Spectral characteristics of mechanical heart valve closing sounds

BACKGROUND AND AIMS OF THE STUDY: In-vivo evaluation of cavitation is based on the registration of high-frequency pressure fluctuations that represent a mixture of both cavitation and valve closing sounds, and are difficult to separate. In order to extract the cavitation signal, a high-pass filter removing the closing sound is applied. Importantly, the cut-off frequency should be chosen based on the valve's resonance pattern. This could be determined in a cavitation-free, air-operated set-up. As air and water/blood have different physical properties that could influence resonance frequencies, it is necessary to correlate the frequency content of the closing sounds recorded in air to represent expected findings in fluid. The study aim was to characterize the impact of the surrounding media on resonance frequency of a sound source, and to develop a method capable of evaluating the spectral characteristics of mechanical heart valves. METHODS: Five different valves were investigated. An in-vitro set-up was developed where the valves were operated in an airflow-controlled setting without cavitation. The valve closing sounds were recorded and a spectral analysis was performed. The resonance frequency of a simple sound source was also recorded in water and air in order to evaluate the impact of the surrounding media. RESULTS: Resonance frequencies from the sound source measured in air increased 14% compared with corresponding measurements in water. These data were used to correct findings from the five valves that showed different spectral characteristics in air. The frequency at which 97.5% of the signal energy was contained ranged from 40.9 to 65.8 kHz. CONCLUSION: Using an airflow in-vitro model, it was possible to determine the frequency signature of different mechanical heart valves. This might provide the information needed to design the optimal high-pass filter when evaluating cavitation.     

Evaluation technique for bileaflet mechanical valves

BACKGROUND AND AIMS OF THE STUDY: Several techniques were utilized to evaluate the performance of bileaflet mechanical heart valves and improve upon current valve designs. METHODS: Particle image velocimetry (PIV), computational fluid dynamics (CFD) and video analysis were used to evaluate St. Jude Medical (SJM) and ATS valves in an abrupt enlargement outlet chamber. Video analysis and PIV provided information on the opening angle of the leaflets and 2D velocity fields. PAM-FLOW CFD software was used to predict flow through the valves with leaflets in the fully open position, which corresponds to steady flow conditions in the experimental study. The ATS valve was also modeled at its reported incomplete leaflet opening angle, and with an orifice modification. Additional CFD techniques were employed to find the natural opening angle of the ATS valve. RESULTS: In steady and pulsatile flow, the SJM leaflets opened completely. PIV showed three fairly uniform jets passing through the SJM valve. CFD analysis of the pressure distribution across the SJM valve showed a resultant moment in the open direction, indicating that the leaflets were held against the open stops at 85 degrees. In both steady and pulsatile flow, the leaflets in the ATS valve did not open to the designed opening angle. From video analysis, the ATS leaflet opening angle was estimated at 72-76 degrees. PIV showed that the incomplete opening angle of the ATS leaflets produced a large wake region behind the leaflets, and lateral jets that impinged on the side walls of the test chamber. CFD analysis showed a pressure distribution across the ATS valve which produced a moment causing the leaflets not to open to 85 degrees. The equilibrium position of the leaflet was approximately 75 degrees. The leaflets in the CFD model opened to the full 85 degrees when the orifice was increased by 2 mm on the outflow side. CONCLUSION: The combination of techniques used in this study provided a method that will be useful in evaluating new valve designs. It was learned that leaflet position, pivot location and orifice height influence pressure distribution across the leaflets, thereby affecting the opening angle.     

Lower anticoagulation for mechanical heart valves: experience with the ATS bileaflet valve

Background: The design of the bileaflet ATS (ATS Medical Inc., Minneapolis, USA) mechanical valve incorporates an open pivot at the hinge mechanism. Total washout of the blood at the pivot area was observed using 3-D computational fluid dynamics modelling. This phenomenon could make the valve less vulnerable to clot formation in patients without major thromboembolic risk factors.

Methods: From January 1993 to June 1999, 286 consecutive patients had the ATS valve inserted in the aortic position. Patients were divided into two groups. Group 1 comprised all patients in regular sinus rhythm with good left ventricular function (144 patients). Group 2 included patients in non-sinus rhythm and/or with large hypocontractile left ventricles (142 patients). The anticoagulation regime in group 1 was used to obtain an international normalised ratio (INR) between 1.5 and 2.5. This contrasts with our regular aim to maintain the INR strictly between 2.5 and 3.5 for all mechanical valves, as achieved in group 2.

Results: The follow-up period (99% completeness) ranged from 18 to 84 months. Survival (Kaplan-Meier) was 97 and 98% and 92 and 81% at 1 and 5 years in group 1 and group 2, respectively (P = 0.12). Log rank analysis failed to detect a statistical difference in thromboembolism or bleeding between both groups (P > 0.05). However, trends were in favour of group 1. Univariate analysis selected poor ventricular function and an ‘erratic’ INR value (P = 0.002) as risk factors for death. The sole independent risk factor for bleeding was the use of aspirin (P = 0.025).

Conclusions: The excellent group 1 data and outcome encouraged us to continue our low intensive anticoagulation regime and perhaps should be regarded as a new concept for treatment of selected mechanical valve patients.     

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.     

Flow characterization of the ADVANTAGE and St. Jude Medical bileaflet mechanical heart valves

BACKGROUND AND AIM OF THE STUDY: The study aim was to characterize time-dependent flow fields and flow structures within the ADVANTAGE (ADV) and St. Jude Medical (SJM) prosthetic bileaflet mechanical heart valves. METHODS: Three-dimensional unsteady computational fluid dynamic simulations were conducted in the aortic position for both valves. Flow boundary conditions were acquired from an in-vitro experiment. The governing equations were solved by a finite volume method that employed a moving cell technique to simulate the motion of the valve leaflet in the cardiac cycle. The computed velocities were subsequently validated using the velocities measured in the in-vitro experiment. RESULTS: Both valves had similar flow phenomena at the geometric symmetry plane of the valve housing, and both experienced a waterhammer effect upon closure. However, flow characteristics in the pivots differed distinctively between both valves. More dynamic flow activity was observed at the bi-level butterfly pivots of the ADV valve. Flow vena contracta and large flow boundary separation zones at the central flow orifice were captured adjacent to the pivots of the SJM valve. During valve opening, retrograde systolic flow at the bottom of the pivot was observed. No persistent flow stases were seen in the pivots of either valves. CONCLUSION: Although overall flow characterization for both valves was similar, flow features within each valve's pivots correlated to the pivot design. The bi-level butterfly pivot design of the ADV valve appeared to provide relatively easy passages for pivot flow washing.     

Lower-intensity anticoagulation for mechanical heart valves: a new concept with the ATS bileaflet aortic valve

BACKGROUND AND AIM OF THE STUDY: The design of the bileaflet ATS mechanical valve incorporates an open pivot at the hinge mechanism. Total washout of blood at the pivot area seen using three-dimensional computational fluid dynamics modeling may make the valve less vulnerable for clot formation in patients without major thromboembolic risk factors. METHODS: Between January 1993 and June 1999, the ATS valve was implanted in the aortic position in 286 consecutive patients. Patients were allocated prospectively to two groups: group 1 comprised patients in regular sinus rhythm with good left ventricular (LV) function (n = 144); group 2 included patients in non-sinus rhythm and/or with large hypocontractile left ventricles (n = 142). The anticoagulation regime in group 1 was to achieve an INR of 1.5-2.5, rather than to maintain INR strictly at 2.5-3.5 for mechanical valves (as in group 2). RESULTS: Follow up was 99% complete and ranged from 50 to 120 months. Survival (Kaplan-Meier) was respectively 95% and 90% and 90% and 83% at 2 and 5 years in favor of group 1 (p = 0.0055). Multivariate analysis selected advanced age, poor LV function and 'erratic' INR as risk factors for death. Log rank analysis failed to detect any statistical difference in thromboembolism. Bleeding occurred more frequently in group 2 (p = 0.018); independent risk factors for bleeding were the presence of aspirin (p = 0.0164) and advanced age (p = 0.02). CONCLUSION: The excellent group 1 data and outcome encouraged continuation of the low-intensity anticoagulation regime, and should be regarded as a new concept for the treatment of mechanical valve patients.     

Effect of hinge gap width on the microflow structures in 27-mm bileaflet mechanical heart valves

BACKGROUND AND AIM OF THE STUDY: Most bileaflet mechanical heart valves (BMHVs) incorporate some retrograde flow through their hinge mechanism to prevent flow stasis and inhibit microthrombus formation. This reverse flow is characterized by high velocities and shear stresses, thereby promoting platelet activation and hemolysis inside the hinge region. In the present study, the thromboembolic potential of three 27-mm BMHVs with varying hinge gap widths was assessed via in-vitro characterization of the hinge microflow structures. METHODS: Three 27-mm BMHV prototypes with different hinge gap widths (50, 100, and 200 microm) were provided by St. Jude Medical Inc. The valves were mounted in the mitral position of a left heart flow simulator, and two-dimensional laser Doppler velocimetry was used to measure the hinge velocity fields. RESULTS: All three valve prototypes revealed Reynolds shear stress (RSS) levels above 2000 dynes/cm2, which exceeded the threshold for platelet activation and hemolysis. The hinge flow fields were characterized by leakage jets during systole, and a strong vortical flow during diastole. The leakage jet size and corresponding RSS levels were found to increase with the hinge gap width. All three gap widths had RSS >4000 dynes/cm2 (range: 5640 to 13,315 dynes/cm2). The hinge with the smallest gap width registered the highest jet velocity magnitude (2.08 m/s) during systole. CONCLUSION: The study results showed that the hinge gap width influences washout and RSS levels inside the hinge recess. The 100-microm hinge gap width provided optimum fluid dynamic performance. In contrast, the two valves with large and small hinge gap widths may have higher thromboembolic potential.     

Anticoagulation for pregnant patients with mechanical heart valves

Management of a pregnant patient with mechanical heart valve is a complex issue for all health care providers involved in the care of such patients. Complications may arise at any stage due to the increased haemodynamic load imposed by pregnancy or because of impaired cardiac performance often seen in these patients. In addition, the use of various cardiovascular drugs in pregnancy (especially anticoagulants) may lead tofoetal loss or teratogenic complications. Additionally, the risk of thrombo-embolic complications in the mother is increased by the hypercoagulable state of pregnancy. In this review, we have attempted to draw inferences to guide management of such patients based on the available literature. It seems that in pregnant women with mechanical heart valves, recent data support warfarin use throughout pregnancy, followed by a switch to heparin and planned induction of labour. However, the complexity of this situation demands a cafeteria approach where the patient herself can choose from the available options that are supported by evidence-based information. Unfortunately there is no consensus on such data. An overview of the available literature forms the basis of this review. In conclusion, a guideline comprising pragmatic considerations is preffered.     

Flow-induced platelet activation in mechanical heart valves

BACKGROUND AND AIM OF THE STUDY: A study was conducted to measure in vitro the procoagulant properties of platelets induced by flow through mechanical heart valves. METHODS: The procoagulant activity of platelets was measured using a real-time assay of platelet activation state (PAS), which was based on a modification of the prothrombinase method. Acetylated prothrombin was used instead of normal prothrombin in this assay in order to eliminate the positive feedback effect of thrombin. This enabled a direct comparison between thrombin generation rates in the assay and the flow stresses that induce platelet activation. Gel-filtered platelets (10(5) per microliters) were circulated through a left ventricular assist device with two Bj?rk-Shiley mono-leaflet mechanical heart valves mounted in opposition, and platelet activation state was measured over 30-min time courses. The results were compared with two configurations in which the leaflet motion of one of the valves was restricted (severely restricted and mildly restricted), mimicking defective function of a compromised valve in vivo, and with a control lacking valves. RESULTS: The severely restricted valve activated the platelets at a rate eight-fold higher than with unrestricted valves, and three-fold higher than with mildly restricted valves. Both restricted valves activated platelets at rates significantly higher than either the control (no valves) or the unrestricted valve. CONCLUSION: Flow through compromised mechanical heart valves causes platelet activation, which can be measured with a modified prothrombinase assay system. The ability to perform sensitive quantitative measurements in cardiovascular devices in vitro may have a significant impact on the design and development of these devices.     

Dynamic impact stress analysis of a bileaflet mechanical heart valve

BACKGROUND AND AIMS OF THE STUDY: Mechanical heart valves (MHV) are widely used to replace dysfunctional and failed heart valves. The bileaflet MHV is very popular due to its superior hemodynamics. At present, bileaflet MHVs account for about two-thirds of the prosthetic heart valve market. Since their introduction in 1977, the hemodynamics of bileaflet prostheses has been extensively studied. New technologies used to develop MHV include better design concepts, materials, manufacturing processes, and post-design verification. The study aim was to investigate the dynamic impact stress of a newly designed bileaflet MHV under normal physiological conditions. METHODS: Pro/Engineer was used to generate a 3-D model of the designed valve. ANSYS 5.5 and LS-DYNA were used to calculate stress and deformation of the valve. Due to symmetry, a one-half orifice and one leaflet were modeled using the eight-noded hexahedral elements. When valve leaflets are in the fully closed position, the static contact stress between leaflet and orifice was predicated under typical heart valve closing pressure of 80 mmHg. To study the dynamic effects of the closing valve, LS-DYNA was used to simulate leaflet motion. Typical physiological pressure waveform was employed to initiate this leaflet motion. Two types of valve were investigated: Test valve A (size 19, flat leaflet); and test valve B (size 19, tapered leaflet 1.5 degrees, with the same thickness at pivot as valve A). The non-invasive laser sweeping technique was used to measure leaflet closing velocity in a mock flow test rig. The closing velocity of test valve A was compared by experimental and computed results. The corresponding dynamic contact stress on the leaflet was obtained for different modes of loading, simulated under angular velocity, acceleration, and especially under representative pressure waveform. RESULTS: The experimental closing velocity of test valve A was 1.07 +/- 0.05 m/s; the computed value was 1.130 m/s. During full closure, the leaflets showed a slight rebound, and this was also seen experimentally. For test valve B, the computed closing velocity was 1.039 m/s. In the dynamic impact analysis, the physiological pressure waveform was obtained at a normal heart rate of 70 beats/min from the mock flow test rig. Dynamic stress and displacement of the model valve were calculated as the valve was closing. The time step of calculation was determined by the wave propagation velocity and element size. With an interhinge distance of 4.966 mm based on the geometric design of the valve, maximum dynamic von Mises stress appeared near the hinge of the leaflet (26.92 MPa for valve A; 22.36 MPa for valve B). By varying the position of the hinge/pivoting axis (+/- 10%), an optimized valve geometry could be obtained based on minimal impact stress on the valve leaflet. CONCLUSION: Based on closing velocity comparison of valve A, the calculated model and loading conditions were seen to be reasonable. Computational accuracy was satisfied. The tapering feature of the leaflet is designed especially for minimal impact stress at the leaflet contact areas upon impact with the inner walls of the BMHV. These points provide an optimum structure design for the Nanyang Technological University BMHV.     

Microbubbles associated with mechanical heart valves: their relationwith serum lactic dehydrogenase levels

BACKGROUND: There has been no consensus about the prevalence and mechanism of generation of microbubbles in mechanical prosthetic heart valves (PHV). The aim of this study was to determine the prevalence of microbubbles in PHV and their relation to serum lactic dehydrogenase (LDH) levels. METHODS AND RESULTS: We prospectively studied 150 normally functioning mitral PHV (98 bileaflet and 52 monoleaflet) in 150 patients with the use of transesophageal echocardiography with a multiplane 5-MHz probe. None of the patients had an aortic prosthetic valve. Blood was drawn to determine serum LDH level. None of the patients had any factors that might affect the LDH level other than the PHV-related hemolysis. Patients with spontaneous echo contrast in the left atrium that might affect the assessment of the microbubbles were excluded. We devised a method to determine the amount of microbubbles for each PHV. Microbubbles were detected in 118 (79%) of 150 PHV, including 97 (99%) of 98 bileaflet valves and 21 (40%) of 52 monoleaflet (tilting disk) valves (P <. 0001). Intraobserver variability was not statistically significant for the determination of the amount of microbubbles (z = 1.7, P =. 08). There was a strong correlation between serum LDH levels and the amount of microbubbles (rs = 0.69, P <.001). CONCLUSIONS: Microbubbles were detected in more patients than reported previously. They were found to be associated more with the bileaflet than the monoleaflet PHV. Sorin monoleaflet valves were associated with microbubbles significantly less often than the others. There was a strong correlation between serum LDH levels and microbubble counts, which suggests that hemolysis may be related to microbubble formation.     

Unalloyed pyrolytic carbon for implanted mechanical heart valves.

Materials for implanted heart valves face challenges unmatched for most mechanical applications. The valves must function without interruption for lifetimes in the order of 10(9) cycles in a corrosive, hostile environment. A particular carbon has been widely used for the past three decades. The mechanical behavior of this isotropic pyrolytic carbon (PyC), with and without silicon alloying, was studied in this research program. Several questions were addressed: the fatigue strength at lifetimes of 10(9) cycles, the threshold for crack growth, the validity of fracture mechanics for PyC, and the statistics of ultra-high survival under cyclic stress. It was found that cyclic stressing within the scatter band of the static strength did not initiate cracks. Furthermore, artificially induced cracks did not grow under cyclic stressing below a threshold stress intensity factor. This threshold stress intensity factor was valid over a wide range of crack lengths. No failures occurred when batches of 29 specimens were tested above the service stress at 6 x 10(8) cycles. Cyclic stressing did not reduce the static strength. The service stress is a small fraction of the fracture strength; thus, a very high probability of survival can be ensured by a proof test of the assembly.     

In-vitro testing of three totally supra-annular bileaflet mechanical valves: hydrodynamics in the Sheffield pulse duplicator

BACKGROUND AND AIM OF THE STUDY: In-vivo echocardiographic studies are limited by several confounding factors and technical pitfalls, and consequently the hypothetical differential hydraulic behavior between different prosthetic heart valves has not been identified. However, for surgeons it is essential to know the functional and geometric characteristics of the prostheses to be used. Herein, the in-vitro performance of two new supra-annular bileaflet prostheses--the Medtronic Advantage Supra and Sorin Bicarbon Overline--was compared with that of the 21-mm St. Jude Medical (SJM) Regent valve. METHODS: Three high-performance, production-quality prostheses, including the sewing-ring cuffs, were tested in the aortic chamber of a Sheffield pulse duplicator. The sizes of the prostheses which fitted the 21-mm valve holder were: 21 mm Advantage Supra Medtronic; 19 mm SJM Regent, and 18 mm Sorin Bicarbon Overline. The tests were carried out at a fixed pulse rate (70 beats/min), and at increasing cardiac output (CO) of 2, 4, 5, and 7 l/min. Forward-flow pressure drop, total regurgitant volume, closing and leakage volumes, effective orifice area (EOA) and stroke work loss (SWL) were recorded while the valve was operated at each CO. RESULTS: The SJM Regent and Sorin Bicarbon Overline valves each showed, at increasing CO, significantly lower mean and peak gradients. The calculated EOA and SWL were significantly better with the SJM Regent and Sorin Bicarbon Overline prostheses. The Medtronic Advantage Supra valve showed comparable results only while performing at 2 l/min CO. With regards to the regurgitant fraction, lowest values were observed with the Medtronic Advantage Supra valve. CONCLUSION: This hydrodynamic evaluation model allowed a comparison to be made of the efficiency of recently commercialized bileaflet prostheses, among which the older SJM Regent and the newer Sorin Bicarbon Overline valves demonstrated the best performances.     

Two-dimensional fluid-structure interaction simulation of bileaflet mechanical heart valve flow dynamics

BACKGROUND AND AIM OF THE STUDY: Mechanical heart valve implantation requires long-term anticoagulation because of thromboembolic complications. Recent studies have indicated that the relatively high wall shear stresses and negative pressure transients developed during the valve closing phase may be dominant factors inducing thrombus initiation. The study aim was a two-dimensional (2D) functional simulation of flow past bileaflet heart valve prosthesis during the closing phase, incorporating the fluid-structure interaction analysis to induce motion of the leaflets. METHODS: The fluid-structure interaction model used was based on unsteady 2D Navier-Stokes equations with the arbitrary Lagrangian-Eulerian method for moving boundaries, coupled with the dynamic equation for leaflet motion. Parametric analysis of the effect of valve size, leaflet density, and the coefficient of resilience at the instant of impact of the leaflet with the housing were also performed. RESULTS: Comparing the predicted motion of the leaflet with previous experimental results validated the simulation. The results showed the presence of negative pressure transients near the inflow side of the leaflet at the instant of valve closure, and the negative pressure transients were augmented during the leaflet rebound process. Relatively high velocities and wall shear stresses, detrimental to the formed elements in blood were present in the clearance region between the leaflet and valve housing at the instant of valve closure. CONCLUSION: The simulation can be potentially applied to analyze the effects of valve geometry and dimensions, and the effect of leaflet material on the flow dynamics past the valve prosthesis during the opening and closing phases for design improvements in minimizing problems associated with thromboembolic complications.     

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.     

Videoassisted thrombectomy of mechanical prosthetic heart valves.

BACKGROUND AND AIM OF THE STUDY: Thrombosis of a bileaflet mechanical heart valve is a life-threatening clinical event. Surgical thrombectomy of bileaflet mechanical prostheses remains an appropriate treatment in selected patients. METHODS: Between 1996 and 1998, five patients (three men, two women; average age 56 +/- 1 years; range: 56 to 66 years) with thrombosis of left-sided bileaflet mechanical valves were treated with videoassisted thrombectomy of the prosthesis. Four patients had thrombosis of a bileaflet mitral mechanical valve, and one patient had thrombosis of an aortic valve prosthesis. Preoperatively, patients were in either NYHA functional class IV (n = 4) or class I (n = 1). Surgery was performed through a right anterior thoracotomy or a median sternotomy. A rigid 30 degrees thoracoscope was inserted into the left atrium or aorta to visualize the thrombosed valve. The thrombus was extracted and the prosthesis under-surface examined and cleaned. Leaflet mobility, assessed with transesophageal echocardiography, was normal following surgical thrombectomy. RESULTS: Mean cardiopulmonary bypass time was 102 +/- 30 min; mean aortic cross-clamping time was 47 +/- 25 min. There was no hospital mortality; mean hospital stay was 9 +/- 1 days (range: 6 to 11 days). Anticoagulation with intravenous heparin was resumed 24 h after surgery. Three patients were discharged on coumarin treatment alone; two patients received aspirin plus coumarin. Mean postoperative follow up was 7 +/- 8 months (range: 1 to 21 months). One patient died 21 months after thrombectomy of a mitral prosthesis, with an unconfirmed diagnosis of recurrent mitral valve dysfunction. At 1-15 months after surgery, four patients are in NYHA class I, without evidence of prosthesis dysfunction. CONCLUSION: Videoassisted thrombectomy of a bileaflet mechanical heart valve is a treatment option in patients with acute thrombosis of the prosthesis.     

Anticoagulation of pregnant women with mechanical heart valves

The management of pregnant women with mechanical heart valves is challenging. Recently, based on small numbers of patients and poor-quality data, correspondence from Aventis Pharmaceuticals Inc has described treatment "failures" and concerns about teratogenicity with the use of the low-molecular-weight heparin (LMWH) enoxaparin. The company issued a "Warning" that enoxaparin should not be used in patients with prosthetic heart valves and a "Precaution" about potential teratogenicity. This has created a huge problem for physicians managing pregnant women with prosthetic heart valves because the alternatives, unfractionated heparin and warfarin, are problematic. There have been case reports of failures (including death from thrombosed valves) with unfractionated heparin, whereas the package insert for warfarin states that the drug is contraindicated during pregnancy because of potential teratogenicity. Initially, LMWHs appeared suitable for pregnant patients with prosthetic heart valves. Unfortunately, the company correspondence, presumably supported by the Food and Drug Administration (FDA), raises medicolegal concerns with use of any LMWH. We believe that pharmaceutical companies and the FDA should not endorse scientifically unsupported claims that eliminate acceptable therapeutic options. This correspondence has created considerable confusion among patients and treating physicians and is likely to lead to frivolous lawsuits and preclude the performance of properly designed trials in pregnant women. We believe a consensus conference among experts in the field to identify key unresolved issues and a commitment by the FDA and industry to perform appropriate studies are now critical.