Steady Blood Flow Research Articles

A CFD investigation of steady and pulsatile blood flow through a life-like, three-dimensional bypass graft model

A CFD investigation of steady and pulsatile blood flow through a life-like, three-dimensional bypass graft model

QUANTITATIVE LEG BLOOD FLOW BY DOPPLER ULTRASOUND IN UPRIGHT AND SUPINE EXERCISE 818

It has been suggested that a limitation to work performance in supine cycle ergometry is an impairment of peripheral circulation to the working muscles. We evaluated the effects of perfusion pressure on muscle blood flow using Doppler ultrasound to measure femoral artery mean blood velocity (MBV) and diameter (D). Nine healthy subjects were studied at rest and constant load knee extension (50W) while seated upright (UP) and supine (SU). Average resting MBV was 6.37±0.61 cm/sec (mean±SE) in UP and 8.28±0.96 cm/sec in SU. Average MBV at 50W was 36.6±3.58 cm/sec in UP and 44.2±5.69 cm/sec in SU. Average D was greater (P<0.05) in UP (10.1±0.32 mm) than in SU (9.06±0.31 mm) at rest. Average D was not different in UP (10.2±0.28 mm) vs SU (9.67±0.24 mm) during exercise. Leg blood flow was 0.31±0.04 l/min in UP rest and 0.32±0.04 l/min in SU rest and increased to 1.79±0.15 l/min in UP and 1.94±0.25 l/min in SU exercise. ˙VO2 was similar during rest (326±14 vs 341±24 ml/min) and exercise(902±36 vs 940±26) in UP and SU, respectively. Under the assumption the increase in ˙VO2 with exercise was in the legs only, we estimate an (a-v)O2 difference of 161 ml O2/l in UP and 154ml O2/l in SU exercise at 50W. These estimates of flow suggest that steady state blood flow to exercising muscle is not compromised in submaximal knee extension exercise in the supine position.

Platelet adhesion onto the wall of a flow chamber with an obstacle

In the present study, the data of the initial adhesion of platelets onto the wall of a flow chamber with an obstacle in steady human blood flows were obtained. The flowfields and the distribution of stress-related factors were simulated numerically by a finite volume method and the fluid dynamic effect on the platelet adhesion is discussed. In addition to the wall shear effect, the normal stress effect was also taken into account. A parameter Vn/¦Vt¦ was devised to assess the combined effect of both shear and normal forces in platelet adhesion. It was found that the peak adhesion occurred next to, but not on, the impingement point on the obstacle where the value of Vn/¦Vt¦ was negative. In these regions, direct impact played a major role in platelet adhesion. On the other hand, near the separation point before the obstacle where Vn/¦Vt¦ was insignificant, the mechanism was believed to be different from that in the direct impact region. Denser adhesion there might be caused by the accumulation and frequent collision of particles due to flow retardation and/or detour of the flow path. Interestingly, relatively low adhesion was found inside the recirculation regions. These results show that the normal stress effect (impingement) should be considered in platelet adhesion in addition to the shear effect.

The tonic sympathetic input to the cochlear vasculature in guinea pig

Vascular tone is an essential component in maintaining steady regional blood flow and dynamic responsiveness of a vascular bed. Sympathetic innervation can contribute to vascular tone. Although certain studies have reported evoked changes in cochlear blood flow (CBF) with activation of the sympathetic fibers to the cochlear vasculature, other studies have failed to show evidence of sympathetic contribution to CBF regulation when the cervical sympathetic fibers were unilaterally sectioned. We hypothesized that the bilateral ‘sympathectomy of the stellate ganglia’ would remove sufficient sympathetic input to the cochlea to yield a change in CBF resting level. To test this hypothesis a new technique was used to expose the stellate ganglia (SG) bilaterally and induce a chemical sympathectomy. We observed that unilateral SG blockade with 2 μl of 4 mM lidocaine hydrochloride on either side produced a 5–10% increase in CBF, which recovered to baseline during the following 2 min. A subsequent blockade of the contralateral SG produced a rapid 25–35% increase, which then recovered partially during the following 3–4 min, remaining 5–15% above the baseline over a 20 min measurement period. Superior cervical ganglion transection did not affect CBF. Our results provide evidence for the existence of a tonic sympathetic component in the control of vascular tone in guinea pig cochlea. This neural effect is derived bilaterally from SG. This result is consistent with previous anatomical studies showing the bilateral innervation of the cochlea by the SG sympathetic fibers and with previous physiological studies on the bilaterality of evoked changes in CBF due to electric stimulation of SG.

Optimum kinetic energy dissipation in blood flow in glass capillaries by flow field determination: Analysis of curvature effect by axial tomographic and image velocimetry techniques

Based on the variation in the optical density due to erythrocyte concentration and movement, the axial tomographic and image velocimetry techniques are respectively applied to determine the flow field, i.e., the distribution of erythrocytes and axial and radial velocity components, in steady blood flow through a curved glass capillary with a diameter of 180 μm. The data at four positions (two straight and two curved segments of the capillary) are recorded by a video-microscopic system on a video cassette. The erythrocyte and velocity distribution profiles change from symmetric at the straight position to an asymmetric shape at the curved sections. These profiles become symmetric again at the straight section of the capillary. The increase in the radial velocity component at curved portions is attributed to the secondary flow. The tomograms obtained by concentration profiles show respective changes in the cellular population at various cross-sectional positions. The kinetic energy dissipation, as calculated based on a determination of the flow field, is the minimum for the observed profiles. Any deviation towards parabolic form leads to the dissipation of a higher amount of energy.

A model for pulsatile and steady ocular blood flow.

The aim was to elucidate two controversial questions. (1) is the pulsatile inflow dominant or is there a sizeable steady inflow component? (2) Is the outflow of blood steady or pulsatile in induced and in glaucomatous hypertension? The OBF system (Langham) was used for pulse recording. A simple electric flow model was conceived and analysed in terms of Fourier series, using linear circuit theory. Possible explanations of the seemingly inconsistent observations are indicated by the model. (1) A steady inflow of similar magnitude to the pulsatile one is predicted. The total flow can be determined at present theoretically from recordings of the pulsatile inflow and of the systolic and diastolic intraocular blood pressure. Some methodological improvements are desirable in order to obtain reliable estimates in practice. (2) The outflow is normally steady, probably even in ocular hypertension and glaucoma. In hypertension induced with the suction cup this may be doubted. The eye has similarities with the device called the Starling resistor which describes the behaviour of collapsible vessels. Acute pressure increase (plus the pulsations of the IOP) may put the collapsible vessels into a state which disturbs the steady outflow. The effects of changes in IOP and in other components on the size and shape of the flow curve are demonstrated.

Hemolysis During Membrane Plasma Separation With Pulsed Flow Filtration Enhancement

The use of pulsed blood flow in membrane plasmapheresis permits enhancement of plasma filtration yet may result in high levels of hemolysis due to large increases in instantaneous transmembrane pressure (TMP). This work investigates the occurrence of hemolysis as a function of TMP and wall shear rates (gamma w) for both steady and pulsed blood flow conditions. Two types of hollow fiber filters with identical polypropylene membranes but different lengths and membrane areas (0.1 and 0.25 m2) were tested. Fresh citrated bovine blood was circulated through the fibers at various blood flowrates and TMP in a single pass circuit using a pulsation generator, made of a single roller peristaltic pump. The free hemoglobin concentration of the plasma, Hbm, was measured from permeate samples collected at each set of TMP and gamma w conditions. It was found that the net hemolysis generated by the filtration was proportional to the membrane area. This justified the introduction of an hemolysis index, IH, equal to the plasma hemoglobin per unit membrane area. The boundary for the occurrence of hemolysis was thus defined by setting IH = 30 mg/dl.m2. For both steady and pulsed flow conditions the hemolysis boundaries were found to be straight lines in the TMP-gamma w plane. They were identical for the two filters under steady flow but different for pulsed flow. At the same time mean wall shear rates hemolysis occurred at a lower time mean TMP under pulsed flow conditions than under steady flow conditions.

EFFECT OF MODIFYING NITRIC OXIDE PATHWAY ON LIVER CIRCULATION IN A RABBIT ENDOTOXIN SHOCK MODEL

The role of nitric oxide (NO) inhibition on liver circulation during sepsis is unknown. To answer this question, we studied the effects of L-arginine (the substrate for the NO synthase), linsidomine (a direct NO donor), and N omega-nitro-L-arginine (an NO inhibitor) on the liver circulation in anesthetized rabbits previously injected with endotoxin (Escherichia coli, Salmonella enteridis, and Salmonella minnesota, 400 micrograms each). After endotoxin administration, and without fluid resuscitation, rabbits showed a hypodynamic shock with decrease in mean arterial pressure (MAP) and aortic blood flow velocity. Portal vein blood flow velocity decreased, whereas hepatic artery blood flow velocity increased. Saline or treatments were injected, 75 min after endotoxin administration. In saline-treated rabbits, MAP, aortic and portal vein blood flow velocities remained steady but hepatic artery blood flow velocity decreased. Only N omega-nitro-L-arginine (7.5 mg/kg, intravenously) significantly increased MAP compared to saline treatment. However, aortic, portal vein, and hepatic artery blood flow velocities were lower in rabbits treated with N omega-nitro-L-arginine than in saline-treated rabbits. L-Arginine (600 mg/kg, intravenously) increased aortic blood flow and portal vein blood flow velocity with no change on hepatic artery blood flow velocity. In contrast, linsidomine (1 mg) increased both hepatic flows. These results show that NO inhibition after endotoxin injection reduces systemic and liver flows, while NO release from linsidomine improves them. These findings question the usefulness of NO inhibition during septic shock, particularly as hepatic failure frequently occurs in the evolution of the disease.

Development of a new laser-Doppler microvelocimetering and its application in patients with coronary artery stenosis.

A new Laser-Doppler microvelocimetering (LDMV) was developed to study blood flow in individual vessels in patients with coronary artery stenosis. The LDMV measures steady blood flow velocity, diameter of vessels and pulsation flow. In addition, the effects of China's traditional herbs--Salvia Mitiorrhyza and other drugs--on the blood flow in the patients were investigated. LDMV constitutes a new quantitative procedure and is a promising tool for rapid monitoring of tissue perfusion in cardiovascular disease and estimating the action of drugs on microcirculation.

A monoclonal antibody against rabbit tissue factor inhibits thrombus formation in stenotic injured rabbit carotid arteries.

Tissue factor (TF) is a transmembrane protein that binds factor VII/VIIa, thus activating the extrinsic blood coagulation pathway. Since this pathway appears to be involved in the formation of intravascular thrombi, the anti-rabbit TF monoclonal antibody, AP-1, was produced and tested as an antithrombotic agent in a rabbit model of recurrent intravascular thrombosis. In this model, a plastic constrictor is positioned around the injured rabbit carotid arteries, and flow is monitored with a Doppler flow probe. This produces cyclic flow variation (CFV) in the carotid artery, which is caused by recurrent formation and dislodgment of thrombi at the site of the stenosis. After monitoring CFV pattern for 30 minutes, AP-1 was infused intravenously into nine rabbits at doses of 0.05 to 1.5 mg/kg body weight, and a control monoclonal antibody that does not react with rabbit TF was infused into four additional rabbits. In all rabbits receiving AP-1, CFV was abolished, and a steady normal blood flow was restored, indicating that thrombus formation had been blocked by AP-1. By contrast, in all rabbits that received the control monoclonal antibody, CFV continued unaltered. There was no change in the partial thromboplastin time and ex vivo platelet aggregation to several different agonists after infusion of AP-1, indicating an absence of systemic effects on the coagulation process. We conclude that activation of the extrinsic coagulation pathway has a key role in triggering intravascular thrombosis and that an anti-TF monoclonal antibody is an effective antithrombotic agent that could have therapeutic potential for humans.

Experimental evaluation of intrinsic and nonstationary ultrasonic Doppler spectral broadening in steady and pulsatile flow loop models

Intrinsic and nonstationary Doppler spectral broadening, and the skewness of the spectral representation, were evaluated experimentally using porcine red cell suspensions as ultrasonic scatterers. Theoretically, the relative Doppler bandwidth, defined as the intrinsic bandwidth divided by the mean Doppler frequency shift, should be velocity independent. The relative Doppler bandwidth invariance theorem was experimentally verified with an in vitro steady laminar blood flow model. It is shown that the relative bandwidth is both independent of the flow velocity and blood hematocrit. Using a pulsatile laminar flow model, the authors demonstrated that the relative Doppler bandwidth invariance theorem did not hold during flow acceleration and deceleration. In addition, a positive skewness of the Doppler spectra was observed during acceleration while a negative skewness was measured during the deceleration of blood. The effect of the window duration used in the Fourier spectral computation, on nonstationary broadening, is characterized.

Computer simulation of blood flow patterns in arteries of various geometries

The purpose of this study is to illustrate the application of computer simulation to the study of blood flow through arteries and to demonstrate the relationship between geometry of the vessels and local flow patterns. A finite element computer program was developed to simulate steady and pulsatile blood flow by solving the continuity and Navier-Stokes equations. The accuracy of the computational method has been confirmed by comparing the numeric results to analytic solutions and to published experimental data from physical models. The results are presented as plots of the velocity vectors, streamlines, and pressure contours. The computational model has been applied to illustrate flow patterns in the following situations: pulsatile flow in a cylindric artery and an artery with an axisymmetric stenosis, steady flow in cylindric arteries with stenoses of varying severity and with different flow rates, steady flow in an artery containing a fusiform aneurysm, steady flow in a two-dimensional model of a symmetric Y-shaped bifurcation, and steady flow in a two-dimensional model of the carotid bifurcation. Regions that are commonly associated with arterial disease often coincide with zones of reversed or stagnant flow. In conclusion, the versatility and feasibility of computational simulation of blood flow is illustrated by this study. Although this mathematic model is a simplication of the real flow phenomena, it yields results that provide useful insights into the understanding of local blood flow patterns for a variety of complex geometries.

Computer simulation of blood flow patterns in arteries of various geometries

The purpose of this study is to illustrate the application of computer simulation to the study of blood flow through arteries and to demonstrate the relationship between geometry of the vessels and local flow patterns. A finite element computer program was developed to simulate steady and pulsatile blood flow by solving the continuity and Navier-Stokes equations. The accuracy of the computational method has been confirmed by comparing the numeric results to analytic solutions and to published experimental data from physical models. The results are presented as plots of the velocity vectors, streamlines, and pressure contours. The computational model has been applied to illustrate flow patterns in the following situations: pulsatile flow in a cylindric artery and an artery with an axisymmetric stenosis, steady flow in cylindric arteries with stenoses of varying severity and with different flow rates, steady flow in an artery containing a fusiform aneurysm, steady flow in a two-dimensional model of a symmetric Y-shaped bifurcation, and steady flow in a two-dimensional model of the carotid bifurcation. Regions that are commonly associated with arterial disease often coincide with zones of reversed or stagnant flow. In conclusion, the versatility and feasibility of computational simulation of blood flow is illustrated by this study. Although this mathematic model is a simplification of the real flow phenomena, it yields results that provide useful insights into the understanding of local blood flow patterns for a variety of complex geometries.

Gas Transport in Serpentine Microporous Tubes Under Steady and Pulsatile Blood Flow Conditions

A serpentine gas exchange unit was built with cylindrical tubular microporous membranes featuring periodic arcs with a fixed curvature ratio (ratio of tube radius to radius of curvature) of 1/14 and circular angles between 30 and 360 deg. Oxygen transfer was measured under steady and pulsatile blood flow conditions in vitro and ex vivo to assess the design features which most effectively augment gas transfer. Under steady blood flow conditions, oxygen transfer increased with circular angles beyond 70 deg. Under pulsatile conditions, a wide range of geometrical and fluid mechanical parameters could be combined to enhance gas transfer performance, which eventually depended upon the secondary Reynolds number and the Womersley parameter.

Theoretical investigation of estimation of steady and pulsatile blood flow and blood vessel cross section by CW NMR excitation

The author shows theoretically that when a magnetised blood bolus enters a CW NMR excitor coil of length Le at resonance and the signal from the T2-decaying, precessing transverse magnetisation of the flowing blood spins is subsequently detected by a detector coil of length L separated from the excitor coil by a distance Delta l, then by recording CW NMR signals at three positions such as Delta l=0, 0.5 and 1.0 cm one can eliminate the static tissue signal and measure non-invasively the steady component V0 as well as the total vessel cross section, beta accurately. The time dependent part of the CW NMR signal which depends on Vpulse(t), is also dependent on V0 non-linearly unless both L and Le are greater than 50 cm and Delta l is zero. Finally, methods of obtaining true Vpulse(t) from the CW NMR signal after applying proper correction due to the steady flow are discussed.

Ventilatory effects of high and pulsatile blood flow in the pulmonary circulation

The mechanism of ventilatory stimulation that accompanies increases in cardiac output is unknown. Previous studies addressing this issue have been inconclusive. However, only steady pulmonary blood flow was used. The effect of flow pulsatility merits consideration, because increasing cardiac output raises not only mean pulmonary arterial pressure but also pulse pressure; mechanoreceptors with an important dynamic component to their responses may cause a response to pulsatile, but not steady, flow. Studies were done on anesthetized cats (n = 4) and dogs (n = 4). The right pulmonary artery was cannulated within the pericardium, and systemic blood was pumped from the left atrium to the right pulmonary artery. The right pulmonary circulation was perfused at different levels of flow, which was either steady or pulsatile. Steady-state flow of up to 150 ml.kg-1.min-1 (270 ml.kg-1.min-1 when corrected for the proportion of lung tissue perfused) did not affect breathing pattern. When high pulmonary flow was made pulsatile (pulse pressure approximately 23 mmHg), breath duration decreased from 3.7 +/- 0.72 to 3.4 +/- 0.81 (SD) s (P less than 0.01), representing a change in frequency of only 9%. There was no change in peak inspiratory activity. It was concluded that pulmonary vascular mechanoreceptors are not likely to contribute significantly to the increase in ventilation in association with increases in cardiac output.

Estimation of pulsatile ocular blood flow from intraocular pressure

A relationship has been derived between intraocular pressure and pulsatile blood flow in the eye. Measurements of intraocular pressure show a time variation that is associated with the pulsatile component of arterial pressure. Experimental results provide a means of transforming intraocular pressure changes into ocular volume changes. The eye is represented by a chamber with elastic walls, a pulsatile incoming flow of incompressible fluid (blood), and a steady outgoing flow of blood. Under these conditions, the rate of pulsatile blood flow through the eye can be approximated from the instantaneous intraocular pressure measurements. Data from a healthy human eye are used to illustrate the analysis.

Formation of a boundary layer in steady-state blood flow

This paper is devoted to a study of boundary layer formation in the steady flow of blood through the human aorta. Blood is treated as an incompressible fluid. Consideration is given to a flat-top velocity profile which combines the potential flow with the boundary layer; expressions for the displacement thickness, skin-friction and pressure in the entry region are derived.

Blood flow in tapered tubes with biorheological applications

A steady laminar flow of blood in a uniform tapered tube has been examined. Blood rheology is assumed to be described by a polar fluid. The analytical expressions for velocities (both axial and radial), total angular velocity, wall shear and pressure drop have been obtained. In literature, the parameters N (coupling number) and L (length ratio) have been chosen independently. But, in the present analysis, it is found that they are interrelated. Variation of the flow variables with suspension concentration and tapered angle have been investigated. Some of the theoretical models for the flow through tapered tubes have been critically examined. The pressure-flow relationship has been studied numerically over the flow rate range 0.01-0.1 cc/sec and compared with experimental results. It has been shown that the existing experimental results are for the tapered tubes of larger diameter which correspond to the flow under Newtonian conditions. Finally, some biological implications and future developments of this theory have been indicated.

A comparative computational study of blood flow through prosthetic heart valves using the finite element method

The steady flow of blood through three common types of prosthetic heart valves was simulated numerically using the finite element method. The velocity, pressure and stress fields were obtained for the disk-type, tilting-disk and ball-type prosthetic heart valves in aortic position, for increasing Reynolds numbers up to 900, 1500 and 2000 respectively. Computer graphics of calculated velocities are presented, showing in detail the accelerated flow, recirculation and stagnation areas developed in the prosthesis. Maximum wall shear stresses were found at 0.5, 1.4, and 1.2 diameters from the sewing ring downstream for the disk, tilting-disk and ball valves being the values of 55, 18 and 33 dyn cm −2 respectively. In the vicinity of the occluder, maximum shear stresses of 38, 30 and 47 dyn cm −2 respectively were computed. The flow characteristics and performance for each valve are compared, the results are presented in terms of energy loss and maximum shear stress. The velocity and stress fields are compared with in vitro evaluations found in the literature.