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PubMed2001

Pulsatile flow in an end-to-side vascular graft model: comparison of computations with experimental data.

Various hemodynamic factors have been implicated in vascular graft intimal hyperplasia, the major mechanism contributing to chronic failure of small-diameter grafts. However, a thorough knowledge of the graft flow field is needed in order to determine the role of hemodynamics and how these factors affect the underlying biological processes. Computational fluid dynamics offers much more versatility and resolution than in vitro or in vivo methods, yet computations must be validated by careful comparison with experimental data. Whereas numerous numerical and in vitro simulations of arterial geometries have been reported, direct point-by-point comparisons of the two techniques are rare in the literature. We have conducted finite element computational analyses for a model of an end-to-side vascular graft and compared the results with experimental data obtained using laser-Doppler velocimetry. Agreement for velocity profiles is found to be good, with some clear differences near the recirculation zones during the deceleration and reverse-flow segments of the flow waveform. Wall shear stresses are determined from velocity gradients, whether by computational or experimental methods, and hence the agreement for this quantity, while still good, is less consistent than for velocity itself from the wall shear stress numerical results, we computed four variables that have been cited in the development of intiimal hyperplasia-the time-averaged wall shear stress, an oscillating shear index, and spatial and temporal wall shear stress gradients in order to illustrate the versatility of numerical methods. We conclude that the computational approach is a valid alternative to the experimental approach for quantitative hemodynamic studies. Where differences in velocity were found by the two methods, it was generally attributed to the inability of the numerical method to model the fluid dynamics when flow conditions are destabilizing. Differences in wall shear, in the absence of destabilizing phenomena, were more likely to be caused by difficulties in calculating wall shear from relatively low resolution in vitro data.

Anastomosis, Surgical↗
PubMed1990

Pulsatile flow in the choroidal circulation: a preliminary investigation.

A preliminary investigation has been made of choroidal blood flow using a computer-aided image analysis approach to interpretation of indocyanine green (ICG) dye choroidal angiograms. The goal of the study was to characterise blood flow through the choroidal arteries vs. choroidal capillaries and veins. The methods of analysis used are briefly reviewed, and preliminary data obtained mainly from monkey eyes are presented. Preliminary conclusions are made regarding the relationship between compliance of choroidal arterial vessels and blood flow through them.

Angiography↗
PubMed1993

Influence of pulsatile flow on the behaviour of human fibroblasts adhered to glass.

In the human body, cells contacting biomaterials surfaces are frequently exposed to pulsatile shear stresses, e.g. blood vessel prostheses. Most studies involving shear, however, try to achieve a steady, pulse-free shear stress in studying cell-biomaterial interactions. In this study, human fibroblasts adhering to glass were exposed to an applied 0.5 Hz square-wave pulsatile shear, created in a parallel plate flow chamber by a computer-driven pump. Cells were also exposed to the single lower and higher shear stresses making up the square wave in the presence or absence of pressure variations due to the peristaltic roller pump. Results indicate that cells exposed to the applied square-wave regime showed a detachment rate in between that of the single shear stresses making up the square wave. Furthermore, the presence of the pulsations gave rise to elongated cell shapes in the direction of flow and the formation of a more extensive filopodial network than in the absence of pulsations. This was also true for cells exposed to the high shear component in the presence of pressure variations. Cells exposed to the high shear component without any pressure variations, however, adapted spherical shapes after the onset of flow.

Cell Adhesion↗
PubMed2005

Numerical study on the pulsatile flow characteristics of proximal anastomotic models.

Haemodynamics was widely believed to correlate with anastomosis restenosis. Utilizing the haemodynamic parameters as indicator functions, distal anastomosis was redesigned by some researchers so as to improve the long-term graft patency rate. However, there were few studies upon the proximal anastomosis. Therefore, in this study, flow characteristics and distributions of the haemodynamic parameters in proximal anastomosis under physiological flow condition have been investigated numerically for three different grafting angles: namely, 45 degrees forward facing, 45 degrees backward facing, and 90 degrees anastomotic joints. The simulation results showed a flow separation region along the graft inner wall immediately after the heel at peak flow phase and it decreased in size with the grafting angle shifting from 45 degrees forward facing to 45 degrees backward facing. At the same time, a pair of vortex was found in the cross-sectional planes of the 45 degrees backward facing and 90 degrees grafts. In addition, stagnation point was found along the graft outer wall with small shifting during the physiological cycle. High spatial and temporal wall shear stresses gradients (WSSG) were observed around the anastomotic joint. Low time-averaged wall shear stress (WSS) with elevated oscillation shear index (OSI) was found near the middle of anastomosis at the aorta wall and along the graft inner wall respectively, while high time-averaged WSS with low OSI was found at the heel, the toe, and the region downstream of the toe. These regions correlated to early lesion growth. Elevated time-averaged WSSG was found at the same region, where the elevated low-density lipoprotein (LDL) permeability was observed as reported in the literature. The existence of nearly fixed stagnating location, flow separation, vortex, high time-averaged WSS with low OSI, low time-averaged WSS with elevated OSI, and high time-averaged WSSG may lead to graft stenosis. Moreover, the simulation results obtained were consistent with those of experimental measurements. Based on the validated simulation results, the 45 degrees backward-facing graft was found to have the lowest variation range of time-averaged WSS and the lowest segmental average of WSSG among the three models investigated. The 45 degrees backward-facing graft is thus recommended for the bypass operation with expected higher patency rate.

Anastomosis, Surgical↗
PubMedDate not supplied

Can cardiac catheterization accurately assess the severity of aortic stenosis? An in vitro pulsatile flow study.

An accurate hemodynamic assessment of aortic stenosis has important clinical implications. In clinical practice, cardiac catheterization is often used to assess the severity of aortic stenosis. However, in conducting catheterization, the precise position of the catheter tip is often not known or controlled. From the standpoint of hydrodynamics, the position of the catheter tip may affect pressure measurement due to the complicated flow fields distal to the valve. This fact is particularly true when the diagnosed valve is stenotic. The study was aimed to investigate how the position of the catheter tip in catheterizing aortic stenosis affects pressure measurement. The experiments were conducted in an in vitro pulse duplicator system. Laser flow visualization was used to examine the flow fields in the vicinity of varying degrees of aortic stenosis, and a pressure transducer with a side-hole catheter was used to measure pressures. Minimal variation in transvalvular pressure drop measured along the radial direction was observed for varying degrees of valvular aortic stenosis. This implies that, in catheterization, the placement of the catheter tip along the radial direction does not seem to affect pressure measurement. However, along the axial direction, pressure recovery was observed for all the cases studied. Therefore, within the region of pressure recovery, the position of the catheter tip may affect the pressure drop measurement. This may cause inaccuracy in assessing the severity of aortic stenosis. However, this concern may be overcome by pulling the catheter slightly further downstream, so that the position of the catheter tip is outside of the pressure recovery region.

Adult↗
PubMedDate not supplied

Hemodynamics in rigid and distensible saccular aneurysms: a numerical study of pulsatile flow characteristics.

Flow characteristics are examined in two lateral model aneurysms by means of numerical simulation. The study concentrates on basic flow and stress patterns in a rigid wall and in a distensible wall aneurysm. The numerical solution of the governing Navier-Stokes equations describing incompressible, pulsatile, three-dimensional non-Newtonian flow is accomplished with the use of a finite element method together with a pressure correction technique. The inflow into the aneurysm is seen to arise from the downstream lip of the orifice and to be directed backward to the center. Backflow to the parent vessel takes place along the walls of the aneurysm. The intra-aneurysmal flow is found to be low compared with the flow velocity in the parent vessel, and even stagnation of flow occurs in the dome of the aneurysm. With a distensible wall, the basic flow characteristics are changed during systolic flow. The intra-aneurysmal secondary flow increases significantly. The increase and decrease of the flow velocity at the downstream lip reflect the expansion and contraction of the aneurysm wall where the maximal wall displacement during systolic acceleration is about 6% of the aneurysm diameter.

Aneurysm↗
PubMed1992

Influence of 4 different membrane oxygenators on inflammation-like processes during extracorporeal circulation with pulsatile and non-pulsatile flow.

The influence of four different membrane oxygenators (HF 4000, BOS-CM 50, CML 2, Maxima) on leucocyte count, concentrations of PMN-elastase, clotting factor XII, AT-III, C1-INH, alpha 2-antiplasmin and C3a was registered before, during and after CPB with pulsatile and nonpulsatile flow in 80 male patients aged between 36 and 67 years. With all systems tested, there was a drop in the concentrations of clotting factor XII, AT-III, C1-INH and alpha 2-antiplasmin in the early extracorporeal circulation (ECC) phase, exceeding the average hematocrit reduction accounted for by dilution. This drop was the least distinct with CML 2 systems, both with pulsatile and nonpulsatile perfusion, indicating system-inherent influences. Leucocyte cound and PMN-elastase concentration rose significantly during ECC irrespective of oxygenator tested of flow type applied. The rise in leucocyte count even continued for about 4 h after ECC. During the first 40 min of ECC, these changes were paralleled by a significant rise in C3a concentration, suggesting complement activation as a main cause for PMN activation. However, there is reason to suppose involvement of further mechanisms operating in PMN activation, since the elevated C3a-concentrations began to fall off while leucocyte count and PMN-elastase concentrations were still increasing.

Acute-Phase Proteins↗
PubMed1990

[Detection of cerebral microembolisms during extracorporeal circulation with pulsatile flow using transcranial Doppler monitoring].

The incidence of focal neurological deficits and diffuse neuropsychiatric disorders in patients undergoing open-heart surgery still remains unacceptably high. The consensus is increasing that diffuse microembolism is the most important cause. Changes in blood flow velocity in the middle cerebral artery were investigated by transcranial Doppler sonography in 26 patients regarding the effect of continuous and pulsatile perfusion during extracorporeal circulation. The quotient of systolic and diastolic flow velocity was significantly increased in patients with pulsatile perfusion. Abnormal high-frequent Doppler-signals were registered in 8 patients using pulsatile perfusion without diastolic basic flow. All the other patients in whom continuous pump flow was applied (13) did not reveal any signs of hemodynamic disturbances. In the group treated with pulsatile perfusion the systolic pressures before and behind the oxygenator were significantly increased in patients with high-frequent Doppler-signals which are likely to be caused by gaseous microemboli. Investigating one type of four different oxygenators no cerebral hemodynamic alterations were recorded even during pulsatile perfusion. The use of membrane oxygenators and arterial filters during pulsatile perfusion cannot completely prevent the occurrence of gaseous microemboli; their principle causes are supposed to be the level of perfusion pressure, the flow velocity and the design of oxygenator.

Blood Flow Velocity↗
PubMed1993

Two-dimensional velocity measurements in a pulsatile flow model of the normal abdominal aorta simulating different hemodynamic conditions.

The infrarenal abdominal aorta and aortic bifurcation are frequent sites of atherosclerosis. The local hemodynamics are considered to be an atherogenetic factor, and a detailed description of the flow fields in this region of the arterial tree is therefore essential. The aim of this study was to provide quantitative two-dimensional data on the velocity fields in the abdominal aorta, using a realistic flow model of the abdominal aorta and its main branches, under various physiologic flow conditions (i.e. rest and exercise). Velocities in the suprarenal abdominal aorta were antegrade, with very little retrograde and radial velocity components present. In the infrarenal abdominal aorta, velocity profiles were not fully developed, and large-scale retrograde flow was present during part of diastole for the rest condition. For the exercise conditions small-scale retrograde velocities were present during diastole, especially at the distal posterior vessel wall, but not at the distal anterior vessel wall. For the rest and medium exercise conditions, secondary flows were created in the distal abdominal aorta during diastole, most prominent near the posterior wall. Calculated wall shear stress directions revealed the presence of both oscillatory and multidirectional wall shear stresses mainly in parts of the infrarenal abdominal aorta, and were found to correlate well with the published data on the distribution of early atherosclerotic lesions. This quantitative study demonstrates the necessity of carefully modeling both the anatomy and the physiology in order to understand the complex hemodynamics present in the abdominal aorta.

Adult↗