Conduit arterial wave reflection promotes pressure transmission but impedes hydraulic energy transmission to the microvasculature

An in silico study of the effects of cardiovascular aging on carotid flow waveforms and indexes in a virtual population

Cardiovascular aging is strongly associated with increased risk of cardiovascular disease and mortality. Moreover, health and lifestyle factors may accelerate age-induced alterations, such as increased arterial stiffness and wall dilation, beyond chronological age, making the clinical assessment of cardiovascular aging an important prompt for preventative action. Carotid flow waveforms contain information about age-dependent cardiovascular properties, and their ease of measurement via noninvasive Doppler ultrasound (US) makes their analysis a promising tool for the routine assessment of cardiovascular aging. In this work, the impact of different aging processes on carotid waveform morphology and derived indexes is studied in silico, with the aim of establishing the clinical potential of a carotid US-based assessment of cardiovascular aging. One-dimensional (1-D) hemodynamic modeling was employed to generate an age-specific virtual population (VP) of N = 5,160 realistic carotid hemodynamic waveforms. The resulting VP was statistically validated against in vivo aging trends in waveforms and indexes from the literature, and simulated waveforms were studied in relation to age and underlying cardiovascular parameters. In our study, the carotid flow augmentation index (FAI) significantly increased with age (with a median increase of 50% from the youngest to the oldest age group) and was strongly correlated to local arterial stiffening (r = 0.94). The carotid pulsatility index (PI), which showed less pronounced age variation, was inversely correlated with the reflection coefficient at the carotid branching (r = -0.88) and directly correlated with carotid net forward wave energy (r = 0.90), corroborating previous literature where it was linked to increased risk of cerebrovascular damage in the elderly. There was a high correlation between corrected carotid flow time (ccFT) and cardiac output (CO) (r = 0.99), which was not affected by vascular age. This study highlights the potential of carotid waveforms as a valuable tool for the assessment of cardiovascular aging.NEW & NOTEWORTHY An age-specific virtual population was generated based on a 1-D model of the arterial circulation, including newly defined literature-based specific age variations in carotid vessel properties. Simulated carotid flow/velocity waveforms, indexes, and age trends were statistically validated against in vivo data from the literature. A comprehensive study of the impact of aging on carotid flow waveform morphology was performed, and the mechanisms influencing different carotid indexes were elucidated. Notably, flow augmentation index (FAI) was found to be a strong indicator of local carotid stiffness.

Carotid wave analysis in young adults with a history of adolescent anorexia nervosa: a case control study

BACKGROUND

Anorexia nervosa (AN) is associated with abnormalities that may increase the risk of future cardiovascular disease. This study assessed the cardiovascular health of individuals who recovered from AN during adolescence by conducting wave power analysis.

METHODS

Former AN patients discharged from the Royal Children's and Monash Children's Hospitals (N = 17) in Melbourne, Australia underwent ultrasound imaging of the right carotid artery. Wave power analysis was conducted to assess biomechanical interactions of the cardiovascular system. Patient measures were compared to healthy controls (N = 51).

RESULTS

Eighty-eight percent of the former AN patients and controls were female, aged approximately 25 years, with a healthy body mass index. Mean carotid flow and pulsatility index were not different between groups. Carotid arterial strain and distensibility were lower, and the wave speed and beta stiffness index higher in the former AN patients. Characteristic impedance was not different nor were the forward and backward wave amplitudes. However, wave reflection indices (ratios of backward-to-forward compression wave area, and wave-related effect on pressure and hydraulic power) were 12-18% lower in the former AN patients (p < 0.05).

CONCLUSIONS

Increased carotid artery stiffness and reduced wave reflection are evident in young adults who recovered from adolescent AN. This may relate to an adaptive process that helps to maintain or restore flow and characteristic impedance despite increased vessel stiffness, with this warranting future investigation.

Transport of nitrite from large arteries modulates regional blood flow during stress and exercise

Background

Acute cardiovascular stress increases systemic wall shear stress (WSS)-a frictional force exerted by the flow of blood on vessel walls-which raises plasma nitrite concentration due to enhanced endothelial nitric oxide synthase (eNOS) activity. Upstream eNOS inhibition modulates distal perfusion, and autonomic stress increases both the consumption and vasodilatory effects of endogenous nitrite. Plasma nitrite maintains vascular homeostasis during exercise and disruption of nitrite bioavailability can lead to intermittent claudication.

Hypothesis

During acute cardiovascular stress or strenuous exercise, we hypothesize enhanced production of nitric oxide (NO) by vascular endothelial cells raises nitrite concentrations in near-wall layers of flowing blood, resulting in cumulative NO concentrations in downstream arterioles sufficient for vasodilation.

Confirmation and implications

Utilizing a multiscale model of nitrite transport in bifurcating arteries, we tested the hypothesis for femoral artery flow under resting and exercised states of cardiovascular stress. Results indicate intravascular transport of nitrite from upstream endothelium could result in vasodilator-active levels of nitrite in downstream resistance vessels. The hypothesis could be confirmed utilizing artery-on-a-chip technology to measure NO production rates directly and help validate numerical model predictions. Further characterization of this mechanism may improve our understanding of symptomatic peripheral artery occlusive disease and exercise physiology.

Arterial stiffness and augmentation index are associated with balance function in young adults

OBJECTIVE

Arterial stiffness and pulsatile central hemodynamics have been shown to affect various aspects of physical function, such as exercise capacity, gait speed, and motor control. The aim of this study was to examine the potential association between arterial stiffness and balance function in healthy younger men and women.

METHODS

112 participants (age = 21 ± 4 years, n = 78 women) underwent measures of arterial stiffness, pulsatile central hemodynamics, balance function and physical fitness in this cross-sectional study. Postural sway was measured in triplicate while participants stood on a foam surface with their eyes closed for 20 s. The average total center of pressure path length from the three trials was used for analysis. Measures of vascular function were estimated using an oscillometric blood pressure device while at rest and included pulse wave velocity (PWV), augmentation index (AIx), and pulse pressure amplification. Measures of physical fitness used as covariates in statistical models included handgrip strength determined from a handgrip dynamometer, lower-body flexibility assessed using a sit-and-reach test, estimated maximal aerobic capacity (VO₂max) using heart rate and a step test, and body fat percentage measured from air displacement plethysmography.

RESULTS

The results from linear regression indicated that after considering sex, mean arterial pressure, body fat, estimated VO₂max, handgrip strength, and sit-and-reach, PWV (β = 0.44, p < 0.05) and AIx (β = - 0.25, p < 0.01) were significant predictors of postural sway, explaining 10.2% of the variance.

CONCLUSION

Vascular function is associated with balance function in young adults independent of physical fitness. Increased arterial stiffness may negatively influence balance, while wave reflections may be protective for balance.

Optimised design of an arterial network model reproduces characteristic central and peripheral hemodynamic waveform features in young adults

Abstract

The arterial network in healthy young adults is thought to be structured to optimize wave reflection in the arterial system, producing an ascending aortic pressure waveform with three key features: early systolic peak, negative systolic augmentation and diastolic hump. One‐dimensional computer models have provided significant insights into arterial haemodynamics, but no previous models of the young adult have exhibited these three features. Given that this issue was likely to be related to unrepresentative or non‐optimized impedance properties of the model arterial networks, we developed a new ‘YoungAdult’ model that incorporated the following features: (i) a new and more accurate empirical equation for approximating wave speeds, based on area and relative distance to elastic–muscular arterial transition points; (ii) optimally matched arterial junctions; and (iii) an improved arterial network geometry that eliminated ‘within‐segment’ taper (which causes wave reflection in conduit arteries) whilst establishing ‘impedance‐preserving’ taper. These properties of the model led to wave reflection occurring predominantly at distal vascular beds, rather than in conduit arteries. The model predicted all three typical characteristics of an ascending aortic pressure waveform observed in young adults. When compared with non‐invasively acquired pressure and velocity measurements (obtained via tonometry and Doppler ultrasound in seven young adults), the model was also shown to reproduce the typical waveform morphology observed in the radial, brachial, carotid, temporal, femoral and tibial arteries. The YoungAdult model provides support for the concept that the arterial tree impedance in healthy young adults is exquisitely optimized, and it provides an important baseline model for investigating cardiovascular changes in ageing and disease states.

Key points

The origin of wave reflection in the arterial system is controversial, but reflection properties are likely to give rise to characteristic haemodynamic features in healthy young adults, including an early systolic peak, negative systolic augmentation and diastolic hump in the ascending aortic pressure waveform, and triphasic velocity profiles in peripheral arteries. Although computational modelling provides insights into arterial haemodynamics, no previous models have predicted all these features.

An established arterial network model was optimized by incorporating the following features: (i) a more accurate representation of arterial wave speeds; (ii) precisely matched junctions; and (iii) impedance‐preserving tapering, thereby minimizing wave reflection in conduit arteries in the forward direction. Comparison with in vivo data (n = 7 subjects) indicated that the characteristic waveform features in young adults were predicted accurately.

Our findings strongly imply that a healthy young arterial system is structured to optimize wave reflection in the main conduit arteries and that reflection of forward waves occurs primarily in the vicinity of vascular beds.

Inter-arm differences in regional arterial stiffness and geometry lead to inter-arm systolic blood pressure differences: A modelling study

An inter-arm systolic blood pressure difference (ISBPD), if substantial in magnitude (typically defined as ≥10 mmHg), is a potential cardiovascular risk factor in adults, due to its association with cardiovascular events/mortality. A substantial ISBPD occurs in approximately 10% of the adult population, and, although associations with vascular disease and elevated stiffness have been reported, the mechanisms underlying ISBPD remain unknown. The aim of this study was to investigate whether inter-arm differences in segmental pulse wave velocity, cross-sectional area, or vascular bed compliance/resistance could give rise to substantial differences in brachial pressures between arms; for example, due to differences in pulse wave transmission and reflection. Using an established one-dimensional model of the major systemic arteries, pulse wave velocity (PWV) was uniformly increased or decreased in arteries of 1) the supra-aortic region leading up to the arm, 2) the brachial region, 3) the forearm, and 4) all of these (entire arm pathway); for the left arm, right arm, and both arms. Cross-sectional area and vascular bed compliance and resistance of the arms were similarly varied. Inter-arm differences in segmental PWV and cross-sectional area (but not bilateral changes) led to associated substantial inter-arm SBP differences, which were observed with changes to brachial, forearm and/or entire arm pathways and were related to altered transmission of forward waves and amplitude/timing of reflected waves. Vascular bed compliance and resistance had minimal influence. We conclude that inter-arm differences in arterial stiffness and geometry may contribute to inter-arm systolic blood pressure differences, warranting further investigation.

Impact of acute changes in blood pressure and arterial stiffness on cerebral pulsatile haemodynamics in young and middle-aged adults

NEW FINDINGS

What is the central question of this study? Does cerebrovascular pulsatility respond differently to acute increases in arterial stiffness in middle-aged compared with young adults? What is the main finding and its importance? Compared with young adults, middle-aged adults exhibited similar changes in cerebral pulsatile damping despite attenuated changes in carotid diameter and cerebrovascular pulsatility during blood pressure-dependent, but not blood pressure-independent, increases in large artery stiffness.

ABSTRACT

Acute manipulation of arterial stiffness through interventions that increase sympathetic activity might provoke cerebral pulsatility and damping and reveal whether cerebrovascular haemodynamics respond differently to transient elevations in arterial stiffness in middle-aged compared with young adults. We compared cerebral pulsatility and damping in middle-aged versus young adults during two different sympathetic interventions [cold pressor test (CP) and lower-body negative pressure (LBNP)] that increase arterial stiffness acutely. Cerebrovascular haemodynamics were assessed in 15 middle-aged (54 ± 7 years old; 11 female) and 15 sex-matched young adults (25 ± 4 years old) at rest and during the CP test (4 min, 6.4 ± 0.8°C) and LBNP (6 min, -20 mmHg). Mean blood pressure was measured continuously via finger photoplethysmography. Carotid-femoral pulse wave velocity (cfPWV) and carotid stiffness were measured via tonometry and ultrasound. Blood velocity pulsatility index (PI) was measured at the middle cerebral (MCA) and common carotid artery (CCA) using Doppler, with pulsatile damping calculated as CCA PI divided by MCA PI. Increases in cfPWV were driven by changes in mean pressure during CP but not during LBNP in both groups (P < 0.05). Pulsatile damping decreased in both groups (P < 0.05) despite reductions in MCA PI and greater carotid dilatation during CP in young compared with middle-aged adults (P < 0.05). Pressure-independent increases in cfPWV during LBNP did not alter pulsatile damping but decreased MCA PI in both young and middle-aged adults (P < 0.05). These data suggest that changes in carotid diameter and cerebrovascular pulsatility differ between young and middle-aged adults despite similar changes in cerebral pulsatile damping during blood pressure-dependent, but not blood pressure-independent, increases in large artery stiffness.

Wave Reflection at the Origin of a First-Generation Branch Artery and Target Organ Protection: The AGES-Reykjavik Study

[Figure: see text].

Age, sex, and the vascular contributors to cerebral pulsatility and pulsatile damping

Cerebral pulsatility reflects a balance between the transmission and damping of pulsatility in the cerebrovasculature. Females experience greater cerebral pulsatility with aging, which may have implications for sex differences in stroke risk and cognitive decline. This study sought to explore vascular contributors to cerebral pulsatility and pulsatile damping in men and women. Adults (n = 282, 53% female) underwent measurements of cerebral (middle cerebral artery) pulsatility, pulsatile damping (ratio of cerebral to carotid pulsatility), large artery stiffening (ratio of aortic to carotid pulse wave velocity), and carotid wave transmission/reflection dynamics using wave intensity analysis. Multiple regression revealed that older age, female sex, greater large artery stiffening, higher carotid pulse pressure, and greater forward wave energy was associated with increased cerebral pulsatility (adjusted R² = 0.44, P < 0.05). Contributors to decreased cerebral pulsatile damping included older age, female sex, and lower wave reflection index (adjusted R² = 0.51, P < 0.05). Our data link greater large artery stiffening, carotid pulse pressure, and forward wave energy to greater cerebral pulsatility, while greater carotid wave reflection may enhance cerebral pulsatile damping. Lower cerebral pulsatile damping among females may contribute to greater age-associated cerebral pulsatile burden compared with males.

NEW & NOTEWORTHY Cerebral pulsatility contributes to brain health and depends on a balance between transmission and damping of pulsatile hemodynamics into the cerebrovasculature. Our data indicate that cerebral pulsatility increases with age, female sex, extracranial artery stiffening, forward wave energy, and pulse pressure, whereas pulsatile damping decreases with age and female sex and increases with greater carotid wave reflections. These novel data identify pulsatile damping as a potential contributor to sex differences in cerebral pulsatile burden.