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At least 667 records · Page 37Linked to original sources
PubMed2002

Effect of acquisition parameters on the accuracy of velocity encoded cine magnetic resonance imaging blood flow measurements.

PURPOSE: To investigate the effect of acquisition parameters on the accuracy of 2D velocity encoded cine magnetic resonance imaging (VEC MRI) flow measurements. MATERIALS AND METHODS: Using a pulsatile flow phantom, through-plane flow measurements were performed on a flexible vessel made of polyvinyl alcohol cryogel (PVA), a material that mimics the MR signal and biomechanical properties of aortic tissue. RESULTS: Repeated VEC MRI flow measurements (N = 20) under baseline conditions yielded an error of 0.8 +/- 1.5%. Slice thickness, angle between flow and velocity encoding directions, spatial resolution, velocity encoding range, and radio frequency (RF) flip angles were varied over a clinically relevant range. Spatial resolution had the greatest impact on accuracy, with a 9% overestimation of flow at 16 pixels per vessel cross-section. CONCLUSION: VEC MRI proved to be an accurate and reproducible technique for pulsatile flow measurements over the range of acquisition parameters examined as long as sufficient spatial resolution was prescribed.

Analysis of Variance↗
PubMed2005

Effects of flow patterns on the localization and expression of VE-cadherin at vascular endothelial cell junctions: in vivo and in vitro investigations.

Atherosclerosis occurs preferentially at vascular curvature and branch sites where the vessel walls are exposed to fluctuating shear stress and have high endothelial permeability. Endothelial permeability is modulated by intercellular adhesion molecules such as VE-cadherin. This study was designed to elucidate the effects of different flow patterns on the localization and expression of VE-cadherin in endothelial cells (ECs) both in vivo and in vitro. VE-cadherin staining at EC borders was much stronger in the descending thoracic aorta and abdominal aorta, where the pulsatile flow has a strong net forward component than in the aortic arch and the poststenotic dilatation site beyond an experimental constriction, where the flow near the wall is complex and reciprocating with little net flow. With the use of flow chambers the effects of pulsatile flow (12 +/- 4 dyn/cm2 at 1 Hz) and reciprocating flow (0.5 +/- 4 dyn/cm2 at 1 Hz) on VE-cadherin organization in endothelial monolayers were studied in vitro. VE-cadherin staining was continuous along cell borders in static controls. Following 6 h of either pulsatile or reciprocating flow, the VE-cadherin staining at cell borders became intermittent. When the pulsatile flow was extended to 24, 48 or 72 h the staining around the cell borders became continuous again, but the staining was still intermittent when the reciprocating flow was similarly extended. Exposure to pulsatile or reciprocating flow for 6 and 24 h neither change the expression level of VE-cadherin nor its distribution between membrane and cytosol fractions as determined by Western blot and compared with static controls. These findings suggest that the cell junction remodeling induced by different flow patterns may result from a redistribution of VE-cadherin within the cell membrane. Both the in vivo and in vitro data indicate that pulsatile and reciprocating flow patterns have different effects on cell junction remodeling. The lack of junction reorganization in regions of reciprocating flow in vivo and in vitro may provide a mechanistic basis for the high permeability and the preferential localization of atherosclerosis in regions of the arterial stress with complex flow patterns and fluctuating shear stress.

Animals↗
PubMedDate not supplied

Experimental analysis of the influence of stenotic geometry on steady flow.

We studied the flow behavior under steady flow conditions in four models of cylindrical stenoses at Reynolds numbers from 150 to 920. The flow upstream of the constrictions was always fully developed. The constriction ratios of the rigid tubes (D) to the stenoses (d) were d/D = 0.273; 0.505; 0.548; 0.786. The pressure drop at various locations in the stenotic models was measured with water manometers. The flow was visualized with a photoelasticity apparatus using an aqueous birefringent solution. We also studied the flow behavior at pulsatile flow in a dog aorta with a constriction of 71%. The flow through stenotic geometries depends on the Reynolds number of the flow generated in the tube and the constriction ratio d/D. At low d/D ratios, (with the increased constriction), the flow separation zones (recirculation zones, so-called reattachment length) and flow disturbances increased with larger Reynolds numbers. At lower values, eddies were generated. At high Re, eddies were observed in the pre-stenotic regions. The pressure drop is a function of the length and internal diameter of the stenosis, respective ratio of stenosis to the main vessel and the Reynolds numbers. At low Re-numbers and low d/D, distinct recirculation zones were found close to the stenosis. The flow is laminar in the distal areas. Further experiments under steady and unsteady flow conditions in a dog aorta model with a constriction of 71% showed similar effects. High velocity fluctuations downstream of the stenosis were found in the dog aorta. A videotape demonstrates these results.

Animals↗
PubMed1995

Changes in contractility and afterload have only slight effects on subendocardial systolic flow impediment.

To test the hypothesis that contractility did not greatly influence systolic impediment to subendocardial flow we perfused the coronary artery at 70 mmHg and altered aortic pressure in 7 dogs and contractility in another 7. We measured myocardial systolic flow impediment (SFI) by comparing regional flows while beating and during asystole. Cardiac contraction impeded 29% of subendocardial asystolic flow, which was not affected by either intervention. In subepicardium, contraction increased flow by 25%, but dobutamine impeded systolic flow. Subepicardial SFI was only 16% of subendocardial SFI. Dobutamine slightly decreased estimated percent systolic myocardial blood flow (%SMBF) in subendocardium (+/- 12%) but decreased subepicardial %SMBF (45.5 to 17.4%). Phasic coronary flow pulsatility increased more with dobutamine than increased afterload, and pulsatility and SFI correlated only in subepicardium. Systolic-to-total coronary flow ratio and %SMBF did not correlate closely in subendocardium. SFI was most prominent in the subendocardium, whereas subepicardial SFI mainly determined epicardial coronary flow pulsatility. We conclude that the effects of contractility changes differ when evaluating regional SFI vs. phasic flow pulsatility.

Animals↗
PubMed1994

Clinical evaluation of a new type of centrifugal pump.

The major problems with existing centrifugal pumps are leakage, mechanical trauma, and thrombus formation. In consideration of these problems, a new compact centrifugal pump system was developed. The purpose of this study was to evaluate the new centrifugal pump system clinically. Ten patients underwent open heart surgery with a centrifugal pump or a roller pump. During surgery, hemodynamic and hematological data were obtained. A pulsatile assist device in the pump circuit was used in patients with severe heart disease. There was neither operative death nor hospital mortality, and there was no difference with regard to hemodynamic data between the two groups. The centrifugal pump groups, however, had significantly lower hemolysis, especially during prolonged cardiopulmonary bypass. This centrifugal pump could also create sufficient pulsatile flow with a pulsatile assist device. Postoperative macroscopic and microscopic findings demonstrated the smooth surface of the pump without thrombus formation. This centrifugal pump system might be useful for prolonged cardiopulmonary bypass.

Adolescent↗
PubMed1999

Flow patterns in the radiocephalic arteriovenous fistula: an in vitro study.

A significant number of late failures of arteriovenous fistulae for haemodialysis access are related to the progression of intimal hyperplasia. Although the aetiology of this process is still unknown, the geometry of the fistula and the local haemodynamics are thought to be contributory factors. An in-vitro study was carried out to investigate the local haemodynamics in a model of a Cimino-Brescia arteriovenous (AV) fistula with a 30 degrees anastomotic angle and vein-to-artery diameter ratio of 1.6. Flow patterns were obtained by planar illumination of micro-particles suspended in the fluid. Steady and pulsatile flow studies were performed over a range of flow conditions corresponding to those recorded in patients. Quantitative measurements of wall shear stress and turbulence were made using laser Doppler anemometry. The flow structures in pulsatile flow were similar to those seen in steady flow with no significant qualitative changes over the cardiac cycle. This was probably the result of the low pulsatility index of the flow waveform in AV fistulae. Turbulence was the dominant feature in the vein, with relative turbulence intensity > 0.5 within 10 mm of the suture line decreasing to a relatively constant value of about 0.10-0.15 between 40 and 70 mm from the suture line. Peak and mean Reynolds shear stress of 15 and 20 N/m2, respectively, were recorded at the suture line. On the floor of the artery, peak values of temporal mean and oscillating wall shear stress of 9.22 and 29.8 N/m2, respectively. In the vein, both mean and oscillating wall shear stress decreased with distance from the anastomosis.

Arteriovenous Shunt, Surgical↗
PubMed1995

Comparison of valvular resistance, stroke work loss, and Gorlin valve area for quantification of aortic stenosis. An in vitro study in a pulsatile aortic flow model.

BACKGROUND: Valvular resistance and stroke work loss have been proposed as alternative measures of stenotic valvular lesions that may be less flow dependent and, thus, superior over valve area calculations for the quantification of aortic stenosis. The present in vitro study was designed to compare the impacts of valvular resistance, stroke work loss, and Gorlin valve area as hemodynamic indexes of aortic stenosis. METHODS AND RESULTS: In a pulsatile aortic flow model, rigid stenotic orifices in varying sizes (0.5, 1.0, 1.5 and 2.0 cm2) and geometry were studied under different hemodynamic conditions. Ventricular and aortic pressures were measured to determine the mean systolic ventricular pressure (LVSPm) and the transstenotic pressure gradient (delta Pm). Transvalvular flow (Fm) was assessed with an electromagnetic flowmeter. Valvular resistance [VR = 1333.(delta Pm/Fm)] and stroke work loss [SWL = 100.(delta Pm/LVSPm)] were calculated and compared with aortic valve area [AVA = Fm/(50 square root of delta Pm)]. The measurements were performed for a large range of transvalvular flows. At low-flow states, flow augmentation (100-->200 mL/s) increased calculated valvular resistance between 21% (2.0 cm2 orifice) and 66% (0.5-cm2 orifice). Stroke work loss demonstrated an increase from 43% (2.0 cm2) to 100% (1.0 cm2). In contrast, Gorlin valve area revealed only a moderate change from 29% (2.0 cm2) to 5% (0.5 cm2). At physiological flow rates, increase in transvalvular flow (200-->300 mL/s) did not alter calculated Gorlin valve area, whereas valvular resistance and stroke work loss demonstrated a continuing increase. Our experimental results were adopted to interpret the results of three clinical studies in aortic stenosis. The flow-dependent increase of Gorlin valve area, which was found in the cited clinical studies, can be elucidated as true further opening of the stenotic valve but not as a calculation error due to the Gorlin formula. CONCLUSIONS: Within the physiological range of flow, calculated aortic valve area was less dependent on hemodynamic conditions than were valvular resistance and stroke work loss, which varied as a function of flow. Thus, for the assessment of the severity of aortic stenosis, the Gorlin valve area is superior over valvular resistance and stroke work loss, which must be indexed for flow to adequately quantify the hemodynamic severity of the obstruction.

Aortic Valve↗
PubMed2001

The influence of ventricular input impedance on the hydrodynamic performance of bioprosthetic aortic roots in vitro.

BACKGROUND AND AIM OF THE STUDY: Hydrodynamic function testing using pulsatile flow simulators provides a valuable means of comparative assessment of heart valves in vitro. The majority of pulsatile flow simulators consist of modular rigid chambers and a positive displacement pump with an infinite input impedance, in which the inertia of the test fluid results in pressure oscillations when the valves under test are opening and closing. For mechanical and stented bioprosthetic valves these pressure oscillations decay quickly. However, due to the highly compliant nature of tissue roots, the resulting pressure and flow oscillations are extreme and extend throughout systole. With increasing interest in the use of free-sewn roots and valves it is most desirable to improve this hydrodynamic model. The aim of this study was to investigate the influence in changes in ventricular input impedance on the hydrodynamic characteristics of free-sewn aortic roots and stented valves. METHODS: The Leeds pulsatile flow simulator was modified to incorporate additional compliance chambers in the form of a viscoelastic impedance adaptor (VIA) at the pump/ventricular interface. Six 23 mm bioprosthetic aortic roots fixed with 0.5% buffered glutaraldehyde at zero pressure, and a size 23 mm stented porcine aortic bioprosthesis were tested in this modified simulator, at the conditions of maximum and minimum input compliance. RESULTS: The pressure and flow waveforms for the fixed aortic roots showed considerable differences at the conditions of maximum and minimum input compliance. Indeed, the extreme pressure oscillations observed at minimum compliance (infinite input impedance) were not present at maximum compliance, and the forward flow waveform was much smoother. In contrast, for the stented valve, the differences in the pressure and flow waveforms between maximum and minimum input compliance were minimal, but this was expected due to the lack of compliance in the stented valve itself. In addition, the flow and pressure waveforms at maximum compliance in the VIA were comparable for the fixed aortic roots and the stented bioprosthesis, thus allowing direct comparison of the characteristics of these two different devices. Using test conditions of maximum input compliance, effective orifice area for the roots was 1.69 cm2 compared with 1.47 cm2 for the stented valve. CONCLUSION: An appropriate physiological model for the hydrodynamic testing of compliant tissue roots has been established.

Aortic Valve↗
PubMed1997

Relation of the flow field distal to a moderate stenosis to the Doppler power.

An experimental investigation was undertaken to establish how different flow regimes affect the Doppler signal. A rigid tube model consisting of a 70% asymmetric area stenosis was used with steady and pulsatile flow conditions. The characteristics of the flow field at various sites was determined using a photochromic flow visualization method. Continuous-wave Doppler measurements were made using a 41% suspension of human red blood cells (RBCs) in saline as well as a dilute suspension of 4% fixed RBCs. For steady flow, the photochromic results indicated that for Reynolds numbers (Re) of 545 and 1410, turbulence was generated and the length of the turbulent region was found to increase with increasing Re. Under pulsatile flow conditions, turbulence was triggered around peak systole and began to dissipate in late deceleration, and by the end of diastole the flow field almost relaminarized. During the turbulent phase of the flow cycle, the poststenotic flow field was seen to consist of four distinct flow regimes similar to those observed for steady flow. For higher Womersley parameters and Reynolds numbers the turbulent zone was found to be larger and to occupy a greater fraction of the flow cycle. These flow visualization results were compared with the Doppler power measurements made at the same locations and under similar flow conditions. At physiological hematocrits (41%) the onset of turbulence for both steady and pulsatile flow increased the backscattered Doppler power. The location of the peak Doppler power coincided with the region of maximum turbulence observed using the photochromic technique.

Constriction, Pathologic↗
PubMedDate not supplied

The relationship between blood flow and pulsatility index in the superior mesenteric artery at rest and during constrictor stimuli in normal subjects.

The relationships between superior mesenteric artery blood flow (SMABF) and pulsatility index (PI) measurement during rest (25 subjects) and stimuli constricting the SMA (16 subjects) have been studied in normal subjects. At rest and during constrictor stimuli, SMABF and PI were highly reproducible (r = 0.89, p < 0.01 for SMABF, and r = 0.97, p < 0.001 for PI) between two observers. There was significant correlation between changes in SMABF, PI, and SMA vascular resistance during the constrictor stimuli, except during head-up tilt when PI was unchanged. Both PI and SMABF measurements are reproducible and can be used to monitor physiological changes in suitable (18 of 25) subjects. PI measurement, although semiquantitative, by itself can also be used to monitor these changes. This may be also of importance in pathological situations such as intestinal ischemia, where measurement of volume blood flow may be less accurate due to irregularities of the vessel wall. PI measurement, however, ideally should not be used in studies involving postural change.

Adult↗
PubMed1999

Model studies of the flow in abdominal aortic aneurysms during resting and exercise conditions.

Pulsatile flow in abdominal aortic aneurysm (AAA) models has been examined in order to understand the hemodynamics that may contribute to growth of an AAA. The model studies were conducted by experiments (flow visualization and laser Doppler velocimetry) and by numerical simulation using physiologically realistic resting and exercise flow conditions. We characterize the flow for two AAA model shapes and sizes emulating early AAA development through moderate AAA growth (mean and peak Reynolds numbers of 362 < Re(mean) < 1053 and 3308 < Re(peak) < 5696 with Womersley parameter 16.4 < alpha < 21.2). The results of our investigation indicate that AAA flow can be divided into three flow regimes: (i) Attached flow over the entire cycle in small AAAs at resting conditions, (ii) vortex formation and translation in moderate size AAAs at resting conditions, and (iii) vortex formation, translation and turbulence in moderate size AAAs under exercise conditions. The second two regimes are classified in the medical literature as disturbed flow conditions that have been correlated with atherogenesis as well as thrombogenesis. Thus, AAA disturbed hemodynamics may be a contributing factor to AAA growth by accelerating the degeneration of the arterial wall. Our investigation also concluded that vortex development is considerably weaker in an asymmetric AAA. Furthermore, turbulence was not observed in the asymmetric model. Finally, our investigation suggests a new mode of transition to turbulence: vortex ring instability and bursting to turbulence. The transition process depends on a combination of the pulsatile flow conditions and the tube cross-sectional area change.

Aortic Aneurysm, Abdominal↗
PubMed1993

Three-dimensional display of calculated velocity profiles for physiological flow waveforms.

PURPOSE: To improve the understanding of the nature of pulsatile flow, three-dimensional idealized velocity profiles corresponding to measured physiological mean flow velocity waveforms were displayed at selected instants throughout the flow cycle. METHODS: The Fourier harmonics for each waveform were determined, and their corresponding velocity profiles at each instant of time were calculated with the Womersley equations. Velocity profiles were calculated by summing the contributions from each harmonic. RESULTS: Calculated profiles were displayed in a three-dimensional perspective for both normal carotid and femoral arteries and for simple sinusoidal flow with a superimposed steady component. CONCLUSION: The potential value of such displays is discussed in terms of gaining an improved understanding of the nature of pulsatile flow and clarifying the interpretation of Doppler ultrasound recordings.

Adult↗
PubMed2000

Using one rotary blood pump to produce separate pulsatile circulations in the upper and lower halves of the body.

Separate systemic circulations with pulsatile flow were obtained using 1 rotary blood pump as a left ventricular assist device. The outlet of the pump was divided into 2 conduits, 1 connected to the upper half of the body and the other connected to the lower half. An electric actuator that clamped the 2 outlet conduits alternately provided pulsatile flows. An in vitro experiment showed that the pulsatility phases of the upper and lower halves of the body were complementary with pulsatile flow, and an in vivo experiment showed that controlled flow distributions of continuous flows could be obtained.

Animals↗
PubMed1993

Magnetic resonance imaging of blood flow with a phase subtraction technique. In vitro and in vivo validation.

RATIONALE AND OBJECTIVES: One promising approach to flow quantification uses the velocity-dependent phase change of moving protons. A velocity-encoding phase subtraction technique was used to measure the velocity and flow rate of fluid flow in a phantom and blood flow in volunteers. METHODS: In a model, the authors measured constant flow velocities from 0.1 to 270.0 cm/second with an accuracy (95% confidence intervals) of +/- 12.5 cm/second. There was a linear relationship between the magnetic resonance imaging (MRI) measurement and the actual value (r2 = .99; P = .0001). RESULTS: Measuring mean pulsatile flow from 125 to 1,900 mL/minute, the accuracy of the MRI pulsatile flow measurements (95% confidence intervals) was +/- 70 mL/minute. There was a linear relationship between the MRI pulsatile flow measurement and the actual value (r2 = .99; P = .0001). In 10 normal volunteers, the authors tested the technique in vivo, quantitating flow rates in the pulmonary artery and the aorta. The average difference between the two measurements was 5%. In vivo carotid flow waveforms obtained with MRI agreed well with the shape of corresponding ultrasound Doppler waveforms. CONCLUSIONS: Velocity-encoding phase subtraction MRI bears potential clinical use for the evaluation of blood flow. Potential applications would be in the determination of arterial blood flow to parenchymal organs, the detection and quantification of intra- and extra-cardiac shunts, and the rapid determination of cardiac output and stroke volume.

Adult↗