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At least 1,711 records · Page 95Linked to original sources
PubMed1982

Toward a non-traumatic determination of the rheological properties of the vascular wall.

It is possible to show the existence of a biunivocal relationship, at a given time, between the longitudinal velocity profile of a pulsatile flow in a flexible tube on the one hand and the pressure gradient which induces the flow and the mechanical behavior of the wall on the other hand, for an assumed rheological model. From experimental data of velocity profiles, only pressure gradients can be correctly calculated. To determine the elasticity of the material, a theory is developed which requires a knowledge of the time dependence of the wall motion. This theory is tested first on numerically simulated velocity data then verified experimentally with a hydromechanical device.

Animals↗
PubMed1983

Water transport in the arterial wall--a theoretical study.

Water transport in the arterial wall is studied using a mathematical model based on the theory for the consolidation of water saturated soils (Biot, 1941; Kenyon, 1976a). The intimal pressure is considered to be harmonic in time. Analytical results are obtained for both large and small consolidation times since both the situations are of physiological relevance. For large consolidation times, the filtration is confined to a thin boundary layer. Large pressure gradients exist within the boundary layer while the pressure gradient is negligible in the intermediate layer. Thus, the pulsatile flow is found to be confined to the boundary layer while a smaller mean flow exists throughout the wall.

Arteries↗
PubMed1984

In-vitro wall shear measurements at aortic valve prostheses.

Wall shear distributions during the cardiac cycle at the valve rings of Starr-Edwards, Björk-Shiley and Lillehei-Kaster aortic valves are measured and compared with thresholds reported for shear-induced trauma of blood components. Further, for the disk valves, the influence of pulse rate on wall shear stresses is evaluated. Hot film anemometry with flush-mounted wall shear probes is used as measurement technique in a pulsatile flow mock circuit. The experimental systolic data support the better hemodynamic characteristics of the disk valves over the ball valve also with respect to the threshold shear stresses of flow induced blood trauma. These results are confirmed by postoperative clinical studies, where lower LDH-values are found with the disk than with the ball valves. During diastole, however, high shear stresses are measured and calculated at the valve ring of the Björk-Shiley prosthesis, which can be referred to the non-overlapping closing mechanism. This result is discussed with respect to the often observed thrombus formation at the disk downstream of the smaller orifice of the Björk-Shiley valve.

Aortic Valve↗
PubMed1987

On the paths of fluid particles in an axisymmetrical aneurysm.

The aim of this study is the characterization of the pulsatile flow field by demonstration of the paths of single particles in a model of an axisymmetric aneurysm. The detailed analysis of the flow field can give additional information on the flow pattern and the time of transition of blood particles in the segment. The basis of the calculations is the system of the Navier-Stokes equations for incompressible Newtonian fluid flow. To solve these equations numerically the finite element method was used. The trajectory equations for a fluid particle were solved by use of a predictor-corrector procedure. The results of the computer simulation demonstrate the development, shift and disappearance of vortices in the excavation and give references to zones of stasis. This behavior can be an important factor in thrombogenesis.

Aneurysm↗
PubMed1989

Experimental determination of wall shear rate in canine carotid arteries perfused in vitro.

The mathematical model of Hung (Tsai and Hung, 1984) is employed to determine the wall shear rate acting on canine carotid arteries perfused in vitro. Model equations for pulsatile flow in a deformable vessel are coupled with experimental data of dynamic pressure drop, flow rate, vessel radius and radial wall motion. Derived quantities, e.g. velocity profiles and wall shear, are obtained for vessels exposed to 'normotensive' hemodynamics, 'hypertension' simulations and perfusions in which the compliance of the vessel wall is deliberately altered. Our results indicate that wall shear varies markedly as a function of the hemodynamic environment. The effects of vessel radius vs flow rate on the development of wall shear are also demonstrated. It is found that convective processes correlate with the magnitude of wall shear in the 'hypertension' simulations. The present findings and complementary published data may explain, at least in part, the variations in vessel wall transport and endothelial cell biology we observe as a function of the hemodynamic environment. For example we have documented that the exposure of canine carotids to 'hypertensive' (vs 'normotensive') hemodynamics is associated with an increased flux of lipoproteins (LDL) into the intima and luminal media. Alternations in wall compliance, on the other hand, profoundly influence endothelial shape, orientation and cytoskeletal array.

Animals↗
PubMed1977

Complications and sequelae of temporal artery catheterization in the high-risk newborn.

One hundred high-risk newborns underwent a total of 115 temporal artery catheterizations using direct arterial exposure. Early catheter-related complications occurred in 10 patients: retrograde thrombosis of the posterior auricular artery (6), small area of skin slough at catheterization site (2), and positive blood cultures. (2). Twenty-five surviving infants were examined 6-10 mo after catheter removal: 5 patients showed scarring of minor cosmetic importance; 20 patients had barely perceptible incisional scars; and 21 patients had a return of pulsatile flow in the catheterized temporal artery.

Bacterial Infections↗
PubMed1988

Comparison of static incubation versus physiologic perfusion techniques for quantitation of luminal release of prostacyclin and thromboxane in canine arteries and veins.

Intraluminal release of 6-keto-PGF1 alpha and TxB2 in ex vivo canine arteries and veins was assessed during five consecutive 15-min periods using static incubation and physiologic perfusion techniques. Arterial segments were perfused with 90 ml/min pulsatile flow at 100 mm Hg and vein segments with 90 ml/min nonpulsatile flow at 7 mm Hg. During the final 15-min period vessels were stimulated with arachidonic acid (AAS). Perfusion of vein segments caused a higher release of 6-keto-PGF1 alpha during the first 30 min (P less than 0.05) and following AAS (P less than 0.05) than did static incubation. Perfused arterial segments exhibited a higher release than did incubated segments of 6-keto-PGF1 alpha for 45 min (P less than 0.01) as well as following AAS (P less than 0.01). TxB2 release was higher during the entire observation period in perfused arteries and veins compared to incubated vessels (P less than 0.01 and less than 0.05, respectively). There was no correlation between the amounts of 6-keto-PGF1 alpha or TxB2 released when comparing values obtained by one technique to values obtained by the other (P greater than 0.1). These data suggest that flow related shear stress alters vascular prostanoid production, and that such should be accounted for when interpreting results of studies on prostacyclin and thromboxane release from intact vessels.

6-Ketoprostaglandin F1 alpha↗
PubMed1985

Haemodynamics of coronary artery-saphenous vein bypass.

The velocity distribution of a suspension of red blood cell ghosts in an idealized model of the coronary artery-saphenous vein bypass has been investigated with the aid of laser Doppler anemometry. Pulsatile flow simulated pressure variations in the ascending aorta and ghost cell velocities were determined by the Doppler shift of scattered laser light. Using four different model bypasses it was demonstrated that turbulent flow at the graft-coronary intersection can be delayed by decreasing the discontinuity in diameter between the bypass vein and coronary artery, and also by reducing the bypass vein and host coronary artery intersection angle.

Biomedical Engineering↗
PubMed1987

Results of a comparative in vitro study of Duromedics and Björk-Shiley monostrut mitral heart valve prostheses.

Two different mechanical heart valves with annulus diameters 21-29 mm, (five Björk-Shiley monostrut tilting disc valves and five Duromedics bileaflet valves) have been tested in pulsatile flow in the mitral position of a mock circulation. Reflux, pressure, and orifice area have been measured while cardiac output was varied between 2 and 6 1 min-1. Insufficiency, mean orifice area, discharge coefficient, and performance and efficiency indices have been calculated. Mean values of insufficiency for the Björk-Shiley monostrut valves varied between 4.8 and 17.2% while the corresponding values for the Duromedics valves were in the range 6.1-17.3%. Mean values for orifice areas of the Björk-Shiley monostrut valves increased with the larger valve sizes from 101.1 to 210.2 mm2; for the Duromedics valves the area range was 134.5-262.9 mm2. Because of the larger orifice areas the values of discharge coefficient and performance index for the Duromedic valves were higher than those for the Björk-Shiley monostrut valves. As the insufficiency of the two mechanical valves was similar, and the orifice area of the bileaflet valves was greater than that of the tilting disc valves, Duromedics valves gave higher valves for the efficiency index, which varied between 0.31 and 0.39; for Björk-Shiley monostrut valves the index varied between 0.24 and 0.28 under the same test conditions. This hydrodynamic in vitro comparison of mechanical heart valves showed that the Duromedics bileaflet valves were superior to the Björk-Shiley tilting disc valves.

Biomedical Engineering↗
PubMed1982

Quantitative analysis of methods for reducing physiological brain pulsations.

Normal movements of the mammalian brain, caused by the arterial and venous pressure fluctuations of each cardiac and respiratory cycle, have made obtaining stable intracellular recordings from neurons difficult. This study quantitated the movements of the cats' brainstem and examined the effects of traditional neurophysiological techniques used to reduce pulsation. Two components of brain movement were recorded: (1) an arterial component--relatively low amplitude (110-266 micrometers) and short duration (330-400 ms) excursions corresponding to the pressure wave of each cardiac systole [A-wave]; and (2) a pulmonary component--slower (10-12/min), high amplitude plateau-like displacement (300-950 micrometers) lasting for a time (2.4-5.1 s) corresponding to the inspiration of each respiratory cycle [P-wave]. Pneumothoraces and mechanical ventilation combined with elevating the animal's head reduced the pulmonary component by an average of 68% and the arterial component by 40%. Cerebrospinal fluid drainage could reduce the P-wave component of movement by as much as 50%. To reduce arterial pulsations below 100 micrometers, the mean arterial pressure (MAP) had to be lowered to less than 40 mm Hg, which was not compatible with maintaining normal brainstem auditory evoked responses. Residual movements at MAPs greater than 50 mm Hg were still sufficient to make stable intracellular penetration of small neurons difficult. The authors suggest the solution to this problem is the development of a cardiopulmonary bypass system which generates a non-pulsatile flow of oxygenated blood, described in a companion paper.

Animals↗
PubMed1987

A miniaturised roller pump.

A roller pump is described which is able to pump microlitre quantities of solution in either direction. It depends upon a caged roller bearing where the individual rollers press against silicon rubber tubing having an internal diameter of 0.5 mm. This assembly was housed in a cylindrical cavity shaped to produce a non-pulsatile flow. The pump is driven by a stepping motor to produce a constant flow rate. It is used to obtain small samples of bathing solution from near the surface of brain slices to monitor efflux of amino acids.

Equipment Design↗
PubMedDate not supplied

An in vitro model and its application for the study of carotid Doppler spectral broadening.

A new in vitro model has been developed for studying the changes in the ultrasound Doppler spectrum that occur in the region of a stenosis. Pulsatile flow in rigid acrylic tubes was produced by means of a modified hemodialysis pump. The Doppler spectral waveforms were measured using a continuous wave Doppler system, a probe of a known field pattern, a real-time high resolution frequency analyzer, and a video display and recording system. The flow velocity waveforms were found to be nearly identical to those seen in the human carotid. Measurements were made to determine the critical stenosis and the results are similar to those reported from in vivo studies. In a preliminary study, the extent of spectral broadening was found to be dependent on the recording site in relation to the stenosis, the severity of the stenosis, and the flow rate. Using qualitative methods it was not possible to determine either the influence of the shape of the stenosis or the phase of the cardiac cycle on spectral broadening.

Carotid Arteries↗
PubMed1986

A Doppler flowmeter for use in theatre.

We have developed a Doppler flowmeter based on a 10 MHz pencil probe and mean frequency estimator which overcomes many of the limitations of existing electromagnetic and ultrasonic flowmeters. The output of the flowmeter, which is proportional to the first moment of the Doppler power spectrum and hence mean blood velocity is linear from 1.3 to over 50 cm s-1 for pulsatile flow. Variation in vessel diameter and angle of insonation, which are the common sources of error in Doppler flowmetry, are minimised by constraining the vessel in a plastic cuff which fixes the probe angle at 50 degrees. A simple gauge is used to compress the vessel flat, before the cuff is applied, to measure the wall thickness to within 0.25 mm. The vessel internal diameter and hence blood flow can then be calculated using an experimentally determined calibration factor to compensate for non-even insonation. A range of sterilizable cuffs from 3-12 mm diameter have been built and the flowmeter is now being used routinely during all arterial reconstructive surgery. The accuracy and reproducibility of the system was tested for range of different sized silastic tubes on a hydraulic model and found to be less than 12% for vessels greater than 2 mm internal diameter. Satisfactory signals were easily obtained from all prosthetic materials with the exception of PTFE. The instantaneous output was compared to an electromagnetic flowmeter using a fast Fourier transform algorithm; the moduli of the harmonics were virtually identical but the Doppler system produced a smaller phase shift with increasing harmonics.(ABSTRACT TRUNCATED AT 250 WORDS)

Humans↗
PubMed1988

Automated calculation of stenosis diameters from the width of the velocity jet with the use of a multi-gate pulsed Doppler system.

The aim of this study was to evaluate an algorithm for automated estimation of the width of a jet stream originating from a stenosis. The evaluation was performed in a pulsatile flow model. The width of the jetstream was assessed by measuring the diameter of the region with relatively high velocities (the jet) in the velocity profiles, as recorded with a multi-gate pulsed Doppler system. Measurements were performed at 3, 6, and 9 mm downstream of three different stenoses (stenosis diameter: 3, 5, or 8 mm) at different Reynolds numbers (200-1600) based on time averaged flow velocity for a tube of diameter 15 mm. The developed algorithm was used successfully for automated detection and quantification of jet flow diameters downstream to a stenosis. The algorithm can be used for calculating the stenosis diameter notwithstanding a theoretically predictable overestimation of about 1 mm, depending on the Reynolds number and the distance from the stenosis.

Algorithms↗
PubMedDate not supplied

A new concept of renal hemodynamics.

Observations which show that urine output can vary with changes in the contour of the pulse cure, that it can even increase while the mean blood pressure falls during counterpulsation, that the well known hypertensive effect of a renal artery stenosis can also be produced by an aneurysm of the renal artery, cannot be explained by orthodox concepts of renal hemodynamics. The anatomical fact that the afferent glomerular arteriole is about twice as large as the efferent has so far not been adequately explained. The hypothesis that the combination of pulsatile flow and the unique anatomical configuration of the glomerulus permit a hydraulic switching mechanism which modifies profoundly its perfusion explains these questions.

Arterioles↗
PubMed1986

Reducing motion artifacts in two-dimensional Fourier transform imaging.

The effects of motion in two-dimensional Fourier transform imaging (2DFT) are considered. Specific calculations describing the case of periodic motion are presented. The results predict the commonly seen artifact of image replication, sometimes referred to as ghosting. Expressions for both position and amplitude of these ghosts are derived. Simulated examples illustrate the image degradation for pulsatile flow and in plane motion. Several methods of reducing motion artifacts are then suggested. These include: randomization of views, averaging views, matching repeat times to the respiratory period, hybrid imaging, ROPE and COPE. The latter two methods reorder the data acquisition to destroy the coherence of the motion. They do not increase the data acquisition time and promise to be part of the standard approach to remove motion artifacts. The final step in actually recovering ideal resolution can be accomplished by using a model of the motion and a generalized transform inversion technique.

Fourier Analysis↗
PubMed1992

Carbomedics and St. Jude Medical bileaflet valves: an in vitro and in vivo comparison.

The hydrodynamic and haemodynamic function of 29 mm mitral and 23 mm aortic St Jude Medical (SJM) and CarboMedics (CM) heart valves have been compared in a pulsatile flow simulator in the laboratory and by doppler ultrasound in vivo. The laboratory studies showed that there was no significant difference in the pressure drop across the valves. The valves also had similar regurgitant volumes. Doppler ultrasound confirmed these results with a mean pressure drop of 12.9 +/- 5.5 mmHg across the CM aortic valves and 12.6 +/- 5.5 mmHg for the SJM aortic valves. The mean mitral diastolic pressure drops were 3.8 +/- 1.8 and 4.6 +/- 1.4 mmHg for the CM and SJM valves, respectively. Mild aortic regurgitation was detected in 5 of 14 patients in the CM group and 1 out of 6 patients in the SJM group, though none of the cases were clinically significant. Both the in vitro and in vivo results show these valves to have acceptable and comparable function.

Aortic Valve↗