity was assessed as the relative peak increase in artery diameter. Hierarchical multiple regression determined the relationships between age, sex, and the age by sex interaction on all baseline and CVR outcomes. There was no relationship between ICA reactivity (%) with age (P=0.07), sex (P=0.56) or a moderator effect of sex on the age effect (P=0.24). MCAv CVR showed no relationship with age (P=0.59), sex (P=0.09), or an age by sex moderator effect (P=0.90). We observed a positive relationship of MCAv CVR time-constant with age (P=0.013), such that the speed of the MCA response was slower with advancing age. The present study provides comprehensive data on age and sex specific relationships with intracranial and extracranial cerebrovascular responses to hypercapnia. Despite similar MCAv CVR and ICA reactivity between sexes, kinetic responses of the MCA revealed a slower rate of adjustment with advancing age.

Cerebrovascular reactivity of the middle cerebral artery velocity (CVR MCAv) to carbon dioxide (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 four minutes 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 hours 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 second, peak 30 second) (between-day: P>0.14, ηp20.22, d>0.27). Absolute, rather than relative CVR, yielded the most reproducible parameters (coefficient of variation: 8.1-13.2% versus 14-83% respectively). There were significant differences between CVR outcomes (P0.89) 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 (kappa=0.15, P=0.09). No mean differences were present for novel kinetic outcomes (amplitude, time-delay, time constant) (between-day: P>0.05, d0.38, d

New Findings: What is the central question of this study? We sought to investigate whether peripheral and cerebrovascular function are impaired in early and late postmenopausal females compared with premenopausal females, while also accounting for nitric oxide and estradiol levels. What is the main finding and its importance? We observed no differences in peripheral vascular and cerebrovascular function between healthy and physically active premenopausal females and early and late postmenopausal females. Our findings contradict previous cross-sectional observations of vascular and cerebrovascular dysfunction across menopause. Longitudinal studies assessing vascular and cerebrovascular outcomes across the menopausal transition are warranted. Abstract: the risk of cardiovascular and cerebrovascular disease increases in ageing females, coinciding with the onset of menopause. Differences in peripheral and cerebrovascular function across menopausal stages, however, are poorly characterized. The aim of this study was to compare peripheral and cerebrovascular function between healthy premenopausal (PRE), early (1–6 years after final menstrual period; E-POST) and late (>6 years after final menstrual period; L-POST) postmenopausal females. We also explored the association between reproductive hormones, NO bioavailability and cerebrovascular function. In 39 females (40–65 years of age), we measured arterial stiffness, brachial artery flow-mediated dilatation, and cerebrovascular reactivity (CVR) to hypercapnia in the middle (MCAv) and internal (ICA) carotid arteries. Follicle-stimulating hormone, estradiol, progesterone and plasma nitrate and nitrite concentrations were also measured. Years since final menstrual period (PRE, 0 ± 0 years; E-POST, 3 ± 1 years; L-POST, 11 ± 4 years; P 

Understanding the contribution of the autonomic nervous system to cerebral blood flow (CBF) control is challenging, and interpretations are unclear. The identification of calcium channels and adrenoreceptors within cerebral vessels has led to common misconceptions that the function of these receptors and actions mirror those of the peripheral vasculature. This review outlines the fundamental differences and complex actions of cerebral autonomic activation compared with the peripheral circulation. Anatomical differences, including the closed nature of the cerebrovasculature, and differential adrenoreceptor subtypes, density, distribution and sensitivity, provide evidence that measures on peripheral sympathetic nerve activity cannot be extrapolated to the cerebrovasculature. Cerebral sympathetic nerve activity seems to act opposingly to the peripheral circulation, mediated at least in part by changes in intracranial pressure and cerebral blood volume. Additionally, heterogeneity in cerebral adrenoreceptor distribution highlights region-specific autonomic regulation of CBF. Compensatory chemo- and autoregulatory responses throughout the cerebral circulation, and interactions with parasympathetic nerve activity are unique features to the cerebral circulation. This crosstalk between sympathetic and parasympathetic reflexes acts to ensure adequate perfusion of CBF to rising and falling perfusion pressures, optimizing delivery of oxygen and nutrients to the brain, while attempting to maintain blood volume and intracranial pressure. Herein, we highlight the distinct similarities and differences between autonomic control of cerebral and peripheral blood flow, and the regional specificity of sympathetic and parasympathetic regulation within the cerebrovasculature. Future research directions are outlined with the goal to further our understanding of autonomic control of CBF in humans.

The acute effect of exercise intensity on cerebrovascular reactivity, and whether this mirrors changes in peripheral vascular function, has not been investigated. The aim of this study was to explore the acute effect of exercise intensity on cerebrovascular reactivity (CVR) and peripheral vascular function in healthy young adults (n=10, 6 females, 22.7 ± 3.5 years). Participants completed four experimental conditions on separate days: high intensity interval exercise (HIIE) with intervals performed at 75% maximal oxygen uptake (V̇O2max; HIIE1), HIIE with intervals performed at 90% V̇O2max (HIIE2), continuous moderate intensity exercise (MIE) at 60% V̇O2max and a sedentary control condition (CON). All exercise conditions were completed on a cycle ergometer and matched for time (30 min) and average intensity (60% V̇O2max). Brachial artery flow-mediated dilation (FMD) and CVR of the middle cerebral artery were measured before exercise, and one- and three hours post-exercise. CVR was assessed using transcranial Doppler ultrasonography to both hypercapnia (6% carbon dioxide breathing) and hypocapnia (hyperventilation). FMD was significantly elevated above baseline one and three hours following both HIIE conditions (P0.33). CVR to both hypercapnia and hypocapnia, and when expressed across the end-tidal CO2 range, was unchanged in all conditions, at all time points (all P>0.14). In conclusion, these novel findings show that the acute increases in peripheral vascular function following HIIE, compared to MIE, were not mirrored by changes in cerebrovascular reactivity, which was unaltered following all exercise conditions in healthy young adults.

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

Cardiovascular diseases (CVD) are the leading cause of non-communicable diseases worldwide, with the underlying atherosclerotic process originating in youth. Children and adolescents with CVD risk factors have impaired endothelial function, which is implicated in the process of atherosclerosis. Habitual sugar sweetened beverage (SSB) consumption is associated with the progression of CVD risk factors in youth, and adolescents consume the highest quantities of SSBs. Acute SSB consumption results in vascular dysfunction in adults, though the effects in youth are unknown. It is thought that exposure to CVD risk factors in youth may impair cerebrovascular reactivity (CVR), possibly having implications for future CVD risk. It is also unknown whether the types of sugar in SSBs have different consequences on vascular function. This thesis aimed to investigate the effect of sugar moiety on cerebrovascular function in adolescents, following consumption of a sugary drink and subsequent meal. Data on the reliability of CVR in a paediatric population was needed to first establish if this was a reliable measure of endothelial function. The purpose of this thesis was to: 1) examine the within and between-day reliability of a breath-hold protocol to assess CVR in adolescents. 2) examine the acute effect of sugar moiety (fructose, sucrose, glucose) on CVR and putative blood outcomes, and 3) examine the effects of SSB consumption on postprandial health in adolescents. Chapter 3 examined the reliability of a breath-hold protocol to assess CVR in youth, determined via transcranial Doppler ultrasonography of the middle cerebral artery (MCA). CVR was calculated as the percentage increase in MCAv mean following three breath-hold attempts. This outcome yielded acceptable levels of within and between-day reliability for use in multiple visit experiments to assess CVR in adolescents. Chapter 4 investigated the effect of sugar moiety on cerebrovascular function, measured through breath-hold induced CVR, in adolescents following SSB consumption and a subsequent challenge meal. This study found that the glucose and sucrose drinks resulted in elevated blood glucose levels compared to fructose and water. With consumption of fructose, elevations in uric acid were present, however the sugar moieties all presented similar increases in TAG concentrations following meal consumption. Despite these different metabolic responses, no significant impairments in CVR were present following the drink or challenge meal.
This thesis demonstrated that consumption of SSBs led to increases in glucose and uric acid concentrations, which have previously been shown to be atherogenic. This thesis also provided data on the reliability of CVR as a non-invasive and easy to administer tool for measurement of endothelial function in youth. This is the first study to demonstrate that breath-hold induced CVR can be reliably measured in youth, as a practical, affordable and non-invasive method. These findings provide valuable data that will inform the implementation and analysis of a breath-hold protocol for reliable CVR assessment in youth in future research. Having established that CVR was reliable within and between-day, it was not possible to determine if it was sensitive to change, with no effects seen on CVR following acute SSB consumption. To build on these findings, future research should explore the acute and chronic effects of SSB consumption, with consideration of measuring a range of different vascular outcomes such as changes in peripheral microvascular and macrovascular functions. As this thesis did not include another measure of peripheral endothelial function, it is not certain whether endothelial function was impaired, or if CVR was not sensitive to change in the present study. In order to determine if CVR is sensitive to change, future investigation is needed with established measures of peripheral endothelial function (i.e. flow mediated dilation) alongside measures of CVR.