Aging is associated with an earlier arrival of reflected waves without a distal shift in reflection sites

Background-Despite pronounced increases in central pulse wave velocity (PWV) with aging, reflected wave transit time (RWTT), traditionally defined as the timing of the inflection point (T-INF) in the central pressure waveform, does not appreciably decrease, leading to the controversial proposition of a "distal-shift" of reflection sites. T-INF, however, is exceptionally prone to measurement error and is also affected by ejection pattern and not only by wave reflection. We assessed whether RWTT, assessed by advanced pressure-flow analysis, demonstrates the expected decline with aging. Methods and Results-We studied a sample of unselected adults without cardiovascular disease (n=48; median age 48 years) and a clinical population of older adults with suspected/established cardiovascular disease (n=164; 61 years). We measured central pressure and flow with carotid tonometry and phase-contrast MRI, respectively. We assessed RWTT using wave-separation analysis (RWTTWSA) and partially distributed tube-load (TL) modeling (RWTTTL). Consistent with previous reports, T-INF did not appreciably decrease with age despite pronounced increases in PWV in both populations. However, aging was associated with pronounced decreases in RWTTWSA (general population -15.0 ms/decade, P<0.001; clinical population -9.07 ms/decade, P=0.003) and RWTTTL (general -15.8 ms/decade, P<0.001; clinical -11.8 ms/decade, P<0.001). There was no evidence of an increased effective reflecting distance by either method. TINF was shown to reliably represent RWTT only under highly unrealistic assumptions about input impedance. Conclusions-RWTT declines with age in parallel with increased PWV, with earlier effects of wave reflections and without a distal shift in reflecting sites. These findings have important implications for our understanding of the role of wave reflections with aging

Development, Validation, and Clinical Application of a Numerical Model for Pulse Wave Velocity Propagation in a Cardiovascular System with Application to Noninvasive Blood Pressure Measurements

High blood pressure blood pressure is an important risk factor for cardiovascular disease and affects almost one-third of the U.S. adult population. Historical cuff-less non-invasive techniques used to monitor blood pressure are not accurate and highlight the need for first principal models. The first model is a one-dimensional model for pulse wave velocity (PWV) propagation in compliant arteries that accounts for nonlinear fluids in a linear elastic thin walled vessel. The results indicate an inverse quadratic relationship (R^2=.99) between ejection time and PWV, with ejection time dominating the PWV shifts (12%). The second model predicts the general relationship between PWV and blood pressure with a rigorous account of nonlinearities in the fluid dynamics, blood vessel elasticity, and finite dynamic deformation of a membrane type thin anisotropic wall. The nonlinear model achieves the best match with the experimental data. To retrieve individual vascular information of a patient, the inverse problem of hemodynamics is presented, calculating local orthotropic hyperelastic properties of the arterial wall. The final model examines the impact of the thick arterial wall with different material properties in the radial direction. For a hypertensive subject the thick wall model provides improved accuracy up to 8.4% in PWV prediction over its thin wall counterpart. This translates to nearly 20% improvement in blood pressure prediction based on a PWV measure. The models highlight flow velocity is additive to the classic pressure wave, suggesting flow velocity correction may be important for cuff-less, non-invasive blood pressure measures. Systolic flow correction of the measured PWV improves the R2 correlation to systolic blood pressure from 0.81 to 0.92 for the mongrel dog study, and 0.34 to 0.88 for the human subjects study. The algorithms and insight resulting from this work can enable the development of an integrated microsystem for cuff-less, non-invasive blood pressure monitoring

An inverse transmission line model of the lower limb arterial system

Includes bibliography. Includes disk in pocket at back of book

Aging is Associated With an Earlier Arrival of Reflected Waves Without a Distal Shift in Reflection Sites

To access publisher's full text version of this article, please click on the hyperlink in Additional Links field or click on the hyperlink at the top of the page marked Files. This article is open access.Background-—Despite pronounced increases in central pulse wave velocity (PWV) with aging, reflected wave transit time (RWTT), traditionally defined as the timing of the inflection point (TINF) in the central pressure waveform, does not appreciably decrease, leading to the controversial proposition of a “distal-shift” of reflection sites. TINF, however, is exceptionally prone to measurement error and is also affected by ejection pattern and not only by wave reflection. We assessed whether RWTT, assessed by advanced pressure-flow analysis, demonstrates the expected decline with aging. Methods and Results-—We studied a sample of unselected adults without cardiovascular disease (n=48; median age 48 years) and a clinical population of older adults with suspected/established cardiovascular disease (n=164; 61 years). We measured central pressure and flow with carotid tonometry and phase-contrast MRI, respectively. We assessed RWTT using wave-separation analysis (RWTTWSA) and partially distributed tube-load (TL) modeling (RWTTTL). Consistent with previous reports, TINF did not appreciably decrease with age despite pronounced increases in PWV in both populations. However, aging was associated with pronounced decreases in RWTTWSA (general population 15.0 ms/decade, P<0.001; clinical population 9.07 ms/decade, P=0.003) and RWTTTL (general 15.8 ms/ decade, P<0.001; clinical 11.8 ms/decade, P<0.001). There was no evidence of an increased effective reflecting distance by either method. TINF was shown to reliably represent RWTT only under highly unrealistic assumptions about input impedance. Conclusions-—RWTT declines with age in parallel with increased PWV, with earlier effects of wave reflections and without a distal shift in reflecting sites. These findings have important implications for our understanding of the role of wave reflections with agingNIH/ R56 HL-124073-01A1, 5-R21-AG-043802-02, PPG/1P01-1HL09430

Validation of a 1D Algorithm That Measures Pulse Wave Velocity to Estimate Compliance in Blood Vessels

The purpose of this research is to determine if it is possible to validate the new 1D method for measuring pulse wave velocity in the aorta in vivo and estimate compliance. Arterial pressure and blood flow characterize the traveling of blood from the heart to the arterial system and have played a significant role in the evaluation of cardiovascular diseases. Blood vessel distensibility can give some information on the evolution of cardiovascular disease. A patient’s aorta cannot be explanted to measure compliance; therefore we are using a flow phantom model to validate the 1D pulse wave velocity technique to estimate compliance

Toward simultaneous flow and pressure assessment in large arteries using non-invasive ultrasound

Ultrageluid wordt in de kliniek vaak toegepast om op een niet-invasieve manier geometrische eigenschappen van grote vaten, zoals diameter en wanddikte en hemodynamische variabelen zoals bloedstroomsnelheid te bepalen. Om biomechanische parameters en hemodynamische variabelen die karakteristiek zijn voor de ontwikkeling van hart en vaatziekten, zoals compliantie en impedantie, te bepalen, is de bepaling van geometrie en bloedstroomsnelheid alleen onvoldoende. Daarvoor is een gelijktijdige en bij voorkeur niet invasieve meting van debiet en druk vereist. Met de huidige ultrageluidstechnieken is het onmogelijk om gelijktijdig debiet en druk nauwkeurig te bepalen. Debiet wordt vaak bepaald aan de hand van twee metingen: een diametermeting (geluidsbundel loodrecht op het vat) en een meting van de maximale axiale bloedstroomsnelheid met behulp van Doppler ultrageluid (geluidsbundel onder een hoek met het vat). Door een theoretische snelheidsverdeling aan te nemen, bijvoorbeeld een Poiseuille of Womersley profiel, wordt hieruit vervolgens het debiet berekend. In-vivo zijn vaten zelden recht: vaten zijn taps toelopend, gekromd en hebben vertakkingen. Dientengevolge zijn er secundaire snelheidscomponenten aanwezig die de axiale snelheidverdeling be¨invloeden. Dit resulteert in asymmetrische axiale snelheidsverdelingen. Omdat de aangenomen snelheidsverdelingen slechts geldig zijn voor rechte vaten, geeft een dusdanige bepaling een onnauwkeurige afschatting van het debiet. Verder is het onmogelijk om gelijktijdig met de snelheidsmeting nauwkeurig de wandbeweging te bepalen, waardoor de debietmeting nog verder verslechtert en het gelijktijdig bepalen van druk uit wandbeweging en debiet onmogelijk wordt. In dit onderzoek worden Particle Image Velocimetry (PIV) gebaseerde algoritmen toegepast op RF-data die verkregen zijn met behulp van een commercieel beschikbaar, voor klinische toepassing goedgekeurd ultrageluidssysteem. Dit maakt het mogelijk om snelheidscomponenten loodrecht op de ultrageluidbundel, en dus gelijktijdig wandpositie en axiale snelheid nauwkeurig te meten. Deze snelheidsmeettechniek is gevalideerd door metingen van het snelheidsprofiel in een experimentele opstelling te vergelijken met resultaten van computational fluid dynamics (CFD) berekeningen, voor stationaire en instationaire stromingen in een recht vat. Er werd een goede overeenstemming gevonden voor het axiale snelheidsprofiel. Integratie van het gemeten axiale snelheidsprofiel leverde een nauwkeurige afschatting van het debiet op. Omdat in de praktijk de meeste vaten gekromd zijn is de snelheids meetmethode vervolgens gevalideerd voor toepassing op stromingen in dit soort geometrieën. In de experimentele opstelling zijn axiale snelheidsprofielen gemeten voor stationaire en instationaire stroming in kromme buizen. Opnieuw zijn de gemeten profielen vergeleken met resultaten van CFD-berekeningen. Ook hier werd een goede overeenstemming gevonden tussen de gemeten profielen en de met behulp van CFD berekende snelheidsprofielen. Om nauwkeurig debiet te bepalen op basis van de gemeten asymmetrische axiale snelheidsprofielen, is een analytische en een op CFD gebaseerde studie gedaan naar stroming in kromme vaten. Deze studie heeft geresulteerd in de cos ¿-methode. Toepassing van de cos ¿-methode op de gemeten asymmetrische axiale profielen gaf een nauwkeurige afschatting van het debiet, voor stationaire en instationaire flow. Vergeleken met de huidig toegepaste afschattingsmethode voor het debiet werd een grote verbetering gevonden. Voor een fysiologisch relevant debiet gaf de cos ¿-methode een gemiddelde afwijking van 5% ten opzichte van het referentiedebiet terwijl deze voor de huidig toegepaste Poiseuille en Womersley benaderingen gelijk was aan 20%. Tenslotte is getracht om de lokale druk te bepalen uit enkel een niet-invasieve ultrageluidsmeting door een meting van de diameter te combineren met een gelijktijdige bepaling van de lokale compliantie. De lokale compliantie is bepaald door de lokale golfsnelheid (PWV) te meten. Verschillende methoden om lokaal de PWV te meten zijn getest in de experimentele opstelling. Hieruit bleek dat de QA-methode, een methode waarbij de lokale PWV bepaald wordt uit de verhouding tussen veranderingen in debiet en veranderingen in oppervlak van de dwarsdoorsnede van het vat, het mogelijk maakt om lokaal nauwkeurig PWV te meten. Door de PWV meting te combineren met een gelijktijdige meting van de diameter werd de lokale druk nauwkeurig afgeschat. Dit geeft aan dat het haalbaar is om op een niet-invasieve manier in-vivo druk te meten met behulp van ultrageluid. Hoewel de meettechnieken besproken in deze studie alleen getest zijn voor toepassing in een gecontroleerde experimentele omgeving, zijn de vooruitzichten voor klinische toepassing veelbelovend. De gepresenteerde methoden maken het mogelijk om de toestand van het vaatbed nauwkeuriger te bepalen, waardoor in de toekomst informatie verkregen kan worden over het effect van therapeutische ingrepen en factoren ge¨identificeerd kunnen worden die karakteristiek zijn voor de ontwikkeling van hart- en vaatziekten

On the Estimation of Total Arterial Compliance from Aortic Pulse Wave Velocity

Total arterial compliance (C T) is a main determinant of cardiac afterload, left ventricular function and arterio-ventricular coupling. C T is physiologically more relevant than regional aortic stiffness. However, direct, in vivo, non-invasive, measurement of C T is not feasible. Several methods for indirect C T estimation require simultaneous recording of aortic flow and pressure waves, limiting C T assessment in clinical practice. In contrast, aortic pulse wave velocity (aPWV) measurement, which is considered as the "gold standard” method to assess arterial stiffness, is noninvasive and relatively easy. Our aim was to establish the relation between aPWV and C T. In total, 1000 different hemodynamic cases were simulated, by altering heart rate, compliance, resistance and geometry using an accurate, distributed, nonlinear, one-dimensional model of the arterial tree. Based on Bramwell-Hill theory, the formula $$C_{\text{T}} = k \cdot {\text{aPWV}}^{ - 2}$$ was found to accurately estimate C T from aPWV. Coefficient k was determined both analytically and by fitting C T vs. aPWV data. C T estimation may provide an additional tool for cardiovascular risk (CV) assessment and better management of CV diseases. C T could have greater impact in assessing elderly population or subjects with elevated arterial stiffness, where aPWV seem to have limited prognostic value. Further clinical studies should be performed to validate the formula in viv

Review of Zero-D and 1-D Models of Blood Flow in the Cardiovascular System

<p>Abstract</p> <p>Background</p> <p>Zero-dimensional (lumped parameter) and one dimensional models, based on simplified representations of the components of the cardiovascular system, can contribute strongly to our understanding of circulatory physiology. Zero-D models provide a concise way to evaluate the haemodynamic interactions among the cardiovascular organs, whilst one-D (distributed parameter) models add the facility to represent efficiently the effects of pulse wave transmission in the arterial network at greatly reduced computational expense compared to higher dimensional computational fluid dynamics studies. There is extensive literature on both types of models.</p> <p>Method and Results</p> <p>The purpose of this review article is to summarise published 0D and 1D models of the cardiovascular system, to explore their limitations and range of application, and to provide an indication of the physiological phenomena that can be included in these representations. The review on 0D models collects together in one place a description of the range of models that have been used to describe the various characteristics of cardiovascular response, together with the factors that influence it. Such models generally feature the major components of the system, such as the heart, the heart valves and the vasculature. The models are categorised in terms of the features of the system that they are able to represent, their complexity and range of application: representations of effects including pressure-dependent vessel properties, interaction between the heart chambers, neuro-regulation and auto-regulation are explored. The examination on 1D models covers various methods for the assembly, discretisation and solution of the governing equations, in conjunction with a report of the definition and treatment of boundary conditions. Increasingly, 0D and 1D models are used in multi-scale models, in which their primary role is to provide boundary conditions for sophisticate, and often patient-specific, 2D and 3D models, and this application is also addressed. As an example of 0D cardiovascular modelling, a small selection of simple models have been represented in the CellML mark-up language and uploaded to the CellML model repository <url>http://models.cellml.org/</url>. They are freely available to the research and education communities.</p> <p>Conclusion</p> <p>Each published cardiovascular model has merit for particular applications. This review categorises 0D and 1D models, highlights their advantages and disadvantages, and thus provides guidance on the selection of models to assist various cardiovascular modelling studies. It also identifies directions for further development, as well as current challenges in the wider use of these models including service to represent boundary conditions for local 3D models and translation to clinical application.</p