Pulse Wave Velocity Prediction and Compliance Assessment in Elastic Arterial Segments

Critical role of arterial constitutive model in predicting blood pressure from pulse wave velocity

BACKGROUND

A promising approach to cuff-less, continuous blood pressure monitoring is to estimate blood pressure (BP) from Pulse Wave Velocity (PWV). However, most existing PWV-based methods rely on empirical BP-PWV relations and have large prediction errors, which may be caused by the implicit assumption of thin-walled, linear elastic arteries undergoing small deformations. Our objective is to understand the BP-PWV relationship in the absence of such limiting assumptions.

METHOD

We performed Fluid-Structure Interaction (FSI) simulations of the radial artery and the common carotid artery under physiological flow conditions. In these dynamic simulations, we employed two constitutive models for the arterial wall: the linear elastic model, implying a thin-walled linear elastic artery undergoing small deformations, and the Holzapfel-Gasser-Ogden (HGO) model, accounting for the nonlinear effects of collagen fibers and their orientations on the large arterial deformation.

RESULTS

Despite the changing BP, the linear elastic model predicts a constant PWV throughout a cardiac cycle, which is not physiological. The HGO model correctly predicts a positive BP-PWV correlation by capturing the nonlinear deformation of the artery, showing up to 50 % variations of PWV in a cardiac cycle.

CONCLUSION

Dynamic FSI simulations reveal that the BP-PWV relationship strongly depends on the arterial constitutive model, especially in the radial artery. To infer BP from PWV, one must account for the varying PWV, a consequence of the nonlinear arterial response due to collagen fibers. Future efforts should be directed towards robust measurement of time-varying PWV if it is to be used to predict BP.

Pulse Wave Morphology Changes in Aortic Valve Stenosis Detected with Cardio-Ankle Vascular Index

Background

Cardio-ankle vascular index (CAV) is a measure of systemic arterial stiffness and has been shown to increase after aortic valve surgery. However, change in CAVI-derived pulse wave morphology has not previously been addressed.

Case Study

A 72-year-old female was transferred to a large center for heart valve interventions for evaluation of her aortic stenosis. Few co-morbidities were detected on medical history, other than previous radiation treatment for breast cancer, and no signs of other concomitant cardiovascular disease. The patient was accepted for surgical aortic valve replacement due to severe aortic valve stenosis and arterial stiffness was assessed with CAVI, as part of an ongoing clinical study. The pre-operative CAVI was 4.7 which after surgery increased almost 100% to 9.35. In tandem, the slope of systolic upstroke pulse morphology captured from brachial cuffs was changed from a prolonged flattened pattern to a steeper.

Conclusion

After aortic valve replacement surgery due to aortic valve stenosis, in addition to increased CAVI-derived measures of arterial stiffness, the slope of the CAVI-derived upstroke pulse wave morphology changes to a steeper slope. This finding could have implications in the future of aortic valve stenosis screening and utilization of CAVI.

Reproduction of human blood pressure waveform using physiology-based cardiovascular simulator

This study presents a cardiovascular simulator that mimics the human cardiovascular system's physiological structure and properties to reproduce the human blood pressure waveform. Systolic, diastolic blood pressures, and its waveform are key indicators of cardiovascular health. The blood pressure waveform is closely related to the pulse wave velocity and the overlap of the forward and reflected pressure waves. The presented cardiovascular simulator includes an artificial aorta made of biomimetic silicone. The artificial aorta has the same shape and stiffness as the human standard and is encased with a compliance chamber. The compliance chamber prevents distortion of the blood pressure waveform from strain-softening by applying extravascular pressure. The blood pressure waveform reproduced by the simulator has a pressure range of 80-120 mmHg, a pulse wave velocity of 6.58 m/s, and an augmentation index of 13.3%. These values are in the middle of the human standard range, and the reproduced blood pressure waveform is similar to that of humans. The errors from the human standard values are less than 1 mmHg for blood pressure, 0.05 m/s for pulse wave velocity, and 3% for augmentation index. The changes in blood pressure waveform according to cardiovascular parameters, including heart rate, stroke volume, and peripheral resistance, were evaluated. The same pressure ranges and trends as in humans were observed for systolic and diastolic blood pressures according to cardiovascular parameters.

Cardiovascular deconditioning and impact of artificial gravity during 60-day head-down bed rest—Insights from 4D flow cardiac MRI

Microgravity has deleterious effects on the cardiovascular system. We evaluated some parameters of blood flow and vascular stiffness during 60 days of simulated microgravity in head-down tilt (HDT) bed rest. We also tested the hypothesis that daily exposure to 30 min of artificial gravity (1 g) would mitigate these adaptations. 24 healthy subjects (8 women) were evenly distributed in three groups: continuous artificial gravity, intermittent artificial gravity, or control. 4D flow cardiac MRI was acquired in horizontal position before (−9 days), during (5, 21, and 56 days), and after (+4 days) the HDT period. The false discovery rate was set at 0.05. The results are presented as median (first quartile; third quartile). No group or group × time differences were observed so the groups were combined. At the end of the HDT phase, we reported a decrease in the stroke volume allocated to the lower body (−30% [−35%; −22%]) and the upper body (−20% [−30%; +11%]), but in different proportions, reflected by an increased share of blood flow towards the upper body. The aortic pulse wave velocity increased (+16% [+9%; +25%]), and so did other markers of arterial stiffness (CAVI; CAVI0). In males, the time-averaged wall shear stress decreased (−13% [−17%; −5%]) and the relative residence time increased (+14% [+5%; +21%]), while these changes were not observed among females. Most of these parameters tended to or returned to baseline after 4 days of recovery. The effects of the artificial gravity countermeasure were not visible. We recommend increasing the load factor, the time of exposure, or combining it with physical exercise. The changes in blood flow confirmed the different adaptations occurring in the upper and lower body, with a larger share of blood volume dedicated to the upper body during (simulated) microgravity. The aorta appeared stiffer during the HDT phase, however all the changes remained subclinical and probably the sole consequence of reversible functional changes caused by reduced blood flow. Interestingly, some wall shear stress markers were more stable in females than in males. No permanent cardiovascular adaptations following 60 days of HDT bed rest were observed.

A structural approach to 3D-printing arterial phantoms with physiologically comparable mechanical characteristics: Preliminary observations

Pulse wave behavior is important in cardiovascular pathophysiology and arterial phantoms are valuable for studying arterial function. The ability of phantoms to replicate complex arterial elasticity and anatomy is limited by available materials and techniques. The feasibility of improving phantom performance using functional structure designs producible with practical 3D printing technologies was investigated. A novel corrugated wall approach to separate phantom function from material properties was investigated with a series of designs printed from polyester-polyurethane using a low-cost open-source fused filament fabrication 3D printer. Nonpulsatile pressure-diameter data was collected, and a mock circulatory system was used to observe phantom pulse wave behavior and obtain pulse wave velocities. The measured range of nonpulsatile Peterson elastic strain modulus was 5.6–19 to 12.4–33.0 kPa over pressures of 5–35 mmHg for the most to least compliant designs respectively. Pulse wave velocities of 1.5–5 m s⁻¹ over mean pressures of 7–55 mmHg were observed, comparing favorably to reported in vivo pulmonary artery measurements of 1–4 m s⁻¹ across mammals. Phantoms stiffened with increasing pressure in a manner consistent with arteries, and phantom wall elasticity appeared to vary between designs. Using a functional structure approach, practical low-cost 3D-printed production of simple arterial phantoms with mechanical properties that closely match the pulmonary artery is possible. Further functional structure design development to expand the pressure range and physiologic utility of dir"ectly 3D-printed phantoms appears warranted.

Black beans and red kidney beans induce positive postprandial vascular responses in healthy adults: A pilot randomized cross-over study

BACKGROUND AND AIMS

Consuming pulses (dry beans, dry peas, chickpeas, lentils) over several weeks can improve vascular function and decrease cardiovascular disease risk; however, it is unknown whether pulses can modulate postprandial vascular responses. The objective of this study was to compare different bean varieties (black, navy, pinto, red kidney) and white rice for their acute postprandial effects on vascular and metabolic responses in healthy individuals.

METHODS AND RESULTS

The study was designed as a single-blinded, randomized crossover trial with a minimum 6 days between consumption of the food articles. Vascular tone (primary endpoint), haemodynamics and serum biochemistry (secondary endpoints) were measured in 8 healthy adults before and at 1, 2, and 6 h after eating ¾ cup of beans or rice. Blood pressure and pulse wave velocity (PWV) were lower at 2 h following red kidney bean and pinto bean consumption compared to rice and navy bean, respectively (p < 0.05). There was greater vasorelaxation 6 h following consumption of darker-coloured beans, as shown by decreased vascular tone: PWV was lower after consuming black bean compared to pinto bean, augmentation pressure was lower after consuming black bean compared to rice and pinto bean, and wave reflection magnitude was lower after consuming red kidney bean and black bean compared to rice, navy bean, and pinto bean (p < 0.05). LDL-cholesterol concentrations were lower 6 h after black bean consumption compared to rice (p < 0.05).

CONCLUSION

Overall, red kidney and black beans, the darker-coloured beans, elicited a positive effect on the tensile properties of blood vessels, and this acute response may provide insight for how pulses modify vascular function.

In-Home Cardiovascular Monitoring System for Heart Failure: Comparative Study

Background

There is a pressing need to reduce the hospitalization rate of heart failure patients to limit rising health care costs and improve outcomes. Tracking physiologic changes to detect early deterioration in the home has the potential to reduce hospitalization rates through early intervention. However, classical approaches to in-home monitoring have had limited success, with patient adherence cited as a major barrier. This work presents a toilet seat–based cardiovascular monitoring system that has the potential to address low patient adherence as it does not require any change in habit or behavior.

Objective

The objective of this work was to demonstrate that a toilet seat–based cardiovascular monitoring system with an integrated electrocardiogram, ballistocardiogram, and photoplethysmogram is capable of clinical-grade measurements of systolic and diastolic blood pressure, stroke volume, and peripheral blood oxygenation.

Methods

The toilet seat–based estimates of blood pressure and peripheral blood oxygenation were compared to a hospital-grade vital signs monitor for 18 subjects over an 8-week period. The estimated stroke volume was validated on 38 normative subjects and 111 subjects undergoing a standard echocardiogram at a hospital clinic for any underlying condition, including heart failure.

Results

Clinical grade accuracy was achieved for all of the seat measurements when compared to their respective gold standards. The accuracy of diastolic blood pressure and systolic blood pressure is 1.2 (SD 6.0) mm Hg (N=112) and –2.7 (SD 6.6) mm Hg (N=89), respectively. Stroke volume has an accuracy of –2.5 (SD 15.5) mL (N=149) compared to an echocardiogram gold standard. Peripheral blood oxygenation had an RMS error of 2.3% (N=91).

Conclusions

A toilet seat–based cardiovascular monitoring system has been successfully demonstrated with blood pressure, stroke volume, and blood oxygenation accuracy consistent with gold standard measures. This system will be uniquely positioned to capture trend data in the home that has been previously unattainable. Demonstration of the clinical benefit of the technology requires additional algorithm development and future clinical trials, including those targeting a reduction in heart failure hospitalizations.

Manufacturing Abdominal Aorta Hydrogel Tissue-Mimicking Phantoms for Ultrasound Elastography Validation

Ultrasound (US) elastography, or elasticity imaging, is an adjunct imaging technique that utilizes sequential US images of soft tissues to measure the tissue motion and infer or quantify the underlying biomechanical characteristics. For abdominal aortic aneurysms (AAA), biomechanical properties such as changes in the tissue's elastic modulus and estimates of the tissue stress may be essential for assessing the need for the surgical intervention. Abdominal aortic aneurysms US elastography could be a useful tool to monitor AAA progression and identify changes in biomechanical properties characteristic of high-risk patients. A preliminary goal in the development of an AAA US elastography technique is the validation of the method using a physically relevant model with known material properties. Here we present a process for the production of AAA tissue-mimicking phantoms with physically relevant geometries and spatially modulated material properties. These tissue phantoms aim to mimic the US properties, material modulus, and geometry of the abdominal aortic aneurysms. Tissue phantoms are made using a polyvinyl alcohol cryogel (PVA-c) and molded using 3D printed parts created using computer aided design (CAD) software. The modulus of the phantoms is controlled by altering the concentration of PVA-c and by changing the number of freeze-thaw cycles used to polymerize the cryogel. The AAA phantoms are connected to a hemodynamic pump, designed to deform the phantoms with the physiologic cyclic pressure and flows. Ultra sound image sequences of the deforming phantoms allowed for the spatial calculation of the pressure normalized strain and the identification of mechanical properties of the vessel wall. Representative results of the pressure normalized strain are presented.

PWPSim: A new simulation tool of pulse wave propagation in the human arterial tree

Hemodynamic simulation enhances investigations of pulse wave propagation phenomena and enables validation of new methods of pulse wave analysis. However, such simulation systems or tools are not readily available nor easily accessible. In this study, a new simulation tool of pulse wave propagation in the human arterial tree was developed based on a transmission line model (TLM). This paper describes the theory of TLM of the human arterial tree used by this simulation. The results are a display of the main functions and simulation results of this tool. This tool allows simulation of pulse wave propagation with capability to change the range of parameters, such as heart rate, mean flow and left ventricular ejection time, body height, arterial radius and wall thickness, arterial viscoelasticity, peripheral resistance and compliance. It also accounts for the nonlinear elasticity of arteries. The simulation results are displayed as 2D and 3D figures of blood pressure and flow waveforms, input impedance and pressure transfer function between aorta and femoral artery, including systolic blood pressure, diastolic blood pressure, pulse pressure, carotid-femoral pulse wave velocity, brachial-ankle pulse wave velocity and ankle-brachial index. It is a useful and interactive simulation tool of pulse wave propagation in the systematic arterial tree.

Detecting Regional Stiffness Changes in Aortic Aneurysmal Geometries Using Pressure-Normalized Strain

Transabdominal ultrasound elasticity imaging could improve the assessment of rupture risk for abdominal aortic aneurysms by providing information on the mechanical properties and stress or strain states of vessel walls. We implemented a non-rigid image registration method to visualize the pressure-normalized strain within vascular tissues and adapted it to measure total strain over an entire cardiac cycle. We validated the algorithm's performance with both simulated ultrasound images with known principal strains and anatomically accurate heterogeneous polyvinyl alcohol cryogel vessel phantoms. Patient images of abdominal aortic aneurysm were also used to illustrate the clinical feasibility of our imaging algorithm and the potential value of pressure-normalized strain as a clinical metric. Our results indicated that pressure-normalized strain could be used to identify spatial variations in vessel tissue stiffness. The results of this investigation were sufficiently encouraging to warrant a clinical study measuring abdominal aortic pressure-normalized strain in a patient population with aneurysmal disease.

Effects of cardiac timing and peripheral resistance on measurement of pulse wave velocity for assessment of arterial stiffness

To investigate the effects of heart rate (HR), left ventricular ejection time (LVET) and wave reflection on arterial stiffness as assessed by pulse wave velocity (PWV), a pulse wave propagation simulation system (PWPSim) based on the transmission line model of the arterial tree was developed and was applied to investigate pulse wave propagation. HR, LVET, arterial elastic modulus and peripheral resistance were increased from 60 to 100 beats per minute (bpm), 0.1 to 0.45 seconds, 0.5 to 1.5 times and 0.5 to 1.5 times of the normal value, respectively. Carotid-femoral PWV (cfPWV) and brachial-ankle PWV (baPWV) were calculated by intersecting tangent method (cfPWV_tan and baPWV_tan), maximum slope (cfPWV_max and baPWV_max), and using the Moens-Korteweg equation ([Formula: see text] and [Formula: see text]). Results showed cfPWV and baPWV increased significantly with arterial elastic modulus but did not increase with HR when using a constant elastic modulus. However there were significant LVET dependencies of cfPWV_tan and baPWV_tan (0.17 ± 0.13 and 0.17 ± 0.08 m/s per 50 ms), and low peripheral resistance dependencies of cfPWV_tan, cfPWV_max, baPWV_tan and baPWV_max (0.04 ± 0.01, 0.06 ± 0.04, 0.06 ± 0.03 and 0.09 ± 0.07 m/s per 10% peripheral resistance), respectively. This study demonstrated that LVET dominates the effect on calculated PWV compared to HR and peripheral resistance when arterial elastic modulus is constant.

Baseline Cardiovascular Characteristics of Adult Patients with Chronic Kidney Disease from the KoreaN Cohort Study for Outcomes in Patients With Chronic Kidney Disease (KNOW-CKD)

Cardiovascular disease (CVD) is the most common cause of death in patients with chronic kidney disease (CKD). We report the baseline cardiovascular characteristics of 2,238 participants by using the data of the KoreaN Cohort Study for Outcomes in Patients With Chronic Kidney Disease (KNOW-CKD) study. The cohort comprises 5 subcohorts according to the cause of CKD: glomerulonephritis (GN), diabetic nephropathy (DN), hypertensive nephropathy (HTN), polycystic kidney disease (PKD), and unclassified. The average estimated glomerular filtration rate (eGFR) was 50.5 ± 30.3 mL/min⁻¹/1.73 m⁻² and lowest in the DN subcohort. The overall prevalence of previous CVD was 14.4% in all patients, and was highest in the DN followed by that in the HTN subcohort. The DN subcohort had more adverse cardiovascular risk profiles (higher systolic blood pressure [SBP], and higher levels of cardiac troponin T, left ventricular mass index [LVMI], coronary calcium score, and brachial-ankle pulse wave velocity [baPWV]) than the other subcohorts. The HTN subcohort exhibited less severe cardiovascular risk profiles than the DN subcohort, but had more severe cardiovascular risk features than the GN and PKD subcohorts. All these cardiovascular risk profiles were inversely correlated with eGFR. In conclusion, this study shows that the KNOW-CKD cohort exhibits high cardiovascular burden, as other CKD cohorts in previous studies. Among the subcohorts, the DN subcohort had the highest risk for CVD. The ongoing long-term follow-up study up to 10 years will further delineate cardiovascular characteristics and outcomes of each subcohort exposed to different risk profiles.

Improved Blood Pressure Prediction Using Systolic Flow Correction of Pulse Wave Velocity

Hypertension is a significant worldwide health issue. Continuous blood pressure monitoring is important for early detection of hypertension, and for improving treatment efficacy and compliance. Pulse wave velocity (PWV) has the potential to allow for a continuous blood pressure monitoring device; however published studies demonstrate significant variability in this correlation. In a recently presented physics-based mathematical model of PWV, flow velocity is additive to the classic pressure wave as estimated by arterial material properties, suggesting flow velocity correction may be important for cuff-less non-invasive blood pressure measures. The present study examined the impact of systolic flow correction of a measured PWV on blood pressure prediction accuracy using data from two published in vivo studies. Both studies examined the relationship between PWV and blood pressure under pharmacological manipulation, one in mongrel dogs and the other in healthy adult males. Systolic flow correction of the measured PWV improves the R² correlation to blood pressure from 0.51 to 0.75 for the mongrel dog study, and 0.05 to 0.70 for the human subjects study. The results support the hypothesis that systolic flow correction is an essential element of non-invasive, cuff-less blood pressure estimation based on PWV measures.