RESUMEN
Biomechanical relationships involving lingual myoanatomy, contractility, and bolus movement are fundamental properties of human swallowing. To portray the relationship between lingual deformation and bolus flow during swallowing, a weakly one-way solid-fluid finite element model (FEM) was derived employing an elemental mesh aligned to magnetic resonance diffusional tractography (Q-space MRI, QSI) of the human tongue, an arbitrary Lagrangian-Eulerian (ALE) formulation with remeshing to account for the effects of lingual surface (boundary) deformation, an implementation of patterned fiber shortening, and a computational visualization of liquid bolus flow. Representing lingual tissue deformation in terms of its 2D principal Lagrangian strain in the mid-sagittal plane, we demonstrated that the swallow sequence was characterized by initial superior-anterior expansion directed towards the hard palate, followed by sequential, radially directed, contractions of the genioglossus and verticalis to promote lingual rotation (lateral perspective) and propulsive displacement. We specifically assessed local bolus velocity as a function of viscosity (perfect slip conditions) and observed that a low viscosity bolus (5 cP) exhibited maximal displacement, surface spreading and local velocity compared to medium (110 cP, 300 cP) and high (525 cP) viscosity boluses. Analysis of local nodal velocity revealed that all bolus viscosities exhibited a bi-phasic progression, with the low viscosity bolus being the most heterogeneous and fragmented and the high viscosity bolus being the most homogenous and cohesive. Intraoral bolus cohesion was depicted in terms of the distributed velocity gradient, with higher gradients being associated with increased shear rate and bolus fragmentation. Lastly, we made a sensitivity analysis on tongue stiffness and contractility by varying the degree of extracellular matrix (ECM) stiffness through effects on the Mooney-Rivlin derived passive matrix and by varying maximum tetanized isometric stress, and observed that a graded increase of ECM stiffness was associated with reduced bolus spreading, posterior displacement, and surface velocity gradients, whereas a reduction of global contractility resulted in a graded reduction of obtainable accommodation volume, absent bolus spreading, and loss of posterior displacement. We portray a unidirectionally coupled solid-liquid FEM which associates myoarchitecture-based lingual deformation with intra-oral bolus flow, and deduce that local elevation of the velocity gradient correlates with bolus fragmentation, a precondition believed to be associated with aspiration vulnerability during oropharyngeal swallowing.
Asunto(s)
Deglución , Lengua , Humanos , Imagen por Resonancia Magnética , Rotación , Lengua/diagnóstico por imagen , ViscosidadRESUMEN
BACKGROUND AND AIM OF THE STUDY: Percutaneous mitral valve (MV) dilatation is routinely performed for mitral stenosis using either a single balloon (SB) or double balloon (DB) technique. The study aim was to compare the two techniques using the finite element (FE) method. METHODS: An established FE model of the MV was modified by fusing MV leaflet edges at commissure level to simulate a stenotic valve (orifice area = 180 mm2). FE models of a 30 mm SB (low-pressure, elastomeric balloon) and an 18 mm DB system (high-pressure, non-elastic balloon) were created. RESULTS: Both, SB and DB simulations, resulted in the splitting of commissures and consequent relief of stenosis (final MV areas of 610 mm2 and 560 mm2, respectively). Stresses induced by the two balloon systems varied across the valve. At full inflation, SB showed a higher stress in the central part of the leaflets and at the commissures compared to DB simulation, which demonstrated a more uniform stress distribution. This was due to mismatch of the round shape of the SB within an oval mitral orifice. Due to its high compliance, commissural splitting was not easily accomplished with the SB. Conversely, the DB guaranteed commissural splitting, even when a high force was required to break the commissure welds. CONCLUSION: The FE model demonstrated that MV dilatation can be accomplished by both SB and DB techniques. However, the DB method resulted in a higher probability of splitting the fused commissures, with less potential for damage to the MV leaflets by overstretching, even at higher pressures.
Asunto(s)
Cateterismo/métodos , Estenosis de la Válvula Mitral/terapia , Fenómenos Biomecánicos , Cateterismo/instrumentación , Análisis de Elementos Finitos , Humanos , Técnicas In Vitro , Válvula Mitral/fisiología , Modelos CardiovascularesRESUMEN
Although dietary modification is a common treatment strategy used to manage dysphagic patients who aspirate thin liquids, there are no standard definitions for thickened liquid preparation. This lack of standardization leads to variability in practice and points to the need for a simple tool for clinicians to assess thickened liquid consistency. The current study analyzed the utility of the Line Spread Test (LST) in this regard. Twenty-six liquids (10 powder-thickened "nectar" juices, 10 powder-thickened "honey" juices, and 6 barium mixtures) were assessed using both a viscometer for objective measurement of viscosity and the LST. Whereas the LST was able to separate the juices into nectar and honey categories, it was not able to separate barium mixtures into these categories nor compare barium to juices. Furthermore, the LST was not predictive of viscosity. Thus, the results of the current study suggest that the LST may be useful in the broad categorization of fluids into therapeutically significant groupings but that it cannot be used more specifically to measure fluid viscosity. Further studies of this and other tools are necessary to identify inexpensive practical tools for quantification of thickened liquid consistency.
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Trastornos de Deglución/diagnóstico , Deglución/fisiología , Trastornos de Deglución/fisiopatología , Humanos , Valores de Referencia , ViscosidadRESUMEN
We summarize from previous works the functions of circular vs. longitudinal muscle in esophageal peristaltic bolus transport using a mix of experimental data, the conservation laws of mechanics and mathematical modeling. Whereas circular muscle tone generates radial closure pressure to create a local peristaltic closure wave, longitudinal muscle tone has two functions, one physiological with mechanical implications, and one purely mechanical. Each of these functions independently reduces the tension of individual circular muscle fibers to maintain closure as a consequence of shortening of longitudinal muscle locally coordinated with increasing circular muscle tone. The physiological function is deduced by combining basic laws of mechanics with concurrent measurements of intraluminal pressure from manometry, and changes in cross sectional muscle area from endoluminal ultrasound from which local longitudinal shortening (LLS) can be accurately obtained. The purely mechanical function of LLS was discovered from mathematical modeling of peristaltic esophageal transport with the axial wall motion generated by LLS. Physiologically, LLS concentrates circular muscle fibers where closure pressure is highest. However, the mechanical function of LLS is to reduce the level of pressure required to maintain closure. The combined physiological and mechanical consequences of LLS are to reduce circular muscle fiber tension and power by as much as 1/10 what would be required for peristalsis without the longitudinal muscle layer, a tremendous benefit that may explain the existence of longitudinal muscle fiber in the gut. We also review what is understood of the role of longitudinal muscle in esophageal emptying, reflux and pathology.
Asunto(s)
Esófago/fisiología , Modelos Teóricos , Músculo Liso/fisiología , Peristaltismo/fisiología , Fenómenos Biomecánicos , Trastornos de la Motilidad Esofágica/fisiopatología , Esófago/diagnóstico por imagen , Reflujo Gastroesofágico/fisiopatología , Humanos , Modelos Biológicos , Contracción Muscular/fisiología , Músculo Liso/diagnóstico por imagen , UltrasonografíaRESUMEN
As individuals age, one of the objective changes that occurs in the oropharyngeal swallow is the development of a delay between bolus entry into the pharynx and the initiation of airway protection mechanisms. For longer delays, this phenomenon is sometimes referred to as "premature spillage," and it has been suggested that such spillage, which is a risk factor for dysphagia, may be associated with pre-swallow lingual gestures, or "tongue pumping." The goal of the current study was to develop a simplified two-dimensional computational model of the oropharynx to simulate the containment of a Newtonian fluid bolus within the oral cavity in response to a given pattern of lingual gestures for different viscosities. An arbitrary Lagrangian-Eulerian simulation was performed using the commercial finite element software package, LS-Dyna. It was found that for a given lingual motion, higher viscosity Newtonian boluses, consistent with those offered therapeutically, were able to be contained within the simulated oral cavity while a lower viscosity bolus would be "spilled," suggesting a potential mechanisim by which thickened liquids may reduce aspiration. Although the current data must be validated with more realistic, three-dimensional geometric information and for a wider range of bolus rheologies, they represent an exciting first step towards realistic modeling of oropharyngeal bolus flow.
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Deglución/fisiología , Modelos Biológicos , Orofaringe/fisiología , Simulación por Computador , Trastornos de Deglución/fisiopatología , Análisis de Elementos Finitos , Alimentos , Humanos , Modelos Anatómicos , Orofaringe/anatomía & histología , Neumonía por Aspiración/etiología , Neumonía por Aspiración/fisiopatología , Reología , Lengua/fisiología , ViscosidadRESUMEN
It has been hypothesized that repetitive flexural stresses contribute to the fatigue-induced failure of bioprosthetic heart valves. Although experimental apparatuses capable of measuring the bending properties of biomaterials have been described, a theoretical framework to analyze the resulting data is lacking. Given the large displacements present in these bending experiments and the nonlinear constitutive behavior of most biomaterials, such a formulation must be based on finite elasticity theory. We present such a theory in this work, which is capable of fitting bending moment versus radius of curvature experimental data to an arbitrary strain energy function. A simple finite element model was constructed to study the validity of the proposed method. To demonstrate the application of the proposed approach, bend testing data from the literature for gluteraldehyde-fixed bovine pericardium were fit to a nonlinear strain energy function, which showed good agreement to the data. This method may be used to integrate bending behavior in constitutive models for soft tissue.
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Tejido Conectivo/fisiología , Modelos Cardiovasculares , Pericardio/fisiología , Animales , Bovinos , Simulación por Computador , Elasticidad , Estrés Mecánico , Resistencia a la TracciónRESUMEN
BACKGROUND AND AIM OF THE STUDY: The dynamics of the mitral valve result from the synergy of left heart geometry, local blood flow and tissue integrity. Herein is presented the first coupled fluid-structure computational model of the mitral valve in which valvular kinematics result from the interaction of local blood flow and a continuum representation of valvular microstructure. METHODS: The diastolic geometry of the mitral valve was assembled from previously published experimental data. Anterior and posterior leaflets were modeled as networks of entangled collagen fibers, embedded in an isotropic matrix. The resulting non-linear continuum description of mitral tissue was implemented in a three-dimensional membrane formulation. Chordal tension-only behavior was defined from experimental tensile tests. The computational model considered the valve immersed in a domain of Newtonian blood, with an experimentally determined viscosity corresponding to a shear rate of 180 s(-1) at 37 degrees C. Ventricular and atrial pressure curves were applied to ventricular and atrial surfaces of the blood domain. RESULTS: Peak closing flow and volume were 51 ml/s and 1.17 ml, respectively. Papillary muscle force ranged dynamically between 0.0 and 2.6 N. Acoustic pressure (RMS) was found to be 3.3 Pa, with a peak frequency of 72 Hz at 0.064 s from the onset of systole. Model predictions showed excellent agreement with available transmitral flow, papillary force and first heart sound (S1) acoustic data. CONCLUSION: The addition of blood flow and an experimentally driven microstructural description of mitral tissue represent a significant advance in computational studies of the mitral valve. This model will be the foundation for future computational studies on the effect of pathophysiological tissue alterations on mitral valve competence.
Asunto(s)
Simulación por Computador , Válvula Mitral/fisiología , Modelos Cardiovasculares , Dinámicas no Lineales , Algoritmos , Animales , Función del Atrio Izquierdo/fisiología , Fenómenos Biomecánicos , Fenómenos Fisiológicos Sanguíneos , Viscosidad Sanguínea/fisiología , Gasto Cardíaco/fisiología , Volumen Cardíaco/fisiología , Cuerdas Tendinosas/fisiología , Colágeno , Diástole/fisiología , Ruidos Cardíacos/fisiología , Imagenología Tridimensional , Válvula Mitral/anatomía & histología , Músculos Papilares/fisiología , Reología , Ovinos , Función Ventricular Izquierda/fisiología , Presión Ventricular/fisiologíaRESUMEN
BACKGROUND: Many diseases that affect the mitral valve are accompanied by the proliferation or degradation of tissue microstructure. The early acoustic detection of these changes may lead to the better management of mitral valve disease. In this study, we examine the nonstationary acoustic effects of perturbing material parameters that characterize mitral valve tissue in terms of its microstructural components. Specifically, we examine the influence of the volume fraction, stiffness and splay of collagen fibers as well as the stiffness of the nonlinear matrix in which they are embedded. METHODS AND RESULTS: To model the transient vibrations of the mitral valve apparatus bathed in a blood medium, we have constructed a dynamic nonlinear fluid-coupled finite element model of the valve leaflets and chordae tendinae. The material behavior for the leaflets is based on an experimentally derived structural constitutive equation. The gross movement and small-scale acoustic vibrations of the valvular structures result from the application of physiologic pressure loads. Material changes that preserved the anisotropy of the valve leaflets were found to preserve valvular function. By contrast, material changes that altered the anisotropy of the valve were found to profoundly alter valvular function. These changes were manifest in the acoustic signatures of the valve closure sounds. Abnormally, stiffened valves closed more slowly and were accompanied by lower peak frequencies. CONCLUSION: The relationship between stiffness and frequency, though never documented in a native mitral valve, has been an axiom of heart sounds research. We find that the relationship is more subtle and that increases in stiffness may lead to either increases or decreases in peak frequency depending on their relationship to valvular function.
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Diagnóstico por Computador/métodos , Enfermedades de las Válvulas Cardíacas/diagnóstico , Enfermedades de las Válvulas Cardíacas/fisiopatología , Válvula Mitral/fisiopatología , Modelos Cardiovasculares , Fonocardiografía/métodos , Espectrografía del Sonido/métodos , Animales , Simulación por Computador , Reproducibilidad de los Resultados , Sensibilidad y Especificidad , Ovinos , Estadística como AsuntoRESUMEN
Lingual pressure generation plays a crucial role in oropharyngeal swallowing. To more discretely study the dynamic oropharyngeal system, a 3-bulb array of pressure sensors was designed with the Kay Elemetrics Corporation (Lincoln Park, NJ). The influence of the device upon normal swallowing mechanics and boluses representative of flow relative to age and bolus condition was the focus of this study. Twelve healthy adults in two age groups (31 +/- 5 years, 2 males and 4 females, and 78 +/- 7 years, 2 males and 4 females) participated. Each subject was instructed to swallow four boluses representative of conditions with and without three pressure sensors affixed to the hard palate. Post-swallow residue at four locations, Penetration/Aspiration Scale scores, and three bolus flow timing measures were assessed videofluoroscopically with respect to age and bolus condition. The only statistically significant influences attributable to the presence of the pressure sensors were slight increases in residue in the oral cavity and upper esophageal sphincter with some bolus consistencies, 8% more frequent trace penetration of the laryngeal vestibule predominantly with effortful swallowing, and variances in oral clearance duration. We conclude that the presence of the pressure sensors does not significantly alter normal swallowing patterns of healthy individuals.
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Deglución/fisiología , Boca/fisiología , Presión , Adulto , Factores de Edad , Anciano , Anciano de 80 o más Años , Estudios de Cohortes , Intervalos de Confianza , Esfínter Esofágico Superior/fisiología , Femenino , Humanos , Masculino , Manometría , Orofaringe/fisiología , Probabilidad , Valores de Referencia , Sensibilidad y Especificidad , Umbral SensorialRESUMEN
Our hypothesis states that variceal pressure and wall tension increase dramatically during esophageal peristaltic contractions. This increase in pressure and wall tension is a natural consequence of the anatomy and physiology of the esophagus and of the esophageal venous plexus. The purpose of this study was to evaluate variceal hemodynamics during peristaltic contraction. A simultaneous ultrasound probe and manometry catheter was placed in the distal esophagus in nine patients with esophageal varices. Simultaneous esophageal luminal pressure and ultrasound images of varices were recorded during peristaltic contraction. Maximum variceal cross-sectional area and esophageal luminal pressures at which the varix flattened, closed, and opened were measured. The esophageal lumen pressure equals the intravariceal pressure at variceal flattening due to force balance laws. The mean flattening pressures (40.11 +/- 16.77 mmHg) were significantly higher than the mean opening pressures (11.56 +/- 25.56 mmHg) (P < or = 0.0001). Flattening pressures >80 mmHg were generated during peristaltic contractions in 15.5% of the swallows. Variceal cross-sectional area increased a mean of 41% above baseline (range 7-89%, P < 0.0001) during swallowing. The peak closing pressures in patients that experience future variceal bleeding were significantly higher than the peak closing pressures in patients that did not experience variceal bleeding (P < 0.04). Patients with a mean peak closing pressure >61 mmHg were more likely to bleed. In this study, accuracy of predicting future variceal bleeding, based on these criteria, was 100%. Variceal models were developed, and it was demonstrated that during peristaltic contraction there was a significant increase in intravariceal pressure over baseline intravariceal pressure and that the peak intravariceal pressures were directly proportional to the resistance at the gastroesophageal junction. In conclusion, esophageal peristalsis in combination with high resistance to blood flow through the gastroesophageal junction leads to distension of the esophageal varices and an increase in intravariceal pressure and wall tension.
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Várices Esofágicas y Gástricas/fisiopatología , Esófago/irrigación sanguínea , Esófago/fisiología , Contracción Muscular/fisiología , Peristaltismo/fisiología , Adulto , Deglución , Várices Esofágicas y Gástricas/diagnóstico por imagen , Femenino , Humanos , Masculino , Manometría , Persona de Mediana Edad , Modelos Anatómicos , Presión , UltrasonografíaRESUMEN
BACKGROUND AND AIM OF THE STUDY: The study aim was to develop a three-dimensional coupled fluid-structure finite element model of the aortic valve and root. This model extends previous purely structural finite element models, and represents a significant step toward realistic simulation of the complex interactions among tissue material properties and valvular function. METHODS: The aortic root and valve geometry were extracted from magnetic resonance images and imported into the LS-Dyna explicit finite element package. Leaflet and root tissue were modeled with elastic material properties, and blood was modeled as a Newtonian liquid. A dynamic, fully unsteady analysis was performed in which blood flow through the valve was computed along with the motion of the leaflets and root in response to standard physiologic pressure wave profiles. RESULTS: The opening and closing of the aortic valve under physiological loading conditions was successfully simulated, and feasibility of the model illustrated. The motion of the simulated leaflets was consistent with that seen in intact hearts. Analysis of fluid flow patterns revealed eddy structures in the sinus regions and flow into the coronary circulation. CONCLUSION: The addition of blood flow to structural models of the aortic valve and root is a significant advance in modeling, and allows a closer simulation of valvular function. The model will be used to further assess normal and abnormal physiology as well as the effects of surgical intervention.
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Aorta Torácica/fisiología , Válvula Aórtica/fisiología , Bioprótesis , Análisis de Elementos Finitos , Prótesis Valvulares Cardíacas , Fenómenos Biomecánicos , Simulación por Computador , Humanos , Modelos Cardiovasculares , Diseño de Prótesis , Falla de Prótesis , Flujo Pulsátil , Sensibilidad y Especificidad , Estrés MecánicoRESUMEN
We present a model of esophageal wall muscle mechanics during bolus transport with which the active and "passive" components of circular muscle tension are separately extracted from concurrent manometric and videofluoroscopic data. Local differential equations of motion are integrated across the esophageal wall to yield global equations of equilibrium which relate total tension within the esophageal wall to intraluminal pressure and wall geometry. To quantify the "passive" (i.e. inactive) length-tension relationships, the model equations are applied to a region of the esophagus in which active muscle contraction is physiologically inhibited. Combining the global equations with space-time-resolved intraluminal pressure measured manometrically and videofluoroscopic geometry data, the passive model is used to separate active and "passive" components of esophageal muscle tension during bolus transport. The model is of general applicability to probe basic muscle mechanics including the space-time stimulation of circular muscle, the relationship between longitudinal muscle tension and longitudinal muscle shortening, and the contribution of the collagen matrix surrounding muscle fibers to passive tension during normal human esophageal bolus transport and in pathology. Example calculations of normal esophageal function are given where active tone is found to extend only over a short intrabolus segment near the bolus tail and segmental regions of active muscle squeeze are demonstrated.
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Deglución/fisiología , Esófago/fisiología , Contracción Muscular/fisiología , Fenómenos Biomecánicos , Humanos , Modelos BiológicosRESUMEN
BACKGROUND AND AIM OF THE STUDY: Biaxial mechanical properties have been reported for porcine aortic valve leaflets, but not for the aortic root wall. These data are important for understanding the relationship between tissue material properties and function, providing a baseline for diseased tissue, and for providing a basis for numerical models of aortic mechanics. The study aim was to determine the biaxial material properties of porcine aortic root wall tissue. METHODS: Tissue samples (20 mm x 20 mm) were obtained from the aortic root walls of 18 pigs (anterior and posterior samples from each pig) and tested with a custom-built biaxial tensile testing apparatus. The data were fitted to the strain energy formulation: W = ?[a(long)E11(2) + a(circ)Ecc(2) + 2a(int)E11Ecc], where W is the strain energy, E11 = longitudinal strain, Ecc = circumferential strain, along, a(circ), and aint are the constants that were determined, and represent the longitudinal and circumferential elastic moduli, and interaction between the two axes, respectively. RESULTS: The root wall tissue was less stiff in the longitudinal direction (along = 115.8 +/- 8.4 kPa) than the circumferential direction (a(circ) = 169.9.3 +/- 7.4 kPa). As expected, there was mechanical interaction between the two axes (a(int) = 45.7 +/- 3.4 kPa). Additionally, anterior tissue samples were less stiff than posterior samples. All tissue samples exhibited a linear stress-strain relationship up to 40% strain, in contrast to aortic leaflet tissue, which was highly non-linear. CONCLUSION: These results demonstrated that the porcine aortic root wall tissue is an anisotropic material with linear elastic properties, in contrast to leaflet tissue. Additionally, the data suggest that a finite element model using an isotropic material as a basis for the aorta is insufficient for a physiologically accurate representation.