A note on the critical flow to initiate closure of pivoting disc mitral valve prostheses

Newton's second law of motion for rotating bodies and potential flow theory is used to mathematically model the closing process of a pivoting disc prosthetic heart valve in mitral position. The model predicts closure to be dependent upon disc curvature, eccentricity, mass, diameter, density, opening angle and fluid properties. Experiments using two commercially available prostheses are shown to give good correlation with the theory for large opening angles. Divergence between theory and experiment occur at small opening angles because of the limitation of the potential flow assumption.     

Investigation of left heart flow using a numerical correlation to model heart wall motion

A three-dimensional computational fluid dynamics (CFD) method has been developed to model the flow in the left heart including atrium and ventricle. Since time resolution of the medical scans does not fit the requirements of the CFD calculations, the main challenge in a numerical simulation of heart chambers is wall motion modeling. This study employs a novel three-dimensional approximation scheme to correlate the wall boundary and grid movement in systole and diastole. It uses a geometry extracted from medical images in the literature and deformed based on the reported flow rates. The opening and closing of the mitral (MV) and the aortic valve (AV) considered as simultaneous events. Unstructured tetragonal grids were used for the meshing of the domain. The calculation was performed by a Navier–Stokes solver using the arbitrary Lagrange–Euler (ALE) formulation. Results show that the proposed correlation for the wall motion could predict the main features of heart flows.     

DIASTOLIC OPENING OF THE AORTIC VALVE (AV) IN A CASE OF AORTIC INSUFFICIENCY DUE TO AV FENESTRATION

A patient with severe aortic insufficiency due to fenestration of the non-coronary aortic valve leaflet is described. A preoperative echocardiogram demonstrated early closure of the mitral valve and early diastolic separation of the aortic valve leaflets. These findings disappeared after partial surgical correction and subsequent hemodynamic improvement. Premature opening of the aortic valve is common in severe aortic insufficiency.     

Low pulse pressure with high pulsatile external left ventricular power: influence of aortic waves

Elevated pulse pressure (pp) is considered to be a risk factor for adverse cardiovascular events since it is directly related to an elevated myocardial workload. Information about both pressure and flow wave must be provided to assess hemodynamic complexity and true level of external left ventricular power (ELVP). pp value as a single feature of aortic waves cannot identify true level of ELVP. However, it is generally presumed that ELVP (and consequently LV workload) is positively correlated with pp. This study examined this positive correlation. The aim of this study was to test the hypothesis that aortic wave dynamics can create destructive hemodynamic conditions that increase the ELVP even though pp appears to be normal. To test this hypothesis, a computational model of the aorta with physiological properties was used. A Finite Element Method with fluid-structure interaction was employed to solve the equations of the solid and fluid. The aortic wall was assumed to be elastic and isotropic. The blood was assumed to be an incompressible Newtonian fluid. Simulations were performed for various heart rates (HR) and different aortic compliances while keeping the shape of the inlet flow and peripheral resistance constant. As expected, in most of the cases studied here, higher pp was associated with higher LV power demand. However, for a given cardiac output, mean pressure, and location of total reflection site, we have found cases where the above-mentioned trend does not hold. Our results suggest that using pp as a single index can result in an underestimation of the LV power demand under certain conditions related to the altered wave dynamics. Hence, in hypertensive patients, a full analysis of aortic wave dynamics is essential for the prevention and management of left ventricular hypertrophy (LVH) and congestive heart failure.     

Experimental evaluation of an elastic foundation model to predict contact pressures in knee replacements

Computational wear prediction is an attractive concept for evaluating new total knee replacement designs prior to physical testing and implementation. An important hurdle to such technology is the lack of in vivo contact pressure predictions. To address this issue, this study evaluates a computationally efficient simulation approach that combines the advantages of rigid and deformable body modeling. The hybrid method uses rigid body dynamics to predict body positions and orientations and elastic foundation theory to predict contact pressures between general three-dimensional surfaces. To evaluate the method, we performed static pressure experiments with a commercial knee implant in neutral alignment using flexion angles of 0, 30, 60, and 90 degrees and loads of 750, 1500, 2250, and 3000N. Using manufacturer CAD geometry for the same implant, an elastic foundation model with linear or nonlinear polyethylene material properties was implemented within a commercial multibody dynamics software program. The model's ability to predict experimental peak and average contact pressures simultaneously was evaluated by performing dynamic simulations to find the static configuration. Both the linear and nonlinear material models predicted the average contact pressure data well, while only the linear material model could simultaneously predict the trends in the peak contact pressure data. This novel modeling approach is sufficiently fast and accurate to be used in design sensitivity and optimization studies of knee implant mechanics and ultimately wear.     

Study on Tribological Properties of Irradiated Crosslinking UHMWPE Nano-Composite

Ultra High Molecular Weight Polyethylene(UHMWPE)has been widely used as a bearing material for artificial joint replacementover forty years.It is usually crosslinked by gamma rays irradiation before its implantation into human body.In thisstudy,UHMWPE and UHMWPE/nano-hydroxyapatite(n-HA)composite were prepared by vacuum hot-pressing method.Theprepared materials were irradiated by gamma rays in vacuum and molten heat treated in vacuum just after irradiation.The effectof filling n-HA with gamma irradiation on tribological properties of UHMWPE was investigated by using friction and wearexperimental machine(model MM-200)under deionized water lubrication.Micro-morphology of worn surface was observedby metallographic microscope.Contact angle and hardness of the materials were also measured.The results show that contactangle and hardness are changed by filling n-HA and gamma irradiation.Friction coefficient and wear rate under deionized waterlubrication are reduced by filling n-HA.While friction coefficient is increased and wear rate is reduced significantly by gammairradiation.The worn surface of unfilled material is mainly characterized as adhesive wear and abrasive wear,and that of n-HAfilled material is mainly characterized as abrasive wear.After gamma irradiation,the degrees of adhesive and abrasive wear forunfilled material and abrasive wear of n-HA filled material are significantly reduced.Unfilled and filled materials after irradiationare mainly shown as slight fatigue wear.The results indicate that UHMWPE and UHMWPE/n-HA irradiated at the doseof 150 kGy can be used as bearing materials in artificial joints for its excellent wear resistance compared to original UHMWPE.     

Choice of a hemodynamic model for occlusive thrombosis in arteries

Intravascular thrombosis can lead to heart attacks and strokes that together are the leading causes of death in the US (Kochanek, K.D., Murphy, S.L., Xu, J.Q., 2014). The ability to identify the offending biofluid mechanical conditions and predict the timescale of thrombotic occlusion in vessels and devices may improve patient outcomes. A computational model was developed to describe the growth of thrombus based on the local hemodynamic shear rate. The model predicts thrombus deposition based on initial geometric and fluid mechanical conditions, which are updated throughout the simulation to reflect the changing lumen dimensions. Thrombus growth and occlusion from whole blood was measured in in vitro experiments using stenotic glass capillary tubes, a PDMS microfluidic channel, and a PTFE stenotic aorto-iliac graft. Comparison of the predicted occlusion times to experimental results shows excellent agreement. The results indicate that local shear rate plays a critical role in acute thrombosis, and that hemodynamic characterization may have clinical utility.     

Myocardial oxygenation in isolated hearts predicted by an anatomically realistic microvascular transport model

An anatomically realistic model for oxygen transport in cardiac tissue is introduced for analyzing data measured from isolated perfused guinea pig hearts. The model is constructed to match the microvascular anatomy of cardiac tissue based on available morphometric data. Transport in the three-dimensional system (divided into distinct microvascular, interstitial, and parenchymal spaces) is simulated. The model is used to interpret experimental data on mean cardiac tissue myoglobin saturation and to reveal differences in tissue oxygenation between buffer-perfused and red blood cell-perfused isolated hearts. Interpretation of measured mean myoglobin saturation is strongly dependent on the oxygen content of the perfusate (e.g., red blood cell-containing vs. cell-free perfusate). Model calculations match experimental values of mean tissue myoglobin saturation, measured mean myoglobin, and venous oxygen tension and can be used to predict distributions of intracellular oxygen tension. Calculations reveal that approximately 20% of the tissue is hypoxic with an oxygen tension of <0.5 mmHg when the buffer is equilibrated with 95% oxygen to give an arterial oxygen tension of over 600 mmHg. The addition of red blood cells to give a hematocrit of only 5% prevents tissue hypoxia. It is incorrect to assume that the usual buffer-perfused Langendorff heart preparation is adequately oxygenated for flows in the range of < or =10 ml. min-1. ml tissue-1.     

Dynamic modelling of prosthetic chorded mitral valves using the immersed boundary method

Current artificial heart valves either have limited lifespan or require the recipient to be on permanent anticoagulation therapy. In this paper, effort is made to assess a newly developed bileaflet valve prosthesis made of synthetic flexible leaflet materials, whose geometry and material properties are based on those of the native mitral valve, with a view to providing superior options for mitral valve replacement. Computational analysis is employed to evaluate the geometric and material design of the valve, by investigation of its mechanical behaviour and unsteady flow characteristics. The immersed boundary (IB) method is used for the dynamic modelling of the large deformation of the valve leaflets and the fluid-structure interactions. The IB simulation is first validated for the aortic prosthesis subjected to a hydrostatic loading. The predicted displacement fields by IB are compared with those obtained using ANSYS, as well as with experimental measurements. Good quantitative agreement is obtained. Moreover, known failure regions of aortic prostheses are identified. The dynamic behaviour of the valve designs is then simulated under four physiological pulsatile flows. Experimental pressure gradients for opening and closure of the valves are in good agreement with IB predictions for all flow rates for both aortic and mitral designs. Importantly, the simulations predicted improved physiological haemodynamics for the novel mitral design. Limitation of the current IB model is also discussed. We conclude that the IB model can be developed to be an extremely effective dynamic simulation tool to aid prosthesis design.     

In vitro hemodynamics and valve imaging in passive beating hearts

Due to their high complexity, surgical approaches to valve repair may benefit from the use of in vitro simulators both for training and for the investigation of those measures which can lead to better clinical results. In vitro tests are intrinsically more effective when all the anatomical substructures of the valvular complexes are preserved. In this work, a mock apparatus able to house an entire explanted porcine heart and subject it to pulsatile fluid-dynamic conditions was developed, in order to enable the hemodynamic analysis of simulated surgical procedures and the imaging of the valvular structures. The mock loop's hydrodynamic design was based on an ad-hoc defined lumped-parameter model. The left ventricle of an entire swine heart was dynamically pressurized by an external computer-controlled pulse duplicator. The ascending aorta was connected to a hydraulic circuit which simulated the input impedance of the systemic circulation; a reservoir passively filled the left atrium. Accesses for endoscopic imaging were located in the apex of the left ventricle and in the aortic root. The experimental pressure and flow tracings were comparable with the typical in vivo curves; a mean flow of 3.5±0.1l pm and a mean arterial pressure of 101±2 mmHg was obtained. High-quality echographic and endoscopic video recordings demonstrated the system's excellent potential in the observation of the cardiac structures dynamics. The proposed mock loop represents a suitable in vitro system for the testing of minimally-invasive cardiovascular devices and surgical procedures for heart valve repair.     

On estimating intraventricular hemodynamic forces from endocardial dynamics: A comparative study with 4D flow MRI

Intraventricular pressure gradients or hemodynamic forces, which are their global measure integrated over the left ventricular volume, have a fundamental importance in ventricular function. They may help revealing a sub-optimal cardiac function that is not evident in terms of tissue motion, which is naturally heterogeneous and variable, and can influence cardiac adaptation. However, hemodynamic forces are not utilized in clinical cardiology due to the unavailability of simple non-invasive measurement tools.Hemodynamic forces depend on the intraventricular flow; nevertheless, most of them are imputable to the dynamics of the endocardial flow boundary and to the exchange of momentum across the mitral and aortic orifices. In this study, we introduce a simplified model based on first principles of fluid dynamics that allows estimating hemodynamic forces without knowing the velocity field inside the LV.The model is validated with 3D phase-contrast MRI (known as 4D flow MRI) in 15 subjects, (5 healthy and 10 patients) using the endocardial surface reconstructed from the three standard long-axis projections. Results demonstrate that the model provides consistent estimates for the base-apex component (mean correlation coefficient r = 0.77 for instantaneous values and r = 0.88 for root mean square) and good estimates of the inferolateral-anteroseptal component (r = 0.50 and 0.84, respectively).The present method represents a potential integration to the existing ones quantifying endocardial deformation in MRI and echocardiography to add a physics-based estimation of the corresponding hemodynamic forces. These could help the clinician to early detect sub-clinical diseases and differentiate between different cardiac dysfunctional states.     

Numerical Modeling of Intraventricular Flow during Diastole after Implantation of BMHV

This work presents a numerical simulation of intraventricular flow after the implantation of a bileaflet mechanical heart valve at the mitral position. The left ventricle was simplified conceptually as a truncated prolate spheroid and its motion was prescribed based on that of a healthy subject. The rigid leaflet rotation was driven by the transmitral flow and hence the leaflet dynamics were solved using fluid-structure interaction approach. The simulation results showed that the bileaflet mechanical heart valve at the mitral position behaved similarly to that at the aortic position. Sudden area expansion near the aortic root initiated a clockwise anterior vortex, and the continuous injection of flow through the orifice resulted in further growth of the anterior vortex during diastole, which dominated the intraventricular flow. This flow feature is beneficial to preserving the flow momentum and redirecting the blood flow towards the aortic valve. To the best of our knowledge, this is the first attempt to numerically model intraventricular flow with the mechanical heart valve incorporated at the mitral position using a fluid-structure interaction approach. This study facilitates future patient-specific studies.     

Development of aortic and mitral valve continuity in the human embryonic heart

The anatomic relationship of the aortic and mitral valves is a useful landmark in assessing congenital heart malformations. The atrioventricular and semilunar valve regions originate in widely separated parts of the early embryonic heart tube, and the process by which the normal fibrous continuity between the aortic and mitral valves is acquired has not been clearly defined. The development of the aortic and mitral valve relationship was studied in normal human embryos in the Carnegie Embryological Collection, and specimens of Carnegie stages 13, 15, 17, 19, and 23, prepared as serial histologic sections cut in the sagittal plane, were selected for reconstruction. In stage 13, the atrioventricular valve area is separated from the semilunar valve area by the large bend between the atrioventricular and outflow-tract components of the single lumen heart tube created by the left interventricular sulcus. In stages 15 and 17, the aortic valve rotates into a position near the atrioventricular valves with development of four chambers and a double circulation. In stage 19, there is fusion of aortic and mitral endocardial cushion material along the endocardial surface of the interventricular flange, and this relationship is maintained in subsequent stages. Determination of three-dimensional Cartesian coordinates of the midpoints of valve positions shows that, while there is growth of intervalvular distances up to stage 17, the aortic to mitral distance is essentially unchanged thereafter. During the period studied, the left ventricle increases in length over threefold. The relative lack of growth in the saddle-shaped fold between the atrioventricular and outflow tract components of the heart, contrasting with the rapid growth of the outwardly convex components of most of the atrial and ventricular walls, may be attributed to the different mechanical properties of the two configurations. It is postulated that the pathogenesis of congenital heart malformations, which characteristically have failure of development of aortic and mitral valve continuity, may involve abnormalities of rotation of the aortic region or malpositioning of the fold in the heart tube.     

Protein dynamics and distance determination by NOE measurements

Present analysis procedures for NMR structure determination of macromolecules presuppose fixed internuclear distances. Improvement of the precision of the requisite NOE information has stimulated the use of more quantitative distance constraints thus necessitating examination as to whether the assumption of a rigid model systematically biases the distance estimates. Analysis using the simple (r-6) dependence of NOE buildup rates seriously underestimates the correct distance for spatially proximal proton pairs having fluctuations comparable to those observed in X-ray temperature factor analysis. However, by calculating the proper generalized order parameter it is shown that for nuclei undergoing rapid isotropic uncorrelated fluctuations the effective distance is identical to the distance between the mean positions of the nuclei. Similar analysis of molecular dynamics simulation data from bovine pancreatic trypsin inhibitor indicates that the distance obtained from the generalized order parameter predicts the distance between the mean positions to within a few percent regardless of the degree of correlation of the pairwise motion for virtually all main chain and dynamically constrained side chain protons.     

Mitral valve prolapse detected during hemodynamic studies

To estimate frequency of the posterior mitral valve leaflet prolapse in routinely performed left ventriculography, 1000 consecutive ventriculograms of the right anterior oblique projection were analyzed. A group of patients consisted of 511 women and 489 men at mean age 46,5 years. Clinical diagnosis of heart lesions, myocardial disease, pulmonary hypertension or arrhythmias were indications for hemodynamic studies. In the investigated group of patients, there were no patients with clinical diagnosis of the coronary artery disease. Prolapse of the posterior mitral valve leaflet was diagnosed in 59 patients. Idiopathic mitral valve prolapse was diagnosed in 10 patients. Prolapse of the posterior mitral valve leaflet was most frequent in atrial septal defect (16.6%), myocardial lesion (12.5%), and after mitral commissurotomy (8.9%). Posterior mitral valve leaflet prolapse is not a frequent anomaly in routinely performed left ventriculography. Relatively often occurrence of the mitral valve prolapse in atrial septal defect and only occasional in the aortic lesions and dilated cardiomyopathy seems to point out at a role of the left ventricle size in pathogenesis of this syndrome.     

Numerical simulation of cardiovascular dynamics with healthy and diseased heart valves

This paper presents a new concentrated parameter model for cardiovascular dynamics that includes an innovative model of heart valve dynamics, which is embedded in the overall model of the four chambers of the heart and the systemic and pulmonary circulation loops. The heart chambers are described with a variable elastance model, and the systemic and pulmonary loops are described with modified Windkessel models. In modelling the heart valve dynamics, the various factors that influence the valve motion are examined, and the governing differential equation for valve motion is derived. The heart valve model includes the influence of the blood pressure effect, the friction effect from the tissue, and from blood motion. As improvement from previous works, the contribution of the blood vortex effect in the vicinity of the valve leaflets to valve motion is specially considered. The proposed model is then used in simulation of healthy and certain pathological conditions such as mitral valve stenosis and aortic regurgitation. The predicted results agree well with results illustrated in cardiology textbooks.     

Diastolic gradient of pressure and the effective artificial mitral valve section area in terms of its dysfunction

A seven-year experience of treatment of 126 patients with mitral heart disease who were implanted monocuspid MIKS and bicuspid MEDINZh-2 and ROSKARDIKS prostheses (Russia) is presented. The comparative assessment of hemodynamic efficiency and the analysis of the rate of the occurrence of dysfunction of these mechanical prostheses revealed that the MIKS and MEDINZh-2 implants have advantages of hemodynamic characteristics over ROSKARDIKS, despite the priority of the standard size. It was shown that the initially low diastolic pressure gradient on the mitral valve prosthesis and the initially larger area of the prosthetic effective mitral valve aperture are of crucial importance for preventing valve complications and reducing the number of open heart reoperations.     

Langevin dynamics of the choline head group in a membrane environment.

Computer simulations of dipalmitoylphosphatidylcholine (DPPC) have been performed using Langevin dynamics and a Marcelja-type mean field. This work has focused on the dynamics of the choline head group to parameterize the empirical constraints against phosphorus-carbon dipolar couplings (Dp-c) as measured by nuclear magnetic resonance (13C-NMR). The results show good agreement with experimental values at constraints equivalent to the choline tilt observed in joint refinement of x-ray diffraction and neutron diffraction scatterings. Quadrupolar splittings for the alpha and beta positions are also calculated and compared with 2H-NMR experiments. The model predicts torsional transition rates around the alpha-beta bonds and for the two C-O-P-O torsions. It also predicts T1 relaxation times for the alpha and beta carbons.     

Hemodynamics of conscious unrestrained baboons, including cardiac output

A method is described for comprehensive hemodynamic study of undisturbed baboons (Papio hamadryas) that incorporates cardiac output measurement by thermodilution. Instrumentation includes arterial, aortic, and central venous catheterization by a surgical technique that does not require entry to peritoneal or thoracic cavities. It provides a means for right atrial indicator delivery with aortic temperature recording of thermodilution curves. Accuracy was confirmed by comparison to measurement by Swan-Ganz catheters. Diurnal variations of systemic arterial pressure in long-term study of conscious baboons were shown to result from significant increases in cardiac output by day (P less than 0.001), despite concomitant falls in systemic vascular resistance. The cardiac output values obtained were 0.13 l.min-1.kg-1 at night and 0.16 l.min-1.kg-1 by day. Comparison of these results to previous reports of cardiac output in baboons highlights the inadequacies of methods that require physical restraint or anesthesia. This technique also leaves the baboons intact for subsequent breeding or experimental use after catheter removal without the need for further surgery.