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PubMed1998

Utility of three-dimensional ultrasound Doppler flow reconstruction of the proximal jet to quantify effective orifice area: in vitro steady and pulsatile flow studies.

We examined the utility of three-dimensional (3D) reconstruction of two-dimensional color Doppler images of the proximal jet to quantify the effective area of an orifice in an in vitro model. Steady and pulsatile flows were directed through various orifices; orifice vena contracta areas were quantified with laser flow visualization, thus providing gold standard effective orifice areas. Three-dimensional areas followed vena contracta areas well, although variations in color Doppler gain and 3D gray levels for thresholding produced significant changes in reconstructed images. These variations were minimized by using minimum color gain and 50% gray level threshold. At these settings, 3D areas still overestimated vena contracta areas by approximately 25% because of the poor lateral resolution of the color Doppler system, which caused bleeding of the flow signal past the edges of the proximal jet. Nevertheless, 3D flow images provided a superior format for qualitative and quantitative appreciation of proximal jet shape and dimensions.

Heart Valve Diseases↗
PubMedDate not supplied

Artifacts from pulsatile flow in MR imaging.

Previous investigators have examined the effect of blood flow on the apparent blood vessel signal intensity. These studies reported flow brightening and darkening effects within vessels. In this paper we have investigated another type of flow artifact, which originates from the pulsatile nature of blood flow. These flow artifacts have characteristic bright and dark "ghosting" patterns which appear close to small vessels, usually arteries, which are bright in slow flow. Similar to the amplitude-of-motion artifacts caused by patient motion (e.g., breathing and cardiac motion) the ghosting artifacts due to pulsatile flow are best characterized as frequency modulated spectral sidebands. The pulsatile artifacts can have both dark and bright structures and usually appear close to the "moving" vessel that generates the artifact. In this paper we present a study of the chief features of these pulsatile flow artifacts, and we develop a theoretical description of their origins in terms of "accidental" velocity-encodings that occur strongly in most magnetic resonance imaging sequences.

Blood Flow Velocity↗
PubMed1983

[Pulsatile flow dynamics of the ductus arteriosus, thoracic aorta and pulmonary artery in patients with patent ductus arteriosus].

In 20 infants or children with an isolated or complicated patent ductus arteriosus (PDA), we qualitatively and quantitatively studied pulsatile flow dynamics of the ductus, descending thoracic aorta and pulmonary artery by means of a catheter-tip electromagnetic flow velocity probe. They were divided into four groups according to ductal shunt states as follows: 14 patients with a continuous left-to-right (L-R) shunt (Group I), three patients with a bidirectional but a dominant L-R shunt (Group IIA), two patients with a bidirectional but dominant right-to-left (R-L) shunt (Group IIB), and one patient without a significant ductal flow (Group III). In Group I, the ductal flow was pulsatile and showed continuous L-R shunting. The timing of a peak flow velocity was coincident with the peak aortic pressure at the mid-ductus, and it shifted to diastolic phase as the flow sensor approached the pulmonary end of the ductus arteriosus. In Group IIA, the peak velocity of a L-R shunt flow was located at mid-diastole and a transiently reversed R-L shunt flow was seen during systole. Two patients of Group IIB showed that the peak flow velocity of a dominantly reversed shunt was at mid-diastole, while a low grade L-R shunt flow was seen over a wide range of diastolic period. One patient of Group III who underwent operation for aortic arch interruption did not show any significant ductal flow because of a narrow PDA. In most cases of the present study, a diastolic backflow reflecting a L-R ductal shunt during diastole was demonstrated both in the descending thoracic aorta and main pulmonary artery. The peak flow velocity of the thoracic aorta was correlated with the ductal L-R shunt ratio determined by the Fick method (r = 0.46), and the diastolic regurgitant flow fraction of the thoracic aorta was increased in patients with a larger L-R shunt or with a reversed shunt. Therefore, it was suggested that a net forward flow of the thoracic aorta is reduced in these patients. On the other hand, the quantitative evaluation of a pulmonary flow during systole was found unreliable and expected to be underestimated because of the occurrence of turbulence at the site of the main pulmonary artery by the confluence of ejection stream from the right ventricle and a shunted flow from the aorta.(ABSTRACT TRUNCATED AT 400 WORDS)

Aorta, Thoracic↗
PubMed1988

Decrease in pulsatile flow in the internal carotid artery in fetal hydrocephalus.

The effect of ventriculomegaly on pulsatile flow in the internal carotid arteries has been studied by Doppler ultrasound in four hydrocephalic fetuses. The pulsatility index showed progressive elevation proportional to the developing ventriculomegaly and thus may be a valuable index for the study of the mechanism of brain injury and for the determination of optimal timing of intervention.

Carotid Artery, Internal↗
PubMed1985

Experimental study of physiological pulsatile flow past valve prosthesis in a model of human aorta--II. Tilting disc valves and the effect of orientation.

In Part II of this two paper sequence, pulsatile flow development past a tilting disc valve in a model human aorta has been studied using quantitative laser Doppler techniques. The valve was mounted in three different orientations with respect to the aortic root in this study. Under pulsatile flow, the region of flow reversal induced near the wall of the minor flow orifice extends to more than one tissue annulus diameter downstream from the valve into the ascending aorta. In a plane perpendicular to the tilt axis, a bi-helical secondary flow is induced distal to the valve. This secondary flow is further compounded by the multiple curvatures in the aorta. Hence the valve orientation affects the velocity profiles as far downstream as the mid-arch region as well as in the brachio-cephalic arterial branch. In the mid-arch region, a flow reversal along the entire cross-section is observed in early diastole for all the three orientations of the disc valve.

Aorta↗
PubMed2000

Pulsatile flow enhances endothelium-derived nitric oxide release in the peripheral vasculature.

The effects of pulsatility in blood flow on endothelium-derived nitric oxide (EDNO) release in the peripheral vasculature were investigated. The basal and flow-stimulated EDNO release were compared between pulsatile and nonpulsatile systemic flows before and after the administration of NO synthase inhibitor N(G)-monomethyl-L-arginine (L-NMMA). Peripheral vascular resistance (PVR) was significantly lower in pulsatile flow than in nonpulsatile flow, but this difference disappeared after L-NMMA. The percent increase in PVR by L-NMMA was significantly larger in pulsatile flow. In reactive hyperemia in the hindlimb, the peak flow did not differ; however, both the repayment flow and the duration were significantly larger in pulsatile flow. Percent changes of these parameters by L-NMMA were significantly larger in pulsatile flow. These data indicated that pulsatility significantly enhances the basal and flow-stimulated EDNO release in the peripheral vasculature under in vivo conditions. We also studied the involvement of the Ca(2+)-dependent and Ca(2+)-independent pathways in flow-induced vasodilation using calmodulin inhibitor calmidazolium and tyrosine kinase inhibitor erbstatin A. PVR was significantly elevated by erbstatin A but not by calmidazolium, suggesting that flow-induced vasodilation was largely caused by tyrosine kinase inhibitor-sensitive activation of NO synthase.

Angiotensin II↗
PubMed2006

Modeling pulsatile flow in aortic aneurysms: effect of non-Newtonian properties of blood.

Pulsatile flow in an axisymmetric rigid-walled model of an abdominal aorta aneurysm was analyzed numerically for various aneurysm dilations using physiologically realistic resting waveform at time-averaged Reynolds number of 300 and peak Reynolds number of 1607. Discretization of the governing equations was achieved using a finite element scheme based on the Galerkin method of weighted residuals. Comparisons with previously published work on the basis of special cases were performed and found to be in excellent agreement. Our findings indicate that the velocity fields are significantly affected by non-Newtonian properties in pathologically altered configurations. Non-Newtonian fluid shear stress is found to be greater than Newtonian fluid shear stress during peak systole. Further, the maximum shear stress is found to occur near the distal end of AAA during peak systole. The impact of non-Newtonian blood flow characteristics on pressure compared to Newtonian model is found insignificant under resting conditions. Viscous and inertial forces associated with blood flow are responsible for the changes in the wall that result in thrombus deposition and dilation while rupture of AAA is more likely determined by much larger mechanical stresses imposed by pulsatile pressure on the wall of AAA.

Aortic Aneurysm, Abdominal↗
PubMed1984

Magnetic resonance signal intensity patterns obtained from continuous and pulsatile flow models.

The purpose of this investigation was to extend previous steady state flow studies with magnetic resonance (MR) to pulsatile flow measurements obtained with gated cardiac techniques in man with a pulsatile artificial heart device. Bovine blood and a solution of MnCl2 were studied. Correlation was made with complex and time-varying MR signals observed in the descending aorta on cardiac gated images obtained through the midthorax. MR signals from flowing fluids represent velocity distribution as shown in a velocity profile, and laminar flow is distinguished from nonlaminar. At very slow flow rates, signal intensity is lower than background. As the rate is increased, paradoxical enhancement occurs followed by loss of signal, which is complete at 7 liters per minute with fluid and 15 liters per minute with blood. These areas correspond to maximum fluid velocities of 41 and 88 cm/sec, respectively.

Animals↗
PubMed1990

Pulsatile flow of power-law fluid model for blood flow under periodic body acceleration.

A mathematical model has been proposed to study the pulsatile flow of a power-law fluid through rigid circular tubes under the influence of a periodic body acceleration. Numerical solutions have been obtained by using finite difference method. The accuracy of the numerical procedure has been checked by comparing the obtained numerical results with other numerical and analytical solutions. It is found that the agreement between them is quite good. Interaction of non-Newtonian nature of fluid with the body acceleration has been investigated by using the physiological data for two particular cases (coronary and femoral arteries). The axial velocity, fluid acceleration, wall shear stress and instantaneous volume flow rate have been computed and their variations with different parameters have been analyzed. The following important observations have been made: (i) The velocity and acceleration profiles can have more than one maxima, this is in contrast with usual parabolic profiles where they have only one maximum at the axis. As n increases, the maxima shift towards the axis; (ii) For the flow with no body acceleration, the amplitude of both, wall shear and flow rate, increases with n, whereas for the flow with body acceleration, the amplitude of wall shear (flow rate) increases (decreases) as n increases; (iii) In the absence of body acceleration, pseudoplastic (dilatant) fluids, with low frequency pulsations, have higher (lower) value of maximum flow rate Qmax than Newtonian fluids, whereas for high frequencies, opposite behavior has been observed; for flow with body acceleration pulsations gives higher (lower) value of Qmax for pseudoplastic (dilatant) fluids than Newtonian fluids.

Acceleration↗
PubMed2000

Numerical simulation of pulsatile flow in a compliant curved tube model of a coronary artery.

The endothelial cells (ECs) lining a blood vessel wall are exposed to both the wall shear stress (WSS) of blood flow and the circumferential strain (CS) of pulsing artery wall motion. These two forces and their interaction are believed to play a role in determining remodeling of the vessel wall and development of arterial disease (atherosclerosis). This study focused on the WSS and CS dynamic behavior in a compliant model of a coronary artery taking into account the curvature of the bending artery and physiological radial wall motion. A three-dimensional finite element model with transient flow and moving boundaries was set up to simulate pulsatile flow with physiological pressure and flow wave forms characteristic of the coronary arteries. The characteristic coronary artery curvature and flow conditions applied to the simulation were: aspect ratio (lambda) = 10, diameter variation (DV) = 6 percent, mean Reynolds number (Re) = 150, and unsteadiness parameter (alpha) = 3. The results show that mean WSS is about 50 percent lower on the inside wall than the outside wall while WSS oscillation is stronger on the inside wall. The stress phase angle (SPA) between CS and WSS, which characterizes the dynamics of the mechanical force pattern applied to the endothelial cell layer, shows that CS and WSS are more out of phase in the coronaries than in any other region of the circulation (-220 deg on the outside wall, -250 deg on the inside wall). This suggests that in addition to WSS, SPA may play a role in localization of coronary atherosclerosis.

Animals↗
PubMed1989

[Pulsatile flow in an elastic circular tube. 1. The application of the finite Hankel transforms to the equations for the flow].

The finite Hankel transforms are applied to the linealized equations of motion for the pulsatile flow in an elastic circular tube. In this paper, the time dependency of the pressure is the known function which is represented by Fourier series expansion. The Fourier transforms are applied to the quantities of the axial components of the pressure and flow velocities, and the finite Hankel transforms are applied to the radial components of them. It is shown that the solutions of the flow velocities are adequate forms for computer calculation. Using the Fourier series coefficients given by the data of the pressure in time, we can calculate the flow pattern in the steady state.

Fourier Analysis↗
PubMed1984

Hypoxic pulmonary vasoconstriction during steady and pulsatile flow in ferrets.

We examined the effects of hypoxia and pulsatile flow on the pressure-flow relationships in the isolated perfused lungs of Fitch ferrets. When perfused by autologous blood from a pump providing a steady flow of 60 ml/min, the mean pulmonary arterial pressure rose from 14.6 to 31.3 Torr when alveolar PO2 was reduced from 122 to 46 Torr. This hypoxic pressor response was characterized by a 10.1-Torr increase in the pressure-axis intercept of the extrapolated pressure-flow curves and an increase in the slope of these curves from 130 to 240 Torr X l-1 X min. With pulsatile perfusion from a piston-type pump, mean pulmonary arterial pressure increased from 17.5 to 36.3 Torr at the same mean flow. This hypoxic pressor response was also characterized by increases in the intercept pressure and slope of the pressure-flow curves. When airway pressure was raised during hypoxia, the intercept pressure increased further to 25 +/- 1 Torr with a further increase in vascular resistance to 360 Torr X l-1 X min. Thus, in contrast to the dog lung, in the ferret lung pulsatile perfusion does not result in lower perfusion pressures during hypoxia when compared with similar mean levels of steady flow. Since the effects of high airway pressure and hypoxia are additive, they appear to act at or near the same site in elevating perfusion pressure.

Animals↗
PubMed1989

Pulsatile flow index for qualitative measurements of blood flow with duplex ultrasound. An experimental study.

We determined the accuracy of Doppler blood flow measurements in an experimental investigation using a tissue-simulating phantom, pulsatile flow pumps and heparinized blood. A new index for qualitative assessment of blood flow, the pulsed flow index (PFI) is described. The PFI takes advantage of the area under the flow velocity curve between the true zero line and the diastolic baseline. Under conditions of continuous flow, the PFI ranged from 0.82 to 0.94 (mean value 0.90). The PFI was found to be relatively independent of the transducer/vessel angle (+/- 8%) and the inter/intra-operator variation was small (+/- 7.5%, or +/- 7%, respectively).

Blood Flow Velocity↗
PubMed1987

Fluid particle motion and Lagrangian velocities for pulsatile flow through a femoral artery branch model.

A flow visualization study using selective dye injection and frame by frame analysis of a movie provided qualitative and quantitative data on the motion of marked fluid particles in a 60 degree artery branch model for simulation of physiological femoral artery flow. Physical flow features observed included jetting of the branch flow into the main lumen during the brief reverse flow period, flow separation along the main lumen wall during the near zero flow phase of diastole when the core flow was in the downstream direction, and inference of flow separation conditions along the wall opposite the branch later in systole at higher branch flow ratios. There were many similarities between dye particle motions in pulsatile flow and the comparative steady flow observations.

Adult↗
PubMed1997

The assignment of velocity profiles in finite element simulations of pulsatile flow in arteries.

In this paper we present a new method for the assignment of pulsatile velocity profiles as input boundary conditions in finite element models of arteries. The method is based on the implementation of the analytical solution for developed pulsatile flow in a rigid straight tube. The analytical solution provides the fluid dynamics of the region upstream from the fluid domain to be investigated by means of the finite element approach. In standard fluid dynamics finite element applications, the inlet developed velocity profiles are achieved assuming velocity boundary conditions to be easily implementable-such as flat or parabolic velocity profiles-applied to a straight tube of appropriate length. The tube is attached to the inflow section of the original fluid domain so that the flow can develop fully. The comparison between the analytical solution and the traditional numerical approach indicates that the analytical solution has some advantages over the numerical one. Moreover, the results suggest that subroutine employment allows a consistent reduction in solving time especially for complex fluid dynamic model, and significantly decreases the storage and memory requirements for computations.

Algorithms↗
PubMed1996

Doppler ultrasound simulation model for pulsatile flow with nonaxial components.

A numerical model that can produce pulsed Doppler signals for nonaxial, pulsatile flow is presented. The model takes into account both hemodynamic and acoustic factors that affect the Doppler signal, such that a wide range of flow patterns and arbitrary transducer types can be simulated. The physics of blood flow is modeled by solving the Navier-Stokes equations utilizing a finite element technique, and the acoustic field is modeled using the acoustic impulse response method. The model was validated by comparison to the Womersley theory. The median deviation was 3.45%. Doppler signals from flow through a 50% stenosis were also simulated. The calculated spectra demonstrated the changing flow patterns from jets and vortices. This new computer model can be used to test spectral analysis tools on simulated Doppler signals, whose underlying flow patterns are of clinical importance.

Acoustics↗
PubMed2003

The effect of asymmetry in abdominal aortic aneurysms under physiologically realistic pulsatile flow conditions.

In the abdominal segment of the human aorta under a patient's average resting conditions, pulsatile blood flow exhibits complex laminar patterns with secondary flows induced by adjacent branches and irregular vessel geometries. The flow dynamics becomes more complex when there is a pathological condition that causes changes in the normal structural composition of the vessel wall, for example, in the presence of an aneurysm. This work examines the hemodynamics of pulsatile blood flow in hypothetical three-dimensional models of abdominal aortic aneurysms (AAAs). Numerical predictions of blood flow patterns and hemodynamic stresses in AAAs are performed in single-aneurysm, asymmetric, rigid wall models using the finite element method. We characterize pulsatile flow dynamics in AAAs for average resting conditions by means of identifying regions of disturbed flow and quantifying the disturbance by evaluating flow-induced stresses at the aneurysm wall, specifically wall pressure and wall shear stress. Physiologically realistic abdominal aortic blood flow is simulated under pulsatile conditions for the range of time-average Reynolds numbers 50 < or = Rem < or = 300, corresponding to a range of peak Reynolds numbers 262.5 < or = Repeak < or = 1575. The vortex dynamics induced by pulsatile flow in AAAs is depicted by a sequence of four different flow phases in one period of the cardiac pulse. Peak wall shear stress and peak wall pressure are reported as a function of the time-average Reynolds number and aneurysm asymmetry. The effect of asymmetry in hypothetically shaped AAAs is to increase the maximum wall shear stress at peak flow and to induce the appearance of secondary flows in late diastole.

Anisotropy↗
PubMed1978

Pulsatile flow characteristics of aortic valve porcine heterografts in man.

Pulsatile flow characteristics in the ascending aorta were examined in 14 patients 12 months after aortic valve replacement with the glutaraldehyde-fixed porcine aortic heterograft. Analysis of the velocity signals permitted measurement of peak velocity, maximal acceleration, stroke volume, and ejection time in addition to the standard measurement of systolic aortic valvar gradient. Angiographic techniques permitted visualization of a pattern of blood flow in the ascending aorta. Valvar aortic gradients were minimal, and, when the phasic ascending aortic blood flow velocity parameters were compared to those in a group of eight patients with normal aortic valves, these characteristics of flow were nearly indistinguishable. This remarkable similarity to the flow characteristics of normal valves may be important to long-term performance of a prosthetic aortic valve. Our results suggest that the glutaraldehyde-fixed porcine heterograft warrants serious consideration when selecting a substitute aortic valve for man.

Animals↗