In vitro hydrodynamic characteristics among three bileaflet valves in the mitral position

The non-fully open phenomenon of the advancing standard medical bileaflet heart valves (the ATS valve) are frequently observed in clinical cases, even though there is no problem with their hemodynamic function. The movement of the leaflets was affected easily by the transvalvular flow because of the unique open pivot design of the ATS valve. In this paper, a comparative in vitro hydrodynamic test was conducted among 3 different types of bileaflet valves, and the effect of different shapes of downstream conduits, which induce different transvalvular flow, on hydrodynamic performance was studied. Three bileaflet valves, the ATS valve, CarboMedics valve (CM), and St. Jude Medical valve (SJM), with an annulus diameter of 29 mm for the mitral position were chosen throughout our experiments. First, pressure drops across the valves under steady flow were measured. Then, the valves were tested at the mitral position with our pneumatically driven pulsatile pump. In this pulsatile flow study, 2 different conduits (straight shape and abrupt enlargement shape) were in turn incorporated at the downstream portion of the mitral valve. A high-speed video camera was employed to observe leaflet movements. In a steady-flow test, the ATS and the SJM produced the same pressure drop, but the CM recorded a higher value. In the pulsatile study, it was observed that the ATS leaflets did not open fully in the mitral position when the downstream conduit with an abrupt enlargement shape was incorporated. However, the CM and the SJM always indicated a fully open movement regardless of the shape of downstream conduits. When the straight downstream conduit was incorporated, the ATS produced a similar pressure drop to that of the SJM, which coincided with the steady test results. When the enlargement conduit was incorporated, however, the ATS presented the lowest pressure drop despite the non-fully open movement. The conduit shape at the valve downstream had a significant influence on the closing volume. These findings indicate that the conduit shape at the valve outlet can affect the hydrodynamic characteristics of bileaflet valves.     

Prosthetic heart valve evaluation by magnetic resonance imaging.

OBJECTIVE: To evaluate the potential of magnetic resonance imaging (MRI) for evaluation of velocity fields downstream of prosthetic aortic valves. Furthermore, to provide comparative data from bileaflet aortic valve prostheses in vitro and in patients. METHODS: A pulsatile flow loop was set up in a 7.0 Tesla MRI scanner to study fluid velocity data downstream of a 25 mm aortic bileaflet heart valve prosthesis. Three dimensional surface plots of velocity fields were displayed. In six NYHA class I patients blood velocity profiles were studied downstream of their St. Jude Medical aortic valves using a 1.5 Tesla MRI whole-body scanner. Blood velocity data were displayed as mentioned above. RESULTS: Fluid velocity profiles obtained from in vitro studies 0.25 valve diameter downstream of the valve exhibited significant details about the cross sectional distribution of fluid velocities. This distribution completely reflected the valve design. Blood velocity profiles in humans were considerably smoother and in some cases skewed with the highest velocities toward the anterior-right ascending aortic wall. CONCLUSION: Display and interpretation of fluid and blood velocity data obtained downstream of prosthetic valves is feasible both in vitro and in vivo using the MRI technique. An in vitro model with a straight tube and the test valve oriented orthogonally to the long axis of the test tube does not entail fluid velocity profiles which are compatible to those obtained from humans, probably due to the much more complex human geometry, and variable alignment of the valve with the ascending aorta. With the steadily improving quality of MRI scanners this technique has significant potential for comparative in vitro and in vivo hemodynamic evaluation of heart valves.     

Discrepancy between Doppler and catheter measurements of pressure gradients across small-size prosthetic valve.

OBJECTIVES: The exact role of pressure gradient across the prosthetic valve estimated from Doppler flow velocity remains controversial. This in-vivo study was designed to assess the actual discrepancy between Doppler and catheter measurements of the pressure gradients for small bileaflet prosthetic valves in the aortic position. METHODS: Bileaflet prosthetic valves (19 mm-ATS) were implanted into the aortic position in pigs, and pressure gradients across the valves were examined by volume loading under right heart bypass. The pressure gradient obtained by catheter was defined as the conventional peak-to-peak gradient between the left ventricle and aorta. The peak Doppler gradients were calculated from the maximal instantaneous Doppler velocity with the ultrasound probe positioned on the diaphragm at the level of the cardiac apex. RESULTS: There were strong correlations between pressure gradients and cardiac output. The Doppler gradient was constantly higher than the catheter values, and the resultant discrepancy between Doppler and catheter measurements was directly dependent on cardiac output (y=9.9x+0.6, r2=0.55). For cardiac output > or = 5.0 L/min, the difference between Doppler and catheter gradients reached 40 mmHg, and maximum differences of up to 80 mmHg were observed. CONCLUSIONS: In view of the presence of striking overestimation of catheter gradient by Doppler measurement, Doppler ultrasound should be used cautiously to assess small-size bileaflet prosthetic valve function with consideration of the patient's hemodynamic state.     

Hydrodynamic characteristics of bileaflet mechanical heart valves in an artificial heart: cavitation and closing velocity

The aim of this study was to investigate the possibility of using the bileaflet valves in an electrohydraulic total artificial heart (EHTAH). Three kinds of bileaflet valves, namely the ATS valve (ATS Medical Inc., Minneapolis, MN, USA), the St. Jude valve (St. Jude Medical Inc., St. Paul, MN, USA), and the Sorin Bicarbon valve (Sorin Biomedica, Vercelli, Italy), were mounted in the mitral position on an inclined 45 degrees plane in an EHTAH. The pressure waves near the valve surface, the valve-closing velocity, and a high-speed camera were employed to investigate the mechanism for bileaflet valve cavitation. The cavitation bubbles in the bileaflet valves were concentrated along the leaflet tip. The cavitation intensity increased with an increase in the valve-closing velocity. It was established that squeeze flow holds the key to bileaflet valve cavitation. At lower heart rates, the delay time of the asynchronous closure motion between the two leaflets of the Sorin Bicarbon valve was greater than that of the other bileaflet valves. At higher heart rates, no significant difference was observed among the bileaflet valves.     

Experimental investigation of the flow downstream of a dysfunctional bileaflet mechanical aortic valve

Mechanical heart valve replacement is the preferred alternative in younger patients with severe symptomatic aortic valve disease. However, thrombus and pannus formations are common complications associated with bileaflet mechanical heart valves. This leads to risks of valve leaflet dysfunction, a life‐threatening event. In this experimental study, we investigate, using time‐resolved planar particle image velocimetry, the flow characteristics in the ascending aorta in the presence of a dysfunctional bileaflet mechanical heart valve. Several configurations of leaflet dysfunction are investigated and the induced flow disturbances in terms of velocity fields, viscous energy dissipation, wall shear stress, and accumulation of viscous shear stresses are evaluated. We also explore the ability of a new set of parameters, solely based on the analysis of the normalized axial velocity profiles in the ascending aorta, to detect bileaflet mechanical heart valve dysfunction and differentiate between the different configurations tested in this study. Our results show that a bileaflet mechanical heart valve dysfunction leads to a complex spectrum of flow disturbances with each flow characteristic evaluated having its own worst case scenario in terms of dysfunction configuration. We also show that the suggested approach based on the analysis of the normalized axial velocity profiles in the ascending aorta has the potential to clearly discriminate not only between normal and dysfunctional bilealfet heart valves but also between the different leaflet dysfunction configurations. This approach could be easily implemented using phase‐contrast MRI to follow up patients with bileaflet mechanical heart valves.     

Flow characteristics past jellyfish and St. Vincent valves in the aortic position under physiological pulsatile flow conditions

Thrombus formation and hemolysis have been linked to the dynamic flow characteristics of heart valve prostheses. To enhance our understanding of the flow characteristics past the aortic position of a Jellyfish (JF) valve in the left ventricle, in vitro laser Doppler anemometry (LDA) measurements were carried out under physiological pulsatile flow conditions. The hemodynamic performance of the JF valve was then compared with that of the St. Vincent (SV) valve. The comparison was given in terms of mean systolic pressure drop, back flow energy losses, flow velocity, and shear stresses at various locations downstream of both valves and at cardiac outputs of 3.5 L/min, 4.5 L/min, and 6.5 L/min respectively. The results indicated that both valves created disturbed flow fields with elevated levels of turbulent shear stress as well as higher levels of turbulence in the immediate vicinity of the valve and up to 1 diameter of the pipe (D) downstream of the valve. At a location further downstream, the JF valve showed better flow characteristics than the SV in terms of velocity profiles and turbulent shear stresses. The closure volume of the SV valve was found to be 2.5 times higher than that of the JF valve. Moreover, the total back flow losses and mean systolic pressure drop also were found to be higher in the SV than the JF valve.     

In Vitro Fluid Dynamic Characteristics of Ionescu-Shiley and Carpentier-Edwards Tissue Bioprostheses

Abstract: In the study Reported here, the in vitro fluid dynamic characteristics of the Ionescu-Shiley (calf pericardial) and Carpentier-Edwards (porcine) aortic tissue valves were studied. The experiments conducted were pressure drop measurements, leaflet photography, flow visualization, and velocity measurements. The pressure drop studies indicated that both types of tissue valves created relatively large pressure drops. These pressure drops were larger than those observed with the corresponding sizes of Bjork-Shiley, Hall-Kaster, and St. Jude aortic prostheses. The photographs of the opening of the valve leaflets indicated that the tissue valves do not open as ideally as do the natural valves. It was also observed that the Ionescu-Shiley aortic valves opened more symmetrically and with reproducibility than the corresponding Carpentier-Edwards aortic valves. Velocity and shear stress measurements made with a laser-Doppler anemometer indicated that the flow that emerged from the leaflets for both types of tissue valves was like a jet and could lead to turbulent shear stress on the order of 1,000–3,000 dynes/cm². Such turbulent shear stresses could be harmful to blood components. The jet-type flow could also damage the endothelial lining of the wall of the ascending aorta. The velocity measurements also indicated an annular region of stagnant fluid between the outflow surfaces of the leaflets and the flow channel wall. Such a region could lead to the build-up of thrombotic, fibrotic, and/or calcific material on the outflow surfaces of the leaflets. Both types of valve designs, however, created relatively low wall shear stresses and regurgitant volumes.     

Comparative study of the hydrodynamic function of the CarboMedics valve

The hydrodynamic function of each size of the CarboMedics valve was assessed in a pulsatile flow simulator. The mean pressure difference with respect to forward flow, regurgitant volumes, and total energy loss across each valve were analyzed. The results for the 23-mm aortic and 29-mm mitral CarboMedics valves were compared with those for the St. Jude Medical and Bj?rk-Shiley Monostrut valves. Results showed good hydrodynamic function for each CarboMedics valve, although the pressure difference and total energy loss across the 19-mm aortic valve was high. The hydrodynamic function of the CarboMedics valve was comparable with that of the St. Jude Medical valve. Both valves showed similar leakage volumes, which were significantly larger than that for the Bj?rk-Shiley Monostrut valve. On account of this the total energy loss in the Bj?rk-Shiley valve was significantly less than that for the bileaflet valves in the aortic position. Concern remains for the continuing presence of high closed-valve regurgitation in the bileaflet valves.     

Discrepancy between doppler and catheter measurements of pressure gradients across small-size prosthetic valve

Objectives: The exact role of pressure gradient across the prosthetic valve estimated from Doppler flow velocity remains controversial. This in-vivo study was designed to assess the actual discrepancy between Doppler and catheter measurements of the pressure gradients for small bileaflet prosthetic valves in the aortic position. Methods: Bileaflet prosthetic valves (19 mm-ATS) were implanted into the aortic position in pigs, and pressure gradients across the valves were examined by volume loading under right heart bypass. The pressure gradient obtained by catheter was defined as the conventional peak-to-peak gradient between the left ventricle and aorta. The peak Doppler gradients were calculated from the maximal instantaneous Doppler velocity with the ultrasound probe positioned on the diaphragm at the level of the cardiac apex. Results: There were strong correlations between pressure gradients and cardiac output. The Doppler gradient was constantly higher than the catheter values, and the resultant discrepancy between Doppler and catheter measurements was directly dependent on cardiac output (y=9.9x+0.6, r²=0.55). For cardiac output ≧5.0 L/min, the difference between Doppler and catheter gradients reached 40 mmHg, and maximum differences of up to 80 mmHg were observed. Conclusions: In view of the presence of striking overestimation of catheter gradient by Doppler measurement, Doppler ultrasound should be used cautiously to assess small-size bileaflet prosthetic valve function with consideration of the patient’s hemodynamic state.     

Turbulent stress measurements downstream of three bileaflet heart valve designs in pigs

Objective: Mechanical heart valves can cause thromboembolic complications, possibly due to abnormal flow patterns that produce turbulence downstream of the valve. The objective of this study was to investigate whether three different bileaflet valve designs would exhibit clinically relevant differences in downstream turbulent stresses. Methods: Three bileaflet mechanical heart valves (Medtronic Advantage^®, CarboMedics© Orbis™ Universal and St. Jude Medical^® Standard) were implanted into 19 female 90 kg pigs. Blood velocity was measured during open chest conditions in the cross sectional area downstream of the valves with 10 MHz ultrasonic probes connected to a modified Alfred^® Pulsed Doppler equipment. As a measure of turbulence, Reynolds normal stress (RNS) was calculated at three different cardiac output ranges (3–4, 4.5–5.5, 6–7 L/min). Results: Data from 12 animals were obtained. RNS correlated with increasing cardiac outputs. The highest instantaneous RNS observed in these experiments was 47 N/m², and the mean RNS taken spatially over the cross sectional area of the aorta during systole was between 3 N/m² and 15 N/m². In none of the cardiac output ranges RNS values exceeded the lower critical limit for erythrocyte or thrombocyte damage for any of the valve designs. Conclusions: Reynolds normal stress values were below 100 N/m² for all three valve designs and the difference in design was not reflected in generation of turbulence. Hence, it is unlikely that any of the valve designs causes flow induced damage to platelets or erythrocytes.     

Preclinical assessment of a trileaflet mechanical valve in the mitral position in a calf model

BACKGROUND: The bileaflet valve is currently the mechanical replacement valve of choice. Though durable, it does not closely mimic native valve hemodynamics and remains potentially thrombogenic. METHODS: Prototype trileaflet valves (T1 and T2) were implanted in the mitral position in calves. Group I calves received either a T1 valve (n = 12) or a control bileaflet valve (n = 5); Group II, either a T2 valve (n = 7) or a control bileaflet valve (n = 5). Valve function, perivalvular leakage, and transvalvular pressure gradients were evaluated. Also, long-term prototype leaflet wear was evaluated in vivo in one Group I calf (502 days) and two Group II calves (385 and 366 days). Calves were euthanized and necropsied at study termination, and major organs weighed and examined. RESULTS: Valve function was excellent and hematologic parameters remained normal in all calves that survived to study termination. Mean peak transvalvular pressure gradients were 10 +/- 7 mm Hg for T1 valves, 6 +/- 3 mm Hg for T2 valves, and 12 +/- 4 mm Hg for bileaflet control valves. Clinically insignificant valvular regurgitation was observed in both prototypes. Explanted valves showed no thrombus-impaired leaflet motion, except in two T1-fitted calves and one T2-fitted calf. Major organs showed no evidence of clinically significant thromboembolic events. There were no other significant differences between the results of experimental and control groups. CONCLUSIONS: Prototype trileaflet valves performed safely and effectively in the mitral position in calves, even without long-term anticoagulation. This warrants their evaluation as an equivalent alternative to bileaflet valves.     

Non-invasive assessment of differences between bileaflet and tilting-disc aortic valve prostheses by 3D-Doppler profiles

The aim of this study was to analyse flow characteristics of two different prosthetic valves by means of a non-invasive 3D Doppler technique. As previously demonstrated, negative velocity peaks within a 3D-Doppler profile significantly correlate with the severity of aortic stenosis. Transesophageal echocardiography was performed in 42 patients with normal aortic valves and in 35 patients after aortic valve replacement (bileaflet n=23, tilting-disc n=12). Three-dimensional reconstruction of color Doppler data was performed by the EchoAnalyzer software developed at our institution. Cross-section velocity distribution in the ascending aorta was analysed 2 cm distal to the aortic valve in 3 different sectors (non-coronary (NC), left-coronary (LC) or right-coronary (RC)). The percentages of negative velocity values (PNVV) in native aortic valves (6.8+/-6.4%, range: 0-21.8%) were significantly lower (P<0.0001) than in prosthetic valves (bileaflet: 38.5+/-18.5%, range: 13.2-71%; tilting-disc: 47.2+/-17.6%, range: 21.7-78.1%). Significant differences between normal and prosthetic valves were found in all different sectors. Furthermore, Medtronic Hall showed significantly higher PNVV than St. Jude Medical within the LC sector (P=0.03). This method, which allows non-invasive analysis of 3D flow distributions in patients, revealed significant differences between prosthetic valves and native valves as well as among different prosthetic types.     

Microflow fields in the hinge region of the CarboMedics bileaflet mechanical heart valve design

OBJECTIVE: The design of bileaflet mechanical heart valves includes some degree of leakage flow on valve closure for the reverse flow to wash the hinge and pivot region of the valve. It is believed that this reverse flow helps to prevent areas of stasis and inhibit microthrombus formation. However, the magnitude of this retrograde flow may also give rise to unacceptable levels of blood element damage and lead to platelet activation or hemolysis as a result of the increased flow velocities through the hinge region. The purpose of this study was to evaluate the hinge flow dynamics of a 23-mm CarboMedics bileaflet mechanical valve (Sulzer CarboMedics Inc, Austin, Tex) and then to compare the results with those of the St Jude Medical 23-mm Regent (St Jude Medical Inc, Minneapolis, Minn) and Medtronic Parallel (Medtronic, Inc, Minneapolis, Minn) valves studied earlier. This comparison allows new insight into the microflow fields within the hinge region of the CarboMedics bileaflet mechanical valve, which have not been previously assessed during its clinical history. METHODS: Two-dimensional laser Doppler velocimetry was used to measure the velocity and turbulent shear stress fields in the hinge regions. To conduct these measurements, exact dimensional models of the bileaflet hinge regions were cast or machined from transparent plastic materials. The experiment was conducted in a pulsatile flow loop with measurements taken at different levels within the pivot and hinge regions. RESULTS: In the 23-mm CarboMedics valve hinge, the phase-averaged forward velocity obtained at the flat level and levels of 190 microm and 390 microm above flat and 1 mm below flat were 0.54 m/s, 0.77 m/s, 0.3 m/s, and 1.0 m/s, respectively. Corresponding values of the peak phase-averaged leakage velocities were 3.17 m/s, 2.91 m/s, 2.52 m/s, and 0.5 m/s, respectively. Corresponding turbulent shear stresses were 5510 dyne/cm(2), 5640 dyne/cm(2), 4380 dyne/cm(2), and 4810 dyne/cm(2), respectively. CONCLUSIONS: The hinge flow dynamics of the CarboMedics bileaflet design lie somewhere in between those of the St Jude Medical and the Medtronic Parallel valve designs. The fluid dynamics of the investigated valve were found to be similar to those of the St Jude Medical valves, although with slightly higher leakage velocities and turbulent shear stresses. This discrepancy may be a result of the sharper corners associated with the hinge design of the CarboMedics valve. It could also be due to the incremental enlargement of the internal orifice area of the St Jude Medical Regent design.     

Effect of mechanical aortic valve orientation on coronary artery flow: comparison of tilting disc versus bileaflet prostheses in pigs

OBJECTIVE: Orientation for optimal systolic performance of tilting disc and bileaflet aortic valves was defined in previous studies. The present study investigates the influence of valve orientation on coronary artery flow in an animal model. METHODS: A rotation device holding either a Medtronic Hall tilting disc (n = 4; Medtronic, Inc, Minneapolis, Minn), a St Jude Medical bileaflet (n = 4; St Jude Medical, Inc, St Paul, Minn), or a Medtronic Advantage bileaflet (n = 3) aortic valve was implanted. The device allowed rotation of the valve without reopening the aorta. Flow through the left anterior descending coronary artery was measured preoperatively and at normal versus high cardiac output after weaning from extracorporeal circulation. Measurements were performed at the best and worst hemodynamic position, as defined previously. RESULTS: Coronary flow rates were similar in all animals preoperatively (26 +/- 4.1 mL/min). After aortic valve replacement, left anterior descending flow increased significantly to 58.2 +/- 10.6 mL/min. Highest flow rates at normal cardiac output were found in the optimum orientation, especially for the Medtronic valves (Medtronic Hall, 64 +/- 8.7 mL/min; Medtronic Advantage, 64.6 +/- 11.6 mL/min; St Jude Medical, 48.3 +/- 10.3 mL/min), whereas the worst position demonstrated significantly lower left anterior descending flow, with no differences among valves (Medtronic Hall, 37.5 +/- 1.3 mL/min; St Jude Medical, 35.7 +/- 10.7 mL/min; Medtronic Advantage, 39.8 +/- 10 mL/min). Left anterior descending artery flow increased significantly with higher cardiac output. CONCLUSIONS: Coronary blood flow was significantly influenced by mechanical aortic valve implantation and the orientation of prostheses. For both valve designs, the previously defined optimum orientation with respect to pressure gradients and turbulence demonstrated the highest left anterior descending flow rates. Even in its optimum orientation, the St Jude Medical valve showed significantly lower coronary flow than the other valves.     

Orientation of tilting disc and bileaflet aortic valve substitutes for optimal hemodynamics.

Procedures for implantation of mechanical aortic valves have to consider eccentric flow in the aortic root. We describe how to optimize orientation of tilting disc (Medtronic Hall) and bileaflet (St. Jude Medical) valves. In tilting disc valves, an asymmetric design faces an asymmetric flow. Hemodynamic performance of this valve type, regarding turbulence and pressure gradients, is close to normal physiology and superior to the bileaflet valve design. This difference is more pronounced the smaller the valve size.     

The hemodynamic performance of standard bileaflet valves is impaired by a tilted implantation position.

OBJECTIVES: Severe sclerosis of the native aortic annulus can result in a tilted implantation position of mechanical prostheses. In this study, the effects of tilting and rotation on the hemodynamic performance of standard bileaflet valves were assessed in an extracorporeal mock circulatory system. METHODS: A pulsatile mock circulation driven by a Berlin Heart system was developed. Main physiological components of the human circulation were mimicked. SJM-AHPJ prostheses (21, 23, 25 mm) were mounted in an artificial aortic root containing physiologically oriented coronary ostia. All experiments were performed under constant conditions (stroke volume 60 ml, heart rate 70 bpm, systolic pressure 130 mmHg). Hydrostatic pressures were measured via fluid-filled catheters, transvalvular flow by ultrasonic probes. Data were digitally recorded at 50 Hz. Multiple pressure, volume, energy, and dimension parameters were derived off-line. Each valve was tested in a 0 degrees (untilted) versus 20 degrees (tilted) position at three axial rotation angles (0 degrees, 45 degrees, 90 degrees ). Tilting was performed independent of rotation by elevation of the prosthesis in the non-coronary sinus. RESULTS: In all valves and all rotation angles, tilting resulted in a size-dependent significant increase of mean pressure gradient (range, 28-35% [21 mm valve], 59-96% [23 mm valve], 124-220% [25 mm valve]), valvular resistance (39-51, 84-121, 177-332%), regurgitation volume (84-148, 32-131, 93-118%), and systolic energy loss (113-146, 30-132, 69-213%), as well as a decrease of total stroke volume (2-5, 0-11, 3-10%), effective stroke volume (6-11, 9-14, 14-22%), cardiac output (6-11, 8-14, 13-22%), and effective opening area (16-24, 32-37, 47-57%). The strongest impairment of hemodynamic performance was seen at 90 degrees rotation with reference to total and effective stroke volume, cardiac output, mean pressure gradient, and regurgitation fraction. CONCLUSIONS: Tilting of bileaflet valves resulted in a significant impairment of systolic and diastolic hemodynamics. Superiority of larger valves diminished in the tilted position. The strongest tilting effect was seen at 90 degrees rotation. Such a position should therefore be avoided or surgically corrected by rotating the valve.     

Unsteady Fluid Dynamics of Several Mechanical Prosthetic Heart Valves Using a Two Component Laser Doppler Anemometer System

Abstract: Five typical mechanical heart valves (Starr-Edwards, Björk-Shiley convexo-concave (c-c), Björk-Shiley monostrut, Bicer-Val, and St. Jude Medical) were tested in the mitral position under the pulsatile flow condition. The test program included measurements of velocity and turbulent stresses at 5 downstream locations. The study was carried out using a sophisticated cardiac simulator in conjunction with a highly sensitive 2 component laser Doppler anemometer (LDA) system. The continuous monitoring of parametric time histories revealed useful details about the complex flow and helped to establish the locations and times of the peak parameter values. Based upon the nondimensional presentation of data, the following general conclusions can be made. First, all the 5 valve designs created elevated turbulent stresses during the accelerating and peak flow phases, presenting the possibility of thromboembolism and perhaps hemolysis. Second, the difference in valve configuration seemed to affect the flow characteristics; third, the bileaflet design of the St. Jude valve appeared to create a lower turbulence stress level.     

An in vitro assessment by means of laser Doppler velocimetry of the medtronic advantage bileaflet mechanical heart valve hinge flow

OBJECTIVE: The aim of this study was to evaluate the hinge flow field characteristics of the Medtronic Advantage bileaflet valve and compare them with the flow fields of the St Jude Medical standard valve. The present study provides laser Doppler velocimetry results for the Advantage and St Jude Medical valves to make a direct comparison of the flow fields of the 2 valve designs. This study aids in determining the preclinical efficacy of Medtronic's new bileaflet valve hinge design. METHODS: Two-dimensional laser Doppler velocimetry was used to measure the velocities in the hinge regions of size 29 Medtronic Advantage and St Jude Medical standard bileaflet valve designs. Exact dimensional models of the bileaflet valves, including the hinge regions, were cast from transparent plastic materials to conduct these measurements under simulated physiologic conditions. Laser Doppler velocimetry measurements were conducted under physiologic conditions, with the valves placed in the mitral position of a pulsatile flow loop. Measurements were taken at several elevation levels in the hinge region. Multiple measurement locations obtained in each plane provided a grid work by which the flow fields could be detailed. RESULTS: Velocity measurements obtained for each valve design in the hinge recess were used to reconstruct the flow fields. For the Advantage valve, the peak velocities during leakage flow in the hinge at zero depth, one-third depth, and two-thirds depth levels into the hinge recess were 0.9, 1.6, and 1.8 m/s, respectively. Corresponding values for the St Jude Medical valve were 1.3, 1.6, and 2.1 m/s, respectively. From the reconstructed flow fields, the flow patterns seen within the 2 hinge designs exhibited similar features, with more dynamic flow patterns observed in the Advantage hinge during the forward flow phase. CONCLUSIONS: The present study demonstrated that the hinge flow dynamics of the Advantage bileaflet design were similar to those of the St Jude Medical hinge design. The velocities within the hinge were slightly higher for the St Jude Medical valve but not significantly different. There appears to be more dynamic flow through the hinge of the Advantage valve during the forward flow phase.     

Steady and pulsatile flow studies on a trileaflet heart valve prosthesis

The need for better and longer lasting trileaflet valves has led to the design and development of the ABIOMED polymeric trileaflet valve prosthesis. In vitro fluid dynamic studies in the aortic position indicate that overall it has improved leaflet motion characteristics and pressure drop characteristics compared to the Carpentier-Edwards porcine and Ionescu-Shiley pericardial tissue valves in current clinical use. The ABIOMED valve is, however, more stenotic compared to the St. Jude and Medtronic-Hall low profile mechanical valves, at normal cardiac outputs. Steady and pulsatile flow velocity measurements with a laser-Doppler anemometer system indicate that the flow field downstream of the ABIOMED valve is jet-like and leads to elevated shear stresses. These shear stresses are, however, lower than those observed with the Ionescu-Shiley and Carpentier-Edwards tissue valves. The ABIOMED valves tested had been originally configured for use in valved conduits, and it is therefore our opinion that further improvements can be made to the valve and stent design which would enhance its fluid dynamic performance.     

Concomitant aortic valve replacement and coronary bypass: the effect of valve type on the blood flow in bypass grafts.

OBJECTIVE: In cases of aortic valve replacement, the downstream flow profile and turbulence in the ascending aorta differ according to the prosthetic aortic valve implanted. The objective of this work is to study the influence of prosthetic valve type on the flow in the bypass grafts implanted to the ascending aorta in cases of concomitant aortic valve replacement and coronary artery bypass. METHODS: The study is conducted on 456 patients receiving concomitant aortic valve replacement and coronary bypass vein grafts anastomosed to the ascending aorta. The patients included in the study received a total number of 725 vein grafts, 249 biological aortic valves and 207 mechanical aortic valves. Intraoperative transit time flow measurement was done for all bypass grafts and a multiple regression model was calculated for the factors influencing the flow in the bypass grafts. RESULTS: The mean flow in vein grafts in patients receiving biological valves was 49.79+/-26.88 ml/min, while in patients receiving mechanical valves it was 46.54+/-26.68 ml/min. The multiple regression model revealed that receiving a mechanical valve is an independent risk factor for lower flow in the vein grafts. CONCLUSIONS: The type of the aortic valve implanted and consequently the downstream flow profile in the ascending aorta do affect the flow in the vein grafts in cases of concomitant aortic valve replacement and coronary bypass. Receiving a mechanical aortic valve is an independent risk factor for lower flow in the vein grafts.