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PubMed2002

Feasibility study for real time three dimensional Doppler intracardiac echocardiography.

Using catheter-mounted two-dimensional array transducers, we have obtained real-time three-dimensional color flow and spectral Doppler ultrasound images in phantoms. We have constructed several transducers operating at 5 MHz with up to 137 channels inside a 12 French catheter lumen. The transducer configuration may be side scanning or beveled with respect to the long axis of the catheter lumen. We have also included six electrodes to acquire simultaneous electrocardiograms. We have measured Doppler signals in two phantoms, a string phantom and a pulsatile flow phantom. Using an open chest sheep model, we inserted the catheter into the cardiac chambers to study the utility of in vivo 3D intracardiac Doppler echocardiography.

Animals↗
PubMed2003

A novel permanent maglev impeller TAH: most requirements on blood pumps have been satisfied.

Based on the development of an impeller total artificial heart (TAH) (1987) and a permanent maglev (magnetic levitation) impeller pump (2002), as well as a patented magnetic bearing and magnetic spring (1996), a novel permanent maglev impeller TAH has been developed. The device consists of a rotor and a stator. The rotor is driven radially. Two impellers with different dimensions are fixed at both the ends of the rotor. The levitation of the rotor is achieved by using two permanent magnetic bearings, which have double function: radial bearing and axial spring. As the rotor rotates at a periodic changing speed, two pumps deliver the pulsatile flow synchronously. The volume balance between the two pumps is realized due to self-modulation property of the impeller pumps, without need for detection and control. Because the hemo-dynamic force acting on the left impeller is larger than that on the right impeller, and this force during systole is larger than that during diastole, the rotor reciprocates axially once a cycle. This is beneficial to prevent the thrombosis in the pump. Furthermore, a small flow via the gap between stator and rotor from left pump into right pump comes to a full washout in the motor and the pumps. Therefore, it seems neither mechanical wear nor thrombosis could occur. The previously developed prototype impeller TAH had demonstrated that it could operate in animal experiments indefinitely, if the bearing would not fail to work. Expectantly, this novel permanent magnetic levitation impeller TAH with simplicity, implantability, pulsatility, compatibility and durability has satisfied the most requirements on blood pumps and will have more extensive applications in experiments and clinics.

Heart, Artificial↗
PubMed2006

Swirling flow pattern in a non-planar model of an interposition vein cuff anastomosis.

One of the main causes of long-term failure of ePTFE grafts is the development of anastomotic intimal hyperplasia which leads to graft thrombosis. Experimental studies with bypass grafts have shown an inverse relationship between mean wall shear stress and intimal hyperplasia. The geometry of the anastomosis has a strong influence on the flow pattern and wall shear stress distribution. The aim of this in vitro study was to investigate the influence of non-planarity in a model of a distal anastomosis with interposition vein cuff, an anastomosis configuration that is increasingly being used because of improved clinical results. Laser Doppler anemometer measurements were carried out in silicone rubber models of interposition vein cuff anastomoses with planar and non-planar inflow. The pulsatile flow waveforms were typical of those found in femoro-infrapopliteal bypass. Axial and radial velocities were measured in the proximal and distal outflow segments. As expected a symmetrical helical flow pattern (Dean flow) was evident in the planar model. The model with non-planar inflow, however, gave rise to swirling flow in both the distal and proximal artery outflow segments for during the systolic phase. In patients, the anastomosis is usually non-planar. Since the configuration depends in part upon the tunnelling of the graft, this may be altered to some extent. Non-planar anastomotic configurations induce a swirling flow pattern, which may normalise wall shear stress, thereby potentially reducing intimal hyperplasia.

Anastomosis, Surgical↗
PubMed1987

Doppler ultrasound in aortic stenosis: in vitro studies of pressure gradient determination.

Torricelli's equation expresses a simple relationship between fluid velocity and pressure gradient in orifice flow and is currently used in conjunction with noninvasive Doppler ultrasound to determine gradients in mitral stenosis, and aortic stenosis, as well as other cardiovascular orifices. In theory, however, the Torricelli equation overestimates the gradient in aortic stenosis and the Borda equation should be more applicable. A brief tutorial derivation of the Borda and Torricelli equations is presented. The applicability of Torricelli's equation in aortic stenosis was studied experimentally with a rigid wall, pulsatile flow analogue. Doppler ultrasound and manometric data were collected simultaneously. Percent stenosis, peak flow rate and fluid viscosity were varied. The results demonstrated that the Torricelli equation consistently overestimated the pressure gradient. At 61% area stenosis, the overestimation exceeded 100%. In vivo studies are required to determine the relevance of the observations to clinical situations.

Aortic Valve Stenosis↗
PubMed1996

Nitric oxide in brain death related cardiovascular dysfunction.

PURPOSE: Nitric oxide (NO) is a major regulator of vascular tone, blood pressure, and blood flow, and plays a significant role in disease states associated with hemodynamic alterations. However, the role of NO in association with the effects of brain death (BD) has not yet been evaluated. METHODS: In 17 mongrel dogs (23 to 31 kg), right atrial serum measurements of nitrite and L-arginine as well as NO ex vivo tissue nitrite extraction were performed at baseline (0), and 120, 240, and 360 minutes after BD. The hearts were instrumented with micromanometers, transonic flow probes, and ultrasonic dimension transducers to determine systolic function and to analyze the pulmonary vasculature flow characteristics by Fourier analysis. Brain death was induced by inflation of a subdurally placed balloon and validated neuropathologically. The results are expressed as mean and standard error of the mean (+/- SEM) (P < .05, paired t-test). RESULTS: Right and left ventricular function deteriorated significantly (P < .001) by 37% (+/- 10) and 22% (+/- 7) respectively following BD. Pulmonary and systemic vascular resistance as well as pulmonary impedance decreased significantly over 6 hours after BD. Pulsatile flow, a potent stimulant of NO release, converted significantly to more steady flow. Myocardial NO extraction values remained unchanged after BD and serum L-arginine decreased from 12.84 mu g/L (+/- 0.60) to 11.77 mu g/L (+/- 0.55). CONCLUSIONS: The decreases in pulmonary and systemic vascular resistance, pulmonary impedance, and cardiac function associated with BD are not related to major changes in the NO pathway. NO may not play a key role in the early changes after BD.

Animals↗
PubMedDate not supplied

Determination of the hemodynamic factors which influence the carotid Doppler spectral broadening.

In the diagnosis of extracranial carotid arterial disease, quantitative measurements from the continuous wave (CW) Doppler spectrum have the potential for detecting stenoses and occlusions. The measurement of maximum peak Doppler frequency at the site of stenosis has been shown to detect severe, but not minor or moderate, stenoses. Diagnosis of minor or moderate stenoses may be possible by assessing the degree of flow disturbance beyond the stenosis. Such flow disturbances cause the Doppler spectrum at peak systole to be broadened, and it has been suggested that the measurement of spectral broadening may be of diagnostic value. This paper describes the results of an in vitro study aimed at determining the hemodynamic factors that influence the severity of the Doppler spectral broadening. The spectral broadening index (SBI) at peak systole, defined as SBI = 1 - Fmean/Fmax, was used to quantify the instantaneous spectrum. In a pulsatile flow in vitro model that produced spectral waveforms virtually identical to those recorded in the human carotid, we observed a direct linear relationship between SBI and the severity of stenosis, at least for those stenoses having greater than 40% cross-sectional area (R = 0.82 to 0.93). The SBI was found to be maximum when recorded immediately beyond the stenosis and returned to normal 4-5 cm downstream from the stenosis. The SBI was higher for nonsymmetrically shaped stenoses than for symmetrical stenoses for lesions greater than 60%, but not for stenoses less than 60%. In this model, the SBI recorded from both normal or abnormal waveforms was not affected by the flow rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Carotid Arteries↗
PubMed1994

A compliant tubular device to study the influences of wall strain and fluid shear stress on cells of the vascular wall.

PURPOSE: Cellular constituents of the blood vessel wall are continuously subjected, in vivo, to both mechanical and hemodynamic forces, which elicit structural and biologic responses. We have developed a compliant tubular system, the vascular simulating device (VSD), that reproduces these forces, while supporting the attachment and the experimental manipulation of endothelial and smooth muscle cells. METHODS: The VSD consists of a compliant silicone rubber tube coupled to a pump system, which permits the simultaneous application of known levels of pressure and flow, to vascular wall cells cultured on the inner surface of the tube. Seeded cells can be monitored visually under phase contrast or fluorescent optics, as well as harvested and analyzed for biologic responses. RESULTS: The elastic modulus and compliance of the silicone rubber tube are similar to those of canine and human arteries. Endothelial and smooth muscle cells cultured on the lumenal surface of the tubes remain attached and viable after subjecting them to physiologic pulsatile flow and cyclic strain. CONCLUSION: The VSD makes it possible to approximate, in vitro, those forces encountered by vascular wall cells, in vivo and therefore may make it possible to determine whether specific combinations of mechanical and hemodynamic forces are causally associated with specific vascular diseases.

Animals↗
PubMed1997

Finite element methods in the simulation and analysis of intracranial blood flow.

This paper presents an introduction to the use of finite element methods in the simulation and analysis of intracranial blood flow and lays the foundation for more detailed clinically oriented studies. An overview of finite element theory is provided and includes the formulation of both the continuous and discrete equations of viscous fluid flow. A discussion of appropriate assumptions and boundary conditions governing arterial blood flow is presented. Two-dimensional, rigid-walled models are developed for flow in a straight artery, a 90 degrees curved artery and a bifurcated artery. For each model, a description of the finite element mesh, numerical solution and computational results are presented. This paper is the first in a series which will detail computational analysis of the relationship between pressure, velocity development of intracranial aneurysms and therapeutic approaches to aneurysm management. The goals of this research are to investigate the fluid dynamics that arise as a result of pulsatile flow in the arteries of the circle of Willis, relate these hemodynamics to the formation of aneurysms, develop a computational understanding of the effects of various therapies on blood flow related to aneurysms, and to develop and utilize patient specific computer simulations for treatment planning.

Cerebrovascular Circulation↗
PubMed2001

Performance of time-frequency representation techniques to measure blood flow turbulence with pulsed-wave Doppler ultrasound.

The current processing performed by commercial instruments to obtain the time-frequency representation (TFR) of pulsed-wave Doppler signals may not be adequate to characterize turbulent flow motions. The assessment of the intensity of turbulence is of high clinical importance and measuring high-frequency (small-scale) flow motions, using Doppler ultrasound (US), is a difficult problem that has been studied very little. The objective was to optimize the performance of the spectrogram (SPEC), autoregressive modeling (AR), Choi-Williams distribution (CWD), Choi-Williams reduced interference distribution (CW-RID), Bessel distribution (BD), and matching pursuit method (MP) for mean velocity waveform estimation and turbulence detection. The intensity of turbulence was measured from the fluctuations of the Doppler mean velocity obtained from a simulation model under pulsatile flow. The Kolmogorov spectrum, which is used to determine the frequency of the fluctuations and, thus, the scale of the turbulent motions, was also computed for each method. The best set of parameters for each TFR method was determined by minimizing the error of the absolute frequency fluctuations and Kolmogorov spectral bandwidth measured from the simulated and computed Doppler spectra. The results showed that different parameters must be used for each method to minimize the velocity variance of the estimator, to optimize the detection of the turbulent frequency fluctuations, and to estimate the Kolmogorov spectrum. To minimize the variance and to measure the absolute turbulent frequency fluctuations, four methods provided similar results: SPEC (10-ms sine-cosine windows), AR (10-ms rectangular windows, model order = 8), CWD (w(N) and w(M) = 10-ms rectangular windows, sigma = 0.01), and BD (w(N) = 10-ms rectangular windows, alpha = 16). The velocity variance in the absence of turbulence was on the order of 0.04 m/s (coefficient of variation ranging from 8.0% to 14.5%, depending on the method). With these spectral techniques, the peak of the turbulence intensity was adequately estimated (velocity bias < 0.01 m/s). To track the frequency of turbulence, the best method was BD (w(N) = 2-ms rectangular windows, alpha = 2). The bias in the estimate of the -10 dB bandwidth of the Kolmogorov spectrum was 354 +/- 51 Hz in the absence of turbulence (the true bandwidth should be 0 Hz), and -193 +/- 371 Hz with turbulence (the simulated -10-dB bandwidth was estimated at 1256 Hz instead of 1449 Hz). In conclusion, several TFR methods can be used to measure the magnitude of the turbulent fluctuations. To track eddies ranging from large vortex to small turbulent fluctuations (wide Kolmogorov spectrum), the Bessel distribution with appropriate set of parameters is recommended.

Blood Flow Velocity↗
PubMed2001

Ventilatory modes and mechanics of the hedgehog skate (Leucoraja erinacea): testing the continuous flow model.

The movement of water across the gills of non-ram-ventilating fishes involves the action of two pumps: a pressure pump that pushes water across the gills from the oropharyngeal to the parabranchial cavity, and a suction pump that draws water across the gills from the oropharyngeal into the parabranchial cavity. Together, the two are thought to keep water flowing continuously anteroposteriorly through the head of the respiring animal. However, there is evidence that the pressure and suction pumps do not always work in perfect phase in elasmobranch fishes, leading to periods of higher pressure in the parabranchial than in the oropharyngeal cavity. We investigated the existence and consequence of such pressure reversals in the hedgehog skate Leucoraja erinacea using pressure transducers, sonomicrometry and flow visualization including internal visualization using endoscopy. We noted four patterns of respiration in the experimental skates distinguished by the flow pattern at the three openings into the respiratory system: (1) in through the spiracle only, (2) in through the mouth + spiracle, (3) in through the mouth only, and (4) the mouth held open throughout the respiratory cycle. The first two were by far the dominant modes recorded from experimental animals. We determined that pressure reversals exist in the hedgehog skate, and that the gill bars adducted during such pressure reversals. Direct observation confirmed that these pressure reversals do correspond to pulsatile flow across the gills. During mouth+spiracle ventilation the flow completely reversed direction, flowing from the parabranchial chambers back across the gills and into the oropharyngeal cavity. Finally, we addressed the utility of sonomicrometry as a technique for determining kinematics in aquatic animals. Despite some problems involving errors inherent to the system design, we found the technique useful for complementing such techniques as pressure measurements and endoscopy.

Animals↗
PubMedDate not supplied

In vitro evaluation of a stentless porcine aortic bioprosthesis. A preliminary study.

Currently available aortic bioprosthetic heart valves are all relatively suboptimal due to the constraints placed on leaflets by stenting. The constraints on the stenting mechanism may directly or indirectly cause turbulence in the orifice neighborhood, be nidi of tissue calcification, and impart transleaflet stress differentials. To address all these deficiencies, we undertook the development of a stentless porcine aortic bioprosthesis (SPAB). This study evaluates the hemodynamic performance and durability of this design in special chambers with near physiologic compliance. The results are compared to those shown by a stented porcine aortic bioprosthesis. The pressure measurements under steady flow conditions showed that the hemodynamic performance (including pressure gradient and effective orifice area) of SPAB is superior to that of its stented counterpart, especially in the smaller sizes. In addition, it is expected that this stentless design should minimize the mechanical stress to which the leaflets are subjected throughout the cardiac cycle, thus enhancing the durability of the bioprosthesis. In our accelerated durability study for up to 2 equivalent years, no valve failure has been observed. More in vitro studies under pulsatile flow conditions, including color Doppler flow visualization, are currently being conducted.

Aortic Valve↗
PubMed1997

Cerebral circulatory responses of near-term ovine fetuses during sustained fetal placental embolization.

To test the hypothesis that, in response to an increase in placental vascular resistance and progressive fetal asphyxia, the changes in external carotid blood flow waveforms are directly related to changes in external carotid vascular resistance, we embolized the fetal side of the placenta in pregnant sheep and measured cerebral and external carotid artery circulatory changes in relation to changes in external carotid artery flow waveforms. Chronically catheterized fetal sheep at 0.85 of gestation were embolized (n = 11) in the descending aorta for 6 h, until fetal arterial pH fell to approximately 6.90. Fetuses became rapidly hypoxemic (P < 0.0001) and developed a mixed respiratory and metabolic acidosis (P < 0.0001 for PCO2, pH, and base excess). There was a transient 40% increase in external carotid blood flow at pH approximately 7.25 and a parallel 32% increase in fetal arterial blood pressure (both (P < 0.01), whereas the external vascular resistance remained unaltered. Cerebral blood flow increased by 130% (P < 0.0001), and cerebral vascular resistance decreased by 125% (P < 0.0001) throughout the study. The external carotid resistance index (RI) decreased by 32% (P < 0.0001) at the time external carotid vascular resistance remained unchanged. This fall in external carotid RI was due almost entirely to a 110% increase in external carotid fundamental impedance (P < 0.001). We conclude that the poor relationship between the changes in external carotid vascular resistance and RI indicated that other hemodynamic factors such as vascular impedance to pulsatile flow must be measured for correct interpretation of changes in flow waveform shape under hypoxic conditions. In addition, changes in external carotid blood flow were not proportional to changes in cerebral blood flow in this model.

Animals↗
PubMed1990

[Magnetic resonance angiography using a magnetic resonance flow tagging technique].

A direct bolus imaging method, which was developed for flow quantitation, was applied to the cervical regions of four normal volunteers to perform magnetic resonance angiography. A transverse section of 10 mm thickness was selectively excited to tag blood flow in the supraclavicular region and a projected image of the tagged bolus viewed in terms of anteroposterior direction was obtained after the echo time (TE). Like cine magnetic resonance imaging, multiple images representing four to 16 cardiac phases were obtained with repeated excitations. With a relatively long TE, ranging from 50 to 200 msec, we advanced tagged blood farther downstream, so as to elongate the visualized bolus along the vessel. Within the visualized bolus, the outer layer close to the vessel wall, where the blood flow velocity was slow, stretched like long tails behind the central part of the bolus producing arrow-head shapes, and the tails were assumed to represent the vascular structure. Bilateral common carotid and vertebral arteries were visualized in each image size approximately 5 cm obtained at the systolic phase. Since prolonged TE yielded less signal intensity, the bolus was not clearly visualized when TE was longer than 100 msec. The cine display of images with multiple cardiac phases produced good evaluations of dynamic changes of pulsatile flow, and this method is expected to be a useful diagnostic tool which combines the capability of flow quantitation with non-invasive angiography. The accuracy of this method in delineating a stenotic lesion was also evaluated using phantom with steady flow, since it is one of the most important capabilities of a clinically used angiographic method.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiography↗
PubMed1994

Theoretical considerations on the validity of the Stewart-Hamilton principle in measuring cycle-averaged flows via histogram of indicator in the pulsating compartment.

It has been heuristically shown that the Stewart-Hamilton principle, adapted to external counting observables of system indicator histogram, A(t), its cycle-averaged equilibrium count rate, A(equ), and indicator volume of distribution in the body, V(body), is F/V(body) = A(equ)/integral of o infinity A(t)dt, where F is the cycle-averaged cardiac output. Since the method lacks the theoretical plausibility, it remained unclear whether it is an approximation and what conditions warrant its usability. This paper presents an exact derivation of the above equation. To fulfill it the generalizations of the stationary theory of indicator kinetics were set up that allowed for the conditions of pulsatile flows and volumes and the dependence of the distribution of transit times of indicator on the phase of the cardiac cycle. The assumptions utilized were that the tracer enters the compartment well mixed and convectively carried by the blood in concentrations that do not vary in the single cycle to a material extent. The method yields the cardiac output, even when the flow to a compartment is only a part of it, provided that the fraction of indicator that traversed the system equals the fraction of cardiac output that perfuses the compartment. It was shown that, when applied to a regurgitant ventricle, the method obtains the forward flow and that separate application of the method to each of the ventricles provides the theoretical basis for evaluation of the central-circulatory shunts.

Ductus Arteriosus, Patent↗
PubMed2006

Dynamic modelling of prosthetic chorded mitral valves using the immersed boundary method.

Current artificial heart valves either have limited lifespan or require the recipient to be on permanent anticoagulation therapy. In this paper, effort is made to assess a newly developed bileaflet valve prosthesis made of synthetic flexible leaflet materials, whose geometry and material properties are based on those of the native mitral valve, with a view to providing superior options for mitral valve replacement. Computational analysis is employed to evaluate the geometric and material design of the valve, by investigation of its mechanical behaviour and unsteady flow characteristics. The immersed boundary (IB) method is used for the dynamic modelling of the large deformation of the valve leaflets and the fluid-structure interactions. The IB simulation is first validated for the aortic prosthesis subjected to a hydrostatic loading. The predicted displacement fields by IB are compared with those obtained using ANSYS, as well as with experimental measurements. Good quantitative agreement is obtained. Moreover, known failure regions of aortic prostheses are identified. The dynamic behaviour of the valve designs is then simulated under four physiological pulsatile flows. Experimental pressure gradients for opening and closure of the valves are in good agreement with IB predictions for all flow rates for both aortic and mitral designs. Importantly, the simulations predicted improved physiological haemodynamics for the novel mitral design. Limitation of the current IB model is also discussed. We conclude that the IB model can be developed to be an extremely effective dynamic simulation tool to aid prosthesis design.

Biocompatible Materials↗
PubMed2004

From arteriographies to computational flow in saccular aneurisms: the INRIA experience.

Saccular aneurisms illustrate usefulness and possible techniques of image-based modeling of flow in diseased vessels. Aneurism flow is investigated in order to estimate the rupture risk, assuming that the pressure is the major factor and that high-pressure zones are correlated to within-wall strong-stress concentrations. Computational flow is also aimed at providing additional arguments for the treatment strategy. Angiographies of aneurismal vessels of large and medium size are processed to provide three-dimensional reconstruction of the vessel region of interest. Different reconstruction techniques are used for a side and a terminal aneurisms. Reconstruction techniques may lead to different geometries especially with poor input data. The associated facetisation is improved to get a computation-adapted surface triangulation, after a treatment of vessel ends and mesh adaptation. Once the volumic mesh is obtained, the pulsatile flow of an incompressible Newtonian blood is computed using in vivo non-invasive flowmetry and the finite element method. High pressure zones are observed in the aneurism cavity. The pressure magnitude in the aneurism, the location and the size of high pressure zones depend mainly on the aneurism implantation on the vessel wall and its orientation with respect to the blood flux in the upstream vessel. The stronger the blood impacts on the aneurismal wall the higher the pressure. The state of the aneurism neck, where a high-pressure zone can occur, and the location of the aneurism, with an easy access or not, give arguments for the choice between coiling and surgical clipping. Mesh size and 3D reconstruction procedure affect the numerical results. Helpful qualitative data are provided rather than accurate quantitative results in the context of multimodeling.

Angiography↗
PubMed2005

A low-flow adaptation phase improves shear-stress resistance of artificially seeded endothelial cells.

INTRODUCTION: The purpose of this study was to evaluate the effect of different adaptation phases on the shear-stress resistance of endothelial cells seeded artificially onto vascular prostheses and biological heart valves. MATERIAL AND METHODS: Human endothelial cells (EC), fibroblasts (FB), and smooth muscle cells (SMC) were isolated from vena saphena magna pieces and expanded in culture. Group A: 15 polyurethane vascular grafts (20 mm diameter) were seeded with FB and SMC (53 +/- 1.2 million cells), followed by EC seeding (39 +/- 0.9 million cells). Group B: eight stentless porcine valves (Freestyle, Medtronic, USA) were seeded with FB (68 +/- 1.5 million cells) and EC (42 +/- 1.1 million cells). Shear-stress testing was done under pulsatile flow (pulse rate: 80 pulses/min.). Adaptation phase: flow was set to 0.9 +/- 0.3 l/min (systolic pressure: 40 - 50 mm Hg). High flow was 3.2 +/- 0.6 l/min. (systolic pressure: 140 - 160 mm Hg) and lasted over four hours in all groups. The vascular grafts were divided into three groups (n = 5 each): group 1 (high flow immediately), group 2 (adaptation phase of 15 minutes), and group 3 (adaptation phase of 30 minutes). The valves either were given high flow immediately (n = 4) or had an adaptation phase of 30 minutes (n = 4). Specimens were obtained after cell seeding, before, and after perfusion. RESULTS: A confluent EC layer was achieved on all grafts. After perfusion without adaptation, large defects within the cell layer were found. No FB and SMC were seen at the bottom of these defects. In group B, the defects were largest on the ventricular surface of the leaflets. After an adaptation phase of 15 minutes in group A, only a few defects within the EC layer were detected with a still confluent FB and SMC. After a 30-minute adaptation phase defects within the EC layer were very rare and no interruption of the underlying FB and SMC layer was seen. Immunohistochemical staining for factor VIII and CD31 proved the EC to be viable and staining for collagen IV and laminin revealed the formation of a basement membrane. After perfusion, the specimen also stained positive for eNOS. CONCLUSION: An adaptation phase of 30 minutes proved to be sufficient to allow artificially seeded endothelial cells to adapt to shear stress. The formation of a basement membrane was of great importance for the maintenance of a confluent EC layer.

Adaptation, Physiological↗
PubMed1989

Beneficial effects of banding on venous intimal-medial hyperplasia in arteriovenous loop grafts.

Local hemodynamics were modified in a canine arteriovenous loop graft model by placing a flow-limiting band on femoral polytetrafluoroethylene (PTFE) grafts. Banded and unbanded grafts were implanted in a paired fashion. Hemodynamic studies included Reynolds number and phonoangiography as measures of turbulence. Intimal-medial thickness was measured 8 weeks after implantation. Reduction of the volumetric flow rate by 50 percent resulted in significant changes in flow velocity, flow pulsatility, pressure, and turbulence at the venous anastomosis. Hyperplastic lesions developed in a reproducible manner at the venous anastomosis of the unbanded but not the banded grafts, as evidenced by combined intimal-medial thickness measurements: unbanded grafts 0.68 +/- 0.13 mm (p less than 0.01 versus control), banded grafts 0.25 +/- 0.03 mm (p greater than 0.05 versus control). Stepwise regression analysis indicated Reynolds number had the best correlation with the development of hyperplasia (r = 0.915, p less than 0.005), this being the first time this correlation has been quantitatively determined. We conclude that flow disturbance or turbulence is a major factor in the development of venous intimal-medial hyperplasia in arteriovenous loop grafts.

Animals↗