J Artif Organs (2001) 4:113-125 �9 The Japanese Society for Artificial Organs 2001

Toshinosuke Akutsu, PhD �9 Daiki Higuchi, BS

Flow analysis of the bUeaflet mechanical prosthetic heart valves using laser Doppler anemometer: effect of the valve designs and installed orientations to the flow inside the simulated left ventricle

Abstract Ever since the first introduction of the ball-type valve by Hufnagel in 1952, which was installed in the de- scending aorta to correct aortic valve insufficiency, great efforts have been aimed to produce a hemodynamically and structurally superior prosthetic heart valve. Bileaflet valves, commercially initiated by the St. Jude medical (SJM) valve, perform satisfactorily, and now the majority of the mechanical-type prosthetic heart valves used clinically are of this type. The recent trend in bileaflet valve design seems to be concentrated on the hinge mechanism and leaflet design to improve performance against thromboembolic complications and hemolysis. This paper studied the effects of hinge location, leaflet configuration, valve opening angle, and valve installed orientation to the flow field inside the simulated ventricle using laser Doppler anemometry. As a model prosthetic valve, the SJM valve was selected as a reference, and newer bileaflet valves, including the ATS, the Carbomedics (CM), and the Jyros (JR) valves, were selected for comparison. The test program also utilized a flow visualization technique to map the velocity field inside the simulated ventricle to complement the information ob- tained using the LDA system. Comparison of the velocity profiles at corresponding flow phases revealed the effects of the differences in valve design and orientation. Based on precise examination of the data, the following general con- clusions can be made: all valves (SJM, ATS, CM, and JR) show distinct circulatory flow patterns when the valve is installed in the antianatomical orientation. The small dif- ferences in hinge location and leaflet configuration can generate noticeable differences, particularly during the ac- celerating flow phase of the valve. The ATS and the CM valves open less during the forward flow phase, and this

Received: September 28, 2000 / Accepted: December 11, 2000

T. Akutsu (~) �9 D. Higuchi Department of Mechanical Engineering, Kanto Gakuin University, 4834 Mutsuura, Kanazawa-ku, Yokohama 236-8501, Japan Tel. +81-45-786-7114; Fax +81-45-786-7098 e-mail: akutsu@kanto-gakuin.ac.jp

results in generally diverse and less distinct flow patterns and slower velocity. This is particularly noticeable for the flow through the central orifice. The SJM valve maintains a relatively higher velocity through the central orifice. The curved leaflet JR valve generates higher but divergent flow during the accelerating and peak flow phases.

Key words Flow visualization �9 Bio-fluid mechanics. Medi- cal equipment �9 Prosthetic heart valve

Introduction

Heart valve replacements to correct cardiovascular prob- lems, such as stenosis, insufficiency, and regurgitation, are conducted quite regularly and are established as important surgical correction procedures. Ever since the introduction of the ball-type valve by Hufnagel 1 in 1952 to correct aortic valve insufficiency, and particularly the orthotopic design by Starr and Edward 2 in 1960, great efforts have been aimed to produce a hemodynamically and structurally superior prosthetic heart valve. Contemporary mechanical pros- thetic valves reflect over 40 years of these efforts. Although many types of prosthetic heart valves have been used, over 85% of the mechanical-type prosthetic valves commonly used in the United States are St. Jude medical (SJM) valves of bileaflet design, because of their excellent clinical results. The recent trend in newer bileaflet valve design seems to be concentrated on the hinge mechanism, where the SJM valve needs some improvement for better washout to reduce the thrombus problem, and leaflet design to improve overall valve performance.

As Aoyagi et al. 3 reported, although the frequency of mechanical hemolysis associated with the use of the SJM valve is relatively high, valve orientation seems to be one of the primary causes of heavy hemolysis. Because of this find- ing, their group at Kurume Medical School adapted the antianatomical orientation as their choice for the mitral valve replacement (MRV) operation. Baudet et al. 4 also reported, based on their 5 1/2-year experience, that the SJM

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Fig. 1. The bileaflet valves used during the experimental program: (1) St. Jude medical (SJM) valve (85~ (2) ATS valve (85~ (3) Carbomedics (CM) valve (78~ and (4) Jyros (JR) valve (80 ~

bileaflet prosthesis requires an antianatomical orientation to provide optimal hemodynamics and subsequently to reduce turbulence in the downstream areas. Echocardio- graphic studies 5 confirm that valve performance is more satisfactory in the antianatomical orientation. These clinical results suggest that the antianatomical orientation is the optimum for MRV. Our previous in vitro experimental study 6-9 showed that the simple circulatory flow offered by the antianatomical orientation of the SJM valve seems to provide a smoother flow field and generates less turbulence during its cardiac cycle. Therefore, it is easy to speculate that the flow is easier for the blood components, which is in good agreement with clinical experience. 3'4

In view of the newer valve design, slight design differ- ences can produce noticeable differences in the end. Manu- facturers are keen to correct their products' weakness and try to improve their products. Flat and curved leaflet design differences in the bileaflet design valve would provide no- ticeable differences in the downstream area. Opening angle differences among bileaflet valve designs would affect flow as well. An interesting phenomenon reported for the ATS valve, 1~ which does not open fully during forward flow, would further complicate the interpretation of the flow analysis.

In this ongoing in vitro investigation, studies of the effect of valve orientation are now extended to include four differ- ently designed contemporary mechanical mitral bileaflet valves, and the use a of laser Doppler anemometer (LDA) system allows us to take more precise numeric informa- tions. The aim of this paper is to study the effects of hinge location, leaflet configuration, and valve opening angle in addition to valve installed orientation. As a model pros- thetic valve, the St. Jude medical valve is selected as a reference, and newer bileaflet valves, including the ATS, Carbomedics, and Jyros valves, are selected for comparison. These are examined in a model ventricle to assess the effect of valve geometry and orientation on its flow field inside the simulated ventricle, as in previous reports. 6 9

The advantage of this kind of in vitro study is that it is relatively easy to change the associated parameters control-

ling the phenomenon. Once the correlation between in vivo and in vitro experimental study is established, the predic- tion of the possible clinical problem can be done.

Experimental models and test facilities

The models selected for this study are typical bileaflet mechanical-type prosthetic heart valves that have been in successful clinical use for a number of years. All valves were either 27-mm or 29-mm mitral valves, as summarized below (Fig. 1): St. Jude medical valve (SJM, 29mm, opening angle 85 ~ St. Jude Medical, St. Paul, MN, USA); ATS valve (ATS, 29mm, opening angle 85 ~ ATS, Minneapolis, MN, USA); Carbomedics valve (CM, 29mm, opening angle 78 ~ Sulzer Carbomedics, Austin, TX, USA); Jyros valve (JR, 29mm, opening angle 80 ~ Manley-Western, West Sussex, UK).

The SJM valve was selected as a standard valve. The ATS and the CM valves were selected for their new design ap- proaches, which differ in hinge location and configuration from the SJM valve. The ATS, CM, and JR valves have a lower hinge location, which sits within a valve ring area, whereas the SJM valve has a higher hinge location (more upstream location), which sits outside the valve ring. Be- cause of these differences in hinge location, the leaflet loca- tion will also differ when the valve opens fully. The leaflet of the SJM valve sits within a valve ring area when fully open, whereas the leaflets of the other valves sit partially outside. When most of the leaflet sits within a valve ring area, as in the SJM valve, flow around the leaflet is more likely to be fixed. On the other hand, when a large portion of the leaflet sits outside the valve ring area, flow around the leaflet is more likely to be affected by the downstream conduit shape. The JR valve has curved leaflets and has a unique mecha- nism to allow self-rotation of the support and leaflet, and thus is included in this study. The pulsatile flow facility and associated instrumentation were identical to those in the previous study. 6-9 The pulsatile flow facility used in this study

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is essentially a cardiac pulse duplicator designed to permit hydrodynamic performance assessment of both mitral and aortic heart valves. The housing containing the mitral and aortic valves is made from solid Plexiglas in accordance with the anatomical information. Since the present study is lim- ited to the flow inside the simulated left ventricle affected by the leaflet opening angle, the selection of the aortic valve has little influence on the flow in general. Although the aortic valve has very little influence on the flow inside the ventricle during most of the diastolic cycle, change in flow direction, which is likely to happen during the opening and closing phases of the aortic valve, must be avoided. The ball-type aortic valve was selected for its reliability and the nondirectional nature of the forward and backward flow. Attached to the block is a conically shaped, thin, transparent polyurethane film ventricle simulating the shape and the flexible character of the natural ventricle. 1~ The transparent character of the material was essential for the flow visualiza- tion study, to map the velocity field. The flexible ventricle itself is supported inside the Plexiglas box, which, in turn, is connected to a positive displacement piston pump. Motion of the piston is transferred to the flexible ventricle through the essentially incompressible surrounding medium (water) between the ventricle and the Plexiglas box.

The circulation system is an adaptation of the lumped circuit proposed by Westerhof et al. 12 Immediately fol- lowing a straight Plexiglas aorta section is an adjustable windkessel, which provides the required characteristic resis- tance through a filter element of desired porosity and com- pliance by the air chamber above it. Peripheral resistance is provided by either another filter element or a simple water valve. Connected to the system is an adjustable constant head tank to maintain the necessary back pressure. Over- flow blood analogue fluid returns to the left atrium.

The system consists primarily of a model left ventricle, with prosthetic valves under test, whose diastolic and sys- tolic phases are controlled by a positive displacement pump, driven by a stepper motor, which in turn is control- led through a microcomputer system (Toshiba Dynabook- 486EZ, Tokyo, Japan). The LDA test arrangement with associated instrumentation used during the experiment is schematically shown in Fig. 2. LDA measuring locations are shown in Fig. 3.

The measurement of velocity profile and turbulent inten- sity was carried out using a two-component LDA system (Ion Laser Technology, Model 5490ACWC Argon Laser with Model 5400C power supply, Salt Lake City, UT, USA; Dantec 3-beam Two-component Ar-ion Optics; Dantec

116

Fig. 3. Schematic diagram showing the cross section of the simulated heart and traverse locations for laser Doppler anemometry measurement

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Model 55N20 Frequency Tracker) in the forward scatter mode. Velocity measurements were conducted on the cen- tral vertical plane. The velocity vector U at a location is resolved into two components: u and v. Here u is the direc- tion toward the aortic valve at 45 ~ with respect to the local vertical. The v component is at 90 ~ to u. The measuring volume is calculated to be 0.088mm in diameter and 1.30ram in length. Two Bragg cells were used to provide a frequency shift of 400 KHz for each channel, allowing mea- surement of the reverse flow. With the data sampling rate of 500Hz, which provides a maximum frequency response of 250Hz, the choice of bandwidth setting was found to be adequate. The decision to use a sampling rate of 500Hz was based on the examination of the frequency spectrum associ- ated with the valves under steady test, 13 where the dominant turbulent frequency was found to be below 250Hz. Hasenkam et al. 14 also found a rapid decline in the turbulent energy content beyond 100Hz.

Mapping of the velocity field was carried out using a 200 frames/s high-speed video system (Ion Technology, Model 5490ACWC Argon Laser with Model 5400C power supply for light source; Detect D I G - A T I I board and HAS-200R camera with DIG-HASIF interface board for capturing the video image, Tokyo, Japan).

Pressure monitoring was conducted by physiological pressure transducers (Gould Statham P-23 series, Oxnard, CA, USA), and the flow rate was monitored by a pair of magnetic flow transducers (In Vivo Metric Systems Model K, Healdsburg, CA, USA, with Gould Statham Blood Flow- meter Model SP2202, Oxnard, CA, USA).

Experiments were conducted at a pump frequency of 71BPM for a velocity measurement with a stroke volume of 72ml. The average flow rate was about 41/min. The wave form used was sinusoidal. The video capture rate was 200

frames/s with shutter speed of 1/1000s for a duration of 0.96s (192 frames). Captured video images were then ana- lyzed using an image analysis program (Detect, Tokyo, Japan DIPP95) for frame-by-frame analysis, to obtain the velocity pattern.

Since the Reynolds number effect (effect of viscosity change) on the velocity profile for the Reynolds number range experienced during this study is considered to be very small, saline was selected as a working fluid. The systolic and diastolic pressures were adjusted at 130 and 70mmHg, respectively, with a pump frequency of 71BPM.

Results and discussion

Flow field downstream from the SJM valve

SJM valve installed in the anatomical orientation

Under the pulsatile flow condition, three distinct flow phases may be identified: accelerating flow phase, peak flow phase, and deceleration phase. They affect the downstream velocity profile, and the present comprehensive test pro- gram aims at studying the complete time history of these velocity profile changes.

Although the sequential events of the flow inside the simulated ventricle using the particle tracking method were already reported in the our previous paper, 6'7 a summary of events is described based mainly on the velocity vector information obtained using the particle tracking method to illustrate the characteristic nature of the events, and the complementary LDA results are also presented and dis- cussed here.

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At the onset of opening, the flow through the mitral valve is fairly disorganized and directed sideways to form a divergent flow field due to the inclination of the half-discs' opening characteristics and resulting in high levels of insta- bility. The LDA result clearly shows the higher degree of turbulent stresses immediately downstream from the tested mitral valve; the aortic valve side is especially more promi- nent (Fig. 4a). The observed velocity vectors in general are shorter, indicating lower velocity. Subsequently, as the flow accelerated more, the flow was observed to be more orga- nized to form a clearer velocity pattern with increase in instability. Due to the initial directional flow generated by the two half-discs, the flow tends to form two circulatory flows, one located close to the aortic valve and the other located close to the center of the ventricle. This trend gradually decreases but continues until peak flow is reached.

Once the flow reaches its peak value, the velocity pattern becomes more stable with a large circulatory flow pattern with reduction in instability. The L D A results confirm this clearly (Fig. 4b). The turbulent stress level is much lower than that during the accelerating phase. The velocity distri- bution is fairly uniform, indicating that the flow through the central orifice is maintaining its velocity. This large circula- tory flow pattern is slightly disturbed by the inflow from the mitral direction.

The decelerating flow phase is categorized as diminish- ing velocity vectors, as indicated by the shorter velocity vectors.

SJM valve installed in the antianatomical orientation

When the valve is rotated 90 ~ from the anatomical orienta- tion (antianatomical orientation), the general trend experi- enced with the anatomical position is valid in that it tends to form a single circulatory flow at the end. At the onset of opening, the flow through the mitral valve is fairly disorga- nized; however, the directional flow in the sideways direc- tion observed in the anatomical position is absent, since the affected area is now away from the observed plane. The corresponding LDA result is shown in Fig. 5a. It is also worth pointing out that flow through the central orifice (to which these LDA results correspond) has a relatively large velocity, thus contributing to forming a simpler circulatory flow pattern within the simulated ventricle. Note also that the turbulent stress level is much lower than in the case of anatomical orientation. Subsequently, as the flow acceler- ated more, the flow was observed to be more organized and to form a clearer velocity pattern. Here, a single circulatory flow was observed rather than two circulatory flows, as ob- served in the previous case. This trend continues until peak flow is reached. At the peak flow phase, the velocity pattern becomes more stable with a large circulatory flow pattern. The LDA result also shows this very clearly with lower turbulent stresses (Fig. 5b). The circulatory flow pattern is observed to be stabler than in the case of the anatomical position.

Although a slight difference in velocity and turbu- lent stress distribution exists between the anatomical and

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antianatomical orientations during the peak flow phase, uniformly downwards velocity profiles are common in both orientations, suggesting very natural-looking three- dimensionally uniform downwards velocity. Higher turbu- lent stress during the early accelerating phase with the valve in the anatomical orientation seems to correlate well with the clinical experience of Aoyagi 3 at Kurume Medical School that valve orientation seems to be one of the primary causes of heavy hemolysis. This 3 and other <5 results suggest that the antianatomical position is the optimum orientation for MVR. Our observation in our in vitro study using the flow visualization technique 7 shows that the simple circula- tory flow offered by the antianatomical orientation of the SJM valve seems to provide a smoother flow field and gen- erates less turbulence during its cardiac cycle, and hence that the flow is much more easier for the blood components. One can easily speculate that this kind of less disturbed flow will result in less hemolysis and fewer embolic complica- tions. It is also useful to find the suspected area of hemolysis and thromboembolic complication. To complement the flow visualization technique, L D A is useful to find the pre- cise velocity and turbulent stress value.

Flow field downstream from the CM valve

CM valve installed in the anatomical orientation

At the onset of opening, the flow through the mitral valve is fairly disorganized and directed down and sideways, form-

ing a divergent flow field, due to the inclination of the two half-discs. The dominant flow direction is slightly towards the aortic side of the ventricle wall, with subsequent down- wards movement towards the apex area. The L D A results show the distinct divergent flow with higher turbulent stresses (Fig. 6a). The mitral valve side also has strong downwards flow. Subsequently, as the flow accelerates more, the flow is observed to be more organized, forming a clearer velocity pattern. Flow towards the aortic side of the ventricle wall is now directed more downwards and past the lower-central region of the ventricle. Due to the initial di- rectional flow generated by the two half-discs, flow tends to form two flow areas, one area with a circulatory nature located close to the aortic valve, and the other located close to the apex of the ventricle. This trend gradually weakens but continues until peak flow is reached.

Once the flow reaches its peak value, the velocity pattern becomes largely a circulatory flow pattern close to the aortic valve side, with slight directional instability indicated by the slight randomness of the flow directions towards the apex region. The LDA results show two peak flow patterns im- mediately downstream from the mitral valve (Fig. 6b). One possible reason for this less distinct flow pattern is the smaller opening angle of the CM valve (78~ which tends to make the flow diverge more sideways and form two peak types of the velocity distribution.

The decelerating flow phase is categorized by diminish- ing velocity vectors, as indicated by the shorter velocity vectors.

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CM valve installed in the antianatomical orientation

When the valve is rotated 90 ~ from the anatomical orienta- tion (antianatomical orientation), a similar general trend is experienced, in that it tends to form a single circulatory flow at the end. At the onset of opening, the flow through the mitral valve is directionally uniformly downward. Direc- tional flow in the sideways direction observed in the ana- tomical orientation is absent, since the affected area is now away from the observed plane. The LDA results show this trend with much lower velocity values, indicating relatively slower flow through the central orifice (Fig. 7a). Subse- quently, as the flow accelerates, the flow is observed to be more organized and quickly forms a circulatory velocity pattern. Here, the velocity field forms a single circulatory flow rather than two circulatory flows, as observed in the previous case, because of the more downward direction of the main flow. Note, however, that the velocity is much lower than in the corresponding SJM case, indicating that flow through the central orifice is very weak. This trend continues until peak flow is reached. At the peak flow phase, the velocity pattern is generally a large circulatory flow with slight directional instability, as indicated by the randomness of the velocity directions. Since the LDA ve- locity distribution is the result of the average over a number of consecutive cycles, any true random velocity fluctuation will be canceled out. In the LDA result shown in Fig. 7b, this randomness resulted in almost no flow in the lower region because of this characteristic. The circulatory flow

pattern is slightly more stable than in the anatomical orien- tation case, but equally unclear. It is, however, definitely less clear than the pattern observed with the SJM valve, 6'7 which formed a clearly circulatory flow.

Again, the decelerating flow phase is categorized as diminishing velocity vectors, as indicated by the shorter velocity vectors.

Flow field downstream from the ATS valve

A TS valve installed in the anatomical orientation

At the onset of opening, the flow through the mitral valve is mainly downwards and partly directed sideways, to form a divergent flow field due to the inclination of the two half- discs. The dominant flow direction is towards the aortic side of the ventricle wall, similar to that with the CM valve. The LDA results are also very similar to those with the CM valve (Fig. 8a). The velocity vectors in general are shorter, indicating lower velocity. The degree of instability is similar to that with the CM valve. Subsequently, as the flow accel- erated, the flow was observed to be more organized, form- ing a clear velocity pattern that was very similar to that observed with the CM valve. Again, the flow towards the aortic side of the ventricle wall is clearly crossing diagonally at the center of the ventricle. Due to the initial directional flow generated by the two half-discs, the flow tends to form two circulatory flows, one located close to the aortic valve

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and the other located close to the apex of the ventricle, where it becomes unclear. This trend gradually weakens but continues until peak flow is reached.

Once the flow reaches its peak value, the velocity pattern becomes a large, circulatory flow pattern with slight direc- tional instability. This circulatory flow pattern is clearer than that for the CM valve with the same valve orientation. The L D A results show a velocity profile similar to that with the CM valve (Fig. 8b), but flow through the aortic valve side orifice is more dominant, resulting in more uhiform velocity distribution.

The decelerating flow phase is categorized by diminish- ing velocity vectors, as indicated by the shorter velocity vectors, but with slightly clearer flow patterns than the CM valve.

A TS valve installed in the antianatomical orientation

When the valve is rotated 90 ~ to the antianatomical orienta- tion, a similar general trend is experienced, in that it tends to form a single circulatory flow at the end. At the onset of opening, the flow through the mitral valve is predominantly downwards. Flow in the sideways direction, as observed in the anatomical orientation, is present, although the affected area is now away from the observed plane. Surprisingly, the LDA results show almost no velocity (Fig. 9a). This may be caused by the randomness of the velocity profile over a number of cycles. Subsequently, as the flow accelerates, it is observed to be more organized and quickly forms a clear

circulatory velocity pattern similar to that with the SJM valve. 6 Here, a single circulation was observed, rather than two circulatory flows, as observed in the previous case, be- cause of the more downward direction of the main flow. This trend continues until peak flow is reached. At the peak flow phase, the velocity pattern is a clear, large, circulatory flow pattern more distinct than that with the CM valve. This is in fact very similar to the case with the SJM valve, 6 but with much lower velocity. The LDA results show a profile similar to that of the CM valve (Fig. 9b).

It is interesting to discuss the phenomenon observed visually during this experiment. An interesting phenom- enon for the ATS valve during the accelerating phase is the transient phenomenon of the valve leaflet. At the beginning of the accelerating phase, the valve seems to open briefly to a full 85 ~ This, however, does not continue very long. It soon reduces the opening angle somewhat and seems to keep this angle for the rest of the acceleration and peak flow period. This implies that the outer two orifices may have lower pressures than the central orifice, thus causing the higher velocity in that region. The central orifice, however, may have a very low velocity. This may be the reason why the ATS in the antianatomical orientation has lower veloc- ity than the SJM or CM valve. This may also cause random- ness of the valve opening. Because of this, the velocity may be lower if randomness of the valve opening exists. The cause of this phenomenon is not clear yet. It may well be due to the moment effect, as suggested by Feng and oth- ers. 1~ If our view is true, a higher velocity directed sideways

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will generate an uneven velocity profile downstream from this valve. This may cause additional instability in this region.

Flow field downstream from the JR valve

JR valve installed in the anatomical orientation

One of the main features of the JR valve is its unique valve housing, which allows the leaflet to self-rotate during the normal cardiac cycle. 15'16 In our in vitro experimental study, we also observed that the leaflet self-rotates. Since one of the objectives of this experimental study was to find the effect of the valve orientation to the flow field inside the simulated ventricle, we intentionally fixed the valve seat so that the leaflet would not self-rotate.

The characteristic fluid dynamic parameters at four typi- cal instants obtained using the particle tracking method are presented in Fig. 10 for the JR valve installed in the ana- tomical orientation. At the onset of opening, downwards motion of the flow occurred slightly earlier than with the SJM valve, The flow through the curved mitral valve leaflet of the JR valve was mainly downwards but diverted side- ways, to form a divergent flow field due to the curved nature of the two half-discs. However, the dominant directions of flow are more towards the mitral valve side of the ventricle wall, and the flow towards the aortic side of the ventricle walI is weaker initially (Fig. 10a). The LDA result shows this trend more clearly (Fig. l la ) . The velocity vectors in general are longer, indicating higher velocity. The degree of instability is higher than that with other valves. Subse- quently, as the flow accelerates, the aortic side of the flow

gained strength and the flow was observed to be less orga- nized, forming two circulatory regions within the ventricle. Again, the flow towards the aortic side of the ventricle wall is clearly crossing diagonally at the center of the ventricle (Fig. 10b). Due to the initial directional flow generated by the two curved half-discs, flow tends to form two circulatory flows similar to those with the CM valve but clearer, one located close to the aortic valve and the other located close to the apex of the ventricle, where it becomes unclear. Stronger flow on the aortic side continues to grow until peak flow is reached.

Once the flow reaches its peak value (Fig. 10c), the veloc- ity pattern retains this unorganized flow pattern with direc- tional'instability, forming two circulation regions (Fig. 10c). By using LDA, this two-circulation flow pattern can be seen as two peak flow patterns immediately downstream from the valve location (Fig. l lb) .

The decelerating flow phase is categorized by diminish- ing velocity vectors, as indicated by the shorter velocity vectors (Fig. 10d).

JR valve installed in the ant\anatomical orientation

When the valve is rotated 90 ~ to the antianatomical orienta- tion, a similar general trend is experienced, in that it tends to form a single circulatory flow at the end (Fig. 12). At the onset of opening, flow through the mitral valve is predomi- nantly downwards. Flow in the sideways direction, as ob- served in the anatomical orientation, is weak, since the affected area is now away from the observed plane (Figs, 12a and 13a). Subsequently as the flow accelerates, it is observed to be more organized and quickly forms a clear

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circulatory velocity pattern similar to that with the ATS and CM valves. 6'7 Here, a single circulation was observed, rather than two circulatory flows, as observed in the previous case, because of the more downward direction of the main flow (Fig. 12b). This trend continues until peak flow is reached. At the peak flow phase, the velocity pattern is a clear, large, circulatory flow pattern (Figs. 12c and 13b), more distinct than that with the CM valve.

Based on the experimental results obtained for the bileaflet valves, the general trends become clear. Both the ATS and the CM valves and the previous SJM valve 6 show clear circulatory flow patterns when the valve is installed in the antianatomical orientation. It is surprising to find that even the small differences in hinge location and configura- tion among these valves can generate such noticeable differ- ences in the end. Both the ATS and the CM valves tested in this experimental study generate less clear flow patterns and lower velocities than the previously studied SJM valve. 6'7 The fact that the SJM valve installed in the anatomical orientation generates a less clear flow field, higher turbulent normal stress during the accelerating flow phase, and higher Reynolds stress during peak flow phase than the case with the antianatomical orientation seems to correlate well with the higher number of hemolytic cases reported by the Kurume group. 3 If this observation is true, higher number of hemolytic cases are more likely for the other valves, which generally generate much less clear flow fields and higher turbulent stresses. However, it is difficult to rank the valves

from these limitted findings; further detailed study and quantitative assessment are in order.

C o n c l u s i o n

Based on the experimental results obtained for the bileaftet valves, the following general conclusions can be made. All valves (SJM, ATS, CM, and JR) show distinct circu- latory flow patterns when the valve is installed in the antianatomical orientation. The small differences in hinge location and leaflet configuration can generate noticeable differences, particularly during the accelerating flow phase of the valve. The ATS and the CM valves open less, and this results in generally divergent and less distinct flow patterns and slower velocity. This is particularly noticeable for the flow through the central orifice, while the SJM valve main- tains the flow through the central orifice. The curved leaflet JR valve generates a higher but divergent flow during the accelerating and peak flow phases.

R e f e r e n c e s

t. Hufrtagel CA, Harvey WP, RaN1 PJ, McDermott TF. Surgica! correction of aortic insufficiency. Surgery 1954;35:673-683

2. Starr A, Edwards ML, McCord CW, Griswold HE. AorticA re- placement: clinical experience with a semirigid ball-valve prosthe- sis. circulation 1963;27:77%783

3. Aoyagi N, Tanaka I, Nishi Y, Yamashita M, Oryouji A, Hara T, Kosuga K, Ooishi K. Long-term result of MRV by SJM valve. J Jpn Thorac Cardiovasc Surg 1991;39(8):1126-1130 (in Japanese)

4. Baudet EM, Oca CC, Roques XF, Laborde MN, Hafez AS, Collot MA, Ghidoni IM. A 5 1/2 year experience with the St. Jude Medical cardiac valve prosthesis. Early and late results of 737 valve replacements in 671 patients. J Thorac Cardiovasc Surg 1985;90: 137-144

5. Duveau D, Michaud JL, Despins P, Patra P, Train M, Dupon H, Rozo L, Carlier R. Mitral valve replacement with St. Jude Medical prosthesis: 242 cases with clinical results and an evaluation of pros- thesis positioning. In: De Bakey IME (ed) Advances in Cardiac Valves, Clinical Perspectives (Proceedings of the Third Interna- tional Symposium on the St. Jude Valve, November 1982, Scottsdale, Arizona). New York: Yorke Medical Books, 1983;183- 190

6. Akutsu T, Higuchi D. Effect of mechanical prosthetic heart valve orientation on the flow field inside the simulated ventricle: com- parison between St. Jude Medical Valve and Medtronic-Hall Valve. J Artif Organs 1999;2:39-45

7. Akutsu T, Higuchi D. Effect of the mechanical mono and bileaflet heart valve orientation on the flow field inside the simulated ven- tricle. J Artif Organs 2000;3 (in press)

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8. Modi VJ, Bishop WF, Akutsu T_ Unsteady fluid dynamics of three contemporary heart valves using a two component LDA system. Artif Organs 1991;14(4):103-107

9. Akutsu T, Modi VJ. Unsteady fluid dynamics of several mechani- cal prosthetic heart valves using a two component laser Doppler anemometer system. Artif Organs 1997;21:1110-1120

10. Feng Z, Umezu M, Fujimoto T, Tsukahara T, Nurishi M, Kawaguchi D, Masuda S. Analysis of ATS leaflet behavior by in vitro experiment. J Artif Organs 1999;2:46-52

11. Du Plessis LA, Marchand P. The anatomy of the mitral valve and its associated structures. Thorax 1964;19:221-227

12. Westerhof N, Elzinga G, Sipkema P. An artificial arterial system for pumping heart. J Appl Physiol 1971;31(5):776-781

13. Akutsu T. Hydrodynamic performance of mechanical prosthetic heart valves. PhD Thesis, University of British Columbia, BC, Canada (1985)

14. Haseukam JM, Hygaard H, Giersiepen M, Reul H, Stodkilde- Jorgenesen H. Turbulent stress measurements downstream of six mechanical aortic valves in a pulsatile flow model. J Biomech 1988;21(8):631

15. Reece IJ, Tareif HA, Tolia J, Sheth J. Preliminary clinical experi- ence with the Jyros bileaflet mechanical prosthesis. J Card Surg 1995;10:40-45

16. Dihmis WC, Wilde P, Hutter JA. Early experience with the Jyros bileaflet valve. Is leaflet rotation clinically important? J Heart Valve Dis 1994;3:191-196