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At least 19 recordsLinked to original sources
PubMed1979

Dialysis of middle molecules at pulsatile flow.

Pulsatile flow was used in a continuous flow flat-plate dialyzer in order to achieve the more efficient removal of middle molecules. The solutes ranged in the molecular weights from 342 (sucrose) to 1355 (vitamin B-12). The permeability of Cuprophane membrane to sucrose, raffinose and vitamin B-12 were calculated both for pulseless and pulsatile flows. It was found that pulsatile flow is more favorable than steady flow with regard to increase of membrane permeability. However, as the solute molecular weight increases, fluid-phase resistances become relatively less important and the transport rate becomes dominated by the membrane properties.

Dialysis↗
PubMed1985

Interaction of peristaltic flow with pulsatile flow in a circular cylindrical tube.

The effect of pulsatile flow on peristaltic transport in a circular cylindrical tube is analysed. The flow of a Newtonian viscous incompressible fluid in a flexible circular cylindrical tube on which an axisymmetric travelling sinusoidal wave is imposed, is considered. The initial flow in the tube is induced by an arbitrary periodic pressure gradient. A perturbation solution with amplitude ratio (wave amplitude/tube radius) as a parameter is obtained when the frequency of the travelling wave and that of the imposed pressure gradient are equal. The interaction effects of periodic wall induced flow and periodic pressure imposed flow are visualized through the presence of substantially different components of steady and higher harmonic oscillating flow in the first order flow solution. Numerical results show a strong variation of steady state velocity profiles with boundary wave number and Reynolds number and a strong phase shift behaviour of the flow in the radial direction.

Humans↗
PubMed1990

Detection of moving flow separation in pulsatile flow and the degree of stenosis by power of Doppler shift signals.

Power ratios were derived from the principle of ultrasonic Doppler velocimetry to determine a ratio of the volume of vortices to the total vessel volume under the ultrasonic beam. This ratio also equals the ratio of the cross-sectional area of vortices to the vessel lumen. In vitro pulsatile flow experiments were performed in a Reynolds number range of 1,230-4,320 with axisymmetric constrictions with area reductions of 55%, 65%, 70%, and 85% to model carotid stenosis. Flow separation downstream from the model stenoses was detected, and the power ratio fluctuated when vortices with the forward- and reverse-flow velocity components passed by the measurement position. The power ratio estimated the degree of stenosis within 10% of error. Ensemble average of the power ratio was computed to obtain the statistically averaged separated flow region. The moving flow reattachment point was revealed downstream from the 85% stenosis at a Reynolds number of 900.

Blood Circulation↗
PubMed1986

Volume flow measurement of pulsatile flow by digitised cine angiography.

The validation, by phantom experiments, of a method of measuring volumetric, pulsatile blood flow in vessels of about 4 mm calibre is described. 'Instantaneous' as well as integrated flow is obtained from cine angiograms by digitisation of the images and densitometric analysis in an advanced computer vision analysis system. From 48 frames per second cine, pulsatile flow at any moment between 0 and about 400 ml/min was calculated within about 10 per cent of the electromagnetic flow meter reading. Further improvement is expected. It is proposed to advance to 3D and have the methodology available for clinical use in due course.

Angiography, Digital Subtraction↗
PubMed1987

Characteristics of secondary flow in steady and pulsatile flows through a symmetrical bifurcation.

Steady and pulsatile flow in a glass model simulating an arterial bifurcation was investigated by flow visualization techniques. Secondary flow generated at the bifurcation has a similar pattern to a vortex, called the horseshoe vortex, produced around a wall-based protuberance in a circular tube. The same flow disturbance was clearly observed during the decelerating phase of pulsatile flow. The vortex produces a stagnation point on the top and bottom wall just upstream from the bifurcation apex. When aluminium dust was suspended in the test fluid perfusing the blood vessel model, particles deposited over an area spreading from the stagnation point to the lateral corners of the bifurcation. Comparison between the present results and topographical patterns of atherosclerosis reported in the literature suggests that it is in such low shear regions that lipid deposition tends to occur most.

Arteries↗
PubMed1992

Pulmonary pressures at high flows in the intact pulsatile flow perfused lung.

Pulmonary pressure-flow curves can be easily generated in the intact animal by using a combination of systemic arteriovenous (a-v) fistulas and inferior vena cava (IVC) occlusion. By combining this technique with pulmonary artery occlusion, pulmonary pressure-flow curves may be studied over a broader range of pressures than has been previously been done in the intact, resting animal using pulsatile flow. Pressure-flow curves were generated by varying flow through opening and closing of the a-v fistulas in conjunction with inflating and deflating a balloon in the inferior vena cava. The pressure-flow curves were done under two conditions; (1) with both lungs perfused; (2) with the right lung excluded from the circulation (PA occlusion). PA occlusion resulted in no change in alveolar arterial oxygen tension gradient. The pressure-flow relationships for one lung and two lungs were well described by linear equations (r2 = 0.83 +/- 0.03 and 0.82 +/- 0.04 respectively). The slope of the equations increased with PA occlusion (3.6 +/- 0.4 mmHg.L-1 to 5.9 +/- 0.9 mmHg.L-1). There was no change in the pressure axis intercept with PA occlusion (8.34 +/- 0.8 mmHg pre-occlusion and 8.9 +/- 1.3 mmHg post-occlusion). It is concluded that the pulmonary pressure-flow relationship is well described by a linear function above a mean pulmonary artery pressure (PAP) of 10-12 mmHg.

Animals↗
PubMed1990

Post-stenotic core flow behavior in pulsatile flow and its effects on wall shear stress.

Arteries of several species, including man, tend to adjust their diameters such that the mean wall shear stress is in the range of 10-20 dynes cm-2. Additionally, intimal thickening in the human carotid bifurcation correlates well with the reciprocal of wall shear stress as determined in model studies. The correlation indicates that wherever the local mean wall shear stress exceeds approximately 10 dynes cm-2, the artery tends to be spared from intimal thickening. However, it is not known whether mean shear stress, i.e. the time-averaged value, or the instantaneous shear stress is the appropriate correlative variable. Each of these variables suggests different mechanisms for the reaction of the artery wall to its hemodynamic environment. It is therefore important to devise means by which the effects of mean shear and pulsatile shear can be separated in the study of atherogenesis. The present investigation examines the post-stenotic flow field in Plexiglas models under pulsatile conditions approximating those in the aortas of the cynomolgus monkey, an animal often employed in atherogenesis research. Behavior of the core flow and its effects on wall shear stress are studied for stenoses of 75 and 90% area reductions using laser velocimetry. The results show that the post-stenotic field contains regions in which the mean wall shear stress is low, but the pulsatile excursions are large.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

Pulmonary lobar vascular resistances during constant and pulsatile flows.

We measured arterial and venous occlusion pressure profiles in left lower lobes of dog lungs perfused in constant flow and pulsatile flow. Initially, pulmonary arterial pressure, flow rate, and venous pressure were recorded during a series of inflow and outflow occlusions effected during constant flow perfusion. The perfusion was then made pulsatile and while keeping the same flow rate, a second series of vascular occlusions was performed. The arterial occlusion pressures measured with the two modes of perfusion were similar. During pulsatile perfusion, the variation in arterial pressure following an arterial occlusion varied with the timing of occlusion in the pressure cycle. The arterial pressure drop could be recovered by selecting the time-averaged pulmonary arterial pressure as the upstream pressure. During both types of perfusion, the venous occlusion curves were similar. The arterial and venous pressure drops were approximately 4.8 mm Hg and 4.2 mm Hg, respectively. We concluded that the longitudinal distribution of pulmonary vascular pressures can be inferred from occlusion measurements obtained in pulsatile flow.

Animals↗
PubMed1988

Vortex generation in pulsatile flow through arterial bifurcation models including the human carotid artery.

Visualization experiments were performed to elucidate the complicated flow pattern in pulsatile flow through arterial bifurcations. Human common carotid arteries, which were made transparent, and glass-models simulating Y- and T-shaped bifurcations were used. Pulsatile flow with wave forms similar to those of arterial flow was generated with a piston pump, elastic tube, airchamber, and valves controlling the outflow resistance. Helically recirculating flow with a pattern similar to that of the horseshoe vortex produced around wall-based protuberances in circular tubes was observed in pulsatile flow through all the bifurcations used in the present study. This flow type, which we shall refer to as the horseshoe vortex, has also been demonstrated to occur at the human common carotid bifurcation in steady flow with Reynolds numbers above 100. Time-varying flows also produced the horseshoe vortex mostly during the decelerating phase. Fluid particles of dye solution approaching the bifurcation apex diverged, divided into two directions perpendicularly, and then showed helical motion representing the horseshoe vortex formation. While this helical flow was produced, the stagnation points appeared on the wall upstream of the apex. Their position was dependent upon the flow distribution ratio between the branches in the individual arteries. The region affected by the horseshoe vortex was smaller during pulsatile flow than during steady flow. Lowering the Reynolds number together with the Womersley number weakened the intensity of helical flow. A separation bubble, resulting from the divergence or wall roughness, was observed at the outer or inner wall of the branch vessels and made the flow more complicated.

Adult↗
PubMed1988

Turbulent stress measurements downstream of six mechanical aortic valves in a pulsatile flow model.

In a pulsatile flow model aortic Björk-Shiley Standard, Convex-Concave and Monostrut valves were investigated together with the Hall-Kaster (Medtronic-Hall), St Jude Medical and Starr-Edwards Silastic Ball valve using hot-film anemometry. Three-dimensional visualization of average systolic Reynolds normal stresses (RNS) reflected the design of the valves. Mean average RNS were used for comparison of the fluid dynamic performance along with Velocity Energy Ratio (VER100) and Turbulence Energy Ratio (TER) as a relative turbulence intensity for pulsatile flow. Mean average RNS ranged from 13.2 to 37.6 Nm-2 for all the valves with the highest levels for the Björk-Shiley Standard and Starr-Edwards Ball valve and lowest values for the St Jude Medical valve and with the Hall-Kaster (Medtronic-Hall), Björk-Shiley Convex-Concave and Monostrut valves in between.

Aortic Valve↗
PubMedDate not supplied

Cardiac flow measurement by ultrafast CT: validation of continuous and pulsatile flow.

To gauge the accuracy of ultrafast CT in measuring cardiac output and myocardial perfusion in humans, measurements of continuous and pulsatile flow were made in a large asymmetrical phantom. The variation in the relationship between Hounsfield number and contrast concentration was assessed in a human thorax phantom. Radiopaque contrast medium was injected during perfusion of the phantom at a range of flow rates between 1.5 and 8 L/min. The phantom was scanned in two modes (50 and 100 ms) during continuous and pulsatile flow and with the phantom surrounded by air and by water. Flow in the tubes was calculated using indicator dilution theory, and flow in the tissue-equivalent chamber was calculated by applying first-pass distribution principles. The standard deviation of the difference between calculated and measured flow varied from 0.2 to 0.6 L/min, giving 95% limits of agreement from 0.4 to 1.2 L/min. The constant (K) relating Hounsfield unit number to iodine concentration varied widely both in different locations within the phantom and under different scan conditions (17.2-27.6 HU/mg I). Within a human thorax phantom, K varied from 14.15 to 23.18 HU/mg I and was dependent on location within the thorax phantom, the scan mode, and the cross-sectional diameter of the phantom. These data suggest that though the ultrafast CT scanner can measure continuous and pulsatile flow accurately in tubes, precise measurements of cardiac output in humans will require K to be assessed for each subject. Measurements of flow in tissue should be possible.

Cardiac Output↗
PubMedDate not supplied

Effects of flow pulsatility on platelet adhesion to subendothelium.

Platelet adhesion in the annular perfusion system developed by Baumgartner was studied under pulsatile, oscillatory, or steady flow conditions. To investigate in what way pulsatile flow affects platelet adhesion, we developed a flow system that produces a sinusoidal laminar flow superimposed on a constant component in the annular perfusion chamber. Frequencies and amplitudes of this sinusoidal flow were in the physiological range. Pulse frequencies varied between 30 and 120 beats/minute, and different amplitudes of the wall shear rate in the range 75 to 1000 s-1 were studied. Shear rates resulting from the constant flow component were between 500 s-1 and 1800 s-1. Under these conditions, no significant differences in platelet adhesion were observed between steady flow and pulsatile flow. In the case of an oscillatory flow (absence of constant component), a clear dependence of platelet adhesion on the amplitude of the pulse was seen. These data indicate that platelet adhesion in larger blood vessels, such as the aorta and larger arteries where backflow is limited, is not essentially influenced by the pulsatility in these vessels.

Biophysical Phenomena↗
PubMed1992

A flexible blood flow phantom capable of independently producing constant and pulsatile flow with a predictable spatial flow profile for ultrasound flow measurement validations.

The validation of the ultrasound time-domain correlation method of measuring blood flow has required the development of a flexible blood flow phantom capable of generating predictable flow profiles under a wide variety of conditions. The purpose of the phantom is to generate flow with well-known flow properties and not to mimic actual in vivo vessels. This paper describes a flow phantom which can independently generate both constant and pulsatile flow over a wide range of flow rates with a spatially fully developed laminar flow profile. It incorporates a computer-controlled pulsatile pump, which can produce different temporal pulsatile waveforms. The flow phantom also supports multiple vessels, different vessel sizes, as well as different attenuating media. The fluid most commonly used in the phantom is Sephadex mixed with water, and the probability density function of ultrasound reflected from Sephadex is experimentally determined and compared with that of blood. Examples of different constant and pulsatile flow experiments using the phantom are presented.

Adult↗
PubMed1989

Pressure drop and flow rate measurements in a human aortic bifurcation cast for steady and pulsatile flow.

Pressure drop and flow rate measurements in a rigid cast of a human aortic bifurcation under both steady and physiological pulsatile flow conditions are reported. Integral momentum and mechanical energy balances are used to calculate impedance, spatially averaged wall shear stress and viscous dissipation rate from the data. In the daughter branches, steady flow impedance is within 30% of the Poiseuille flow prediction, while pulsatile flow impedance is within a factor of 2 of fully developed, oscillatory, straight tube flow theory (Womersley theory). Estimates of wall shear stress are in accord with measurements obtained from velocity profiles. Mean pressure drop and viscous dissipation rate are elevated in pulsatile flow relative to steady flow at the mean flow rate, and the exponents of their Reynolds number dependence are in accord with available theory.

Aged↗
PubMed1979

Does pulsatile flow influence the incidence of postoperative hypertension?

Twenty patients undergoing primary elective aorta--coronary artery bypass were divided into two equal groups, both receiving identical premedication, anesthetic, and pump primes. The control patients received hypothermic nonpulsatile flow and the study patients received hypothermic pulsatile flow. Hypertension, defined as a pressure of 160/100 mm Hg or higher, was observed in 80% of the control patients and 20% of the patients receiving pulsatile flow (p less than 0.05). Serial renin measurements demonstrated maximum values in the intensive care unit and coincided with the onset of postoperative hypertension in the control patients. Those patients who had received pulsatile flow did not demonstrate notable renin stimulation. Catecholamines were markedly elevated during bypass and in the intensive care unit, but there was no significant difference between the two groups. Peripheral vascular resistance was not significantly lower with pulsatile flow, except in the first study performed in the intensive care unit. We conclude that catecholamines and the renin-angiotensin system contribute to the production of postoperative hypertension and that pulsatile flow diminishes renin stimulation. Pulsatile flow results in a decreased incidence of postoperative hypertension.

Cardiopulmonary Bypass↗
PubMed1990

The decomposition of apparent stresses in disturbed pulsatile flow in the presence of large scale organized structures.

Flow disturbance phenomena that occur in unsteady-in-the-mean flows (i.e. pulsatile or oscillating) at moderate Reynolds numbers are analyzed in both the time domain and the frequency domain. The analysis utilizes variable decomposition into a time-varying underlying waveform and flow disturbances which are composed of large scale organized structures and random fluctuations. A practical technique which incorporates time domain phase conditioning, trend removal, and frequency domain matched filtering, is presented and examined using simulated data of known statistical behavior. The applicability of the method is shown by the decomposition of the simulated data and the technique is then applied to experimental data obtained in pulsatile flow through a constricted tube by means of a laser Doppler anemometer. The cross-sectional area reduction at the constriction throat was 90%. The Womersley parameter in the experiments was 5.3 and the Reynolds number based on the average flow rate per cycle was 300 with a minimum/maximum value of 55/600 based on the instantaneous flow rate. Measurements were taken in the flow region downstream of the constriction throat which included several interesting flow disturbance phenomena. The results of the decomposed flow phenomena demonstrate the significant role of large scale organized structures in such flows. This is particularly important when analyzing blood flow in the large arteries in the presence of severe stenosis or behind prosthetic devices in an attempt to estimate the 'turbulent' stress which act on cellular elements. Estimation of the apparent stress tensor is of importance in an effort to elucidate the mechanical factors which influence the durability of red blood cells under abnormal conditions.

Biomechanical Phenomena↗