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PubMed2001

Pulsatile flow simulation in arterial vascular segments with intravascular ultrasound images.

Previous studies have indicated a correlation between local variation in wall shear stress in arterial blood flow and atheroma development. The purpose of this study was to analyze the hemodynamics in vascular segments from morphologically realistic three-dimensional (3D) reconstruction, and to compare the computed wall shear stress in a compliant vascular segment model and the corresponding rigid walled model. Cross-sectional images of the segments of femoral and carotid arteries in five Yucatan miniswine were obtained using intravascular ultrasound (IVUS) imaging and the segment geometry was reconstructed at different times in the cardiac cycle. The actual measured wall motion from the reconstruction was employed to specify the moving boundaries for simulation of physiological distensibility. Velocity profiles and wall shear stress were computed using unsteady computational fluid dynamics analysis. The computed results revealed that the maximum wall shear stress in the compliant model was approximately 4-17 percent less than that in the rigid model if the wall motion is larger than 10 percent. Our analysis demonstrates that inaccuracies due to inflow velocity profile can be minimized by the extension of the model upstream. The phase angle between the diameter change and wall shear is affected by the local changes in geometry of the arteries. These simulations can be potentially used to analyze the effect of regional wall motion changes in the presence of atherosclerotic lesions on the local fluid dynamics and to correlate the same with subsequent growth of the lesions.

Animals↗
PubMed1992

Effect of synchronous and asynchronous pulsatile flow during left, right, and biventricular bypass.

Ventricular assist devices augment flow from the left atrium to the aorta and/or from the right atrium to the pulmonary artery. Most devices are used in the asynchronous full-to-empty mode (asynchronous) but may also be used in a synchronous counterpulsation mode (synchronous). This study determines the optimal assist modes to reduce myocardial oxygen consumption (MVO2) and metabolism. Twelve pigs were instrumented with carotid artery and Baim coronary sinus catheters for determination of MVO2 and myocardial lactate production (LACT). Six were implanted with a Pierce-Donachy left ventricular assist device (LVAD) and 6 with both right and left ventricular assist devices (BIVAD). Two periods each of control, synchronous, and asynchronous bypass were instituted, the midanterior descending coronary artery (LAD) was ligated, and the sequence was repeated. After each period, MVO2 and LACT were determined and myocardial biopsy specimens were obtained for tissue, lactate, and ATP assay. Following LAD ligation, biopsy specimens were obtained from both the infarct and noninfarct zones of the heart. MVO2 decreased (p < 0.05) in the asynchronous BIVAD mode compared with control. MVO2 was unchanged in synchronous BIVAD or either LVAD mode. Tissue ATP and tissue lactate were unaffected by any mode of bypass. Only BIVAD in the asynchronous mode reduced MVO2. When ventricular assist devices are utilized to aid recovery of the natural heart, two devices should always be inserted to allow biventricular assist. Synchronous counterpulsation offers no advantage.

Adenosine Triphosphate↗
PubMed1992

[Pulsatile flow model with elastic blood vessels for duplex ultrasound studies].

Using ultrasound duplex technique flow phenomena in patients' circulation can be examined. For the interpretation of these examinations it is necessary to have extensive knowledge on flow influencing parameters. This can be easily obtained from simplified flow models. This article describes the components of a flow model that allows examination of ultrasonic contrast media flowing through an artificial heart and vessel mimicking tubes. The artificial heart is the drive which pumps a water glycerol cellulose mixture through the circulation in a pulsatile manner. The shape of the ventricle, the compliance of the aorta, the viscosity of the flow medium and the wall elasticity of the examination vessel were taken into account. The attenuation caused by the surrounding tissue is simulated by a variable layer of castor oil. The flow model is suitable to produce flow profiles that are very similar to physiological profiles.

Blood Flow Velocity↗
PubMed1996

Pulsatile flow in the human left coronary artery bifurcation: average conditions.

The localization of atherosclerosis in the coronary arteries may be governed by local hemodynamic features. In this study, the pulsatile hemodynamics of the left coronary artery bifurcation was numerically simulated using the spectral element method for realistic in vivo anatomic and physiologic conditions. The velocity profiles were found to be skewed in both the left anterior descending and the circumflex coronary arteries. Velocity skewing arose from the bifurcation as well as from the curvature of the artery over the myocardial surface. Arterial wall shear stress was significantly lower in the bifurcation region, including the side walls. The greatest oscillatory behavior was localized to the outer wall of the circumflex artery. The time-averaged mean wall shear stress varied from about 3 to 98 dynes/cm2 in the left coronary artery system. The highly localized distribution of low and oscillatory shear stress along the walls strongly correlates with the focal locations of atheroma in the human left coronary artery.

Blood Flow Velocity↗
PubMed2004

Echogenicity variations from porcine blood I: the "bright collapsing ring" under pulsatile flow.

The temporal and radial variations of the echogenicity from porcine blood were investigated using a linear M12L transducer with a GE LOGIQ 700 Expert system. The "bright collapsing ring" (BRCR) phenomenon, a bright echogenic ring converging from the periphery to the center of the tube wall and eventually collapsing during a pulsatile cycle in cross-sectional B-mode images, was observed from porcine blood in a mock flow loop within a 0.95-cm diameter tube under certain flow conditions. The BRCR phenomenon from porcine blood was stronger as the peak speed was increased from 10 to 25 cm/s, and the mean echogenicity and the "black hole" (BLH) phenomenon, a central echo-poor zone surrounded by a bright hyperechoic zone, became weaker. As stroke rate was increased from 20 to 60 beats/min (bpm), both the BRCR and the BLH phenomena became weaker. These two phenomena were observed at three transmitting frequencies (9, 11 and 13 MHz). As hematocrit was increased from 12 to 45%, the BRCR phenomenon became more apparent. The nonlinear behavior of backscatter as a function of hematocrit reaching a maximum at hematocrit of 10 approximately 20% was observed near the tube wall, but it changed at the center of the tube, indicating the importance of hemodynamics on the ultrasonic backscatter from flowing blood. The combined effects of shear rate and acceleration on red blood cell aggregation are suggested as a possible mechanism for these phenomena.

Animals↗
PubMed1988

Three-dimensional visualization of velocity fields downstream of six mechanical aortic valves in a pulsatile flow model.

Velocity fields downstream of 27 mm Björk-Shiley Standard, Björk-Shiley Convex-Concave, Björk-Shiley Monostrut, Hall-Kaster (Medtronic-Hall), St. Jude Medical and Starr-Edwards Silastic Ball aortic valves were studied in a pulsatile mock circulation. Stroke volume was 70 cm3 and frequency 71 min-1 and 88 min-1. Fluid velocity was measured by a catheter mounted hot-film anemometer probe in a glycerol water mixture one and two diameters downstream of the aortic valve. Velocity fields were dynamically visualized by a three-dimensional technique and revealed qualitative independence of frequency. All profiles were flat in the acceleration phase of systole. From peak systole and throughout the systolic deceleration phase profiles characteristic of the individual valves appeared. The pivoting and tilting disc valves caused a skewed velocity profile with highest velocities downstream of the major orifice and lowest velocities downstream of the minor orifice. The differences between the three investigated Björk-Shiley valves were remarkable. The St. Jude Medical valve generated velocity peaks downstream of the two major orifices and the central slit, and lower velocities in the hinge areas. A rather flat profile with central hollowing was seen downstream of the Starr-Edwards Ball valve. All velocity profiles were more or less dampened two diameters downstream.

Aortic Valve↗
PubMed1993

Pressure propagation in pulsatile flow through random microvascular networks.

A microvascular network with random dimensions of vessels is built on the basis of statistical analysis of conjunctival beds reported in the literature. Our objective is to develop a direct method of evaluating the statistics of the pulsatile hydrodynamic field starting from a priori statistics which mimic the large-scale heterogeneity of the network. The model consists of a symmetric diverging-converging dentritic network of ten levels of vessels, each level described by a truncated Gaussian distribution of vessel diameters and lengths. In each vascular segment, the pressure distribution is given by a diffusion equation with random parameters, while the blood flow rate depends linearly on the pressure gradient. The results are presented in terms of the mean value and standard deviation of the pressure and flow rate waveforms at two positions along the network. It is shown that the assumed statistical variation of vessel lengths results in flow rate deviations as high as 50 percent of the mean, while the corresponding effect of vessel diameter variation is much smaller. For a given pressure drop, the statistical variation of lengths increases the mean flow while the effect on the mean pressure distribution is negligible.

Blood Pressure↗
PubMed1990

In vitro measurement of stenotic human aortic valve orifice area in a pulsatile flow model. Validation of the continuity equation.

Aortic valve orifice area estimation in patients with aortic stenosis may be obtained non-invasively using several Doppler echocardiographic methods. Their validity has been established by correlation with catheterization data using the Gorlin formula, with its inherent limitations, and small discrepancies between the methods are present. To evaluate these differences further, 15 patients with severe aortic stenosis (mean transvalvular gradient 70, range 40-130 mmHg) had aortic valve area estimations by Doppler echocardiography using two variations of the continuity equation. The intact valves removed at valve replacement surgery were then mounted in a pulsatile model and the anatomical area was measured (mean 0.67 +/- 0.17 cm-2) from video recordings during flow at 5.4 l min-1. Aortic valve area calculated using the integrals of the velocity-time curves measured at the left ventricular outflow tract and aortic jet (mean 0.65 +/- 0.17 cm2) correlated best with the anatomical area (r = 0.87, P less than 0.001). The area derived by using the ratio of maximum velocities from the left ventricular outflow tract and aortic jet (mean 0.69 +/- 0.18 cm2) also correlated well with the anatomical area (r = 0.79, P less than 0.001). The index between the left ventricular outflow tract and aortic jet maximum velocities was less than or equal to 0.25 in all. In patients with severe aortic stenosis the aortic valve area can be reliably estimated using Doppler echocardiography.

Adult↗
PubMed1997

A statistical approach to the quantitative comparison of pulsatile flow in vitro data of prosthetic heart valve testing.

BACKGROUND AND AIMS OF THE STUDY: In vitro evaluation and animal testing are fundamental steps in the assessment of prosthetic heart valves before their use in clinical practice. Valve testing under pulsatile conditions is the best in vitro simulation of cardiac valve function, and the ensuing results should support the surgeon's decision concerning the time of valve replacement. However, limits in hydraulic reproduction of cardiac function and differences in protocol implementation of in vitro testing lead to difficulties in obtaining reliable and comparable results. Debate among researchers and standardizing bodies about discrepancies in results becomes critical in light of the European CE certification of implantable medical devices. An interlaboratory environment has been created at the Biomedical Engineering unit of the Istituto Superiore di Sanita in Rome, which uses differing test apparatus to evaluate prosthetic heart valves in vitro, in order to define significant measurement parameters and procedures that produce comparative data. METHODS: Two prosthetic valves-a tilting disc and a bileaflet valve-each sized 29 mm, were tested in the aortic position on two different pulse duplicators (PDs), namely the Dynatek MP1 and a system developed at the University of Sheffield. The common protocol adopted was the FDA Interlaboratory Comparison Testing Protocol. Original software was used to manage all test phases, thus minimizing operator-dependent variability in both systems and imposing strict control of experimental conditions. Statistical analysis performed on the data followed two approaches: (i) separate fitting of the two regression equations of pressure drop-flow rate relationship obtained for each valve on both PDs, and (ii) application of a multiple regression model, to fit a single regression equation of pressure drop-flow rate relationship, using data obtained from both PDs for each valve. In addition, an additional independent (dummy) variable was introduced. RESULTS AND CONCLUSIONS: Using this approach, the valve parameter range was obtained and, by imposing strict control of the experimental set-up, the coincidence of the two valve power laws, estimated by each of the two PDs, was studied.

Heart Valve Prosthesis↗
PubMed2001

Pulsatile flow and atherogenesis: results from in vivo studies.

Compliance mismatch between prosthetic vascular replacement (possibly stented) and native artery is considered to be an important factor in implant failure due, e.g., to vascular remodeling, tissutal growth or intimal hyperplasia (IH). From an in vivo study involving altered vascular mechanics (and, consequently, compliance mismatch), carried out using the Moncada model of atherosclerosis development and smooth muscle cell (SMC) proliferation, the hemodynamic assessment was followed by means of real-time multigated ultrasound profilometry, of collared carotid artery using two different models: non-constrictive and constrictive plastic collars, wrapped around the vessel. The experiments provided the real-time measurement of velocity profiles in vivo and the subsequent estimation of wall shear stresses, locally responsible for the altered hemodynamics. Endothelium modifications were correlated with local hemodynamic alterations by using statistical regression analysis of the development of intimal hyperplasia and the mechanical stimulus applied to the endothelium by means of the two different manipulation models. Different correlations were found between wall shear rate and IH in the two models, showing the importance of the vascular pulsatility in determining SMC proliferation. This result could be useful in minimizing the negative consequences of clinical interventions such as graft and/or stent implantation.

Animals↗
PubMed1999

Arterial pressure, vascular input impedance, and resistance as determinants of pulsatile blood flow in the umbilical artery.

The flow pulsatility index, the ratio of flow pulse amplitude to mean flow over the cardiac cycle, has been used to quantify pulsatility of blood flow in the umbilical artery. In experiments with fetal sheep, we showed that the flow pulsatility index in the umbilical artery is accurately estimated by the ratio of total umbilico-placental vascular resistance (mean arterial pressure divided by mean umbilical flow) divided by fundamental impedance (umbilical vascular impedance at the heart rate frequency) times the pulsatility index (pulse/mean) of the arterial pressure that drives flow through this bed. The pulsatility index of arterial pressure is primarily determined by upstream factors (e.g. heart rate) whereas fundamental impedance depends primarily on the radius and viscoelastic wall properties of the umbilical artery. An increase in resistance in the microcirculation and/or veins causes proportional changes in the flow pulsatility index because these sites have little influence on fundamental impedance. However, an increase in resistance in the highly vasoactive umbilical arteries has offsetting effects on impedance and resistance; consequently, flow pulsatility changes little even when arterial vasoconstriction markedly reduces mean flow. We conclude that when arterial pressure pulsatility is stable, a change in the flow pulsatility index provides a useful indication of a change in resistance in the microcirculation and/or veins but will not reliably detect a resistance change in the artery.

Animals↗
PubMed2005

Multiphase hemodynamic simulation of pulsatile flow in a coronary artery.

A multiphase transient non-Newtonian three-dimensional (3-D) computational fluid dynamics (CFD) simulation has been performed for pulsatile hemodynamics in an idealized curved section of a human coronary artery. We present the first prediction, to the authors' knowledge, of particulate buildup on the inside curvature using the multiphase theory of dense suspension hemodynamics. In this study, the particulates are red blood cells (RBCs). The location of RBC buildup on the inside curvature correlates with lower wall shear stress (WSS) relative to the outside curvature. These predictions provide insight into how blood-borne particulates interact with artery walls and hence, have relevance for understanding atherogenesis since clinical observations show that atherosclerotic plaques generally form on the inside curvatures of arteries. The buildup of RBCs on the inside curvature is driven by the secondary flow and higher residence times. The higher viscosity in the central portion of the curved vessel tends to block their flow, causing them to migrate preferentially through the boundary layer. The reason for this is the nearly neutrally buoyant nature of the dense two-phase hemodynamic flow. The two-phase non-Newtonian viscosity model predicts greater shear thinning than the single-phase non-Newtonian model. Consequently, the secondary flow induced in the curvature is weaker. The waveforms for computed hemodynamic parameters, such as hematocrit, WSS, and viscosity, follow the prescribed inlet velocity waveforms. The lower oscillatory WSS produced on the inside curvature has implications for understanding thickening of the intimal layer.

Biomechanical Phenomena↗
PubMed1979

[Clinical application of a supplement to the extracorporal circulation to produce a pulsatile flow (author's transl)].

Two groups of patients with atherosclerotic coronary artery disease, who underwent aortocoronary bypass operation, were perfused with nonpulsatile flow during extracorporeal circulation (ECC) using membrane oxygenators. One group (MO) was used as a control, while for the other group (PAD) a Pulsatile Assist Device in the arterial line was employed. This apparatus consists of a balloon of 80 ml placed inside a rigid housing. The balloon is compressed by pressurized air or expanded by vacuum supplied by a driving console. The apparatus produced pulse amplitudes between 30 and 50 mm of mercury. Other than a very short-lasting fall in mean arterial pressure, thus showing diminished peripheral resistance, no perceptable advantages were found. Base excess and pH-changes showed no differences, also the given amount of sodium bicarbonate in both groups was the same. On the other hand significantly higher hemolyses took place, increasing with the duration of pulsation. The application of the apparatus as an arterial counterpulsator was possible with limitation in only 5 of 15 patients. In all other patients after a short time massive blood foaming developed in the PAD and the attempts had to be stopped because of the risk of gas embolism. In our opinion this apparatus is an unnecessary supplement to the ECC and as an arterial counterpulsator it seems too dangerous.

Adult↗