Normal cerebral vascular pulsations in humans: changes with age and implications for microvascular disease

Pulsatile microvascular cerebral blood flow waveforms change with intracranial compliance and age

Significance

Diffuse correlation spectroscopy (DCS) is an optical method to measure relative changes in cerebral blood flow (rCBF) in the microvasculature. Each heartbeat generates a pulsatile signal with distinct morphological features that we hypothesized to be related to intracranial compliance (ICC).

Aim

We aim to study how three features of the pulsatile rCBF waveforms: the augmentation index (AIx), the pulsatility index, and the area under the curve, change with respect to ICC. We describe ICC as a combination of vascular compliance and extravascular compliance.

Approach

Since patients with Chiari malformations (CM) (n = 30 ) have been shown to have altered extravascular compliance, we compare the morphology of rCBF waveforms in CM patients with age-matched healthy control (n = 30 ).

Results

AIx measured in the supine position was significantly less in patients with CM compared to healthy controls (p < 0.05 ). Since physiologic aging also leads to changes in vessel stiffness and intravascular compliance, we evaluate how the rCBF waveform changes with respect to age and find that the AIx feature was strongly correlated with age (R healthy subjects = - 0.63 , R preoperative   CM   patient = - 0.70 , and R postoperative   CM   patients = - 0.62 , p < 0.01 ).

Conclusions

These results suggest that the AIx measured in the cerebral microvasculature using DCS may be correlated to changes in ICC.

Neurovascular coupling and cerebrovascular hemodynamics are modified by exercise training status at different stages of maturation during youth

Neurovascular coupling (NVC) is mediated via nitric oxide signaling, which is independently influenced by sex hormones and exercise training. Whether exercise training differentially modifies NVC pre- versus postpuberty, where levels of circulating sex hormones will differ greatly within and between sexes, remains to be determined. Therefore, we investigated the influence of exercise training status on resting intracranial hemodynamics and NVC at different stages of maturation. Posterior and middle cerebral artery velocities (PCAv and MCAv) and pulsatility index (PCAPI and MCAPI) were assessed via transcranial Doppler ultrasound at rest and during visual NVC stimuli. N = 121 exercise-trained (males, n = 32; females, n = 32) and untrained (males, n = 28; females, n = 29) participants were characterized as pre (males, n = 33; females, n = 29)- or post (males, n = 27; females, n = 32)-peak height velocity (PHV). Exercise-trained youth demonstrated higher resting MCAv (P = 0.010). Maturity and training status did not affect the ΔPCAv and ΔMCAv during NVC. However, pre-PHV untrained males (19.4 ± 13.5 vs. 6.8 ± 6.0%; P ≤ 0.001) and females (19.3 ± 10.8 vs. 6.4 ± 7.1%; P ≤ 0.001) had a higher ΔPCAPI during NVC than post-PHV untrained counterparts, whereas the ΔPCAPI was similar in pre- and post-PHV trained youth. Pre-PHV untrained males (19.4 ± 13.5 vs. 7.9 ± 6.0%; P ≤ 0.001) and females (19.3 ± 10.8 vs. 11.1 ± 7.3%; P = 0.016) also had a larger ΔPCAPI than their pre-PHV trained counterparts during NVC, but the ΔPCAPI was similar in trained and untrained post-PHV youth. Collectively, our data indicate that exercise training elevates regional cerebral blood velocities during youth, but training-mediated adaptations in NVC are only attainable during early stages of adolescence. Therefore, childhood provides a unique opportunity for exercise-mediated adaptations in NVC.NEW & NOTEWORTHY We report that the change in cerebral blood velocity during a neurovascular coupling task (NVC) is similar in pre- and postpubertal youth, regardless of exercise-training status. However, prepubertal untrained youth demonstrated a greater increase in cerebral blood pulsatility during the NVC task when compared with their trained counterparts. Our findings highlight that childhood represents a unique opportunity for exercise-mediated adaptations in cerebrovascular hemodynamics during NVC, which may confer long-term benefits in cerebrovascular function.

Second systolic peak in fetal middle cerebral artery Doppler after intrauterine transfusion

OBJECTIVE

To evaluate functional relationship between fetal circulatory response to intrauterine transfusion (IUT) as a circulatory challenge and appearance of second systolic peak (P2) in middle cerebral artery (MCA) based on hemodynamic principles.

METHODS

According to the concept of pulse wave (PW) propagation and reflection in adults, PWs arrive twice at cerebral circulation, as primary wave caused by left ventricle ejection and secondary after reflection in peripheral arteries. Thus adults show a biphasic contour of systolic blood flow in cerebral arteries. Similar waveforms may appear in fetal MCA-Doppler, as a response to IUT as a circulatory challenge. This is a proof-of-principle study, applying classical hemodynamic principles to fetal circulation. Accordingly, appearance of MCA-P2 may indicate vasoconstriction with increased PW reflection and timing of P2(Δt) should agree with the additional PW travel time down to reflection and return (Tr). To test this agreement, we searched our database for IUTs performed for severe fetal anemia, and compared Δt, obtained by Doppler, with Tr, obtained by hemodynamic calculation using human fetal data. Level of agreement was assessed using Bland-Altman-Plots.

RESULTS

We identified 21 fetuses with adequate Doppler quality for Δt evaluation. In four cases (19%) MCA-P2 was observed before the intervention, and in 17 interventions (81%) thereafter; a highly significant association between IUT and P2 appearance (p < 0.001). In these 17 interventions good agreement of P2 timing was found between Doppler assessment: Δt = 80 ± 8 ms, and hemodynamic calculation: Tr = 76 ± 4 ms.

CONCLUSION

P2 appearance in fetal MCA-Doppler seems to indicate PW reflection due to increased vasoconstriction after IUT. Thus hemodynamic considerations might enable Doppler monitoring of fetal vasoconstriction.

Pericytes: Intrinsic Transportation Engineers of the CNS Microcirculation

Pericytes in the brain are candidate regulators of microcirculatory blood flow because they are strategically positioned along the microvasculature, contain contractile proteins, respond rapidly to neuronal activation, and synchronize microvascular dynamics and neurovascular coupling within the capillary network. Analyses of mice with defects in pericyte generation demonstrate that pericytes are necessary for the formation of the blood-brain barrier, development of the glymphatic system, immune homeostasis, and white matter function. The development, identity, specialization, and progeny of different subtypes of pericytes, however, remain unclear. Pericytes perform brain-wide 'transportation engineering' functions in the capillary network, instructing, integrating, and coordinating signals within the cellular communicome in the neurovascular unit to efficiently distribute oxygen and nutrients ('goods and services') throughout the microvasculature ('transportation grid'). In this review, we identify emerging challenges in pericyte biology and shed light on potential pericyte-targeted therapeutic strategies.

Renal Dysfunction Is Associated with Middle Cerebral Artery Pulsatility Index and Total Burden of Cerebral Small Vessel Disease

BACKGROUND AND PURPOSE

Renal dysfunction is known to affect vasculature and lead to systemic arterial stiffness. It also independently increases the risk of cerebral small vessel disease (cSVD) and stroke. We aimed to examine the effect of renal dysfunction on cerebral hemodynamics and the burden of cSVD.

METHODS

Of the 412 patients admitted to Seoul National University Hospital, between May 2015 and 2019, with lacunar infarction and no major intracranial arterial stenosis observed on magnetic resonance angiography, we included 283 patients who had undergone a transcranial Doppler (TCD) ultrasound after 72 h of stroke onset. The patients were divided into renal dysfunction (estimated glomerular filtration rate [eGFR] <60 mL/min/1.73 m2 at admission) and control (eGFR ≥60 mL/min/1.73 m2) groups. We investigated the correlations between renal function, the pulsatility index (PI), and the total MRI burden of cSVD. Furthermore, multivariate analysis was performed to assess the association between renal dysfunction and the PI of the middle cerebral artery (MCA) measured through TCD ultrasound.

RESULTS

Among the total patients, 74 (26.1%) had renal dysfunction (eGFR <60 mL/min/1.73 m2 at admission). Patients with renal dysfunction were significantly older, showed higher pulse pressure, and had a higher prevalence of hypertension, diabetes mellitus, and coronary artery disease. Renal dysfunction was significantly associated with higher distal cerebrovascular flow resistance (median PI 1.12, interquartile range [IQR]: 0.85-1.57 vs. controls 0.84, IQR: 0.54-1.22; p < 0.001). Also, patients with renal dysfunction had a significantly higher total MRI burden of cSVD (median cSVD score 2, IQR: 1-3 vs. controls median score 1, IQR: 0-2; p < 0.001). There was an inverse proportional relationship between the PI and eGFR. Finally, multivariate analysis showed renal dysfunction (adjusted odds ratio: 4.516, 95% confidence interval: 1.051-20.292) and older age (adjusted odds ratio: 1.076, 95% confidence interval: 1.038-1.114) as independent predictors of a high PI.

CONCLUSIONS

Renal dysfunction is independently associated with a high PI of MCA. Renal dysfunction leads to systemic arterial stiffness including stiffness in cerebral arteries, thus increasing the burden of cSVD. Therefore, noninvasive screening for high PI by TCD in kidney failure patients might be helpful.

Respiratory cerebrospinal fluid flow is driven by the thoracic and lumbar spinal pressures

KEY POINTS

Respiration plays a key role in the circulation of cerebrospinal fluid (CSF) around the central nervous system. During inspiration increased venous return from the cranium is believed to draw CSF rostrally. However, this mechanism does not explain why CSF has also been observed to move caudally during inspiration. We show that during inspiration decreased intrathoracic pressure draws venous blood from the cranium and lumbar spine towards the thorax. We also show that the abdominal pressure was associated with rostral CSF displacement. However, a caudal shift of cervical CSF was seen with low abdominal pressure and comparably negative intrathoracic pressures. These results suggest that the effects of epidural blood flow within the spinal canal need to be considered, as well as the cranial blood volume balance, to understand respiratory-related CSF flow. These results may prove useful for the treatment of CSF obstructive pathology and understanding the behaviour of intrathecal drug injections.

ABSTRACT

It is accepted that during inspiration, cerebrospinal fluid (CSF) flows rostrally to compensate for decreased cranial blood volume, caused by venous drainage due to negative intrathoracic pressure. However, this mechanism does not explain observations of caudal CSF displacement during inspiration. Determining the drivers of respiratory CSF flow is crucial for understanding the pathophysiology of CSF flow disorders. To quantify the influence of respiration on CSF flow, real-time phase-contrast magnetic resonance imaging (MRI) was used to record CSF and blood flow, while healthy subjects (5:5 M:F, 25-50 years) performed either a brief expiratory or inspiratory effort between breaths. Transverse images were taken perpendicular to the spinal canal in the middle of the C3 and L2 vertebrae. The same manoeuvres were then performed after a nasogastric pressure catheter was used to measure the intrathoracic and abdominal pressures. During expiratory-type manoeuvres that elevated abdominal and intrathoracic pressures, epidural blood flow into the spinal canal increased and CSF was displaced rostrally. With inspiratory manoeuvres, the negative intrathoracic pressure drew venous blood from C3 and L2 towards the thoracic spinal canal, and cervical CSF was displaced both rostrally and caudally, despite the increased venous drainage. Regression analysis showed that rostral displacement of CSF at both C3 (adjusted R²  = 0.53; P < 0.001) and L2 (adjusted R²  = 0.38; P < 0.001) were associated with the abdominal pressure. However, with low abdominal pressure and comparably negative intrathoracic pressure, cervical CSF flowed caudally. These findings suggest that changes in both the cranial and spinal pressures need to be considered to understand respiratory CSF flow.

Evaluation of cerebral autoregulation performance in patients with arterial hypertension on drug treatment

Cerebral autoregulation (AR) keeps cerebral blood flow constant despite fluctuations in systemic arterial pressure. The final common AR pathway is made up of vasomotor adjustments of cerebrovascular resistance mediated by arterioles. Structural and functional changes in the arteriolar wall arise with age and systemic arterial hypertension. This study evaluated whether AR is impaired in hypertensive patients and whether this impairment differs with disease control. Three groups of patients were prospectively compared: hypertensive patients under treatment with systolic blood pressure (SBP) <140 and diastolic blood pressure (DBP) <90 mm Hg (n = 54), hypertensive patients under treatment with SBP > 140 or DBP > 90 mm Hg (n = 31), and normotensive volunteers (n = 30). Simultaneous measurements of cerebral blood flow velocity (CBFV) and BP were obtained by digital plethysmography and transcranial Doppler, and the AR index (ARI) was defined according to the step response to spontaneous fluctuations in BP. Compared to the uncontrolled hypertension, the normotensive individuals were younger (age 43.42 ± 11.14, P < .05) and had a lower resistance-area product (1.17 ± 0.24, P < .05), although age and greater arteriolar stiffness did not affect the CBFV mean of hypertensive patients, whether controlled or uncontrolled (62.85 × 58.49 × 58.30 cm/s, P = .29), most likely because their ARIs were not compromised (5.54 × 5.91 × 5.88, P = .6). Hypertensive patients under treatment, regardless of their BP control, have intact AR capacity.

Dynamic Effects of Aortic Arch Stiffening on Pulsatile Energy Transmission to Cerebral Vasculature as A Determinant of Brain-Heart Coupling

Aortic stiffness increases with age and is a robust predictor of brain pathology including Alzheimer’s and other dementias. Aging causes disproportionate stiffening of the aorta compared with the carotid arteries, reducing protective impedance mismatches at their interface and affecting transmission of destructive pulsatile energy to the cerebral circulation. Recent clinical studies have measured regional stiffness within the aortic arch using pulse wave velocity (PWV) and have found a stronger association with cerebrovascular events than global stiffness measures. However, effects of aortic arch PWV on the transmission of harmful excessive pulsatile energy to the brain is not well-understood. In this study, we use an energy-based analysis of hemodynamic waves to quantify the effect of aortic arch stiffening on transmitted pulsatility to cerebral vasculature, employing a computational approach using a one-dimensional model of the human vascular network. Results show there exists an optimum wave condition—occurring near normal human heart rates—that minimizes pulsatile energy transmission to the brain. This indicates the important role of aortic arch biomechanics on heart-brain coupling. Our results also suggest that energy-based indices of pulsatility combining pressure and flow data are more sensitive to increased stiffness than using flow or pressure pulsatility indices in isolation.

Carotid Flow Augmentation, Arterial Aging, and Cerebral White Matter Hyperintensities

Objective—

Aortic stiffness and pressure wave reflection are associated with age-related cerebral microvascular disease, but the underlying mechanism remains obscure. We hypothesized that cerebral (carotid) flow alterations potentially mediate these associations.

Approach and Results—

Doppler waveforms were recorded in 286 patients with hypertension to measure the carotid flow augmentation index (FAIx) as the late/early-systolic velocity amplitude ratio. Tonometric waveforms were recorded to estimate the aortic pressure AIx (PAIx), aortic compliance, and carotid-femoral and carotid-radial pulse wave velocities. Additionally, white matter hyperintensities on brain magnetic resonance imaging were evaluated using the Fazekas scale. With increasing age, the carotid late systolic velocity increased, whereas the early systolic velocity decreased, although the aortic augmented pressure increased in parallel with the incident wave height ( P <0.001). Both FAIx and PAIx increased with age, but the age-dependent curves were upwardly concave and convex, respectively. FAIx increased exponentially with increasing PAIx ( r =0.71). Compared with PAIx, FAIx was more closely ( P ≤0.001) correlated with the aortic pulse wave velocity, aortic compliance, and elastic/muscular pulse wave velocity ratio. FAIx was associated with white matter hyperintensities scores independently of confounders including age, sex, diabetes mellitus, hypercholesterolemia, and aortic pulse wave velocity ( P =0.01), and was more predictive of white matter hyperintensities presence than PAIx.

Conclusions—

Carotid FAIx had closer associations with age, aortic stiffness, and cerebral white matter hyperintensities than aortic PAIx. These results indicate that carotid flow augmentation (enhanced by aortic stiffening and pressure wave reflection from the lower body) causes microcerebrovascular injury potentially through increasing cerebral flow pulsations, but this detrimental effect is greater than that estimated from PAIx.

Structure and Function of Systemic Arteries: Reflections on the Arterial Pulse

"Structure and Function of Arteries"-is a topic of great importance to those who deal with arterial hypertension, since it links the source of flow, the left ventricle of the heart (whose output is pulsatile) to the peripheral tissues (whose flow is near continuous). The arterial tree acts passively as a conduit and cushion, and the interaction of heart, arterial tree, and organs is conventionally gauged on the basis of blood pressure measured by cuff in a conveniently located place (the brachial artery). For any precision and perspective to be gained, measurements of brachial systolic and diastolic pressure need be supplemented by other information. When such information is gained, one can understand how beautifully the arterial tree is tuned to the beat of the heart in animals of different size and shape and in humans at age 30 through the first third of a 3 billion beat lifetime. After age 30, the beats themselves progressively destroy the human arteries and their tuning to the heart, with emergence of clinical syndromes. In this review, the subject is tackled quantitatively on the basis of published numerical, physical, physiological, and pathophysiological basis, with principal focus on the beat of the heart, the pulse of the arteries, and their interaction.

Watershed microinfarct pathology and cognition in older persons

Brain microinfarcts are common in aging and are associated with cognitive impairment. Anterior and posterior watershed border zones lie at the territories of the anterior, middle, and posterior cerebral arteries, and are more vulnerable to hypoperfusion than brain regions outside the watershed areas. However, little is known about microinfarcts in these regions and how they relate to cognition in aging. Participants from the Rush Memory and Aging Project, a community-based clinical-pathologic study of aging, underwent detailed annual cognitive evaluations. We examined 356 consecutive autopsy cases (mean age-at-death, 91 years [SD = 6.16]; 28% men) for microinfarcts from 3 watershed brain regions (2 anterior and 1 posterior) and 8 brain regions outside the watershed regions. Linear regression models were used to examine the association of cortical watershed microinfarcts with cognition, including global cognition and 5 cognitive domains. Microinfarcts in any region were present in 133 (37%) participants, of which 50 had microinfarcts in watershed regions. Persons with multiple microinfarcts in cortical watershed regions had lower global cognition (estimate = -0.56, standard error (SE) = 0.26, p = 0.03) and lower cognitive function in the specific domains of working memory (estimate = -0.58, SE = 0.27, p = 0.03) and visuospatial abilities (estimate = -0.57, SE = 0.27, p = 0.03), even after controlling for microinfarcts in other brain regions, demographics, and age-related pathologies. Neither the presence nor multiplicity of microinfarcts in brain regions outside the cortical watershed regions were related to global cognition or any of the 5 cognitive domains. These findings suggest that multiple microinfarcts in watershed regions contribute to age-related cognitive impairment.

Cerebral Haemodynamics: Effects of Systemic Arterial Pulsatile Function and Hypertension

PURPOSE OF REVIEW

Concepts of pulsatile arterial haemodynamics, including relationships between oscillatory blood pressure and flow in systemic arteries, arterial stiffness and wave propagation phenomena have provided basic understanding of underlying haemodynamic mechanisms associated with elevated arterial blood pressure as a major factor of cardiovascular risk, particularly the deleterious effects of isolated systolic hypertension in the elderly. This topical review assesses the effects of pulsatility of blood pressure and flow in the systemic arteries on the brain. The review builds on the emerging notion of the "pulsating brain", taking into account the high throughput of blood flow in the cerebral circulation in the presence of mechanisms involved in ensuring efficient and regulated cerebral perfusion.

RECENT FINDINGS

Recent studies have provided evidence of the relevance of pulsatility and hypertension in the following areas: (i) pressure and flow pulsatility and regulation of cerebral blood flow, (ii) cerebral and systemic haemodynamics, hypertension and brain pathologies (cognitive impairment, dementia, Alzheimer's disease), (iii) stroke and cerebral small vessel disease, (iv) cerebral haemodynamics and noninvasive estimation of cerebral vascular impedance, (v) cerebral and systemic pulsatile haemodynamics and intracranial pressure, (iv) response of brain endothelial cells to cyclic mechanical stretch and increase in amyloid burden. Studies to date, producing increasing epidemiological, clinical and experimental evidence, suggest a potentially significant role of systemic haemodynamic pulsatility on structure and function of the brain.