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At least 1,297 records · Page 72Linked to original sources
PubMed1993

[The use of magnetic tracers in hemodynamics. A proposed model study].

Velocity of blood flow is critical in specific clinical situations. Available non invasive blood velocimeters gives only qualitative informations; quantitative estimates can be obtained invasively. We investigated the possibility of estimating the velocity of fluids using magnetic tracer. A model for the laminar blood flow inside a vessel was developed. A biomagnetometer was used to measure the fluid tracer velocity. The experimental setup was designed to mimic blood flow. A PVC tube simulated the vessel. The tube was connected to a bottle of normal saline. The tracer injection took place 25 cm distal to the saline bottle. The system was calibrated to a constant flow velocity of about 1 cm/s. A paramagnetic fluid (Magnevist, Schering, Berlin) was adopted as a tracer. In each session an amount of 2 cm3 was injected into the tube within 5 seconds. The recording equipment consisted of a second order gradiometer (baseline 7 cm, pick-up coil diameter 1.5 cm) coupled to a rfSQUID magnetometer. The recording bandwidth was dc-3 Hz, the sampling rate 32 Hz with 12 bit digitalization. The biomagnetic system was positioned immediately above the tube 55 cm away from the injection site. The total recording time for each session was 240 seconds. The injection of the tracer took place 60 seconds after the beginning of the data acquisition. Five measuring sessions in the earth magnetic field were performed at first and showed a low signal-to-noise ratio.(ABSTRACT TRUNCATED AT 250 WORDS)

Blood Flow Velocity↗
PubMedDate not supplied

[The hydrodynamic evaluation of some types of aortocoronary bypass].

Liquid flow in cylindrical and converging transplants for aortocoronary bypass, in intricately branching vascular systems were experimentally studied by polarization optic method using "jumping" aortocoronary shunts and different flow regimens, sites and severity of vascular stenosis. Hydrodynamic advantages of conic shunts in comparison with cylindrical ones were revealed, the rest shunting conditions being equal. It was found that when "jumping" shunts are used the distal portion of the coronary artery which is shunted the first should not be dilated by more than 50%. The end anastomosis should be formed with the artery more significant for myocardial blood supply.

Anastomosis, Surgical↗
PubMed1993

[Regulation of the myosin heavy chain gene expression during cardiac hypertrophy induced by pressure-overload].

In response to pressure-overload, the myocardium exhibits a complex molecular changes, characterized by the reexpression of a fetal phenotypes. Although signaling pathways involved in these changes are still largely unknown, studies on the differential expression of the myosin heavy chain (MHC) genes will provided a framework for understanding the molecular mechanisms underlying the reprogramming of cardiac gene expression in response to mechanical load. Recently, in vitro medel of myocardial stretch on the silicone sheet has demonstrated the up regulation of the "cardiac fetal genes", including beta-MHC, skeletal alpha-actin, and atrial natriuretic factor (ANF), which is known to be a hallmark of the altered gene expression in the hypertrophic heart. Transient transfection experiments using this model will be useful for identifying the "stretch-response element" within the promoters of these genes. Furthermore, this approach will eventually identify the stretch-induced intracellular pathways.

Animals↗
PubMed1993

Monte Carlo fluorescence verification of experimental results for the combined ultrasonic and spectroscopic imaging of coronary artery disease.

A system is being tested that combines fluorescence spectroscopy and intravascular ultrasound to image both chemical composition and structure of arterial tissue in vitro. In this system, distances obtained from A-mode ultrasound will be used to compensate for decreases in fluorescence intensity due to the detector-sample separation distance r. For concentrated rhodamine, a fluorescent dye, compensation has been experimentally achieved assuming fluorescence is emitted isotropically and decreases as 1/r2. For dilute rhodamine and human arterial tissue, however, compensation must be achieved with different models, since light penetration into the sample is more significant. Using optical properties consistent with those of the aforementioned samples, experimental results are successfully simulated with a Monte Carlo model for tissue fluorescence. Angular profiles are presented that demonstrate the quantitative difference between the fluorescence of these mediums. The profiles support the hypothesis that, although fluorescence is emitted isotropically within tissue, the angular distribution of light exiting the tissue is not isotropic due to reabsorption events.

Aorta↗
PubMed1993

Heart rate response to exercise in normal women.

Heart rate is known to increase under the influence of increased sympathetic stimulation rates during exertion or stress. Few data are available regarding the rate response to exercise near the fatigue limit, for normal women. The effects on heart rate were studied of fatigue limiting exercise in 87 asymptomatic, normal female subjects employing a bicycle ergometer exercise protocol. The heart rate response to imposed exercise power closely followed a monotonic increasing, negative exponential function. Different age groups followed the same form of function but differed in parameter values. Group fitting was useful to develop parameters with which one group may be compared to another, establish referent ranges, and gain insight into cardiovascular models of exercise physiology.

Adolescent↗
PubMed1993

Accurate local blood flow measurements with dynamic PET: fast determination of input function delay and dispersion by multilinear minimization.

Accurate determination of local blood flow in tissue using the Kety-Schmidt one-compartment model for freely diffusible tracers requires knowledge of the true arterial input function in tissue. Because measured input functions are usually delayed and dispersed with respect to true influx, a correction of the experimental input function is necessary. We describe a technique that uses a fast multilinear least-squares minimization procedure to determine simultaneously the dispersion, the blood flow and the partition coefficient as a function of delay. In this approach, a few multilinear fits are sufficient to determine the complete set of parameters necessary to describe the data. Because of the high speed of the procedure, dispersion effects may be taken into account on a pixel-by-pixel basis in calculating parametric images of blood flow and partition coefficient. The described procedure has been used at our institute for about 1 yr in more than 160 investigations and has proven well suited for routine use in a clinical environment.

Blood Flow Velocity↗
PubMed1993

Accuracy and reproducibility of duplex ultrasound imaging in a phantom model of femoral artery stenosis.

PURPOSE: The improvement of management strategies in patients with intermittent claudication might depend on a better understanding of the natural history of femoral atherosclerosis. The grading of stenoses, the monitoring of their progression, and the assessment of response to treatment are critically dependent on a method's accuracy and variability. Duplex ultrasound imaging provides a noninvasive way of measuring localized disease, but there has been relatively little objective evaluation of its accuracy and reproducibility. The aim of this study was to evaluate the accuracy and variability of duplex velocity ratio measurements of stenosis. METHODS: In a laboratory flow model of the femoral artery, 14 concentric and eccentric stenoses were examined five times by three sonographers. Measurements were then repeated with a standardized technique in which Doppler angle and aperture position were fixed, giving a total of 420 measurements. RESULTS: Velocity ratio showed good correlation with degree of stenosis, R2 = 0.996. Intraobserver variability was low, but interobserver variability was significant with more severe stenosis (p = 0.002, analysis of variance). Standardization of the technique did not improve accuracy or variability. The 95% confidence limit was +/- 20% for a single reading of velocity ratio for stenoses of > 50% diameter reduction. CONCLUSIONS: We conclude that duplex ultrasound imaging can be used to accurately grade arterial stenosis in this range, and the potential exists for noninvasive monitoring of the progression of preocclusive femoral atherosclerosis and its response to treatment. In addition, repeated measurements of velocity ratio over time should be made by the same observer.

Analysis of Variance↗
PubMed1993

Hemodynamics of a side-to-end proximal arterial anastomosis model.

PURPOSE: The purpose of this article was to analyze the fluid mechanical effects of a side-to-end proximal anastomosis and to compare the results with those from our earlier study on the end-to-side distal anastomosis. METHODS: The photochromic tracer technique was used to determine the instantaneous wall shear stress and to visualize the overall flow field under pulsatile flow conditions. The flow consisted of a sinusoid plus a steady component with mean and modulation Reynolds numbers of 355 and 565, respectively, and a Womersley number of 7.9. RESULTS: At the toe and heel of the junction, very high and positive wall shear stresses were seen together with substantial nonperiodic fluctuations. The peak wall shear stress was about four times higher at the toe and about seven times higher at the heel than the maximum values observed at about four tube diameters upstream from the junction. On the bed of the host vessel, nonperiodic fluctuations were also observed, but the shear stresses were mainly negative with magnitudes comparable to those seen upstream. With leakages of 11% and 28% of the mean flow through the blocked end of the host vessel, the shear stress pattern seemed to be significantly affected only at the toe for the higher leakage. Further, when the mean Reynolds number was reduced to 320, the magnitudes of the variations in the wall shear stress were reduced proportionately, except at the heel, where the reduction was much larger than expected. CONCLUSIONS: It appears that the preferential development of intimal hyperplasia at the distal end-to-side anastomosis may be promoted by low wall shear stress at the toe and heel, and probably by high shear stresses or shear stress gradients on the bed.

Anastomosis, Surgical↗
PubMedDate not supplied

Poststenotic signal loss in MR angiography: effects of echo time, flow compensation, and fractional echo.

PURPOSE: To evaluate with steady and pulsatile flow the influence of echo time, gradient strength and duration, and flow compensation on the degree of turbulent signal loss, factors that have been implicated in MR angiography's overestimation of the degree of stenosis. METHODS: We examined poststenotic turbulent flow in two models, one that created a turbulent jet and another that simulated a plaque-like stenosis. The pulse sequence used in these experiments allowed for a single variable (flow compensation, echo time, or gradient strength) to be varied without changing the others. RESULTS: Poststenotic signal loss can lead to overestimation of the degree of a stenosis. The area of signal loss in the turbulent jet was influenced by fractional echo and flow compensation, but not by echo time. We found that the dominant mechanism in poststenotic signal loss is related to the strength and duration of the imaging gradients. CONCLUSIONS: Flow-compensated sequences with reduced gradient strength and duration will reduce poststenotic signal loss and may lead to more accurate estimations of the extent of stenotic lesions.

Arterial Occlusive Diseases↗
PubMed1996

A mathematical model of the first steps of tumour-related angiogenesis: capillary sprout formation and secondary branching.

The growth of a solid tumour is dependent on an adequate supply of nutrients. A tumour can establish a blood supply by inducing neighbouring blood vessels to sprout and grow towards it, a process known as angiogenesis. The tumour cells may secrete a number of diffusible chemicals which stimulate endothelial cell to migrate, to rearrange themselves into capillary tubes or sprouts, and to proliferate. In this paper we focus firstly upon the early stage of angiogenesis wherein the endothelial cells group together in the parent vessel to form the initial capillary-sprout buds. A mathematical model for the formation of the capillary buds is presented which focuses on the potential role that haptotaxis may play. In Section 2 we turn attention to the endothelial cells within the growing and developing capillary sprouts as they migrate towards the tumour cells. Once again the potential role of haptotaxis is focused upon.

Animals↗
PubMed1996

Coronary flow and flow reserve by PET simplified for clinical applications using rubidium-82 or nitrogen-13-ammonia.

UNLABELLED: To validate routine, noninvasive determination of absolute myocardial perfusion and coronary flow reserve (CFR), cardiac PET was performed in animals using a simplified imaging protocol, high-dose dipyridamole and a simplified quantitative algorithm specific for 82Rb and 13N-ammonia. METHODS: One hundred thirty-five PET scans were obtained in eight dogs after intravenous 13N-ammonia or 82Rb using serial dynamic PET or a simple two-image dataset. A simple flow model using the two-image dataset was developed for each radionuclide to account for varying arterial input function, flow-dependent myocardial extraction and increased permeability surface area (PS) product due to capillary recruitment at high flows not incorporated into previous models. Myocardial perfusion by the simple model was compared to standard, complete, two-compartment kinetic models validated by comparison to electromagnetic flow meter. RESULTS: For 13N-ammonia, myocardial perfusion by the simple PET model correlated with that by complete compartmental analysis of multiple serial PET images with r = 0.94, slope = 0.96; CFR by compartmental analysis correlated with CFR by electromagnetic flow meter with r = 0.94, slope = 0.97. For 82Rb, myocardial perfusion determined by the simple model correlated with that determined by complete compartmental analysis of multiple serial PET images with r = 0.98, slope = 1.06; CFR determined by compartmental analysis correlated with CFR by electromagnetic flow meter with r = 0.88, slope = 1.13. CONCLUSION: A simplified PET protocol using 13N-ammonia or 82Rb and simple flow models provide noninvasive measurement of CFR up to six times baseline flow throughout the heart and diagnostic image quality for routine clinical application.

Ammonia↗
PubMed1996

The effect of separate red blood cells on capillary tissue oxygenation calculated with a numerical model.

In simplified models that describe large quantities of capillaries the capillary content is considered to be homogeneous for oxygen transport; but, in reality, the capillaries contain discrete red blood cells (RBCs), and this discreteness will affect oxygen transport from the capillary to the tissue. This was previously investigated with an analytical model, where RBCs were modelled as point-like sources. A numerical approach is used in this investigation, and the results are compared with the analytical model. In both models the effect of the particulate nature of blood depends on the haematocrit and on the RBC velocity. There is only a minor difference between the two models. For rat hearts, the correction factor used in this study, the extraction pressure, can be up to 3 kPa (23 mmHg).

Animals↗
PubMed1996

Dynamic stiffness and implications for assisting the operation of the left ventricle.

The distribution of bending strain and stiffness in the wall of the left ventricle (LV) is relevant to the augmentation of its function by a modified skeletal-muscle wrap in the new surgical procedure of cardiomyoplasty. A novel approach to ventricular mechanics is presented which blends some finite-element results in engineering with new data available on ventricular geometry. Two simplified axisymmetric strip-element models of the LV are used to illustrate aspects of myocardial stiffness in the bending-strain-energy distribution and the effect on wrap synchronization of a change in cross-fibre stiffness when the heart has nonuniform or ectopic beats. The nonlinear and time-dependent nature of both camping and wall stiffness is derived from differential equations governing the dynamic paths from systole to diastole of finite wall elements around the periphery of an oblique LV slice using magnetic resonance imaging (MRI) data. This leads to a geometric method for determining these parameters. Results for time-dependent stiffnesses of elements in their trajectories are presented for a normal heart.

Biomechanical Phenomena↗
PubMed1997

Importance of leaflet elongation in causing systolic anterior motion of the mitral valve.

BACKGROUND AND AIMS OF THE STUDY: There is growing evidence for mitral leaflet elongation in patients with hypertrophic cardiomyopathy. Such elongation could predispose to systolic anterior motion (SAM) of the mitral valve by increasing leaflet mobility and providing a geometry that promotes this condition. METHODS: To test this postulate, five porcine mitral valves were studied in a physiologic left heart pulsatile flow duplicator. They were elongated with patches sutured to the basal posterior leaflet (three sizes per valve) or anterior leaflet (basal, middle, or distal). Each geometry was studied with normal papillary muscle position and with anterior and inward displacement, as seen in hypertrophic cardiomyopathy, to shift the leaflets into the outflow stream. RESULTS: Four points became clear. 1) Leaflet elongation promoted the development of SAM in response to papillary muscle displacement by creating long overlapping residual leaflets capable of moving anteriorly. 2) Posterior leaflet elongation also promoted SAM by shifting leaflet coaptation anteriorly, with progressive increases in SAM. 3) Basal and mid-anterior leaflet elongation caused SAM with prolapse; distal anterior leaflet elongation created SAM with a mobile flap (leaflet elongation without papillary muscle displacement created prolapse). 4) Residual leaflet length correlated well with total leaflet length (r = 0.87-0.98 for each valve), and the degree of SAM in turn correlated well with residual leaflet length (r = 0.62-0.98 for individual valves). CONCLUSIONS: Mitral leaflet elongation, by increasing the residual leaflet length and leaflet mobility, can play an important role in promoting SAM in response to outflow forces, as demonstrated by prospectively altering leaflet length. These findings are consistent with recent observations that reducing leaflet redundancy and posterior leaflet height can reduce obstructive SAM following mitral valve repair in patients with mitral valve prolapse and help relieve obstruction in patients with hypertrophic cardiomyopathy and enlarged leaflets.

Cardiomegaly↗
PubMed1997

Numerical modelling of blood flow behaviour in the valved catheter of the PUCA-pump, a LVAD.

Mechanical heart assistance, performed by the PUlsatile CAtheter (PUCA) pump, chronologically takes place by sucking blood from the left ventricle and ejecting it into the ascending aorta. Within the pump activity the problem of hemolysis and clotting is encountered. In this study the influence of valve geometry on blood cell damage and stagnant zones has been investigated. A variable valve length coupled to a catheter ejection gap and a variable valve angle have been studied. In case of the studied valve, optimal parameter values have been determined. Compared to small catheter ejection gaps with a corresponding valve length, blood damage is found to be less for large ejection gaps with corresponding valve dimensions. In systole a valve positioned in a 0 degree angle proves to be best, whereas in diastole a +20 degree angle is preferable. Because the system is operating in both systole and diastole, a 0 degree angle valve is applied.

Blood Coagulation↗
PubMed1997

Evaluation of right and left ventricular volume and ejection fraction using a mathematical cardiac torso phantom.

UNLABELLED: The availability of gated SPECT has increased the interest in the determination of volume and ejection fraction of the left ventricle (LV) for clinical diagnosis. However, the same indices for the right ventricle (RV) have been neglected. The objective of this investigation was to use a mathematical model of the anatomical distribution of activity in gated blood-pool imaging to evaluate the accuracy of two ventricular volume and ejection fraction determination methods. In this investigation, measurements from the RV were emphasized. METHODS: The mathematical cardiac torso phantom, developed to study LV myocardium perfusion, was modified to simulate the radioactivity distribution of a 99mTc-gated blood-pool study. Twenty mathematical cardiac torso phantom models of the normal heart with different LV volumes (122.3 +/- 11.0 ml), RV volumes (174.6 +/- 22.3 ml) and stroke volumes (75.7 +/- 3.3 ml) were randomly generated to simulate variations among patients. An analytical three-dimensional projector with attenuation and system response was used to generate SPECT projection sets, after which noise was added. The projections were simulated for 128 equidistant views in a 360 degrees rotation mode. RESULTS: The radius of rotation was varied between 24 and 28 cm to mimic such variation in patient acquisitions. The 180 degrees and 360 degrees projection sets were reconstructed using the filtered backprojection reconstruction algorithm with Butter-worth filtering. Comparison was made with and without application of the iterative Chang attenuation correction algorithm. Volumes were calculated using a modified threshold and edge detection method (hybrid threshold), as well as a count-based method. A simple background correction procedure was used with both methods. CONCLUSION: Results indicate that cardiac functional parameters can be measured with reasonable accuracy using both methods. However, the count-based method had a larger bias than the hybrid threshold method when RV parameters were determined for 180 degrees reconstruction without attenuation correction. This bias improved after attenuation correction. The count-based method also tended to overestimate the end systolic volume slightly. An improved background correction could possibly alleviate this bias.

Female↗
PubMed1998

Cellular mechanisms of cardiac mechano-electric feedback in a mathematical model.

BACKGROUND: Cardiac mechanical and electrical activity are closely interrelated. While excitation-contraction coupling is rather well characterized, less is known about cellular mechanisms that promote mechanically induced changes in cardiac electrical activity--mechano-electric feedback. OBJECTIVE: To integrate experimental findings on stretch activation of ion channels and length-dependent changes in intracellular calcium handling into a mathematical description of cardiac cellular activity. METHODS: Simulations are based on the cellular OXSOFT HEART v4.8 models of electrical activity of single cardiac cells of different populations and species. Sarcolemmal stretch-activated channels, mechanically induced changes in the affinity of troponin C to calcium, and length-dependent modulation of calcium handling by the sarcoplasmic reticulum were introduced into the models and linked to a description of sarcomere length or isometric tension. RESULTS: Transient or sustained stretch of cardiomyocytes was simulated during electrical systole and diastole. The electrophysiological response observed in the model depended on timing and severity of mechanical stimulation and on the main subcellular target of the intervention. Responses ranged from triggering of premature action potentials, over changes in action potential shape and duration, to length-dependent variations in contractile behaviour. Modelling findings could be related to experimental observations and may help to explain some of the contradictory data in the literature. The model is sufficiently complete to reproduce experimental findings and to help identify causally linked events.

Animals↗
PubMed1998

Improved guinea-pig ventricular cell model incorporating a diadic space, IKr and IKs, and length- and tension-dependent processes.

The guinea-pig ventricular cell model, originally developed by Noble et al in 1991, has been greatly extended to include accumulation and depletion of calcium in a diadic space between the sarcolemma and the sarcoplasmic reticulum where, according to contempory understanding, the majority of calcium-induced calcium release is triggered. The calcium in this space is also assumed to play the major role in calcium-induced inactivation of the calcium current. Delayed potassium current equations have been developed to include the rapid (IKr) and slow (IKs) components of the delayed rectifier current based on the data of of Heath and Terrar, along with data from Sanguinetti and Jurkiewicz. Length- and tension-dependent changes in mechanical and electrophysiological processes have been incorporated as described recently by Kohl et al. Drug receptor interactions have started to be developed, using the sodium channel as the first target. The new model has been tested against experimental data on action potential clamp, and on force-interval and duration-interval relations; it has been found to reliably reproduce experimental observations.

Animals↗