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At least 1,171 records · Page 65Linked to original sources
PubMed2001

Physiological flow waveform in a rigid elliptical vessel.

A procedure to model the velocity and wall shear stress for a physiological flow in a non-cylindrical vessel is presented. The work describes how a vessel with an elliptical cross section may be used to represent flow in compressed or partially occluded arteries and veins. The procedure was applied to produce a simulation of a physiological flow in a straight rigid vessel with a slightly elliptical cross section (ellipticity, epsilon = 0.8). Fourier analysis was performed on a physiological flow waveform. Flow in the common carotid artery was satisfactorily represented (Pearson correlation coefficient, r > 95%) with a series of five harmonic terms. Expressions involving a linear combination of ordinary and modified Mathieu functions were used to describe the velocity and wall shear stress for each harmonic. An outline of the procedure, and the expressions, which were used to compute the characteristic Mathieu numbers and coefficients is illustrated with their behaviour at the fundamental and Nyquist frequencies. Superposition of the individual contributions to the velocity and wall shear stress allowed the calculation of the overall properties of the flow.

Biophysical Phenomena↗
PubMed2001

Toward a new method of analyzing cardiac performance.

Aside from the traditional diagnostic tools used to assess cardiac performance, such as blood pressure and cardiac output, cardiac source parameters can also be used as an indication of overall cardiac health. The cardiac source parameters under investigation are myocardial visco-elastic properties. The objective of this study is to model the left ventricle using electrical circuits, then use these models to estimate the cardiac source parameters. Two electrical models of the left ventricle have been developed, using a resistor (R) to model viscous losses, a capacitor (C) to represent inverse elastance, and a time-varying pressure source, which models the left ventricular pressure during an isovolumic beat. One model uses a single resistor and the other uses the two elements (R and C) in parallel. The differential equations describing these models have been derived, and a minimization procedure was used to adjust values of R and C in the differential equations until a good agreement between experimental and calculated left ventricular pressure was reached. Preliminary results indicate that the model including a parallel combination of R and C provides a better fit between experimental and calculated pressures. It is hoped that these parameters may one day provide clinicians with yet another diagnostic tool to help discover the source of cardiac disorders.

Animals↗
PubMed2001

A new approach to optimization-based defibrillation.

The purpose of this paper is to develop a new model for optimal cardiac defibrillation, based on simultaneous minimization of energy consumption and defibrillation time requirements. In order to generate optimal defibrillation waveforms that will accomplish the objective stated above, one parameter rho has been introduced as a part of the performance measure to weigh the relative importance of time and energy. All the results of this theoretical study have been obtained for the proposed model, under the assumption that cardiac tissue can be represented by a simple parallel resistor-capacitor circuit. It is well known from modern control theory that the selection of a numerical value of the weight factor is the matter of subjective judgment of a designer. However, it has been shown that defining a cost function can help in selecting a value for rho. Some results of the mathematical development of the algorithm and computer simulations will be included in the paper.

Computer Simulation↗
PubMed2000

The concept of cerebral circulatory-pressure index (CCPI).

The theoretical relations between blood pressure (MABP), intracranial pressure (ICP) and cerebral perfusion pressure (CPP) as well as the new Cerebral Circulation-Pressure Index (CCPI) are presented in the paper. The special nomogram collecting all of these parameters was constructed. CPPI was defined as a quotient of CPP and ICP: [formula: see text] CCPI reflects the relationship between the systemic MABP and ICP and for ensuring the normal conditions of intracranial circulation the MABP must be at least 4 divided by 5 times higher than ICP. The low values of CCPI, if they are not immediately corrected, results on severe brain disability or even death. We didn't find till now what should be the upper limit of CCPI, which overcrossing may be danger for the patient, especially with impaired autoregulation. Based on analysis of patients operated due to intracranial disorders as brain tumours, spontaneous and traumatic haematomas as well as ruptured aneurysms we find that the values of CCPI over 3 have a good prognosis, between 3 and 2 is a field for intensive treatment consisting on decreasing ICP or increasing the systemic blood pressure or both. If CCPI is below 1.5 the prognosis is pessimistic and all the patients died. CCPI together with the nomogram are useful in current evaluation of the state of patients and of the efficacy of treatment and prediction of the treatment results.

Adolescent↗
PubMed2001

A hybrid (numerical-physical) model of the left ventricle.

Hydraulic models of the circulation are used to test mechanical devices and for training and research purposes; when compared to numerical models, however, they are not flexible enough and rather expensive. The solution proposed here is to merge the characteristics and the flexibility of numerical models with the functions of physical models. The result is a hybrid model with numerical and physical sections connected by an electro-hydraulic interface - which is to some extent the main problem since the numerical model can be easily changed or modified. The concept of hybrid model is applied to the representation of ventricular function by a variable elastance numerical model. This prototype is an open loop circuit and the physical section is built out of a reservoir (atrium) and a modified windkessel (arterial tree). The corresponding equations are solved numerically using the variables (atrial and arterial pressures) coming from the physical circuit. Ventricular output flow is the computed variable and is sent to a servo amplifier connected to a DC motor-gear pump system. The gear pump, behaving roughly as a flow source, is the interface to the physical circuit. Results obtained under different hemodynamic conditions demonstrate the behaviour of the ventricular model on the pressure-volume plane and the time course of output flow and arterial pressure.

Heart Ventricles↗
PubMed2001

[A lumped parameter model of the peripheral extra-corporeal circulation].

A three-element model of lumped parameter in peripheral extra-corporeal circulation of an animal has been proposed and set up according to the fluid network theory. GEAR method, appropriate for the computation of the rigid problems, was used to solve the ordinary differential equations, so that a good convergence of the solution to the equations has been achieved. Physiological data of a 20 kg dog, obtained from our animal experiments were used to calculate the pressure and flow rate of important organs. The results of the mathematical model and animal tests showed that the peripheral extra-corporeal circulation could supply enough blood to the important organs, e.g. heart and brain, when the natural heart beating stopped. The results from both theoretical and experiments are critical to the cardiopulmonary cerebral resuscitation for the animals as well as for human beings.

Animals↗
PubMed2001

[Comprehensive evaluation of medical system by using generalized fuzzy set theory].

Based on the theory of generalized fuzzy set, a comprehensive evaluation model has been built, which provides residuals estimate, evaluate function and test methods needed. Verified by the prognosis of 30 patients with myocardial infarction in a motion experiment during rehabilitation, its coincidence rate was found to be above 90%. This model can be developed and applied to medical system and other systems for evaluations as well.

Evaluation Studies as Topic↗
PubMed2001

Different types of aortic stenosis and simulation of their morphological-hydrodynamic interdependence--in vitro study with allografts and stenotic valve models.

Biological valves display a dependence of valve resistance and valve area on flow and a phase shift between systolic flow through the valves and pressure difference across the valves. The pressure-flow relations of stenosed valves raise questions about the "best measure of stenosis". There is a need for quantitative evaluation of the hydrodynamic performance of homografts and allografts. In the present paper, we report on in vitro studies of the hydrodynamic behavior of homografts from human donors, allografts from different animal species as well as three valve models. Valve model I was designed to simulate flow-dependence of valve area, valve model II was designed to simulate restricted valve opening independent of flow, and valve model III was designed to simulate a flow-dependent movement of valve root in flow direction. Among other aspects, the effect of increased viscosity of the test fluid on the pressure difference and the effects of water absorption by valve tissue on valve characteristics were investigated. The results of the present studies clearly indicate that any biological valve may be modelled as a serial connection of a model I type valve and a model II type valve. From the results, the dependence of the characteristic pressure-flow relationship of a valve on valve size and valve distensibility can be clearly seen and the clinical significance of the characteristic coefficients of the pressure-flow relationship of a valve can be elucidated. Further, it was shown that the characteristic phase shift between flow and pressure difference displayed by biological valves is due to their movable valve plane similar to that of valve model III.

Animals↗
PubMed2002

Quantitative assessment of polymerization-binding mechanics of cyanoacrylates: model development and validation.

BACKGROUND AND PURPOSE: Although commonly acknowledged as paramount in significance, the mechanics of cyanoacrylate polymerization remain poorly characterized and quantified for clinical applications. This prompted development of a simplistic model for the systematic study of polymerization and binding behaviors of cyanoacrylates. METHODS: A sliding bed apparatus was constructed that linked a strain gage with a vessel that could be filled with liquid medium, cyanoacrylate, and a microcatheter. As the cyanoacrylate polymerized, the microcatheter was mechanically drawn away very slowly from the fixed vessel, resulting in the development of forces that were recorded to characterize the dynamics of polymerization and binding. Optimization of the model required manipulation of several variables that could influence polymerization. Three different formulations of cyanoacrylate were also tested to determine whether there are significant differences in polymerization dynamics. RESULTS: After experimenting with a few basic physical parameters of the test apparatus, consistent measurements of binding forces during cyanoacrylate polymerization could be recorded and measured. Polymerization produced a multiphasic pattern of binding forces, in which three distinct phases were observed. Furthermore, the rates of polymerization were significantly influenced by a variety of parameters, including the type of fluid within the model vessel, geometry of the model vessel, and rate of injection of acrylic into the well. Furthermore, there were significant differences in the pattern of dynamic binding forces among the various formulations of cyanoacrylate tested. CONCLUSIONS: A standardized bench top testing apparatus has been developed, which can consistently show dynamic binding related to polymerization of cyanoacrylates. This preliminary study shows a clear multiphasic pattern of polymerization binding, which may have important clinical implications. The apparatus may be useful for gaining better insight into a variety of clinically important phenomena related to cyanoacrylate polymerization.

Cyanoacrylates↗
PubMed2002

Monodimensional estimation of maximum Reynolds shear stress in the downstream flow field of bileaflet valves.

BACKGROUND AND AIM OF THE STUDY: Turbulent flow generated by prosthetic devices at the bloodstream level may cause mechanical stress on blood particles. Measurement of the Reynolds stress tensor and/or some of its components is a mandatory step to evaluate the mechanical load on blood components exerted by fluid stresses, as well as possible consequent blood damage (hemolysis or platelet activation). Because of the three-dimensional nature of turbulence, in general, a three-component anemometer should be used to measure all components of the Reynolds stress tensor, but this is difficult, especially in vivo. The present study aimed to derive the maximum Reynolds shear stress (RSS) in three commercially available prosthetic heart valves (PHVs) of wide diffusion, starting with monodimensional data provided in vivo by echo Doppler. METHODS: Accurate measurement of PHV flow field was made using laser Doppler anemometry; this provided the principal turbulence quantities (mean velocity, root-mean-square value of velocity fluctuations, average value of cross-product of velocity fluctuations in orthogonal directions) needed to quantify the maximum turbulence-related shear stress. RESULTS: The recorded data enabled determination of the relationship, the Reynolds stresses ratio (RSR) between maximum RSS and Reynolds normal stress in the main flow direction. The RSR was found to be dependent upon the local structure of the flow field. CONCLUSION: The reported RSR profiles, which permit a simple calculation of maximum RSS, may prove valuable during the post-implantation phase, when an assessment of valve function is made echocardiographically. Hence, the risk of damage to blood constituents associated with bileaflet valve implantation may be accurately quantified in vivo.

Echocardiography, Doppler↗
PubMed1999

Effects of pericytes and various cytokines on integrity of endothelial monolayer originated from blood-nerve barrier: an in vitro study.

In the in vitro blood-brain barrier (BBB) model, astrocytes have been considered to strengthen the barrier function of brain microvascular endothelial cells (BMECs). Because the blood-nerve barrier (BNB) does not comprise cells equivalent to astrocytes, we hypothesized that the integrity of the BNB is supported by endoneurial pericytes, the only cells comprising endoneurial microvessels other than peripheral nerve microvascular endothelial cells (PnMECs). To estimate the barrier function of in vitro BBB/BNB models, we measured the transendothelial electrical resistance (TEER) and permeability for paracellular diffusion of [carboxyl-14C]- inulin across the layers of PnMECs or BMECs co-cultured with pericytes, astrocytes or fibroblasts. When co-cultured with astrocytes, both PnMECs and BMECs showed higher TEER values and significantly lower clearance of inulin. When co-cultured with pericytes, only PnMECs increased the TEER value and lowered the clearance of inulin as effectively as did those co-cultured with astrocytes. Hence, we conclude that endoneurial pericytes may strengthen the BNB function in a PnMEC-specific manner. The TEER values of the PnMEC monolayer challenged by IL-1beta, TNFalpha or VEGF decreased significantly. Based on the present results, the co-culture of PnMECs and pericytes appears to be a useful in vitro BNB model.

Animals↗
PubMed2002

ICP and CBF regulation: a new hypothesis to explain the "windkessel" phenomenon.

The brain tamponade represents the final condition of a progressive intracranial pressure (ICP) increase up to values close to arterial blood pressure (BP) producing a reverberating flow pattern in the cerebral arteries with no net flow. This finding implies intracranial volume changes, therefore a full application of the Monro-Kellie doctrine is impossible. To resolve this contradiction, in eight pigs a reversible condition of brain tamponade was produced by infusing saline into a cerebral ventricle. The following parameters were measured: BP in the common carotid artery, ICP by the same needle utilised for the infusion, arterial and venous blood flow velocity (BFV) at, respectively, internal carotid artery (ICA) and sagittal sinus (SS) site by ultrasound technique. When ICP approached carotid BP values, reverberating BFV waves both at ICA and SS site were simultaneously observed. The arterial and venous reverberating waves appeared to be almost exactly superimposable, with a delay of about 40 msec. This synchronism between the pulsatile arterial and venous BFV indicates that the residual pulsation, still occurring at the arterial proximal level, is compensated by a passive compression-distension of the SS with no blood volume (that is net flow) crossing the intracranial vasculature.

Animals↗
PubMed2002

Hemoglobin/O2 systems: mechanistic discrimination based on Ackers' model.

The kinetics of the reaction of hemoglobin with molecular oxygen, in which rapid mixing is followed by a fast temperature jump, is numerically simulated. We use the system of Ackers (1998) which distinguishes four forms of bi-ligated hemoglobin. The data suggest the involvement of isomerization steps for bi- and triliganded hemoglobin. Our first model assumes a linear addition of oxygen with one path to and from each bi-ligated species. Our second model allows cross-overs between paths, as described by Ackers (1998). Our third model exploits the observation (Perrella et al., 1990) that two of the four bi-ligated forms are at low concentration. We explore whether these models can be distinguished experimentally. We find a narrow oxygen concentration range where Models 1 and 2 can be distinguished by rapid flow experiments. The distinction between Models 2 and 3 is larger in stopped flow experiments within a limited oxygen concentration range but not easily detectable in chemical relaxation following rapid flow. The detection of two special states of free hemoglobin may be possible at low oxygen concentration. However, the step reaction free enthalpy (or Gibbs free energy) values make it more likely that two special states are present in fully ligated hemoglobin.

Computer Simulation↗
PubMed2003

Effect of thrombus on abdominal aortic aneurysm wall dilation and stress.

AIM: Our goal is to understand how a mural thrombus may influence the pressure transmitted to and the dilation experienced by the abdominal aortic aneurysm (AAA) wall. METHODS: Two intact AAAs with mural thrombus were removed from patients and pressurized to 100 mmHg. The pressure was measured using a micro-tip needle transducer inserted in the aneurysm wall and advanced through the thrombus. In 1 patient with AAA, similar measurements were made in vivo. Also, in vitro, in the two aneurysms the dilation as a function of pressure was measured using the markers on the surface before and after the thrombus was removed. RESULTS: Both, in vitro and in vivo, in the presence of the thrombus the pressure transmitted to the aneurysm wall was 91+/-10% of luminal pressure and at 6 mm from the wall it was 96+/-5%. The aneurysm dilated more in the pressure range of 0-40 mmHg (2-8%) than in the range of 40-100 mmHg (0.4-1.8%). Upon removal of the thrombus these dilations increased significantly to 4-15% and 0.9-3.3%, respectively. Overall, the strains (dilation) in the circumferential and longitudinal directions were similar before the thrombus was removed. CONCLUSIONS: Even though the thrombus allows the transmission of luminal pressure to the aneurysm wall, it may prevent the aneurysm from rupture by diminishing the strain on the wall. Consistent with this, a mechanical model of the thrombus proposed is "a thrombus as a fibrous network adherent to the aneurysm wall".

Aged↗
PubMedDate not supplied

[Patterns of cerebral blood flow reactivity in adenosine stress-test in patients with carotid stenosis evaluated with MRI and emission tomography with 99m Tc-HMPAO].

In this study we evaluated the adenosine stress-test combined with 99mTc-HMPAO SPECT as a possible tool for evaluation of brain perfusion reserve in patients with various degrees of cortical neurologic damage due to internal carotid artery stenosis. The investigation comprised 37 patients with atherosclerotic carotid disease, in everybody of whom the stenosis was detected by echo-Doppler study of carotids and later in 28 verified also using invasive intraarterial digital subtractive angiography. In everybody the 99mTc-HMPAO SPECT study was performed twice: at rest and during stress-test with intravenous injection of adenosine with the patient in the supine position. It was gathered three groups of patients with different kinds of cerebral blood flow reactivity in adenosine stress-test. Henceforth, we conclude that, first, the 99mTc-HMPAO SPECT adenosin stress challenge of cerebral blood flow is a reliable functional test for every group of patients with extensive atherosclerosis and can be used for detection of patients with exhausted or decreased cerebral perfusion reserve in whom the risk of future cerebral vascular ischaemic events is elevated.

Adenosine↗
PubMed2003

A wavelet coefficient smoothened RLS adaptive denoising model for ECG.

This paper mainly concentrate on Noise Cancellation methodology for bio-signals. In this paper we have smoothened the Wavelet based signal coefficient and adapted to the MSE using RLS Algorithm. We have applied the Noisy ECG to After reconstruction model (ARM) and Before reconstruction model (BRM) and is implemented and tested. The proposed model combines the advantage of Wavelet Transform (co-efficient smoothening) and Adaptive Filter. This new proposed model performed better and faster. This find many application in the filed of noise elimination.

Algorithms↗
PubMed2003

The maximum A posteriori approach to the inverse problem of electrocardiography.

In this paper we investigate the previously proposed maximum a posteriori (MAP) approach to the problem of determining epicardial potentials from measured body surface potentials, a form of the inverse problem of electrocardiography. The MAP inverse approach uses a priori knowledge of the covariances between epicardial electrograms in its estimate of epicardial potentials. However, in practice, this information is not generally available. In this paper we examined the effectiveness of this method when the covariances are estimated using one depolarization sequence and the MAP method is used with these covariances to estimate the epicardial potentials for a different depolarization sequence.

Algorithms↗