Endothelial function and white matter hyperintensities in older adults with cardiovascular disease

Integration of cerebrovascular CO2 reactivity and chemoreflex control of breathing: mechanisms of regulation, measurement, and interpretation

Cerebral blood flow (CBF) and its distribution are highly sensitive to changes in the partial pressure of arterial CO(2) (Pa(CO(2))). This physiological response, termed cerebrovascular CO(2) reactivity, is a vital homeostatic function that helps regulate and maintain central pH and, therefore, affects the respiratory central chemoreceptor stimulus. CBF increases with hypercapnia to wash out CO(2) from brain tissue, thereby attenuating the rise in central Pco(2), whereas hypocapnia causes cerebral vasoconstriction, which reduces CBF and attenuates the fall of brain tissue Pco(2). Cerebrovascular reactivity and ventilatory response to Pa(CO(2)) are therefore tightly linked, so that the regulation of CBF has an important role in stabilizing breathing during fluctuating levels of chemical stimuli. Indeed, recent reports indicate that cerebrovascular responsiveness to CO(2), primarily via its effects at the level of the central chemoreceptors, is an important determinant of eupneic and hypercapnic ventilatory responsiveness in otherwise healthy humans during wakefulness, sleep, and exercise and at high altitude. In particular, reductions in cerebrovascular responsiveness to CO(2) that provoke an increase in the gain of the chemoreflex control of breathing may underpin breathing instability during central sleep apnea in patients with congestive heart failure and on ascent to high altitude. In this review, we summarize the major factors that regulate CBF to emphasize the integrated mechanisms, in addition to Pa(CO(2)), that control CBF. We discuss in detail the assessment and interpretation of cerebrovascular reactivity to CO(2). Next, we provide a detailed update on the integration of the role of cerebrovascular CO(2) reactivity and CBF in regulation of chemoreflex control of breathing in health and disease. Finally, we describe the use of a newly developed steady-state modeling approach to examine the effects of changes in CBF on the chemoreflex control of breathing and suggest avenues for future research.

Vascular and cognitive functions associated with cardiovascular disease in the elderly

This study examines the relationship between systemic vascular function, neurocognitive performance, and structural brain abnormalities on magnetic resonance imaging (MRI) among geriatric outpatients with treated, stable cardiovascular disease and no history of neurological illness (n = 88, ages 56-85 years). Vascular function was assessed by cardiac ejection fraction and output, sequential systolic and diastolic blood pressures, flow mediated brachial artery reactivity (BAR), and carotid intima media thickness (IMT). White matter hyperintensities (WMH) on MRI were quantified and examined relative to cognitive and vascular function. Principal component analysis revealed two primary vascular components: one associated with cardiac function, the other with atherosclerotic burden/endothelial dysfunction. Both factors were significantly associated with cognitive function and WMH volume. Reduced systolic variability and increased IMT were most strongly related to reduced attention, executive function, and information-processing speed. These findings suggest the possibility that systemic vascular indices may provide proxy measures of cerebrovascular dysfunction and reinforce the importance of achieving greater understanding of interaction between systemic vascular disease and brain dysfunction among elderly people with cardiovascular disease.

White matter hyperintensities and medication adherence

White matter hyperintensities (WMH) are associated with hypertension, age, and cognitive function, but the association between WMH and medication adherence has not been examined. The intent of this investigation was to consider the potential implications of hypertension-related brain morphological changes on medication adherence and thereby improve understanding of the self-management consequences of hypertension. The associations between WMH, blood pressure, age, cognitive function (specifically assessments of prefrontal function), and medication adherence were examined in 16 middle-aged and older adults self-managing at least one prescribed antihypertensive agent. Magnetic resonance imaging using an axial fluid attenuated inversion recovery (FLAIR) sequence was used to assess the presence of WMH. Cognitive assessments included measures of executive function, working memory, attention, and immediate recall. Adherence was monitored for 8 weeks using electronic medication monitoring. More WMH were associated with poorer adherence (rs = -.25) and with higher systolic blood pressure (rs = .46), although these relationships were not statistically significant. WMH were associated with cognitive assessments in the expected direction including Digit Span Backward (rs = -.53, p < .05). Adherence was associated with immediate memory (rs = .54, p < .05) and inversely associated with failure to maintain set on the Wisconsin Card Sorting Test (WCST; rs = -.61, p < .05). These findings provide preliminary evidence for the association between WMH, assessments of prefrontal function, and medication adherence.

Cerebral ischemia mediates the effect of serum uric acid on cognitive function

BACKGROUND AND PURPOSE

High normal concentrations of serum uric acid (UA) are associated with mild cognitive dysfunction and increased cerebral ischemia as indexed by white matter hyperintensity volumes. We hypothesized that individual differences in white matter hyperintensities mediate the association between UA and mild cognitive dysfunction.

METHODS

One hundred eighty community-dwelling adults aged 20 to 96 years completed neuropsychological testing, laboratory blood studies, and a brain MRI scan.

RESULTS

Serum UA was associated (P<0.05) with greater white matter hyperintensities and poorer working memory, processing speed, fluency, and verbal memory. Associations remained after controlling for age, sex, race, education, hypertension, diabetes, alcohol abuse, smoking, and body mass. Adding a term for white matter hyperintensity attenuated these associations such that UA no longer predicted cognitive performance.

CONCLUSIONS

Severity of cerebral ischemia might mediate the association between UA and cognitive dysfunction. Even mild elevations in UA appear to contribute to structural and functional brain changes.

Elevation in cerebral blood flow velocity with aerobic fitness throughout healthy human ageing

It is known that cerebral blood flow declines with age in sedentary adults, although previous studies have involved small sample sizes, making the exact estimate of decline imprecise and the effects of possible moderator variables unknown. Animal studies indicate that aerobic exercise can elevate cerebral blood flow; however, this possibility has not been examined in humans. We examined how regular aerobic exercise affects the age-related decline in blood flow velocity in the middle cerebral artery (MCAv) in healthy humans. Maximal oxygen consumption, body mass index (BMI), blood pressure and MCAv were measured in healthy sedentary (n = 153) and endurance-trained (n = 154) men aged between 18 and 79 years. The relationships between age, training status, BMI and MCAv were examined using analysis of covariance methods. Mean +/- s.e.m. estimates of regression coefficients and 95% confidence intervals (95% CI) were calculated. The age-related decline in MCAv was -0.76 +/- 0.04 cm s(-1) year(-1) (95% CI = -0.69 to -0.83, r(2) = 0.66, P < 0.0005) and was independent of training status (P = 0.65). Nevertheless, MCAv was consistently elevated by 9.1 +/- 3.3 cm s(-1) (CI = 2.7-15.6, P = 0.006) in endurance-trained men throughout the age range. This approximately 17% difference between trained and sedentary men amounted to an approximate 10 year reduction in MCAv 'age' and was robust to between-group differences in BMI and blood pressure. Regular aerobic-endurance exercise is associated with higher MCAv in men aged 18-79 years. The persistence of this finding in older endurance-trained men may therefore help explain why there is a lower risk of cerebrovascular disease in this population.

Vascular smooth muscle function is associated with initiation and processing speed in patients with atherosclerotic vascular disease

We previously reported a relationship between forearm resistance vessel function and global neuropsychological performance in patients with atherosclerotic vascular disease (AVD). This study was conducted to determine the relationships among vascular smooth muscle function, endothelial function, and initiation and processing speed in this sample. Participants were 80 individuals with AVD. Resistance vessel function was measured before and after infusion of vasoactive agents. Neuropsychological assessment included measures of estimated premorbid cognitive function, current global cognitive function, initiation, and processing speed. Vascular smooth muscle function was significantly associated with the initiation/processing speed composite score [R-Square Change = .152; F Change (1,71) = 16.61; p < .001], above and beyond the variance accounted for by age, education, premorbid cognitive function, and endothelium-dependent vascular function. This relationship remained significant when controlling for current level of global cognitive functioning and 10 vascular risk factors. Endothelium-dependent vascular function was not significantly associated with test performance. Decreased vascular smooth muscle function in forearm resistance vessels was significantly associated with relatively poor initiation and processing speed in individuals with AVD. With additional research, measures of vascular function might become useful in the early identification of those individuals at greatest risk for vascular-related cognitive dysfunction.

Vascular Health and Cognitive Function in Older Adults with Cardiovascular Disease

BACKGROUND: We hypothesized that changes in vascular flow dynamics resulting from age and cardiovascular disease (CVD) would correlate to neurocognitive capacities, even in adults screened to exclude dementia and neurological disease. We studied endothelial-dependent as well as endothelial-independent brachial responses in older adults with CVD to study the associations of vascular responses with cognition. Comprehensive neurocognitive testing was used to discern which specific cognitive domain(s) correlated to the vascular responses. METHODS: Eighty-eight independent, community-dwelling older adults (70.02+7.67 years) with mild to severe CVD were recruited. Enrollees were thoroughly screened to exclude neurological disease and dementia. Flow-mediated (endothelial-dependent) and nitroglycerin-mediated (endothelial-independent) brachial artery responses were assessed using 2-d ultrasound. Cognitive functioning was assessed using comprehensive neuropsychological testing. Linear regression analyses were used to evaluate the relationships between the endothelial-dependent and endothelial-independent vascular flow dynamics and specific domains of neurocognitive function. RESULTS: Endothelial-dependent and endothelial-independent brachial artery responses both correlated with neurocognitive testing indices. The strongest independent relationship was between endothelial function and measures of attention-executive functioning. CONCLUSIONS: Endothelial-dependent and endothelial-independent vascular responsiveness correlate with neurocognitive performance among older CVD patients, particularly in the attention-executive domain. While further study is needed to substantiate causal relationships, our data demonstrate that brachial responses serve as important markers of risk for common neurocognitive changes. Learning and behavior-modifying therapeutic strategies that compensate for such common, insidious neurocognitive limitations will likely improve caregiving efficacy.

Alzheimer’s disease in late-life dementia: A minor toxic consequence of devastating cerebrovascular dysfunction

Alzheimer's disease (AD) is thought to be the most common cause of late-life dementia. But pure AD is infrequent whereas AD pathology is often insufficient to explain dementia in the elderly. Conversely, cerebrovascular disease is omnipresent and the crucial role of microvascular alterations increasingly recognized in late dementia or "Alzheimer syndrome". Pathomechanisms of vascular cognitive impairment are still debated but recent data indicate that the initial concept of chronic low grade cerebral hypoxia should not have been abandoned. Thus, it is proposed that windkessel dysfunction is the missing link between vascular and craniospinal senescence on the one hand, and chronic low grade cerebral hypoxia, "senile brain degeneration" and "Alzheimer syndrome" on the other hand. An age-related decrease in the buffering capacity of both the vessels and the craniospinal cavity favours cerebral hypoxia; due to increased capillary pulsatility with disturbances in capillary exchanges or due to a marked reduction in craniospinal compliance with a mechanical reduction in cerebral arterial inflow. "Invisible" windkessel dysfunction, most often related to "hardening of the arteries" may be the most frequent pathomechanism of late-onset dementia whereas associated mild or moderate AD may be merely a toxic manifestation of a primarily hypoxic disease. Structural patterns of arteriosclerotic dementia fit well with an underlying arterial windkessel dysfunction: with secondary mechanical damage to the cerebral small vessels and the brain and predominantly deep hypoxia. The clinical significance of leukoaraïosis, small foci of necrosis, ventricular dilatation, hippocampal and cortical atrophy is in good agreement with their value as indirect markers of windkessel dysfunction. An age-related "invisible" reduction in craniospinal compliance may also contribute to the associations between heart failure, arterial hypotension and cognitive impairment in the elderly and to the high percentage of dementia of unknown origin in the very old. Both neuropathological and clinical overlap between AD and windkessel dysfunction can explain that cerebrovascular dysfunction remains misdiagnosed for AD in the elderly. Evidence of the key role of cerebrovascular dysfunction should markedly facilitate and widen therapeutic research in late-life dementia. Routine MRI including direct assessment of intracranial dynamics should be increasingly used to define etiological subtypes of the "Alzheimer syndrome" and develop a well-targeted therapeutic strategy.

MRI of the 'Alzheimer syndrome'

Interest in the identification of cognitive decline in its earliest manifestations and the heterogeneity of clinically diagnosed Alzheimer's disease (AD) explain the growing number of neuroimaging studies of AD. Alzheimer-type lesions are associated with loss of neurons, and magnetic resonance imaging (MRI) can detect predominantly left atrophic changes in the entorhinal cortex, amygdala and anterior hippocampus several years before the onset of clinical symptoms. Cerebrovascular disease can mimic AD in the elderly whereas MR markers of subcortical vascular disease-leukoaraiosis, lacunar infarcts, microbleeds, ventricular enlargement, cortical and hippocampal atrophy-appear to be structural changes associated with vascular-related cognitive impairment. Furthermore, analysis of prodromal forms of late-onset dementia of Alzheimer's type (DAT) differentiates amnesic single-domain mild cognitive impairment, which shows MR patterns similar to those observed in early-onset DAT, from other predementia patterns without atrophy at the earliest sites of AD pathology. Mesiotemporal atrophy on MRI predicts late-onset DAT, but the current rating scales or measurements of mesiotemporal atrophy do not differentiate anteromesial temporal atrophy that is highly suggestive of AD from predominantly hippocampal atrophy, suggestive of non-AD damage and, usually, vascular disease. The other, most common MRI predictors of late-onset DAT may be considered indirect markers of arterial senescence whereas brain atrophy is diffusely milder and MR markers of small-vessel disease more frequent in late-onset, compared with early-onset, DAT. Thus, MRI suggests an overestimation of AD pathology while underestimating 'arteriosclerotic brain degeneration' in the clinical picture of 'Alzheimer syndrome'.