Mechanical valve closing dynamics: Relationship between velocity of closing, pressure transients, and cavitation initiation

In this study, the closing dynamics of mechanical heart valves was experimentally analyzed with the valves mounted in the mitral position of anin vitro flow chamber simulating a single closing event. The average linear velocity of the edge of the leaflet during the final 2.065° of the traverse before closing was measured using a laser sweeping technique, and the negative pressure transients at 2 mm from the leaflet inflow surface in the fully closed position was recorded at the instant of valve closure. The cavitation number was computed for the various mechanical valves at a range of load at valve closure. The data were correlated with cavitation bubble visualization previously obtained with the same experimental set up. Cavitation incipience with mechanical valves was found to be independent of the flexibility of the valve holder. For the same loading rate at valve closure, valves with flexible (polyethylene) leaflets were found to close with comparable velocity to those with rigid (pyrolytic carbon) leaflets, but the negative pressure transients did not reach magnitudes close to the vapor pressure for the fluid with flexible leaflets. For the same leaflet closing velocity (and hence the cavitation number), valves with a seat stop or a seating lip in the region of maximum leaflet velocity were observed to cavitate earlier, suggesting that the effect of “squeeze flow” may be an important factor in cavitation incipience. This study was presented at the Biomedical Engineering Society's 1996 Annual Fall Meeting, October 1996.     

Negative Pressure Transients with Mechanical Heart-Valve Closure: Correlation between In Vitro and In Vivo Results

Negative pressure transients (NPT) recorded in a single closing event of mechanical valves in the mitral position in an in vitro setup are compared with data recorded in the left atrium in vivo with the valves implanted in the mitral position in an animal model. The loading at valve closure (dP/dt__CL) computed from the in vivo ventricular pressure recording (ranging from 700 to 2300 mm Hg/s) agreed with the magnitudes predicted in our earlier in vitro experiments (750-3000 mm Hg/s). The NPT signals and the corresponding power spectral density plots from the in vivo data were in qualitative agreement with those recorded in vitro. The NPT magnitudes were found to be below the vapor pressure for blood in mechanical valves with rigid occluders suggesting a potential for the valve to cavitate in vivo. Our in vivo results also suggest that the valves with flexible occluders are less likely to cavitate. The correlation of the in vitro and in vivo data also suggests that the flexibility of valve housing used in the in vitro studies is not an important factor in the dynamics of mechanical valve closure in vivo. © 1998 Biomedical Engineering Society. PAC98: 8745Hw, 8790+y     

In vitro squeeze-flow phenomena at the instant of mechanical valve closure

Cavitation bubble formation associated with mechanical valve closure has been investigated in vitro, and the region of bubble formation has been correlated with large negative pressure transients. The region of cavitation bubbles forms in valve designs where leaflets interact with seat stops. It has been postulated that the fluid is squeezed between the leaflet and the seat stop and radially propelled at high velocities, resulting in further pressure reduction below the vapor pressure of fluid and initiating cavitation bubble formation. We conducted in vitro experiments to visualize and detect the presence of squeeze-flow phenomena associated with valve closure of mechanical heart valves. The closing dynamics were studied by simulating a single closing event of the leaflet with the valve mounted at the mitral position. Squeeze flow was detected at the instant of valve closure, when the valve leaflet interacts with the valve seat stop. The use of a high-speed video camera at 1000 frames per second with strobe light at 16000 pulses per second enabled the visualization of cavitation bubbles and its radial motion from the valve's seat stop due to the squeeze-flow effect. Vapor cavitation bubbles were observed to collapse within 0.5 ms after inception. In mechanical valves without seat stops in the major orifice region, bubbles of duration longer than that of the cavitation bubbles were observed. These microbubbles were present for 4 s before collapsing and are believed to be air bubbles whose presence in vivo has been detected with ultrasound imaging.     

Three-Dimensional Fluid-Structure Interaction Simulation of Bileaflet Mechanical Heart Valve Flow Dynamics

The wall shear stress induced by the leaflet motion during the valve-closing phase has been implicated with thrombus initiation with prosthetic valves. Detailed flow dynamic analysis in the vicinity of the leaflets and the housing during the valve-closure phase is of interest in understanding this relationship. A three-dimensional unsteady flow analysis past bileaflet valve prosthesis in the mitral position is presented incorporating a fluid-structure interaction algorithm for leaflet motion during the valve-closing phase. Arbitrary Lagrangian-Eulerian method is employed for incorporating the leaflet motion. The forces exerted by the fluid on the leaflets are computed and applied to the leaflet equation of motion to predict the leaflet position. Relatively large velocities are computed in the valve clearance region between the valve housing and the leaflet edge with the resulting relatively large wall shear stresses at the leaflet edge during the impact-rebound duration. Negative pressure transients are computed on the surface of the leaflets on the atrial side of the valve, with larger magnitudes at the leaflet edge during the closing and rebound as well. Vortical flow development is observed on the inflow (atrial) side during the valve impact-rebound phase in a location central to the leaflet and away from the clearance region where cavitation bubbles have been visualized in previously reported experimental studies.     

The closing velocity of mechanical heart valve leaflets

The closing motion of the occluder leaflets in bileaflet type mechanical heart valves (MHV) was monitored with a laser sweeping technique. The angular displacements of the leaflets were registered with precision of 0.2 μs steps. Experimental measurements were made using five 29 mm Edwards-Duromedics™ including three original specification (EDOS) and two modified specification (EDMS), and two 29 mm St Jude Medical® MHVs. The testing valve was installed in the mitral position of a physiologic pulsatile mock circulatory flow loop using water-glycerine solution as the testing fluid. Each valve was tested by: (1) direct mounting the valve on metal washers, and (2) mounting the valve with its sewing ring. Experiments were carried out at pulse rates of 70, 90, and 120 beats min⁻¹, with the corresponding cardiac output of 5, 6, and 7.5 litres min⁻¹, and maximum left ventricular pressure gradients ( ) of 1,800, 3,000 and 5,600 mm Hg s⁻¹, respectively. The maximum leaflet closing velocity of each of the tested valve types are presented. The difference in leaflet closing movements between the direct rigid mounting and the sewing ring mounting are discussed. The details of the laser sweeping technique are presented.     

Turbulence characteristics downstream of a new trileaflet mechanical heart valve

Design limitations of current mechanical heart valves cause blood flow to separate at the leaflet edges and annular valve base, forming downstream vortex mixing and high turbulent shear stresses. The closing behavior of a bileaflet valve is associated with reverse flow and may lead to cavitation phenomenon. The new trileaflet (TRI) design opens similar to a physiologic valve with central flow and closes primarily due to the vortices in the aortic sinus. In this study, we measured the St. Jude Medical 27 mm and the TRI 27 mm valves in the aortic position of a pulsatile circulatory mock loop under physiologic conditions with digital particle image velocimetry (DPIV). Our results showed the major principal Reynolds shear stresses were <100 N/m2 for both valves, and turbulent viscous shear stresses were smaller than 15 N/m2. The TRI valve closed more slowly than the St. Jude Medical valve. As the magnitudes of the shear stresses were similar, the closing velocity of the valves should be considered as an important factor and might reduce the risks of thrombosis and thromboembolism.     

Instantaneous back flow through peripheral clearance of medtronic hall tilting disc valve at the moment of closure

An investigation of the flow dynamics through the peripheral clearance (the gap formed between the occluder tip and the metal housing in the closed position) of a tilting disc heart valve at the moment of valve closure is presented. A Medtronic Hall valve in the mitral position of anin vitro experimental set up is employed to measure the transient pressure pulses near the entrance (ventricular side) and exit (atrial side) of the peripheral clearance at valve closure. Flow within the peripheral clearance is analyzed employing a two-dimensional quasisteady computational fluid dynamics model with the measured peak pressures specified as the boundary conditions inducing the flow. The valve is visualized from its inflow (atrial) side using a stroboscopic lighting technique to investigate the presence of cavitation bubbles within the clearance. The pressure measurements showed that a relatively large pressure drop exists between the entrance and the exit to the clearance for about 0.5 msec at the moment of valve closure. The numerical simulation resulted in relatively large magnitudes of wall shear stress and pressure reduction within the clearance due to the flow established by the large pressure drop between the entrance and the exit. Cavitation bubbles visualized within the peripheral clearance at higher loading rates for valve closure correlated with the presence of large pressure reduction within the clearance. Analysis of the results of this study indicates that the back flow through the clearance at the instant of valve closure may contribute toward injury to formed elements in blood in spite of the short duration of the flow.     

应用于零维左心血液循环的二尖瓣模型的研究

提出一个可以准确合理地模拟二尖瓣动力学特性的瓣叶运动流阻模型。考虑影响二尖瓣瓣叶运动的跨瓣压差和血流推力,建立二尖瓣运动的控制方程,提出依赖于瓣叶打开角度θ的瓣叶运动流阻模型,把该模型应用于零维左心血液循环系统,得到血液动力学特性。在保持心输出量和反流分数一致的条件下,比较该模型、瞬态关闭的阶梯流阻模型和经验指定的时变流阻模型。结果发现,瓣叶运动流阻模型能反映瓣膜关闭过程中的血液动力学,如压差和流量的滞后性以及关闭流量,同时该模型可以通过调整单位转动惯量跨瓣压差影响系数K_p和血流影响系数K_b的大小,改变瓣膜打开过程和关闭过程所需时间,瓣膜打开和关闭时间分别为50.0和40.2 ms。该模型可弥补阶梯流阻模型中忽略瓣膜运动过程的瞬态关闭的缺点,同时也能避免时变流阻模型中关闭起始时间的不合理性。此模型较为合理准确地模拟二尖瓣关闭过程的动力学特性,且简单易控制。     

Impulsive-Motion Model for Computing the Closing Motion of Mechanical Heart-Valve Leaflets

The speed of mechanical heart-valve leaflets is known to be an important quantity for predicting cavitation, yet no simple computational means exists for predicting the leaflet speed. In this study, a model for simulating the motion of heart-valve leaflets in rigid test systems is presented. The input for the simulations is the ventricular pressure trace, readily measured in heart-valve tests. The model is based upon an impulsive-motion approximation, wherein the motion within the system is produced by rapid acceleration at the boundary, e.g., by a moving piston. A set of quasisteady, linear equations for the pressure field that are decoupled from the leaflet equation of motion is derived. The pressure field and leaflet moment are computed without the need to treat moving boundaries. Model predictions of closing time compared favorably with those measured in a 1994 cavitation study. Computed values of leaflet tip speed were also compared with those of a previous study, at the same value of average pressure slope. The model values were in agreement with measured speeds, given the limitations of using the average pressure slope as a metric for comparison. © 2003 Biomedical Engineering Society. PAC2003: 8719Hh, 8719St, 8768+z, 8710+e     

Improved In Vitro Quantification of the Force Exerted by the Papillary Muscle on the Left Ventricular Wall: Three-Dimensional Force Vector Measurement System

Recent developments indicate that the forces acting on the papillary muscles can be a measure of the severity of mitral valve regurgitation. Pathological conditions, such as ischemic heart disease, cause changes in the geometry of the left ventricle and the mitral valve annulus, often resulting in displacement of the papillary muscles relative to the annulus. This can lead to increased tension in the chordae tendineae. This increased tension is transferred to the leaflets, and can disturb the coaptation pattern of the mitral valve. The force balance on the individual components governs the function of the mitral valve. The ability to measure changes in the force distribution from normal to pathological conditions may give insight into the mechanisms of mitral valve insufficiency. A unique in vitro model has been developed that allows quantification of the papillary muscle spatial position and quantification of the three-dimensional force vector applied to the left ventricular wall by the papillary muscles. This system allows for the quantification of the global force exerted on the posterior left ventricular wall from the papillary muscles during simulation of normal and diseased conditions. © 2001 Biomedical Engineering Society. PAC01: 8719Rr, 8719Ff, 8719Hh, 8719Xx, 8710+e     

Estimation of the Shear Stress on the Surface of an Aortic Valve Leaflet

The limited durability of xenograft heart valves and the limited supply of allografts have sparked interest in tissue engineered replacement valves. A bioreactor for tissue engineered valves must operate at conditions that optimize the biosynthetic abilities of seeded cells while promoting their adherence to the leaflet matrix. An important parameter is shear stress, which is known to influence cellular behavior and may thus be crucial in bioreactor optimization. Therefore, an accurate estimate of the shear stress on the leaflet surface would not only improve our understanding of the mechanical environment of aortic valve leaflets, but it would also aid in bioreactor design. To estimate the shear stress on the leaflet surface, two-component laser-Doppler velocimetry measurements have been conducted inside a transparent polyurethane valve with a trileaflet structure similar to the native aortic valve. Steady flow rates of 7.5, 15.0, and 22.5 L/min were examined to cover the complete range possible during the cardiac cycle. The laminar shear stresses were calculated by linear regression of four axial velocity measurements near the surface of the leaflet. The maximum shear stress recorded was 79 dyne/cm², in agreement with boundary layer theory and previous experimental and computational studies. This study has provided a range of shear stresses to be explored in bioreactor design and has defined a maximum shear stress at which cells must remain adherent upon a tissue engineered construct. © 1999 Biomedical Engineering Society. PAC99: 8719Rr, 8768+z, 8719Hh, 4262Be, 4727Nz, 0630Gv     

Hydrodynamic characteristics of the Edwards MIRA bileaflet valve in a pneumatic ventricular assist device

In the present study, we used a bileaflet valve in our pneumatic ventricular assist device (PVAD). To estimate the effects of the orientation angle of a bileaflet valve on mechanical heart valve cavitation in the PVAD, the valve was rotated from 0 degrees to 90 degrees on an inclined horizontal plane. Tests were conducted under physiological pressure with heart rates of 80 bpm and a systolic ratio of 43%. A 23-mm Edwards MIRA bileaflet valve was installed in the inlet position of the PVAD, and the valve-closing velocity was measured with a closed circuit digital laser displacement sensor. Images of mechanical heart valve cavitation bubbles were recorded with the use of a high-speed video camera. The closing delay time between the two leaflets ranged from 0.88 +/- 0.41 to 0.50 +/- 0.27 ms, which was the largest at a valve orientation angle of 0 degrees . Cavitation bubbles were concentrated along the leaflet tip and were caused by the initial valve closure, valve rebound, and the second valve closure. Even when the valve-closing velocity was slow, stronger cavitation bubbles were observed at the second valve closure and valve rebound. The cavitation event ratio differed from the valve orientation angle, which resulted from the high initial valve closure.     

Surface strains in the anterior leaflet of the functioning mitral valve

The mitral valve (MV) is a complex anatomical structure whose function involves a delicate force balance and synchronized function of each of its components. Elucidation of the role of each component and their interactions is critical to improving our understanding of MV function, and to form the basis for rational surgical repair. In the present study, we present the first known detailed study of the surface strains in the anterior leaflet in the functioning MV. The three-dimensional spatial positions of markers placed in the central region of the MV anterior leaflet in a left ventricle-simulating flow loop over the cardiac cycle were determined. The resulting two-dimensional in-surface strain tensor was computed from the marker positions using a C ⁰ Lagrangian quadratic finite element. Results demonstrated that during valve closure the anterior leaflet experienced large, anisotropic strains with peak stretch rates of 500%–1000%/s. This rapid stretching was followed by a plateau phase characterized by relatively constant strain state. We hypothesized that the presence of this plateau phase was a result of full straightening of the leaflet collagen fibers upon valve closure. This hypothesis suggests that the MV collagen fibers are designed to allow leaflet coaptation followed by a dramatic increase in stiffness to prevent further leaflet deformation, which would lead to valvular regurgitation. These studies represent a first step in improving our understanding of normal MV function and to help establish the principles for repair and replacement. © 2002 Biomedical Engineering Society. PAC2002: 8719Hh, 8719Uv, 8719Rr     

Bioprosthetic Heart Valve Leaflet Deformation Monitored by Double-Pulse Stereo Photogrammetry

A double-pulse stereo photogrammetry technique has been developed for the dynamic assessment of the leaflet deformation of bioprosthetic heart valves under simulated physiological conditions. By using a specially designed triggering technique, which takes the advantage of the field transfer mechanisms of the charge coupled device camera, two consecutive images separated by a time interval as short as 5 ms were captured. This made it possible to investigate the realistic leaflet deformation during the valve opening and closing processes which typically last 25–45 ms. This technique was applied to assess a newly developed pericardial valve leaflet in a physiological pulse flow loop. Quantitative leaflet deformations of the valve opening and closing were generated from sequences of digital images. The results can later be applied to finite element analysis of bioprosthetic heart valve leaflet stress and strain during a complete cardiac cycle. © 2002 Biomedical Engineering Society. PAC02: 8719Hh, 8768+z, 8719Rr, 8719Uv     

Implantation technique and early echocardiographic performance of newly designed stentless mitral bioprosthesis

This article describes the implantation techniques of two new stentless mitral bioprosthesis and their early echocardiographic performance in 12 acute sheep model. The first stentless mitral bioprosthesis (stentless bileaflet valve [SBV]) was designed as a bileaflet valve with sewing ring to suture down to the native mitral annulus. The other one (SBV with chordae) has two chordae-like structures to be attached to the head of the native papillary muscles. Valvar performance and cardiac function were evaluated by epicardial echocardiography at postimplant (Rest) and during dobutamine (DOB) stimulation. Postimplant echocardiography revealed normal leaflet opening with a large orifice area and unrestricted leaflets motion. In both valves, leaflet closure showed no systolic anterior motion, prolapse, or tethering. Mitral regurgitation grade 2 or higher was not detected in any of the experiments. Transvalvar pressure gradients at Rest and DOB were 2.3 ± 1.6 mm Hg and 2.5 ± 2.2 mm Hg in SBV and 1.8 ± 1.1 mm Hg and 2.3 ± 1.2 mm Hg in SBV with chordae, respectively. Both stentless bioprosthesis showed reliable valve performance and preserved cardiac function in the acute phase. Further chronic study is needed to evaluate the reliability of implantation procedures, valvar performance, and biocompatibility.     

Examination of cavitation-induced surface erosion pitting of a mechanical heart valve using closing velocities

 Recently, cavitation on the surface of mechanical heart valves has been studied as a cause of fractures that occur in implanted mechanical heart valves. Several factors, including peak dp/dt of the ventricular pressure, maximum closing velocity of the leaflet, and squeeze flow, have been studied as indices of the cavitation threshold. In the present study, cavitation erosion on the surface of a mechanical valve was examined by focusing on squeeze flow and the water-hammer phenomenon during the closing period of the valve. A simple solenoid-actuated test device that can directly control the valve closing velocity was developed, and opening–closing tests of 3000 and 40 000 cycles were performed at various closing velocities. The results showed that there was a closing velocity threshold above which erosion pitting was induced and that the threshold was about 0.4 m/s in the valves used in this study. Cavitation-induced erosion pits were observed only in regions where squeeze flow occurred immediately before valve closure. On the other hand, the number of the pits was found to be closely related to the area of the water hammer-induced pressure were below the critical pressure defined by water vapor pressure. Therefore, it was concluded that cavitation is initiated and augmented by the two pressure drops due to squeeze flow and the water-hammer phenomenon, respectively. Received: January 15, 2002 / Accepted: May 30, 2002 Correspondence to:H. Lee     

Characterizing the Collagen Fiber Orientation in Pericardial Leaflets Under Mechanical Loading Conditions

When implanted inside the body, bioprosthetic heart valve leaflets experience a variety of cyclic mechanical stresses such as shear stress due to blood flow when the valve is open, flexural stress due to cyclic opening and closure of the valve, and tensile stress when the valve is closed. These types of stress lead to a variety of failure modes. In either a natural valve leaflet or a processed pericardial tissue leaflet, collagen fibers reinforce the tissue and provide structural integrity such that the very thin leaflet can stand enormous loads related to cyclic pressure changes. The mechanical response of the leaflet tissue greatly depends on collagen fiber concentration, characteristics, and orientation. Thus, understating the microstructure of pericardial tissue and its response to dynamic loading is crucial for the development of more durable heart valve, and computational models to predict heart valves' behavior. In this work, we have characterized the 3D collagen fiber arrangement of bovine pericardial tissue leaflets in response to a variety of different loading conditions under Second-Harmonic Generation Microscopy. This real-time visualization method assists in better understanding of the effect of cyclic load on collagen fiber orientation in time and space.     

A synthetic three-leaflet valve.

To increase durability and decrease calcification tendencies, the reduction of mechanical leaflet stress is of prime importance. In order to achieve this for a three-leaflet valve, the leaflets of a new design of prosthesis (the J-3) are manufactured in a medium open, almost flat shaped position, whereby the stent posts are expanded by a cone-shaped mold. Owing to this design, the leaflets have stable closed and open positions, the transition succeeds with low opening pressure. Valves are manufactured by dip-coating in polyurethane. Hydrodynamic evaluation of this polyurethane valve shows minimum pressure drop and very low energy losses compared with other commercially available valves. Very low shear stresses in the flow field downstream of the valve are observed by laser-Doppler-anemometry. In durability tests, prototypes have reached lifetimes equivalent to 17 years. For comparative testing of durability and biocompatibility, six bioprostheses and seven J-3 valves were implanted in mitral position of growing Jersey calves. While animal tests are encouraging, they also reveal necessary manufacturing improvements.     

Cavitation potential of mechanical heart valve prostheses

Just like technical check valves, the function of mechanical heart valve prostheses may presumably also lead to cavitation effects during valve closure. Due to the waterhammer effect, cavitation may primarily occur in the mitral position leading to high mechanical loading of the valve itself and of corpuscular blood elements. Ten different types of commercial mechanical heart valves were investigated in the mitral position of a pulsatile mock loop, to detect cavitation thresholds under physiologically similar conditions by cinematographic techniques. Almost all these valve prostheses show cavitation up to a ventricular pressure gradient of 5000 mmHg/s. The threshold depends on valve type and size and is sometimes within the physiological range below 2000 mmHg/s. Visible cavitation bubbles with a diameter of up to 1.8 mm and a collapse time of less than 0.1 ms suggest that vapour cavitation may play an important role for material and blood damage in mechanical heart valve prostheses.     

Effect of Hypertension on the Closing Dynamics and Lagrangian Blood Damage Index Measure of the B-Datum Regurgitant Jet in a Bileaflet Mechanical Heart Valve

We hypothesize that the formation of the closing vortex and subsequent b-datum regurgitation jet in bileaflet mechanical heart valves is governed by the magnitude of the driving mean aortic pressure (MAP), and that this sensitivity does impact the blood damage index (BDI) corresponding to platelet activation and lysis. High spatial resolution time resolved (1 kHz) as well as phase locked particle image velocimetry techniques captured the dynamic leaflet closure and regurgitation jet of a model 25 mm St. Jude Medical BMHV. Cell trajectories were estimated using Lagrangian particle tracking analysis while the leaflet kinematics was quantified by tracking the leaflet tip-position throughout closure. The non-principal as well as principal shear stress loading histories along each cell trajectory revealed BDI for platelet activation and lysis as a function of cell initial position, release time-point, and blood pressure. Results show that the leaflet closing time reduces by roughly 10 ms, in response to an increase in MAP by 40 mmHg. We report that higher MAP leads to a stronger b-datum vortex and jet formation. Platelet activation BDI lowers with a higher MAP due to reduction in exposure times despite an increase in principal shear stress experienced. Platelet lysis BDI however increases with higher MAP. Maximum BDI may occur for cells initially in the b-datum zone during the onset of leaflet closure. Our results provide a better understanding of BDI of the regurgitant b-datum jet and sheds light on the potential importance of blood damage testing under hypertensive conditions.