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PubMed1990

Determination of volume of vortices in poststenotic pulsatile flow by ultrasonic Doppler power ratio and spectrum analysis.

Based on the principle of ultrasonic Doppler flowmetry, a power ratio was derived from independent forward and reverse flow Doppler shift signals to measure a ratio of the volume of vortices to the total vessel volume in poststenotic separated flow. The ratio was also proportional to the ratio of the cross-sectional areas of vortices to the vessel lumen. In vitro pulsatile flow experiments were performed to test the methodology and to study flow separation and vortex shedding downstream from model stenoses. The averaged flow cross-sectional area ratio linearly correlated (r = 0.91) with the actual area reduction of the stenosis.

Carotid Artery Diseases↗
PubMed1985

Experimental study of physiological pulsatile flow past valve prostheses in a model of human aorta--I. Caged ball valves.

Pulsatile flow development past a caged ball valve in a model human aorta was studied using laser Doppler anemometry. Velocity profiles measured in the ascending aorta and in the mid-arch region were strongly influenced by the geometry of the valve at the root of the aorta. Velocity profiles distal to the valve were asymmetric with jet-like flow in the peripheral region having larger velocity magnitudes towards the left lateral wall. In early diastole, a streamwise vortex motion was observed throughout the model aorta with fluid moving towards the downstream direction along the left lateral wall and reversed flow along the right lateral wall. With the caged ball valve at the root of the aorta, no reversed flow was observed along the inner wall of curvature in the mid-arch region.

Aortic Valve↗
PubMed1994

High speed bolus tagging: time resolved velocity quantification of pulsatile flow in a single breath hold.

We have implemented a high speed method for cardiac-triggered blood velocity quantification within a single breath hold on a conventional MR system. The method, based on bolus tagging, was tested using a pulsatile flow phantom and evaluated in vivo. The image acquisition time was reduced by a factor of N by acquiring N phase encode lines per bolus tag application. The clarity of the flow tag was found to vary with how k-space was covered during data collection. The technique was optimized and multiple bolus tag images were obtained throughout the cardiac cycle within a single breathold.

Aorta, Thoracic↗
PubMedDate not supplied

Study of red cell aggregation in pulsatile flow from ultrasonic Doppler power measurements.

Human red cell aggregability and disaggregability represent important hemorheological parameters of blood. Several techniques have been proposed to evaluate the tendency of red cells to form aggregates and to disrupt in the presence of shear stress. One of the most recent approaches is based on the characterization of the intensity of ultrasonic scattered signals. A pulsatile flow loop model is used in the present study to demonstrate the potential applicability of Doppler ultrasound to detect and characterize the hemodynamic behavior of red cell aggregates. Porcine whole blood specimens collected from 20 different pigs were circulated in the flow model (tube diameter of 0.476 cm) at different mean velocities and pulsation rates. At a pulsation of 70 beats/min for mean velocities of 13 cm/sec and 63 cm/sec, no cyclic variation of the Doppler power was observed, suggesting the absence of rouleaux build-up and rouleaux disruption. At a pulsation of 20 beats/min and mean velocities of 11 cm/sec and 38 cm/sec, statistically significant cyclic variations (p < 0.01) were measured. It is suggested that aggregate size enlargement, rouleaux orientation with the flow field and the effect of shear stress on rouleaux disruption are possible causes for the observed cyclic variation of the Doppler power within the flow cycle at a pulsation of 20 beats/min. A discussion of the potential application of this technique for in vivo study in large vessels is given.

Animals↗
PubMed1999

Real-time 3-dimensional volumetric ultrasound imaging of the vena contracta for stenotic valves with the use of echocardiographic contrast imaging: in vitro pulsatile flow studies.

The purpose of our study was to investigate the utility of real-time 3-dimensional volumetric ultrasound coupled with echo contrast imaging to visualize and quantify effective flow areas for stenotic valves in vitro. Real-time 3-dimensional ultrasound imaging has recently emerged as a promising method for increasing the quantitative accuracy of echocardiography. Since the technique currently does not process Doppler information, its use for quantifying flow has not been studied. However, the use of contrast agents to visualize cardiac flows with the use of echocardiography should allow determination of mass-dependent flow parameters such as effective flow area (vena contracta area) for stenotic lesions. We used real-time 3-dimensional imaging in an in vitro stenotic valve model (areas 0.785 to 1.767 cm2) under pulsatile flow conditions (60 bpm; 40 to 80 mL/beat). An echo contrast agent was used to visualize the distal jet. Real-time 3-dimensional imaging provides simultaneous views of long-axis and short-axis (C-scan) image planes of the jet. The vena contracta was identified and measured by placing the C-scan line immediately distal to the orifice and measuring the cross-sectional flow area. System gain and postprocessing curve shape affected 3-dimensional areas; minimal gain and a custom curve produced best agreement to actual vena contracta areas measured with a previously validated laser method (y = 0.939x + 0.089; r = 0.98; standard error of estimate = 0.158 cm2). We conclude that real-time 3-dimensional ultrasound imaging coupled with a contrast agent can be used as an accurate yet simple clinical means of measuring effective flow areas for stenotic valves.

Albumins↗
PubMed2006

Multiple echo NMR velocimetry: fast and localized measurements of steady and pulsatile flows in small channels.

The understanding of fluid transport in miniaturized flow devices is an important component in the design of flow cells, micromixers, and microreactors. In this manuscript, we employ NMR in the form of a voxel-selective multiple modulation multiple echo sequence (MMMEV) to monitor average velocities in individual microchannels inside a six-channel network. The technique produces average velocities which are consistent with the imposed flow rates. In addition, we take advantage of the short acquisition time (32 ms per velocity component) of the technique to quantitatively track the time evolution of the fluid velocity in a pulsatile flow phantom.

Algorithms↗
PubMed2001

Quantitative analysis of PC MRI velocity maps: pulsatile flow in cylindrical vessels.

The accuracy of MR phase contrast (PC) velocity mapping, and the subsequent derivation of wall shear stress (WSS) values, has been quantitatively assessed. Using a retrospectively gated PC gradient-echo technique, the temporal-spatial velocity fields were measured for pulsatile flow in a rigid cylindrical vessel. The experimental data were compared with values derived from the Womersley solution of the Navier-Stokes equations. For a sinusoidal waveform, the overall root-mean-square (rms) difference between the measured and analytical velocities corresponded to 13% of the peak fluid velocity. The WSS derived from the data displayed a 14% rms difference with the analytical model. As an example of a more complicated flow, a triangular saw-tooth waveform was deconstructed into its Fourier components. Velocity maps and the WSS were calculated by the superposition of the individual solutions, weighted by the Fourier series coefficient, for each harmonic. The velocity and experimentally derived WSS agreed with the analytical results (4% and 12% rms difference, respectively). Evaluation of the analytical models allowed an estimate of the inherent accuracy in the measurement of velocity maps and WSS values.

Arteriosclerosis↗
PubMed1989

Velocity field of pulsatile flow in a porous tube.

This paper describes velocity fields for fully developed periodic laminar flow in a rigid tube with a porous wall. We obtained an analytical solution of the flow by the linear approximation of the Navier-Stokes equation. Unlike the previous works with a constant seepage rate along the axis, we used a wall velocity which contained hydraulic permeation constant Lp. The axial velocity profile shows a local maximum velocity near the wall at a large Womersley number alpha. This suggests that concentration polarization in porous tubular membrane may be reduced at high frequencies if a membrane device is operated under pulsatile flow conditions. The magnitude of wall permeation velocity decreases linearly along the tube axis because the damping of the pressure difference between the inside and the outside of the tube is very small.

Blood Flow Velocity↗
PubMed2005

Combined effects of pulsatile flow and dynamic curvature on wall shear stress in a coronary artery bifurcation model.

A three-dimensional model with simplified geometry for the branched coronary artery is presented. The bifurcation is defined by an analytical intersection of two cylindrical tubes lying on a sphere that represents an idealized heart surface. The model takes into account the repetitive variation of curvature and motion to which the vessel is subject during each cardiac cycle, and also includes the phase difference between arterial motion and blood flowrate, which may be nonzero for patients with pathologies such as aortic regurgitation. An arbitrary Lagrangian Eulerian (ALE) formulation of the unsteady, incompressible, three-dimensional Navier-Stokes equations is employed to solve for the flow field, and numerical simulations are performed using the spectral/hp element method. The results indicate that the combined effect of pulsatile inflow and dynamic geometry depends strongly on the aforementioned phase difference. Specifically, the main findings of this work show that the time-variation of flowrate ratio between the two branches is minimal (less than 5%) for the simulation with phase difference angle equal to 90 degrees, and maximal (51%) for 270 degrees. In two flow pulsatile simulation cases for fixed geometry and dynamic geometry with phase angle 270 degrees, there is a local minimum of the normalized wall shear rate amplitude in the vicinity of the bifurcation, while in other simulations a local maximum is observed.

Animals↗
PubMed1982

Physiological pulsatile flow experiments in a model of the human aortic arch.

An experimental investigation of physiologically relevant pulsatile flow in a model of the human aortic arch has been conducted. The model aortic arch flow chamber was fabricated in clear acrylic from an in situ casting of the human aorta and was incorporated in a mock-circulatory system. The model excluded the coronary sinuses and the three major branching arteries of the mid-arch region in order to concentrate only upon the effects of the multi-dimensional curvatures and tapering in the aorta. Furthermore, a flow straightening section was placed upstream to the flow chamber to eliminate any fluid disturbances created by the prosthetic aortic valve used in these studies. The qualitative flow visualization studies in the model aorta revealed the presence of strong secondary fluid motions near the inner wall. These helical flows dissipated during diastole, being greatly affected by the dramatic flow reversals which occurred along the inner wall at the onset of diastole. Quantitative studies were conducted using a three-sensor hot-film velocity probe to determine the axial, radial and tangential velocity components at various cross-sections in the aorta. The results showed rapid reversal of axial velocity near the inner wall at the onset of diastole.

Aorta, Thoracic↗
PubMed1994

Simulation of three-dimensional pulsatile flow through an asymmetric stenosis.

The main objective of this work was to use desktop workstations to evaluate the computer code HEMO as a tool for predicting coronary blood flows. The flows are usually characterised by complex vortical structures and transitional effects, and as such present challenging computational problems. As the results of the computations shown in the paper demonstrate, we can predict realistic pulsatile flows in constricted tubes using the Sun Sparcstation 1+ in a matter of hours. The results shown in the paper have also demonstrated that the computer simulations can be very useful as a complementary tool for experimental investigations.

Arterial Occlusive Diseases↗
PubMed2001

Pulsatile flow studies of a porcine bioprosthetic aortic valve in vitro: PIV measurements and shear-induced blood damage.

A two-dimensional particle image tracking velocimetry (PIV) system has been used to map the velocity vector fields and Reynolds stresses in the immediate downstream vicinity of a porcine bioprosthetic heart valve at the aortic root region in vitro under pulsatile flow conditions. Measurements were performed at five different time steps of the systolic phase of the cardiac cycle. The velocity vector fields and Reynolds stress mappings at different time steps allowed us to chart a time history of the stress levels experienced by fluid particles as they move across the aortic root. This Lagrangian description of the stresses experienced by individual blood cells enabled us to estimate the propensity of shear-induced damage to platelets and red blood cells. Coupled with flow visualization techniques, the hydrodynamic consequences of introducing a porcine bioprosthetic heart valve into the aortic root was examined. Although the PIV measurements may lack the accuracy of single point measuring systems, the overall view of the flow in the aortic root region compensates for the shortcoming.

Animals↗
PubMedDate not supplied

Pulsatile flow in tubes of elliptic cross sections.

The compression of blood vessels by surrounding tissue is an important problem in hemodynamics, most prominently in studies relating to the heart. In this study we consider a long tube of elliptic cross section as an idealization of the geometry of a compressed blood vessel. An exact solution of the governing equations for pulsatile flow in a tube of elliptic cross section involves Mathieu functions which are considerably more difficult to evaluate than the Bessel functions in the case of a circular cross section. Results for the velocity field, flow rate and wall shear stress are obtained for different values of the pulsation frequency and ellipticity, with emphasis on how the effects of frequency and ellipticity combine to determine the flow characteristics. It is found that in general the effects of ellipticity are minor when frequency is low but become highly significant as the frequency increases. More specifically, the velocity profile along the major axis of the elliptic cross section develops sharp double peaks; the flow rate is reduced in approximately the same proportion as in the case of circular cross section; and the point of maximum shear on the tube wall migrates away from the minor axis where it is located in steady flow.

Animals↗
PubMed1991

Effects of pulsatile flow pattern and each stage of pressure increasing, constant and decreasing, respectively upon inelastic deformation of blood vessel.

The aim of this study is to elucidate the relation of the inelastic expansive deformation of natural blood vessel and the degradation of elasticity to the time pattern of circulating pulsatile pressure flow. When pulsatile pressure amplitude (delta P = Pmax - Pmin) becomes smaller due to Pmin increasing, the blood vessel is subject to creep deformation. In this sense, pulsating pressure will play a role in avoiding creep effect. Fluctuation of maximum pressure Pmax will induce the increase of inelastic deformation and the decrease of rigidity of the blood vessel. The inelastic deformation and the decrease of rigidity in blood vessel is induced at the stage of pressure amplitude rising from a lower one to a higher, but not during pressure amplitude (delta P) kept constant nor at the pressure amplitude decreasing stage. In order to reduce the degradation of mechanical properties of blood vessel, it may be effective to avoid the increase of Pmin and the variability of Pmax.

Biomechanical Phenomena↗
PubMed1990

Pulsatile flow and oscillating wall shear stress in the brachial artery of normotensive and hypertensive subjects.

STUDY OBJECTIVE - The aim of the study was to examine oscillating arterial wall shear stress in hypertension. DESIGN - Pulsatile flow and oscillating wall shear stress were measured in brachial artery in hypertensive v normotensive subjects using pulsed Doppler apparatus. Methods were tested in four subjects using a micrometric procedure of Doppler probe displacement providing instantaneous real time velocity profiles. SUBJECTS - 19 ambulatory male patients with mild to moderate hypertension (diastolic blood pressure 95-114 mm Hg) and 11 normotensive male controls of similar age were studied. All were non-smokers. MEASUREMENTS and RESULTS - Arterial diameter and pulsatile centreline blood velocity were determined with pulsed Doppler, and blood viscosity was measured with a coaxial cylinder viscometer. Shear rates corresponding to maximum (gamma Vmax), minimum (gamma Vmin), and pulse (gamma Vpulse) velocities were evaluated with a simplified method of computation of Womersley equations. Corresponding shear stresses (tau Vmax, tau Vmin, tau Vpulse) were calculated as the product between shear rate and viscosity. The differences in wall shear rates obtained with the Womersley method and with the micrometric procedure were less than 10%. Compared to normotensives, hypertensives had greater arterial diameter [0.508(SEM0.006) v 0.446(0.014), p less than 0.001], lower maximum velocity [36.2(1.5) v 46.3(2.4) cm.s-1, p less than 0.001], lower absolute value of minimum velocity [-8.3(1.2) v -14.3(2.3) cm.s-1, p less than 0.01], lower pulse velocity [44.5(2.2) v 61.2(3.9) cm.s-1, p less than 0.001], and higher blood viscosity [4.77(0.08) v 4.28(0.09) mPa.s, p less than 0.001]. gamma and tau Vmax, Vmin and Vpulse were all lower in absolute value in hypertensives. Overall mean blood pressure in all subjects was negatively correlated to gamma Vmax (r = -0.65), tau Vmax (r = -0.46), gamma Vmin (r = -0.45), tau Vmin (r = -0.37), gamma Vpulse (r = -0.63), and tau Vpulse (r = -0.48). In hypertensives, age was correlated negatively to gamma Vmax (r = -0.44), tau Vmax (r = -0.46), gamma Vmin (r = -0.57), tau Vmin (r = -0.57), gamma Vpulse (r = -0.58), and tau Vpulse (r = -0.58). In normotensives, age was not correlated with shear parameters, except for tau Vmax (r = -0.60) and tau Vpulse (r = -0.66). CONCLUSIONS - The hypertensive state is associated with a reduction in oscillating wall shear in large arteries despite an increase in blood viscosity. Age in combination with hypertension also decreases wall shear conditions.

Adult↗
PubMed1979

The limitation of pulsatile flow through the aqueduct of Sylvius as a cause of hydrocephalus.

The concept is advanced that hydrocephalus results from limitation in the pulsatile flow of CSF downwards through the aqueduct of Sylvius during systole which is necessary to accommodate for the pulsatile pressure and volume increase that accompanies the propagation of the arterial pulse through the brain. Evidence is given to show that flow through the fixed human aqueduct is disturbed and not laminar. Further, with the pressures availalbe, the aqueduct is only just large enough to pass the quantity of fluid which must be vented extracranially during systole. Should the capacity of this systolic venting mechanism be exceeded, physical strain will cause cellular damage in the periventricular and periaqueductal regions which, if prolonged, will lead to tissue destruction and hydrocephalus. There appear to be two main causes for hydrocephalus resulting from this mechanism. Firstly, structural lesions, restricting the lumina of the CSF-venting pathways, especially the aqueduct, will reduce the volume of CSF that can flow through these pathways during systole. The hydrocephalic process will then be continuous and only limited when tissue destruction reduces the systolic volume expansion of the brain such that it can be accomodated by the restricted CSF venting pathways. Secondly, conditions which may increase the amount of the systolic volume expansion of the brain beyond the capacity of the CSF venting pathways. Raised mean intracranial pressure is the most important of these conditions. In such cases the hydrocephalus will be limited by the duration of the causal process and possibly also by the enlargement of the venting pathways, as a result of tissue destruction. This hypothesis also accounts for hydrocephalus resulting from obliteration of the cortical subarachnoid space, obstruction to the cranial venous drainage, deformities in the region of the foramen magnum and arterial encroachment upon the ventricular system.

Acidosis↗
PubMed1991

Albumin recovery enhancement in membrane plasma fractionation using pulsatile flow.

In therapeutic plasmapheresis using cascade filtration, it is important to maximize albumin recovery while rejecting as many gamma-globulins as possible. Several membrane fractionation techniques were investigated using fresh bovine and human plasma and cellulose acetate filters (PF 100, AKZO). In dead end mode the sieving coefficients were found to decrease as transmembrane pressure increased. This was due to membrane plugging during the course of filtration after about 20 minutes which lead to a rapid increase in transmembrane pressure. In single pass mode the albumin recovery factor generally remains around 40% since the permeate flux is much less than the inlet flow. When strong pulsations (4 to 6 Hz) were superposed on the inlet plasma flow in single pass mode, the albumin sieving coefficient remained at about 0.95 while the permeate flux was increased by 106%. As a result a recovery factor of more than 80% could be sustained for at least 90 minutes without membrane plugging. Therefore pulsatile flow plasma fractionation seems to be an interesting approach to combine continuous operation with high albumin recovery.

Animals↗
PubMed1990

Estimation of turbulent shear stresses in pulsatile flow immediately downstream of two artificial aortic valves in vitro.

Measuring turbulent shear stresses is of major importance in artificial heart valve evaluation. Bi- and unidirectional fluid velocity measurements enable calculation of Reynolds shear stress [formula: see text] and Reynolds normal stress [formula: see text]. tau is important due to the relation to hemolysis and thrombus formation, but sigma is the only obtainable parameter in vivo. Therefore, determination of a correlation factor between tau and sigma is pertinent. In a pulsatile flow model, laser Doppler (LDA) and hot-film (HFA) anemometry were used for simultaneous bi- and unidirectional fluid velocity measurements downstream of a Hall Kaster and a Hancock Porcine aortic valve. Velocities were registered in two flow field locations and at four cardiac outputs. The velocity signals were subjected to analog signal processing prior to digital turbulence analysis, as a basis for calculation of tau and sigma. A correlation factor of 0.5 with a correlation coefficient of 0.97 was found between the maximum Reynolds shear stress and Reynolds normal stress, implying [formula: see text]. In vitro estimation of turbulent shear stresses downstream of artificial aortic valves, based on the axial velocity component alone, seems possible.

Aortic Valve↗