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Alessandra Borlotti, Chloe Park, Kim H Parker, Ashraf W Khir
BACKGROUND: A time-domain approach to couple the Windkessel effect and wave propagation has been recently introduced. The technique assumes that the measured pressure in the aorta (P) is the sum of a reservoir pressure (Pr), due to the storage of blood, and an excess pressure (Pe), due to the waves. Since the subtraction of Pr from P results in a smaller component of Pe, we hypothesized that using the reservoir-wave approach would produce smaller values of wave speed and intensities. Therefore, the aim of this study is to quantify the differences in wave speed and intensity using P, wave-only, and Pe, reservoir-wave techniques...
March 2015: Journal of Hypertension
Patrick Segers, Abigail Swillens, Liesbeth Taelman, Jan Vierendeels
Single-point methods such as the PU- and QA-loop methods are used to estimate local pulse wave velocity (PWVPU and PWVQA) in arteries from a combination of pressure (P), flow (Q), velocity (U) or cross-sectional area (A) waveforms. Available data indicate that the PU-loop method tends to overestimate PWV, while the QA-loop method tends to underestimate. Wave reflection has been suggested as a factor playing a role in the agreement between different methods. In this work, we first provide a theoretical basis to (i) demonstrate the interference of wave reflection with the PU-loop method for both solitary sinusoidal waves as well as physiological waveforms; (ii) develop an operator-independent method to correct for the presence of reflections...
May 2014: Physiological Measurement
Alessandra Borlotti, Ye Li, Kim H Parker, Ashraf W Khir
Wave speed (also called pulse wave velocity) is the speed by which disturbance travels along the medium and it depends on the mechanical and geometrical properties of the vessel and on the density of the blood. Wave speed is a parameter of clinical relevance because it is an indicator of arterial stiffness and cardiovascular diseases. The aim of this work is to compare different methods for the determination of local wave speed in bench experiments and investigate their relative accuracy when reflections are present...
January 3, 2014: Journal of Biomechanics
Jordi Alastruey, Anthony A E Hunt, Peter D Weinberg
We present a novel analysis of arterial pulse wave propagation that combines traditional wave intensity analysis with identification of Windkessel pressures to account for the effect on the pressure waveform of peripheral wave reflections. Using haemodynamic data measured in vivo in the rabbit or generated numerically in models of human compliant vessels, we show that traditional wave intensity analysis identifies the timing, direction and magnitude of the predominant waves that shape aortic pressure and flow waveforms in systole, but fails to identify the effect of peripheral reflections...
February 2014: International Journal for Numerical Methods in Biomedical Engineering
Alessandra Borlotti, Ashraf Khir
The Windkessel model, coupled with the wave propagation theory, was applied to data measured in the ascending aorta of 11 anaesthetised dogs during total aortic occlusion at the thoracic and diaphragm levels. Wave speed and wave intensity were calculated using the measured pressure (P) and velocity (U), and separately using the pressure due to the wave (P(ex)) and U in the aorta approximately 1 cm distal to the aortic valve. Results show that wave speed, determined using the PU-loop method, is higher during thoracic than in diaphragm occlusion (p<0...
2011: Conference Proceedings: Annual International Conference of the IEEE Engineering in Medicine and Biology Society
Ye Li, Alessandra Borlotti, Kim H Parker, Ashraf W Khir
Wave speed is directly related to arterial distensibility and is widely used by clinicians to assess arterial stiffness. The PU-loop method for determining wave speed is based on the water hammer equation for flow in flexible tubes and artery using the method of characteristics. This technique determines wave speed using simultaneous measurements of pressure and velocity at a single point. The method shows that during the early part of systole, the relationship between pressure and velocity is generally linear, and the initial slope of the PU-loop is proportional to wave speed...
2011: Conference Proceedings: Annual International Conference of the IEEE Engineering in Medicine and Biology Society
Ye Li, Ashraf W Khir
The relationship between the vessel diameter (D) and fluid velocity (U) in arteries and flexible tubes has been recently characterized as linear in the absence of wave reflections. This relationship allowed for determining local wave speed (C(DU)) using the lnDU-loop method. Using C(DU), it was possible to separate U and D waveforms into their forward and backward components. It was also possible to calculate wave intensity (dI(DU)), using D and U, from which the arrival time of reflected wave (Trw(DU)) could be determined...
April 29, 2011: Journal of Biomechanics
Jordi Alastruey
A local estimation of pulse wave speed c, an important predictor of cardiovascular events, can be obtained at arterial locations where simultaneous measurements of blood pressure (P) and velocity (U), arterial diameter (D) and U, flow rate (Q) and cross-sectional area (A), or P and D are available, using the PU-loop, sum-of-squares (∑(2)), lnDU-loop, QA-loop or new D(2)P-loop methods. Here, these methods were applied to estimate c from numerically generated P, U, D, Q and A waveforms using a visco-elastic one-dimensional model of the 55 larger human systemic arteries in normal conditions...
March 15, 2011: Journal of Biomechanics
Ye Li, Ashraf W Khir
It is well accepted that wave speed is one of the key factors describing wave propagation in arteries [1]. Local wave speed is directly related to the mechanical properties of the arterial wall [2] and is widely used to determine the arterial distensibility [3]. Several methods have been proposed for determining wave speed in arteries, such as foot-to-foot and PU-loop methods. In this paper, we suggest a new method for the determination of wave speed and wall distensibility, using noninvasive measurements. The theoretical foundation of this method is based on the 1-D conservation of mass and momentum equations of flow in flexible tubes...
2009: Conference Proceedings: Annual International Conference of the IEEE Engineering in Medicine and Biology Society
M J P Swalen, A W Khir
It is well established that wave speed can be determined using the initial linear part of the pressure-velocity loop (PU-loop). However, the frequency response of most flow measuring devices is usually slower than that of solid-state pressure transducers; making flow waveforms lagging in time behind pressure waveforms. If this lag, which is traditionally determined by eye, is not corrected prior to the analysis, the PU-loop method may provide inaccurate wave speeds. The main aim of this work is therefore to introduce an objective technique to establish the value of this lag...
July 22, 2009: Journal of Biomechanics
Kim H Parker
Wave intensity analysis applies methods first used to study gas dynamics to cardiovascular haemodynamics. It is based on the method of characteristics solution of the 1-D equations derived from the conservation of mass and momentum in elastic vessels. The measured waveforms of pressure P and velocity U are described as the summation of successive wavefronts that propagate forward and backward through the vessels with magnitudes dP (+/-) and dU (+/-). The net wave intensity dPdU is the flux of energy per unit area carried by the wavefronts...
February 2009: Medical & Biological Engineering & Computing
J Feng, A W Khir
Although the propagation of arterial waves of forward flows has been studied before, that of backward flows has not been thoroughly investigated. The aim of this research is to investigate the propagation of the compression and expansion waves of backward flows in terms of wave speed and dissipation, in flexible tubes. The aim is also to compare the propagation of these waves with those of forward flows. A piston pump generated a flow waveform in the shape of approximately half-sinusoid, in flexible tubes (12 mm and 16 mm diameter)...
May 2008: Proceedings of the Institution of Mechanical Engineers. Part H, Journal of Engineering in Medicine
J Aguado-Sierra, K H Parker, J E Davies, D Francis, A D Hughes, J Mayet
Pulse wave velocity is related to arterial stiffness. Pulse wave velocity changes with age and disease and is a useful indicator of cardiovascular disease. Different methods are used for evaluating pulse wave velocity in systemic vessels, but none is applicable to coronary arteries. In this study we first compare values of wave speed (c) calculated from measurements of pressure (P) and velocity (U) using different analytical methods: PU-loop, beta stiffness parameter, characteristic impedance, foot-to-foot method, and the sum of squares (Sigma(2)), a novel way of calculating the wave speed (calculated from the square root of the sum of the ratio of the dP(2) and dU(2) over a complete cardiac cycle)...
2006: Conference Proceedings: Annual International Conference of the IEEE Engineering in Medicine and Biology Society
A W Khir, M J P Swalen, J Feng, K H Parker
In a previous paper we demonstrated that the linear portion of the pressure-velocity loop (PU-loop) corresponding to early systole could be used to calculate the local wave speed. In this paper we extend this work to show that determination of the time at which the PU-loop first deviates from linearity provides a convenient way to determine the arrival time of reflected waves (Tr). We also present a new technique using the PU-loop that allows for the determination of wave speed and Tr simultaneously. We measured pressure and flow in elastic tubes of different diameters, where a strong reflection site existed at known distances away form the measurement site...
December 2007: Medical & Biological Engineering & Computing
J Feng, Q Long, A W Khir
Earlier work of wave dissipation in flexible tubes and arteries has been carried out predominantly in the frequency domain and most of the studies used the measured pressure waveform for presenting the results. In this work we investigate the pattern of wave dissipation in the time domain using the separated forward and backward travelling waves in flexible tubes. We tested four sizes of latex tubes of 2m in length each, where a single semi-sinusoidal in shape, pressure wave, was produced at the inlet of each tube...
2007: Journal of Biomechanics
A W Khir, A Zambanini, K H Parker
Arterial wave speed is widely used to determine arterial distensibility and has been utilised as a surrogate marker for vascular disease. A comparison between the results of the traditional foot-to-foot method for measuring wave speed to those of the pressure-velocity loop (PU-loop) method is one of the primary objectives of this paper. We also investigate the regional wave speed along the aorta, and the effect of arterial occlusion on the PU-loop measured in the ascending aorta. In 11 anaesthetised dogs, a total occlusion lasting 3 min was produced at four sites: upper thoracic, diaphragm, abdominal and left iliac artery...
January 2004: Medical Engineering & Physics
A W Khir, K H Parker
Wave intensity analysis is a time domain method for studying waves in elastic tubes. Testing the ability of the method to extract information from complex pressure and velocity waveforms such as those generated by a wave passing through a mismatched elastic bifurcation is the primary aim of this research. The analysis provides a means for separating forward and backward waves, but the separation requires knowledge of the wave speed. The PU-loop method is a technique for determining the wave speed from measurements of pressure and velocity, and investigating the relative accuracy of this method is another aim of this research...
June 2002: Journal of Biomechanics
A W Khir, M Y Henein, T Koh, S K Das, K H Parker, D G Gibson
The purpose of this study was to investigate the effect of aortic clamping on arterial waves during peripheral vascular surgery. We measured pressure and velocity simultaneously in the ascending aorta, in ten patients (70+/-5 years) with aortic-iliac disease intra-operatively. Pressure was measured using a catheter tip manometer, and velocity was measured using Doppler ultrasound. Data were collected before aortic clamping, during aortic clamping and after unclamping. Hydraulic work in the aortic root was calculated from the measured data, the reflected waves were determined by wave-intensity analysis and wave speed was determined by the PU-loop (pressure-velocity-loop) method; a new technique based on the 'water-hammer' equation...
December 2001: Clinical Science (1979-)
A W Khir, A O'Brien, J S Gibbs, K H Parker
Considering waves in the arteries as infinitesimal wave fronts rather than sinusoidal wavetrains, the change in pressure across the wave front, dP, is related to the change in velocity, dU, that it induces by the "water hammer" equation, dP=+/-rhocdU, where rho is the density of blood and c is the local wave speed. When only unidirectional waves are present, this relationship corresponds to a straight line when P is plotted against U with slope rhoc. When both forward and backward waves are present, the PU-loop is no longer linear...
September 2001: Journal of Biomechanics
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