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PubMed1997

Effects of racing and gender on viscoelastic properties of horse blood.

Splenic contraction in racing horses increases the hematocrit (hct), thereby increasing blood viscosity. We tested as to whether racing also affects the elastic properties of blood. Mares and geldings were studied for thus purpose. After racing, there was: (i) an increased erythrocyte count independent of gender and race distance (0.32 to 1.7 km): (ii) an increased mean erythrocyte volume in both sexes; (iii) an increased heterogeneity of RBC size in both sexes; (iv) an increased plasma fibrinogen concentration and erythrocyte sedimentation rate in both sexes; and (v) an increased elastic yield stress (EYS). When corrected to a constant hct of 65%, the blood of mares, but not geldings, had increased EYS after racing. Gender differences in fibrinogen response (p = 0.72) did not account for this and the mechanism is not known. Since EYS is analogous to the point at which ketchup starts to flow from a bottle, its increase could be deleterious in vascular beds characterized by pulsatile flow, e.g. the coronary circulation.

Animals↗
PubMed2006

Fluid structure interaction of patient specific abdominal aortic aneurysms: a comparison with solid stress models.

BACKGROUND: Abdominal aortic aneurysm (AAA) is a dilatation of the aortic wall, which can rupture, if left untreated. Previous work has shown that, maximum diameter is not a reliable determinant of AAA rupture. However, it is currently the most widely accepted indicator. Wall stress may be a better indicator and promising patient specific results from structural models using static pressure, have been published. Since flow and pressure inside AAA are non-uniform, the dynamic interaction between the pulsatile flow and wall may influence the predicted wall stress. The purpose of the present study was to compare static and dynamic wall stress analysis of patient specific AAAs. METHOD: Patient-specific AAA models were created from CT scans of three patients. Two simulations were performed on each lumen model, fluid structure interaction (FSI) model and static structural (SS) model. The AAA wall was created by dilating the lumen with a uniform 1.5 mm thickness, and was modeled as a non-linear hyperelastic material. Commercial finite element code Adina 8.2 was used for all simulations. The results were compared between the FSI and SS simulations. RESULTS: Results are presented for the wall stress patterns, wall shear stress patterns, pressure, and velocity fields within the lumen. It is demonstrated that including fluid flow can change local wall stresses slightly. However, as far as the peak wall stress is concerned, this effect is negligible as the difference between SS and FSI models is less than 1%. CONCLUSION: The results suggest that fully coupled FSI simulation, which requires considerable computational power to run, adds little to rupture risk prediction. This justifies the use of SS models in previous studies.

Aged↗
PubMed1991

Quantitation of vascular outflow by measurement of impedance.

One of the most important determinants of graft patency is the degree and character of vascular outflow. This study was designed to evaluate input impedance as a functional assessment of the outflow bed of vascular grafts. Four distinct outflow environments were created for external jugular vein conduits in 42 New Zealand white rabbits. Vein grafts (n = 14) were fashioned as end-to-side common carotid interposition bypass grafts. Arteriovenous fistulas (n = 15) were created by side-to-side anastomosis of the distal common carotid artery and linguofacial vein. Arteriovenous fistulas with outflow obstruction (n = 7) were fistulas with a metal clip partially obstructing the distal outflow channel (1 mm lumen). Vein graft/arteriovenous fistula combinations (n = 6) consisted of a vein graft and arteriovenous fistula in series. Pressure and flow in the external jugular vein were measured, and input impedance spectra were calculated by Fourier methods. By use of a PC-based acquisition and processing system, impedance results for 20 cardiac cycles could be obtained in approximately 10 minutes. The results revealed that vein grafts typically demonstrated high resistance to steady state flow (Rin = 235 +/- 50 x 10(3) dyne . sec/cm-5) and steadily decreasing impedance to pulsatile flow resulting in a characteristic impedance (Z0; average of fourth to tenth harmonics) of 35.5 +/- 8.0 x 10(3) dyne . sec/cm-5. Phase angle values were usually negative, especially at low harmonics (first harmonic phase angle = -1.11 +/- 0.10 radians) indicating that flow led pressure. In contrast, arteriovenous fistula Rin was minimal (6.3 +/- 1.4 x 10(3) dyne . sec/cm-5; p less than 0.05 compared to vein graft, and the impedance was flat across the frequency spectrum (Z0 = 8.5 +/- 1.5 x 10(3) dyne . sec/cm-5; p less than 0.05) with pressure and flow nearly in phase (first harmonic phase angle = -0.05 +/- 0.10 radians). Creation of outflow obstruction in arteriovenous fistulas resulted in significantly elevated Rin (136 +/- 41 x 10(3) dyne/sec . cm-5; p less than 0.05 compared to arteriovenous fistula and Z0 (23 +/- 9 x 10(3) dyne . sec/cm-5, p less than 0.05).(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed2005

Should we discontinue intraaortic balloon during cardioplegic arrest? Splanchnic function results of a prospective randomized trial.

BACKGROUND: Preoperative use of intraaortic balloon pumping (IABP) has increased in high-risk patients. Linear flow during cardiopulmonary bypass (CPB) can induce subclinical damage, whereas automatic IABP mode may maintain pulsatile flow. We sought to evaluate differences between suspending IABP and switching it to an automatic 80 bpm mode during cardioplegic arrest. METHODS: Between January and November 2004, 40 patients undergoing preoperative IABP were randomized to receive either standard nonpulsatile CPB with IABP discontinued during cardioplegic arrest (20 patients; group A) or IABP-induced pulsatile (automatic 80 bpm) CPB (20 patients; group B). Hospital outcome was recorded. Urine output, blood urea nitrogen (BUN), creatine, creatinine clearance, peripheral lactate, recovery of gut motility, alanine-amino-transferase (ALT), aspartate-amino-transferase (AST), lactic dehydrogenase (LDH), bilirubin, and amylase (AMY) were compared. RESULTS: There were no IABP-related complications, nor perioperative renal or liver failures, nor hospital deaths, nor myocardial infarctions. Intensive care and hospital stay, urine output, and recovery of gut motility were comparable. Group B showed lower creatine on the first (p = 0.01) and second (p = 0.005) postoperative days, higher creatinine clearance (first day: p = 0.01; second day: p = 0.03), lower lactate after CPB termination (p = 0.0001) and during the first day (p = 0.001). The ALT, AST, and AMY were lower in group B (first day ALT: p = 0.01; AST: p = 0.04; AMY: p = 0.017; second day ALT: p = 0.01; AST: p = 0.02; AMY: p = 0.027), as well as total bilirubin (first day: p = 0.05; second day: p = 0.02). CONCLUSIONS: Automatic 80 bpm IABP during cardioplegic arrest improves creatinine clearance and splanchnic enzymes. There is no reason to suspend preoperative IABP support during cardioplegic arrest.

Aged↗
PubMed1994

Effect of trabeculectomy on pulsatile ocular blood flow.

Trabeculectomy, despite producing an effective reduction in intraocular pressure, may not prevent continued visual field loss. This may be because of the presence of other factors in the pathogenesis of glaucoma. Vascular factors have been suggested as being particularly important. To study the effect of trabeculectomy on ocular blood flow the technique of ocular pulse analysis has been used to derive a measure of pulsatile ocular blood flow in 17 patients (average age 65.6 (SD 1.8) years) undergoing trabeculectomy. A significant increase in pulsatile ocular blood flow of 29% was observed in the group as a whole in the standing position following operation but in some individuals blood flow changed only slightly despite a large reduction in intraocular pressure. The significance of these findings in relation to the prognosis of visual field preservation following trabeculectomy is discussed.

Aged↗
PubMed2000

Interactions of pulsatile upstream forcing with flow-induced oscillations of a collapsed tube: mode-locking.

Interest in flow through collapsible tubes derives from their prevalence in the human body. In the circulation at least, the driving pressure for flow is pulsatile, and it is accordingly appropriate to investigate the response of a collapsible tube to a pulsatile head. A servo-controlled hydraulically powered piston pump was used to add pulsation to the head propelling aqueous flow through a silicone rubber tube. The pump was commanded to execute a sinusoidal variation of piston position at various frequencies on either side of the natural frequency of an operating point where slow (3.75 Hz) repetitive self-excited oscillations in the absence of such forcing could be guaranteed. The resulting time series were characterised by the ratio of the number of collapse cycles to the number of forcing cycles in one overall interaction waveform period. Nonlinear interaction rather than linear superposition was dominant. Many different mode-locked interactions were recorded, and are here systematised in Farey series and plots which show the extent of entrainment. A few of the time series were aperiodic over the time scales investigated, suggesting possible chaos.

Biomechanical Phenomena↗
PubMedDate not supplied

Development of a miniature intraventricular axial flow blood pump.

A new intraventricular axial flow blood pump has been designed and developed as a totally implantable left ventricular assist device (LVAD). This pump consists of an impeller combined with a guide-vane, a tube housing, and a DC motor. The pump is introduced into the LV cavity through the LV apex, and the outlet cannula is passed antegrade across the aortic valve. Blood is withdrawn from the LV through the inlet ports at the pump base, and discharged to the ascending aorta. Our newly developed axial flow pump system has the following advantages: 1) it is a simple and compact system, 2) minimal blood stasis both in the device and the LV cavity, 3) minimal blood contacting surface of the pump, 4) easy accessibility with a less invasive surgical procedure, and 5) low cost. A pump flow > 5 L/min was obtained against 100 mmHg differential pressure in the mock circulatory system. The pump could produce a passive pulsatile flow effect with a beating heart more efficiently than other non-pulsatile pumps because of minimal pressure drop and inertia along the bypass tract. Anatomic fit studies using dissected hearts of dilated cardiomyopathy (DCM) cadavers showed that this pump could smoothly pass through the aortic valve without any interference with mitral valve function. Recently, a dynamic pressure groove bearing and a miniature lip seal have been developed. The dynamic pressure groove bearing has a simple structure and acts as a pressure resistant sealing mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiomyopathy, Dilated↗
PubMed2005

Effects of transient mild systemic hypoxia on the pulsatile choroidal blood flow in healthy young human adults.

PURPOSE: To investigate the effects of transient mild systemic hypoxia on the pulsatile ocular blood flow (POBF) in the healthy young adult. METHODS: Two measurements of the intraocular pressure (IOP) pulses, used to derive the POBF, were recorded from 19 subjects before, during, and after breathing 12% oxygen in nitrogen. Many physiological variables were also assessed throughout testing. RESULTS: Transient mild systemic hypoxia decreased the hemoglobin oxygen saturation and end-tidal carbon dioxide, increased the heart rate, but did not change the respiratory rate, systemic blood pressure, or ocular perfusion pressure. Mild systemic hypoxia decreased the intraocular pulse volume and the systolic and diastolic times but did not alter the pulse amplitude or the POBF. The IOP was not altered during mild systemic hypoxia. CONCLUSIONS: The absence of change in the POBF during transient mild systemic hypoxia indicated that the global pulsatile choroidal blood flow was not vulnerable to the effects of this transient mild systemic hypoxic stress in the healthy young adult.

Adult↗
PubMed1994

The effect of increased intraocular pressure on pulsatile ocular blood flow in low tension glaucoma.

In the present study the relationship between intraocular pressure (IOP) and pulsatile ocular blood flow (POBF) in low tension glaucoma (LTG) has been investigated. The POBF was measured using the Ocular Blood Flow system of Langham in undisturbed eyes and repeated at IOP increments of about 5 and 10 mmHg in 20 patients affected with bilateral LTG and in 25 healthy subjects matched for age, IOP, refractive error, arterial blood pressure and heart rate. The POBFs were 740.1 (SD 58.83) and 667 (SD 108) microliters/minute (microliter/min) in the control and LTG groups, respectively and statistically significantly different. The POBFs at an IOP increased by 5 mmHg were 658.1 (SD 60.2) and 457.3 (SD 74.8) microliter/min in the control and LTG groups, respectively; in the control group the mean POBF at an IOP increment of 10 mmHg was 552.1 microliters/min (SD 66.9), and in the LTG group was 317.7 microliters/min (SD 85.2). Statistical analysis of the data showed a significant difference of POBFs in the two groups either at an IOP increment of 5 or 10 mmHg (p < 0.0001). The substantial decrease of POBF with increased IOP in LTG eyes compared with normals shows an altered response of the vascular system in LTG. These findings are probably consistent with a lack of the myogenic autoregulation in reply to IOP-induced modifications of the perfusion pressure.

Analysis of Variance↗
PubMed1991

Pulsatile ocular blood flow in patients with low tension glaucoma.

Measurements of the intraocular pressure (IOP) pulse and pulsatile ocular blood flow (POBF) have been made in 22 patients with bilateral low tension glaucoma (LTG) and 29 healthy subjects matched as closely as possible for age, refractive error, IOP, systemic pulse pressure, and heart rate. Recordings were made in both the standing and supine positions. The amplitude of the intraocular pressure pulse was significantly lower in patients with LTG (1.2, SEM 0.1 mmHg standing, and 1.3, SEM 0.1 mmHg lying) than in healthy subjects (1.9, SEM 0.1 mmHg standing, and 2.0 SEM 0.1 mmHg lying): p less than 0.001 standing and p less than 0.002 lying. Measurement of POBF also showed a significant reduction between the healthy subjects (428 (31) SEM microliters/min standing and 345 (28) SEM microliters/min lying) and subjects with LTG (301 (27) SEM microliters/min standing and 249 (24) SEM microliters/min lying), p less than 0.005 standing and p less than 0.02 lying. This represents a difference of approximately 30% between the two groups in either posture. A close non-parametric correlation existed between the level of IOP and the POBF (r = 0.75, p less than 0.001 standing, and r = -0.55, p less than 0.02 lying). Such a correlation was not present in the healthy subjects. A reduction in POBF occurred in both groups on assuming the supine posture (healthy subjects 83 (16) SEM microliters/min, LTG subjects 52 (17) SEM microliters/min). These figures represent reductions of 19% and 17% respectively in comparison with the standing value. The results lend further confirmation to the hypothesis that vascular factors are associated with low tension glaucoma.

Adult↗
PubMedDate not supplied

Determination of volume of arterial blood flow by an ultrasonic device.

This paper describes the results of a reevaluation of the reliability of a newly developed ultrasonic volume flow meter which was developed by Yoshimura and his co-workers (1978). Measurements of volume blood flow (cc/sec) by this new technique were compared with measurement by the electromagnet flow meter. Experiments were performed with an in vitro model as well as with physiological pulsatile flow in an animal model. Results of the study indicated a high degree of correlation between values measured by both techniques with a correlation coefficient of 0.99 I believe that this is an important technical development in the field of noninvasive quantitative determination of blood flow.

Animals↗
PubMed1995

Motion induced phase shifts in MR: acceleration effects in quantitative flow measurements--a reconsideration.

Magnetic resonance phase difference techniques are commonly used to study flow velocities in the human body. Acceleration is often present, either in the form of pulsatile flow, or in the form of convective acceleration. Questions have arisen about the exact time point at which the velocity is encoded, and also about the sensitivity to (convective) acceleration and higher order motion derivatives. It has become common practice to interpret the net phase shifts measured with a phase difference velocity technique as being the velocity at a certain (Taylor) expansion time point, chosen somewhere between the RF excitation and the echo readout. However, phase shifts are developed over the duration of the encoding magnetic field gradient wave form, and should therefore be interpreted as a more or less time-averaged velocity. It will be shown that the phase shift as measured with a phase difference velocity technique represents the velocity at the "gravity" center of the encoding bipolar gradient (difference) function, without acceleration contribution. Any attempt to interpret the measured phase shift in terms of velocity on any other time point than the gradient gravity point will automatically introduce acceleration sensitivity.

Acceleration↗
PubMedDate not supplied

Computational fluid dynamics: hemodynamic changes in abdominal aortic aneurysm after stent-graft implantation.

The aim of this study was to demonstrate quantitatively and qualitatively the hemodynamic changes in abdominal aortic aneurysms (AAA) after stent-graft placement based on multidetector CT angiography (MDCT-A) datasets using the possibilities of computational fluid dynamics (CFD). Eleven patients with AAA and one patient with left-side common iliac aneurysm undergoing MDCT-A before and after stent-graft implantation were included. Based on the CT datasets, three-dimensional grid-based models of AAA were built. The minimal size of tetrahedrons was determined for grid-independence simulation. The CFD program was validated by comparing the calculated flow with an experimentally generated flow in an identical, anatomically correct silicon model of an AAA. Based on the results, pulsatile flow was simulated. A laminar, incompressible flow-based inlet condition, zero traction-force outlet boundary, and a no-slip wall boundary condition was applied. The measured flow volume and visualized flow pattern, wall pressure, and wall shear stress before and after stent-graft implantation were compared. The experimentally and numerically generated streamlines are highly congruent. After stenting, the simulation shows a reduction of wall pressure and wall shear stress and a more equal flow through both external iliac arteries after stenting. The postimplantation flow pattern is characterized by a reduction of turbulences. New areas of high pressure and shear stress appear at the stent bifurcation and docking area. CFD is a versatile and noninvasive tool to demonstrate changes of flow rate and flow pattern caused by stent-graft implantation. The desired effect and possible complications of a stent-graft implantation can be visualized. CFD is a highly promising technique and improves our understanding of the local structural and fluid dynamic conditions for abdominal aortic stent placement.

Aortic Aneurysm, Abdominal↗
PubMed1990

The central axis prosthetic cardiac valve: an in vitro study of pressure drop assessment under steady-state flow conditions.

In this work, a new mechanical prosthetic heart valve, the central axis valve, is presented. This new prosthesis has been tested in vitro, and compared with four other common prosthetic cardiac valves (Starr-Edwards 6120, Bjork-Shiley monostrut, Medtronic-Hall, and St Jude Medical valves). All valves studied have the same orifice diameter of 22 mm. The prostheses were installed inside a transparent mitral test chamber, which enables pressure drop measurement to be made under steady-state flow conditions using a blood analogue fluid. Pressure drop loss is one important factor affecting the overall performance of a prosthetic heart valve. Steady-state flow tests are essential to predict certain flow characteristics and pressure gradient loss before more complicated, expensive, and difficult-to-interpret pulsatile flow tests are conducted. All experiments were performed in vitro and at steady volumetric flow rates of 10 to 30 l/min. The Starr-Edwards SE 6120 showed the highest values for pressure drop. The St Jude Medical valve offers the minimum resistance to flow. The central axis valve comes second to the Starr Edwards valve for this type of measurement. The new valve is promising. A complete valve evaluation programme, covering initial conceptional design through to clinical use, is in progress. Materials for the fabrication of the new valve are also under consideration.

Biocompatible Materials↗
PubMed1988

[The Doppler palm test].

In 384 hands the patency of the arterial arch was investigated by means of both the Allen test and the Doppler palmar arch test as developed by the authors. While the Allen test is purely subjective and gives a high proportion of false results especially in shocked patients, the Doppler palmar arch test is based on two objective criteria: 1. Demonstration of reversed flow in the distal section of the radial artery on proximal compression. 2. Demonstration of pulsatile flow in the radial thumb artery even on compression of the radial artery. The Doppler palmar arch test eliminates all negative aspects of the Allen test and is therefore definitely more reliable than the latter. It is easy to perform for investigators having little experience with Doppler sonography. The time consumed is not greater than with the Allen test.

Arteries↗
PubMed2002

Lattice Boltzmann method for simulating the viscous flow in large distensible blood vessels.

A lattice Boltzmann method for simulating the viscous flow in large distensible blood vessels is presented by introducing a boundary condition for elastic and moving boundaries. The mass conservation for the boundary condition is tested in detail. The viscous flow in elastic vessels is simulated with a pressure-radius relationship similar to that of the pulmonary blood vessels. The numerical results for steady flow agree with the analytical prediction to very high accuracy, and the simulation results for pulsatile flow are comparable with those of the aortic flows observed experimentally. The model is expected to find many applications for studying blood flows in large distensible arteries, especially in those suffering from atherosclerosis, stenosis, aneurysm, etc.

Journal Article↗
PubMed2003

Mixing of a continuous flow of two fluids due to unsteady flow.

In many low-Reynolds number mixing applications, the absence of turbulence makes it difficult to achieve proper mixing of two fluids. In this paper, flow visualization is used to obtain quantitative measurements of mixing that occurs when combining two pulsatile fluid streams at a Y-connection. Mixing results from the interface distortion created by the pulsatile flow. This is generated by combining the action a peristaltic pump, which provides the mean flow, with the action of two pinch valves, one on each arm of the Y-connection, to generate strong pulsations. The action of the pinch valves is be controlled via pulse generators. Apparently chaotic conditions were realized in the confluence region, superimposed with the mean flow. The valve action was optimized to maximize mixing, the latter quantified via image analysis. This work demonstrates a low cost, efficient mixing device for low-Reynolds number conditions, which is therefore suitable for miniaturization.

Biophysics↗