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Biomedical subjects

N H Hwang

Publications and source records attributed to N H Hwang.

At least 19 recordsLinked to original sources
PubMed2001

Digital particle image velocimetry investigation of the pulsating flow around a simplified 2-D model of a bileaflet heart valve.

BACKGROUND AND AIM OF STUDY: Strong interactions are believed to exist between the pulsating valvular flow and the valve leaflet motions. Hinge position, indicated by d/W (d = distance between the two axes of the hinge pivots; W = width of the testing section in the middle plane), plays a critical role in MHV performance. An optimized hinge position for a bileaflet heart valve can be identified as a design criterion for better valve performance. METHODS: A two-dimensional (2-D) digital particle image velocimetry (DPIV) system was used to map the transient flow field of a simplified 2-D model of a bileaflet heart valve with a hydraulic diameter enlarged three-fold under pressure waveforms which was expanded based on Womersley number and Euler number considerations. Six different hinge positions were investigated. RESULTS: At extreme hinge positions (d/W <0.2 or d/W >0.3), large-scale and long-duration stagnation of flow was found in the central orifice, and instability and highly disturbed flow was noted in plots of velocity vectors. CONCLUSION: The transient flow pattern in the vicinity of the valve was greatly affected by the hinge position of moving leaflets. An optimum d/W in the range 0.2-0.3 yielded good velocity field and opening and closing behaviors.

Blood Flow Velocity↗
PubMed2001

Computational fluid dynamics study of a protruded-hinge bileaflet mechanical heart valve.

BACKGROUND AND AIM OF THE STUDY: Following clinical experience with the Medtronic Parallel bileaflet mechanical heart valve, considerable interest has been shown in investigating fluid mechanics inside the hinge socket. Most of these studies involved hinges that are recessed into the valve housing, such as the St. Jude Medical (SJM), CarboMedics, Sorin and On-X bileaflet mechanical heart valves. The aim of this study was to investigate the flow fields of a protruded hinge under steady flow conditions, with the occluder in its fully open position. Computational fluid dynamics (CFD) simulation using the Fluent 4.4.7 commercial solver was applied in this investigation. This protruded hinge mechanism for pivoting the occluder is an in-house design from the Cardiovascular Dynamics Laboratory, Nanyang Technological University. METHODS: The Fluent 4.4.7 code was run on a Silicon Graphic Inc. computer (4-CPUx185 MHz) in the CFD simulation. A body-fitted coordinates (BFC) grid was generated to cover the entire valvular flow domain, including the interior of the hinge and leaflet. Clearance between the leaflet and pivot housing was 50-70 microm. In the vicinity of the protruded hinge, mesh cells were small compared with hinge dimensions. A power law distribution of grid points was applied to optimize the number of cells used to cluster the entire flow field. The overall computational flow domain of the valve channel, including the floating leaflet and immersed hinge, was approximately 170,000 cells in total. Inside the hinge socket, approximately 10,000 cells were generated. A comparative model with recessed hinge that resembled the SJM valve hinge design was modeled. Due to geometric difficulties, an unstructured grid scheme was applied. Great attention was focused within the hinge pocket, in particular to the clearance between the hinge pivot and leaflet. A total of 2 million cells was generated for the whole computational flow domain. RESULTS: Under steady flow conditions, with the leaflet fixed in an open position, the protruded hinge design yielded a pair of small vortices that formed behind the stoppers. A low-magnitude velocity was observed inside the hinge clearance. Vortices developed behind the protruded stopper. Migrating flow was noted beneath the leaflet clearance as a result of pressure difference across the leaflet. For the recessed hinge design, reverse flow dominated the inside of the hinge socket, and developed into a pair of vortices at high Reynolds number. CONCLUSION: The protruded hinge mechanism was designed to expose the overall hinge region to the mainstream flow for a positive washing effect. Flow in this protruded hinge design is, in general, found to be three-dimensional. Initial results under steady flow conditions showed low laminar and turbulent shear stress, while the hinge clearance was well washed.

Computer Simulation↗
PubMed2001

Mechanisms of mechanical heart valve cavitation: investigation using a tilting disk valve model.

BACKGROUND AND AIM OF THE STUDY: The induction of mechanical heart valve (MHV) cavitation was investigated using a 27 mm Medtronic Hall (MH27) tilting disk valve. METHODS: The MH27 valve was mounted in the mitral position of a simulating pulse flow system, and stroboscopic lighting used to visualize cavitation bubbles on the occluder inflow surface at the instant of valve closure. MHV cavitation was monitored using a digital camera with 0.04 mm/pixel resolution sufficient to render the tiny bubbles clearly visible on the computer monitor screen. RESULTS: Cavitation on MH27 valve was classified as five types according to the time, site and shape of the cavitation bubbles. Valve cavitation occurred at the instant of occluder impact with the valve seat at closing. The impact motion was subdivided into three temporal phases: (i) squeezing flow; (ii) elastic collision; and (iii) leaflet rebound. MHV cavitation caused by vortices was found to be initiated by the squeezing jet and/or by the transvalvular leakage jets. By using a tension wave which swept across the occluder surface immediately upon elastic impact, nuclei in the vortex core were expanded to form cavitation bubbles. CONCLUSION: Analysis of the shape and location of the cavitation bubbles permitted a better understanding of MHV cavitation mechanisms, based on the fluid dynamics of jet vortex and tension wave propagations.

Biomechanical Phenomena↗
PubMed2000

Bioprosthetic heart valve leaflet motion monitored by dual camera stereo photogrammetry.

Dual camera stereo photogrammetry (DCSP) was applied to investigate the leaflet motion of bioprosthetic heart valves (BHVs) in a physiologic pulse flow loop (PFL). A 25-mm bovine pericardial valve was installed in the aortic valve position of the PFL, which was operated at a pulse rate of 70 beats/min and a cardiac output of 5 l/min. The systolic/diastolic aortic pressure was maintained at 120/80 mmHg to mimic the physiologic load experienced by the aortic valve. The leaflet of the test valve was marked with 80 India ink dots to form a fan-shaped matrix. From the acquired image sequences, 3-D coordinates of the marker matrix were derived and hence the surface contour, local mean and Gaussian curvatures at each opening and closing phase during one cardiac cycle were reconstructed. It is generally believed that the long-term failure rate of BHV is related to the uneven distribution of mechanical stresses occurring in the leaflet material during opening and closing. Unfortunately, a quantitative analysis of the leaflet motion under physiological conditions has not been reported. The newly developed technique permits frame-by-frame mapping of the leaflet surface, which is essential for dynamic analysis of stress-strain behavior in BHV.

Algorithms↗
PubMed1999

Pressure and flow fields in the hinge region of bileaflet mechanical heart valves.

BACKGROUND AND AIM OF THE STUDY: Recent clinical thrombotic experiences with the Medtronic Parallel (MP) bileaflet heart valve have highlighted the need for new methods to assess preclinical valve hinge flow. The aim of the current study was to investigate hinge pivot flow fields in bileaflet mechanical heart valves using flow visualization in scaled x5 magnification transparent polymer models and computational fluid dynamic (CFD) analysis using CFD 2000 STORM code. METHODS: Polymeric x5 flow models of the On-X, St. Jude Medical (SJM) and MP bileaflet heart valves were constructed using laser stereolithography to replicate the interior geometry while maintaining realistic manufacturing tolerances. Each hinge flow experiment was carried out by installing the transparent x5 model in a pulsatile flow loop, which was designed according to Womersley number similitude requirements. Motions of suspended microparticles in the valve hinge area, recorded by laser imaging techniques, were used to visualize hinge flow. Experimentally measured parameters were used as input for CFD analysis. CFD simulations were made by solving the Navier-Stokes equation using a finite volume method with the pressure-based algorithm for continuity, and a pressure-implicit with splitting of operators (PISO) algorithm for pressure-velocity coupling. Moving grid methodology was employed to simulate periodic motion of the valve leaflets. CFD hinge flow results were visualized on four parallel planes at different depths in the hinge socket. The hinge flow patterns of the three types of bileaflet heart valve design are discussed. RESULTS: Prominent vortex formation and stagnant flow areas were noticed in the pivot region of the MP valve. Vortices persisted throughout both the forward- and reverse-flow phases. These flow structures were not observed in the hinge areas of the SJM and On-X valves. CONCLUSIONS: Vortex formation observed in the MP valve may contribute to the high thrombogenic potential of this valve. The absence of such vortices and areas of stagnant flow in the On-X and SJM valves indicate that hinge flow conditions in these valves do not favor mechanically induced thrombogenesis or thromboembolic events.

Biocompatible Materials↗
PubMed1998

In vitro evaluation of the long-body On-X bileaflet heart valve.

BACKGROUND AND AIMS OF THE STUDY: In order to optimize the length-to-diameter ratio, a series of circular aluminum rings with flared inlet and varying ring lengths, with internal diameters corresponding to that of 19 mm replacement prosthetic heart valve orifices, were tested in a steady-flow hydraulic system. The study aim was to determine the ring length-to-internal diameter ratio that produces the best hydraulic efficiency (i.e. lowest pressure gradient) within the physiologic flow rate range. METHODS: Each ring was tested at flow rates of 10, 15, 20, 25 and 30 l/min and length-to-diameter ratio effect on hydraulic efficiency determined experimentally. The hydraulic effect was most significant for a ratio of about 0.6, with an increase to 1.2 providing little additional benefit. Thus, a ratio of about 0.6 was considered optimum in terms of hydraulic efficiency and incorporated into the design of the On-X bileaflet mechanical heart valve (BHV) series. An in vitro hydrodynamic study of the smallest (19 mm) and largest (25 mm) clinical On-X aortic valves was performed at two independent laboratories. Standard St. Jude Medical BHVs were used as the study controls. RESULTS: Steady-flow experiments showed that the pressure gradient in the On-X valve was about 50% less than that of the comparable size control. The pulsatile flow study demonstrated a similar pressure gradient advantage. Laser Doppler anemometer velocity profiles taken downstream of the On-X valve at the aortic root showed typical characteristics of bileaflet valves, with three velocity peaks. The peak velocity reached 1.6 m/s for the On-X and 1.75 m/s for the control valve. A recirculating vortex was seen in the sinus cavity during the ejection period. This vortex, found in most aortic valves (including bioprostheses), is believed to provide a beneficial wash-out of the valve region and assist in valve closure. CONCLUSIONS: These two independent studies clearly demonstrated that the elongated valve body and comparably larger flow area helped to improve the valve hydrodynamic performance, which is especially beneficial in the smallest (19 mm) size valve.

Aortic Valve↗
PubMed1996

Numerical study of squeeze-flow in tilting disc mechanical heart valves.

BACKGROUND AND AIM OF THE STUDY: The squeeze-flow that develops during valve closure is believed to cause cavitation in mitral mechanical heart valves (MHVs). METHODS: Squeeze-flow was studied in tilting disc MHVs using two different numerical approaches. In the first decoupled analysis, experimental measurements of valve closing velocities were input into a computer model which simulated the resulting flow field. The second coupled approach involved simulation of the occluder motion and housing deformation in response to the surrounding flow. RESULTS: Both models predicted the likelihood of cavitation during the squeeze-flow phase of valve closure. They also indicated that valve mounting compliance influences the squeeze-flow field. The coupled analysis also showed that squeeze-flow is influenced by fluid viscosity and the geometry of the contact region. CONCLUSIONS: These parameters could therefore influence the inception of cavitation in tilting disc mitral MHVs.

Biomechanical Phenomena↗
PubMed1995

Transient pressure at closing of a monoleaflet mechanical heart valve prosthesis: mounting compliance effect.

An in vitro experimental study was performed to investigate the mounting compliance effect on the occluder closing dynamics and the transient pressure at the closing of the mitral Medtronic Hall (MH) mechanical heart valve (MHV). The closing velocity and the transient pressure were simultaneously measured at heart rates of 70, 90, 120, and 140 beats/minute with cardiac outputs of 5.0, 6.0, 7.5, and 8.5 liters/minute, respectively. The experiment was conducted under simulated physiologic ventricular and aortic pressures in a pulsatile mock flow loop. The characteristics of the transient pressure were investigated by detailed mapping of the transient pressure field in the atrial chamber using high frequency pressure transducers. Simultaneous measurements of the occluder closing velocity and the transient pressure around the seat stop of the MH showed that the transient pressure generated on the inflow side dropped below the vapor pressure of liquid during the occluder's sudden deceleration at closing. The amplitude of the transient pressure reduction (TR) was proportional to the occluder approaching velocity. The development of the transient pressure in the rigid and flexible mountings were significantly different. In the rigid mounting (RM), the pressure was reduced below the liquid's vapor pressure and maintained below -350 mmHg for approximately 180 microseconds. Strong signals of high frequency pressure oscillations (HPO) were recorded in the transient pressure traces. The timing of the HPO was found to be consistent with that of the cavitation bubble collapse as observed by others. In the flexible mounting (FM), TR also occurred, but recovered quickly and was followed immediately by a positive pressure spike. Relatively weak HPO appeared in the transient pressure trace. The mapping of the transient pressure field showed that both the transient pressure reduction (on the major orifice side) or rise (on the minor orifice side) as well as the HPO were locally generated near the valve occluder surface. The transient pressure attenuated with distance away from the occluder surface. The HPO were detectable as far as 40 mm away from the occluder surface. The rigid mounting pressure signals showed characteristically two occurrences of high frequency pressure oscillations. The HPO with smaller amplitude occurred first after the initiation of the TR, followed by a burst of strong HPO at about 450 microseconds. It is believed that they were the result of the collapse of cavitation bubbles. The strong HPO did not appear in the flexible mounting signals. The study indicated that the mounting compliance played a significant role in the MHV cavitation inception and the subsequent bubble growth. It also suggested the possibility of detecting the cavitation by using a high frequency pressure transducer positioned in the atrial chamber.

Biomechanical Phenomena↗
PubMed1995

Asynchronous closure and leaflet impact velocity of bileaflet mechanical heart valves.

The effect of gravitational field on the asynchronous closure of four different types of bileaflet heart valves (BHV) were investigated in an in vitro mock circulation system. The experimental study involved the 29 mm St. Jude Medical Standard, 29 mm CarboMedics, 29mm Edwards-Duromedics and 29 mm Edwards-Tekna BHVs. The valves were tested in the mitral position on an inclined 45 degrees plane of the pulsatile mock flow loop. The test valves were orientated with their pivotal axis horizontal so that the gravitational vectors on the two leaflets were clearly disparate. Using a specially designed laser optical system, the time difference at which the closing leaflets made their first contacts with the valve housing was measured. The closing velocity of each leaflet was measured separately by the laser sweeping technique (LST). The experiments were conducted under physiologic ventricular and aortic pressures at the heart rates of 70, 90, and 120 beats/minute with the corresponding flow rates of 5.0, 6.0, and 7.5 liters/minute. Asynchronous closing motions were registered in all four tested BHVs. The leaflet closing motions were random in nature, but indicated clear dependency on the orientations with respect to the gravitational field. The lower leaflet which makes a generally level swing to close, was assisted initially by the gravity and was always found to close earlier than the upper leaflet, which swing upward to close. The initial closure of the upper leaflet was against the gravity which delayed the leaflet motion. Depending on the BHV designs, the time delays between the two leaflets were found to vary from beat to beat but to follow certain probability distributions. In general, the leaflet/housing contact time between the two valve leaflets exhibited the clear trend of decreasing delay time at closure with the increase of the heart rate. For each of the valves tested, the average impact velocity of the first closing leaflet was found always smaller than that of the second closing leaflet at all three heart rates tested. The impact velocities of both the BHV leaflets were found to increase with the heart rate. The difference in the closing velocities between the two leaflets decreases with the increase of the heart rate and is generally proportional to the impact time delay between the two leaflets.

Analog-Digital Conversion↗
PubMed1994

A squeeze flow phenomenon at the closing of a bileaflet mechanical heart valve prosthesis.

In vivo cavitation in cardiovascular flow fields may occur under very unusual circumstances as a localized transient phenomenon which are confined to very small regions in the vicinity of the valve body or leaflet surface. The violent collapse of cavitation bubbles induces local erosion that may lead to structural damage. The fluid mechanical factors that may cause in vivo cavitation inception in mechanical heart valve (MHV) prostheses are investigated. It is established that the closing velocity of the leaflet holds the key to MHV cavitation. During the final phase of valve closing, the fluid mass in the gap space between the closing occluder and the valve's body is squeezed into motion by the rapidly approaching boundaries. The flow pattern created by this motion (termed 'squeeze flow'), is found to be related to the valve geometry, and the impact velocity of the closing leaflet. Given the closing velocity of the leaflet and the geometry of the MHV, computational flow dynamics (CFD) are made to determine the velocity distributions in the gap flow field of a bileaflet MHV in the mitral position. A two dimensional, time dependent model of the gap space show that flow velocity in the gap space can reach values as high as 30 ms-1 in regions near the edge of the inflow surface of the Edwards Duromedics (ED) MHV leaflet. This high speed stream ejected from the gap channel can create the conditions that characterize cavitation. The location of the isolated high speed region corresponds to the surface erosion that was observed in a number of damaged ED-MHV explants.

Acceleration↗
PubMed1994

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 microsecond 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 (dp/dt) 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.

Biomechanical Phenomena↗
PubMed1994

Occluder closing behavior: a key factor in mechanical heart valve cavitation.

A laser sweeping technique developed in this laboratory was found to be capable of monitoring the leaflet closing motion with microsecond precision. The leaflet closing velocity was measured inside the last three degrees before impact. Mechanical heart valve (MHV) leaflets were observed to close with a three-phase motion; the approaching phase, the decelerating phase, and the rebound phase, all of which take place within one to two milliseconds. The leaflet closing behavior depends mainly on the leaflet design and the hinge mechanism. Bileaflet and monoleaflet types of mechanical heart valves were tested in the mitral position in a physiologic mock circulatory flow loop, which incorporated a computer-controlled magnetic drive and an adjustable afterload system. The test loop was tuned to produce physiologic ventricular and aortic pressure wave forms at 70-120 beats/min, with the maximum ventricular dp/dt varying between 1500-5600 mmHg/sec. The experiments were conducted by controlling the cardiac output at a constant level between 2.0-9.0 liters/min. The measured time-displacement curve of each tested MHV leaflet and its geometry were taken as the input for computation of the squeeze flow field in the narrow gap space between the approaching leaflet and the valve housing. The results indicated rapid build-up of both the pressure and velocity in the gap field within microsecs before the impact. The pressure build-up in the gap space is apparently responsible for the leaflet deceleration before the impact. When the concurrent water hammer pressure reduction at closure was combined with the high energy squeeze jet ejected from the gap space, there were strong indications of the environment which favors micro cavitation inceptions in certain types of MHV.

Feedback↗
PubMed1994

An experimental-computational analysis of MHV cavitation: effects of leaflet squeezing and rebound.

A combined experimental-computational study was performed to investigate the flow mechanics which could cause cavitation during the squeezing and rebounding phases of valve closure in the 29 mm mitral bileaflet Edwards-Duromedics (ED) mechanical heart valve (MHV). Leaflet closing motion was measured in vitro, and input into a computational fluid mechanics software package, CFD-ACE, to compute flow velocities and pressures in the small gap space between the occluder tip and valve housing. The possibility of cavitation inception was predicted when fluid pressures dropped below the saturated vapor pressure for blood plasma. The computational analysis indicated that cavitation is more likely to be induced during valve rebound rather than the squeezing phase of valve closure in the 29 mm ED-MHV. Also, there is a higher probability of cavitation at lower values of the gap width at the point of impact between the leaflet tip and housing. These predictions of cavitation inception are not likely to be significantly influenced by the water-hammer pressure gradient that develops during valve closure.

Blood Flow Velocity↗
PubMed1991

Haemodynamics of angioaccess venous anastomoses.

A large percentage of arteriovenous haemodialysis angioaccess loop grafts (AVLG) fail within the first year after surgery, the occlusive lesions being found predominantly at the venous anastomosis site. This paper presents a detailed flow dynamic study of the AVLG system using three elastic, transparent bench-top flow models, which were based on the geometry of silicone rubber casts obtained at different times from a chronic animal model. Each model thus represented a different stage of the lesion development. Flow visualization and laser Doppler anemometer surveys of the flow field confirmed that the hydrodynamic factors favour lesion development near the stagnation point opposite the anastomotic toe, where the momentum of the impinging jet stream, combined with the oscillating wall shear stress generated in the vicinity of the stagnation point, acts in both directions. The accumulation of tracer particles in the region of flow separation is believed to be a combined contribution from the hydraulic forces and the inward motion of the vessel wall. As these hydrodynamic factors are enhanced upon further development of the occlusive lesion, a vicious cycle may be formed.

Animals↗
PubMed1990

Laser assessment of leaflet closing motion in prosthetic heart valves.

The dynamics of leaflet motion in heart valve prostheses (HVP), and in particular the closing velocity, is believed to be related to the valve sound and possibly to the phenomenon of valve cavitation. This paper describes a non-intrusive laser sweeping technique enabling the study of leaflet motion. The principle of measurement and the equipment involved are presented, together with the results of two commercially available, 29 mm bileaflet mitral valves, a St. Jude Medical, and an Edwards Duromedic valve. Experiments were carried out in a pulsatile mock flow testing loop designed to mimic physiological pressure waveforms and ventricular contraction. Measurements of heart rate were made in the range 70-120 beats min-1, with a ventricular pressure slope range of 1800-5600 mm Hgs-1 and a cardiac output range of 5.0-7.5 litres min-1. Motion analysis of the measured data focuses on the velocity of the leaflet immediately before closure.

Equipment Design↗
PubMed1990

Estimation of the rotational undamped natural frequency of bileaflet cardiac valve prostheses.

The angular momentum balance is solved numerically for a size 29 mm CarboMedics prosthetic heart valve. The lift force is estimated from potential flow theory, while the drag force is estimated from the lift force and a blunt body empiricism. Buoyancy and gravitational effects are calculated based on the assumption of homogeneous leaflets. Other assumptions include uniform flow, negligible friction at the pivot axis, negligible viscous damping and fluid inertance, and a symmetry flow condition. Oscillations are predicted in the opening dynamics in the range of 2-25 Hz, for flow rates through one-half of the orifice in the range of 0.1-10.0 l/min. The frequency of these oscillations is dependent upon the orientation of the leaflet in relation to the gravitational field and the magnitude of the flow rate. In vivo and in vitro measurements by other investigators demonstrate similar effects of gravity and flow rate, with flutter frequencies of the order of 10-100 Hz. Excitation frequencies, based on vortex shedding, are estimated to be of the order 2-200 Hz, for the range of flow rates of the theoretical model. These results suggest that the natural frequency of this rotational second order system may, in theory, be a contributing factor to the flutter observed in bileaflet cardiac valve prostheses. The clinical significance of this finding is yet to be established.

Blood Flow Velocity↗