Biomedical subjects

J J Meister

Publications and source records attributed to J J Meister.

At least 37 records · Page 2Linked to original sources

PubMed1998

A model for geometric and mechanical adaptation of arteries to sustained hypertension.

This study aimed to model phenomenologically the dynamics of arterial wall remodeling under hypertensive conditions. Sustained hypertension was simulated by a step increase in blood pressure. The arterial wall was considered to be a thick-walled tube made of nonlinear elastic incompressible material. Remodeling rate equations were postulated for the evolution of the geometric dimensions of the hypertensive artery at the zero-stress state, as well as for one of the material constants in the constitutive equations. The driving stimuli for the geometric adaptation are the normalized deviations of wall stresses from their values under normotensive conditions. The geometric dimensions are modulated by the evolution of the deformed inner radius, which serves to restore the level of the flow-induced shear stresses at the arterial endothelium. Mechanical adaptation is driven by the difference between the area compliance under hypertensive and normotensive conditions. The predicted time course of the geometry and mechanical properties of arterial wall are in good qualitative agreement with published experimental findings. The model predicts that the geometric adaptation maintains the stress distribution in arterial wall to its control level, while the mechanical adaptation restores the normal arterial function under induced hypertension.

Adaptation, Physiological↗

PubMed1998

Arterial vasomotion: effect of flow and evidence of nonlinear dynamics.

Vasomotion has been studied on segments of rat mesenteric and femoral arteries perfused in vitro. We have investigated 1) the effect of perfusion flow on the characteristics of vasomotion and 2) the nature and patterns of vasomotion. We have found that perfusion flow is not a control parameter that contributes to the genesis of vasomotion but that it affects, in most cases only slightly, the frequency and amplitude of vasomotion. We have found evidence that vasomotion is low-dimensional chaotic. The correlation dimension ranged between 2 and 4, and the average Lyapunov's coefficient was approximately 0.1. A great variety of vasomotion patterns was observed with features that are typical of nonlinear deterministic systems: regular and irregular vasomotion, quasiperiodicity, period doubling and higher-order periods, intermittency, mixed modes, and bursting activity. Vasomotion patterns appeared occasionally to be highly sensitive to perturbations in perfusion flow, which also supported the existence of nonlinear dynamics. Finally, entrainment (phase locking) was observed when arteries were perfused with oscillatory flow with frequency in the neighborhood of the frequency of vasomotion.

Animals↗

PubMed1997

Model for the alignment of actin filaments in endothelial cells subjected to fluid shear stress.

Cultured vascular endothelial cells undergo significant morphological changes when subjected to sustained fluid shear stress. The cells elongate and align in the direction of applied flow. Accompanying this shape change is a reorganization at the intracellular level. The cytoskeletal actin filaments reorient in the direction of the cells' long axis. How this external stimulus is transmitted to the endothelial cytoskeleton still remains unclear. In this article, we present a theoretical model accounting for the cytoskeletal reorganization under the influence of fluid shear stress. We develop a system of integro-partial-differential equations describing the dynamics of actin filaments, the actin-binding proteins, and the drift of transmembrane proteins due to the fluid shear forces applied on the plasma membrane. Numerical simulations of the equations show that under certain conditions, initially randomly oriented cytoskeletal actin filaments reorient in structures parallel to the externally applied fluid shear forces. Thus, the model suggests a mechanism by which shear forces acting on the cell membrane can be transmitted to the entire cytoskeleton via molecular interactions alone.

Actins↗

PubMed1997

Residual strain effects on the stress field in a thick wall finite element model of the human carotid bifurcation.

A three-dimensional finite element model of the carotid artery bifurcation was constructed in order to determine the stress field and assess the modification of the stress field when residual strain is taken into account. Residual strain in the carotid bifurcation was characterized by experimental observations. According to these observations, a geometrical model of the carotid artery was constructed to exhibit a state free of strain. Appropriate boundary conditions were applied to yield the correct geometry in the unloaded state, and physiological levels of pressure and axial stretching were applied. The model took into account the varying thickness of the arterial wall along the bifurcation. For modeling purposes, the material was considered to be hyperelastic, incompressible, homogenous and isotropic. For comparison, a similar model of the carotid artery which does not include the effects of residual strain was also created. The results demonstrate that in the model of the carotid artery bifurcation with residual strain, the distribution of maximum principal stress along the inner wall and the circumferential stress throughout the wall is much more uniform than in the model without residual strain. The ratio between the stress at the inner and the outer walls is highest at the lateral wall of the carotid sinus; this is the same location known to be a site of low and oscillatory fluid wall shear stress, and the principal location of early intimal thickening. These results suggest that the localization of atherosclerosis in the carotid artery may be due to local variations in both fluid wall shear stress and solid wall stress.

Animals↗

PubMed1997

Experimental investigation of the distribution of residual strains in the artery wall.

Arterial wall stresses are thought to be a major determinant of vascular remodeling both during normal growth and throughout the development of occlusive vascular disease. A completely physiologic mechanical model of the arterial wall should account not only for its residual strains but also for its structural nonhomogeneity. It is known that each layer of the artery wall possesses different mechanical properties, but the distribution of residual strain among the different mechanical components, and thus the true zero stress state, remain unknown. In this study, two different sets of experiments were carried out in order to determine the distribution of residual strains in artery walls, and thus the true zero stress state. In the first, collagen and elastin were selectively eliminated by chemical methods and smooth muscle cells were destroyed by freezing. Dissolving elastin provoked a decrease in the opening angle, while dissolving collagen and destroying smooth muscle cells had no effect. In the second, different wall layers of bovine carotid arteries were removed from the exterior or luminal surfaces by lathing or drilling frozen specimens, and then allowing the frozen material to thaw before measuring residual strain. Lathing material away from the outer surface caused the opening angle of the remaining inner layers to increase. Drilling material from the inside caused the opening angle of the remaining outer layers to decrease. Mechanical nonhomogeneity, including the distribution of residual strains, should thus be considered as an important factor in determining the distribution of stress in the artery wall and the configuration of the true zero stress state.

Analysis of Variance↗

PubMed1996

On the wave transmission and reflection properties of stenoses.

This study is concerned with the wave reflection properties of arterial stenoses. Two theoretical models have been developed for deriving the reflection coefficient: a linear model resulting from the linearization of the pressure drop-flow equation and an indirect, quasi-nonlinear model, based on the separation of pressure waves into their forward and backward running components proximal and distal to the stenosis. The linear method gave consistently lower values for the reflection coefficient when compared to the quasi-nonlinear model. In vitro experiments in elastic tubes showed that the reflection coefficient is strongly dependent on stenosis severity, mean flowrate, and the elastic properties of the proximal unobstructed artery. For critical stenoses the reflection coefficient is frequency and pulsatility independent. The results suggest that hemodynamically nonsevere stenoses may cause significant wave reflections.

Algorithms↗

PubMed1996

The effects of time-varying curvature on velocity profiles in a model of the coronary arteries.

Blood flow patterns in the coronary arteries are of interest due to their possible involvement in atherosclerosis localization. Spatial variations in hemodynamic flow patterns are predominantly determined by the vessel geometry. The coronary arteries represent a unique situation in the cardiovascular system because their geometry undergoes large dynamic variations during each cardiac cycle due to the contraction of the heart. This study was initiated to analyze the effects of time-varying curvature on flow velocity profiles in a curved tube model of the coronary arteries. An in vitro flow model was constructed, which consisted of a flexible curved tube through which fluid flowed under a steady imposed pressure gradient. The radius of curvature of the tube was varied in time using a stepper motor and carriage. Two different deformation configurations were used to determine if variations in center of curvature displacement affected the velocity profiles. It was found in both cases that the skewing of the axial velocity profile depended on the instantaneous dynamic vessel movement, with maximal skewing occurring when the radius of curvature was in transition from the minimum to the maximum value. The change in skewing was greater for the case where carriage moved obliquely to the main direction of flow than when the carriage moved perpendicularly. Although this study was limited to relatively low values of the radius of curvature and change in curvature, an initial understanding of this flow situation was obtained which may lead to the development of more physiologic models.

Animals↗

PubMed1996

Theoretical study of dynamics of arterial wall remodeling in response to changes in blood pressure.

The dynamics of arterial wall remodeling was studied on the basis of a phenomenological mathematical model. Sustained hypertension was simulated by a step increase in blood pressure. Remodeling rate equations were postulated for the evolution of the geometrical dimensions that characterize the zero stress state of the artery. The driving stimuli are the deviations of the extreme values of the circumferential stretch ratios and the average stress from their values at the normotensive state. Arterial wall was considered to be a thick-walled tube made of nonlinear elastic incompressible material. Results showed that thickness increases montonically with time whereas the opening angle exhibits a biphasic pattern. Geometric characteristics reach asymptotically a new homeostatic steady state, in which the stress and strain distribution is practically identical with the distribution under normotensive conditions. The model predictions are in good agreement with published experimental findings.

Algorithms↗

PubMed1996

Effects of friction and nonlinearities on the separation of arterial waves into their forward and backward components.

In this paper we examine the importance of fluid friction and nonlinearities due to the area-pressure relationship and to the convective acceleration on the separation of arterial pressure and flow waves into their forward and backward components. Experiments were run in straight uniform nonlinearly elastic tubes. Different degrees of fluid friction and nonlinearities, covering the physiological range, have been tested. We predicted the forward and backward running pressure components using two wave separation methods: the classical linear method (Westerhof et al., Cardiovasc. Res, 6,648-656, 1972) and the first order correction (FOC) method (Pythoud et al., Trans ASME J. Biomech. Engng, in press) which takes nonlinearities and fluid friction into account. We found that the two methods yield somewhat different predictions. The differences tend to increase with the degree of fluid friction and nonlinearities and are typically of the order of 4-8%. We further compared the transmission ratio of forward and backward waves predicted by both methods. The transmission ratio was found to be overestimated by 10% by the classical linear method. The nonlinear method gave more accurate estimates, consistent with theory. We conclude that, for in vivo applications, the classical linear method should be the method of choice because it is simpler to use and the erros involved (4-8%) are comparable to measurement erros.

Arteries↗

PubMed1996

Velocity vector reconstruction for color flow Doppler: experimental evaluation of a new geometrical method.

Ultrasonic color flow mapping (CFM) is employed for the diagnosis of a variety of cardiovascular abnormalities. But CFM permits only the analysis of the in-beam component of the velocity distribution. A reconstruction method based on a geometrical approach was developed to calculate the two-dimensional, in-plane velocity vector using the information of two independent CFM measurements obtained from two closely spaced transducers. However, with the CFM measurements being limited by the noise and quantified in a limited number of velocity values, the accuracy of the velocity vector reconstruction is not satisfactory. A simple filtering process was applied to improve the results. The method was tested with simulations and a series of in vitro measurements. The results of the simulations were compared with analytical solutions. The in vitro measurements were compared with the results of laser Doppler anemometry (LDA) and theoretical calculations. The results of in vivo measurements were compared with theoretical calculations based on Womersley's theory. The comparison showed good applicability of the method to different flow fields. The flow vector reconstructed without filtering had an error of more than 20%; whereas our method, including the filtering process, had an error of less than 10%, in comparison with the theoretical or the LDA results. The method could be extended to three-dimensional flows.

Blood Flow Velocity↗

PubMed1996

Separation of arterial pressure waves into their forward and backward running components.

A new separation technique has been developed to determine the forward and backward running arterial pressure wave components. It takes into account friction as well as nonlinear effects due to convective acceleration and to the pressure dependence of the arterial compliance. The new method is a combination of two methods treating friction and nonlinearities separately. The method requires the measurements of pressure and flow at one location as well as the knowledge of the area-pressure relationship. The validity of the method was tested by a simulation experiment in which the forward and backward waves were known a priori. It was shown that the new method is significantly more accurate in the predictions of the forward and backward waves when compared to the classical method assuming linearity and no dissipation. The new wave separation method was also applied to simulated aortic waves for (a) a healthy subject and (b) a subject with decreased compliance. Comparison with the classical linear method showed that neglecting nonlinearities leads to an overestimation of the forward and backward pressure wave amplitudes. The errors, however, were in the order of 5 to 10 percent. We concluded that, for most clinical purposes, the improvement using the nonlinear method is of the same magnitude as experimental errors, and thus the linear method would suffice.

Acceleration↗

PubMed1996

Determinants of stroke volume and systolic and diastolic aortic pressure.

We investigated how parameters describing the heart and the arterial system contribute to the systolic and diastolic pressures (Ps and Pd, respectively) and stroke volume (SV). We have described the heart by the varying-elastance model with six parameters and the systemic arterial tree by the three-element windkessel model, leading to a total of nine parameters. Application of dimensional analysis led to a total of six dimensionless parameters describing dimensionless Ps and Pd, i.e., pressures with respect to venous pressure (Ps/Pv and Pd/Pv). SV was normalized with respect to unloaded ventricular volume (Vd). Sensitivity analysis showed that Ps/Pv, Pd/Pv, and SV/Vd could be accurately described by four, three, and three dimensionless parameters, respectively. With this limited number of parameters, it was then possible to obtain empirical analytical expressions for Ps/Pv, Pd/Pv, and SV/Vd. The analytic predictions were tested against the model values and found to be as follows: Ps predicted = (1.0007 +/- 0.0062) Ps, r = 0.987; Pd predicted = (1.016 +/- 0.0085) Pd, r = 0.992; and SV predicted = (0.9987 +/- 0.0028) SV, r = 0.996. We conclude that aortic Ps, Pd, and SV can be accurately described by a limited number of parameters and that, for any condition of the heart and the arterial system, Ps, Pd, and SV can be presented in analytical form.

Animals↗

PubMed1996

Method for determining distribution of reflection sites in the arterial system.

We developed a new method to determine the location and importance of reflection sites in the arterial system. The method is based on the decomposition of the aortic pressure wave into its forward and backward components, and it provides the reflection profile of the arterial system as a wave reflection site amplitude versus distance from the heart. The reflection profile can be seen as the response of the arterial system to a pressure delta pulse where reflections upstream from the measurement location have been eliminated. The method was successfully tested on a simple model loaded with a pure resistor, a two-element windkessel, and a bifurcating tube system. It was then applied to the aortic pressure and flow signals measured in six mongrel dogs whose aorta was occluded at different levels. The profiles obtained from measurements at control showed two main reflection regions, one located in the vicinity (0.1-0.2 m) of the heart and the other located in the region of the iliac bifurcation. All occlusions, even the most distant one at the iliac bifurcation, could be identified in both amplitude (amount of reflections) and distance from the heart. The spatial resolution of the profiles was approximately 0.1 m as a result of the limited power spectrum contained in the arterial pulse, and the identification of reflection sites decreased rapidly with the distance.

Animals↗

PubMed1995

Evaluation of methods for estimation of total arterial compliance.

Seven classic and recently proposed methods used for the estimation of total arterial compliance have been evaluated for their accuracy and applicability in different physiological conditions. The pressure and flow data are taken from a computer model that provides realistic simulations of the nonlinear-distributed systemic arterial tree. Besides the great flexibility in simulating different physiological or pathological cases, the major advantage of the computer model is that it allows precise knowledge of the pressure-dependent total arterial compliance, which is the variable of interest. The results show that the methods based on the two-element windkessel (WK) model are more accurate than those based on the three-element WK model. The classic exponential decay and the diastolic area method yield essentially similar results, and their compliance estimates are accurate within 10% except at high heart rates. The later part of diastole, i.e., from the time that the systolic pressure wave has reached all peripheral beds, gives the best results. The newly proposed two-area and pulse pressure methods, both based on the two-element WK model, are accurate (errors in general < 10%) and can be applied to other locations in the arterial tree where the decay time and area method cannot. Methods based on the three-element WK model consistently overestimate total arterial compliance (> or = 25%). The errors in the methods based on the three-element WK model arise from the fact that the input impedance in that model deviates significantly from the true input impedance at low frequencies. The strong dependence of compliance on pressure (elastic nonlinearity) does not invalidate the compliance estimates.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1995

Scatter in input impedance spectrum may result from the elastic nonlinearity of the arterial wall.

We have examined the role of the nonlinear elastic properties of the arterial wall on the human aortic input impedance obtained at different heart rates and different pressure and flow wave shapes. Pressure and flow were taken from a computer model that provides realistic simulations of the nonlinear distributed systemic arterial tree. Different wave shapes of ascending aorta pressure and flow and different heart rates were used to derive input impedance moduli and phase angles via Fourier analysis. The results show that the nonlinear elastic properties of the arterial wall are responsible for significant variations in the input impedance spectrum when changes in heart rate and aortic flow wave shape take place. This finding may explain the scatter often observed in experimentally determined input impedance data using different heart rates obtained by pacing.

Arteries↗

PubMed1995

Simultaneous ipsilateral and contralateral measurements of vasomotion in conduit arteries of human upper limbs.

We investigated the patterns of vasomotion in various conduit arteries of the human arm. The internal diameter of the brachial, radial, ulnar, and digital artery was measured noninvasively in 17 healthy volunteers (aged 24-40 yr), using a high-precision ultrasonic echotracking device. Under resting conditions, the radial, ulnar, and digital internal diameter exhibited spontaneous oscillations (vasomotion) with a relative amplitude ranging from 1 to 5% of the mean diameter and a fundamental frequency ranging from 0.01 to 0.05 Hz. This oscillatory behavior was either quasi-periodic or irregular. The low-frequency mode (f < or = 0.05 Hz) present in the diameter signal was identified neither in the heart rate nor in the blood pressure signal. To determine whether the oscillatory activity was propagative, simultaneous measurements of diameter at two sites on the right radial artery were performed and revealed no significant consistent phase shift. Ipsilateral radial and ulnar diameters, measured at the wrist level, exhibited similar and synchronous vasomotion patterns, despite differences in the amplitude. For all subjects, contralateral measurements, performed at two symmetrical sites of the radial arteries, showed similar oscillatory patterns with a strong correlation (0.85 < r < 0.99, n = 12). These results suggested the existence of a global regulatory mechanism that coordinates vasomotion in the large conduit arteries of the human arm.

Adult↗

PubMed1995

Synergistic effects of fluid shear stress and cyclic circumferential stretch on vascular endothelial cell morphology and cytoskeleton.

The development of atherosclerosis is thought to be initiated by a dysfunctional state of the vascular endothelium. The proposal that mechanical forces play a role in the localization of this disease has led researchers to develop in vitro models to assess their effects on cultured endothelial cells. The arterial endothelium is exposed simultaneously to circumferential hoop stretch and wall shear stress, yet previous investigations have focused on the isolated effects of either cyclic stretch or shear stress. The influence of physiological levels of combined shear stress and hoop stretch on the morphology and F-actin organization of bovine aortic endothelial cells was investigated. Cells subjected for 24 hours to shear stresses higher than 2 dyne/cm2 or to hoop stretch greater than 2% elongated significantly compared with unstressed controls and oriented along the direction of flow and perpendicular to the direction of stretch. Exposure to more than 4% stretch significantly enhanced the responses to shear stress. Both shear stress and hoop stretch induced formation of stress fibers that were aligned with the cells' long axes. Simultaneous exposure to both stimuli appeared to enhance stress fiber size and alignment. These results indicate that shear stress and hoop stretch synergistically induce morphological changes in endothelial cells, which suggests that circumferential strain might modulate sensitivity of endothelial cells towards shear stress.

Animals↗

PubMed1995

Forward and backward waves in the arterial system: nonlinear separation using Riemann invariants.

A nonlinear method has been developed to separate arterial pressure and flow waves into their forward and backward running components. It takes into account nonlinearities arising from the area-pressure relationship as well as convective acceleration. The method is based on the treatment of the Riemann invariants and requires measurements of pressure, flow and diameter at one arterial location. The method has been successfully tested by means of a simulation experiment in which the forward and backward waves were known a priori. It was shown that the new method is significantly more accurate in the predictions of the forward and backward waves when compared to the classical linear method. The new wave separation method was then applied to simulated aortic waves for a) a healthy subject and b) a subject with decreased compliance. Comparison with the classical linear method showed that neglecting nonlinearities leads to an overestimation of the forward and backward pressure wave amplitudes of the order of 5 to 10%.

Adult↗