Asian Cardiovasc Thorac Ann 2000;8:3-10
© 2000 Asia Publishing EXchange Pte Ltd
In Vitro Hydrodynamics of Four Bileaflet Valves in Mitral Position
Feng Zhong Gang, PhD,
Mitsuo Umezu, PhD,
Tetsuo Fujimoto, MD,
Toshiya Tsukahara, MS,
Masakazu Nurishi, MS,
Daisuke Kawaguchi, BS
Department of Mechanical Engineering
Umezu Biomedical Engineering Laboratory
Waseda University
Tokyo, Japan
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For reprint information contact: Mitsuo Umezu, PhD or Feng Zhong Gang, PhD Tel: 81 3 5286 3256 Fax: 81 3 3200 2516, email: umezu{at}mn.waseda.ac.jp or iac97050{at}mn.waseda.ac.jp, Department of Mechanical Engineering, Umezu Biomedical Engineering Laboratory, Waseda University, 3-4-1 Ohkobo, Shinjuku-ku, Tokyo 169-8555, Japan.
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Abstract
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Hydrodynamics of St. Jude Medical, Carbomedics, Advancing The Standard, and On-X bileaflet valves with an annular diameter of 25 mm were obtained using an in-vitro test system. Steady flow studies demonstrated different pressure drops due to differences in valve design, particularly the geometric orifice diameter and the opening angle. The On-X valve produced the least pressure drop, whereas the Carbomedics valve had the greatest pressure drop. In pulsatile flow experiments, the On-X and St. Jude Medical valves consistently produced the lowest mean positive pressure gradients, while the Carbomedics valve had the highest gradients. In spite of its parallel leaflets design, the On-X valve showed a closing volume as small as that of Carbomedics valve. The results indicate that a larger orifice diameter and greater opening angle can significantly reduce transvalvular pressure loss. This study also demonstrated that attempts to improve the hydrodynamic efficacy of the On-X valve were successful in reducing the pressure gradient as well as maintaining a low closing volume.
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Introduction
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Modern prosthetic heart valves are generally considered to be safe and efficient as replacements for malfunctioning natural valves, although complications such as thrombosis and thromboembolism are still unsolved problems. It is estimated that nearly 8000 valve prostheses are implanted each year in Japan. The majority (90%) are mechanical, of which approximately 95% are bileaflet valves. Several bileaflet valves are clinically available and it is of interest to compare and evaluate the hemodynamic characteristics of the different valves.1–3
The first bileaflet mechanical valve for clinical use was the St. Jude Medical (St. Jude Medical, Inc., St. Paul, MN, USA) introduced in 1977. Since then, the St. Jude Medical (SJM) bileaflet valve has become the predominant mechanical prosthesis worldwide. The Carbomedics (CM) valve (Carbomedics, Inc., Austin, TX, USA) has been implanted since 1986 and approval for use in the USA was obtained in 1993. In 1992, another pyrolytic carbon bileaflet valve became available: the Advancing The Standard (ATS) valve (ATS Medical, Inc., Minneapolis, MN, USA). The ATS valve is still in the process of obtaining approval for use in the USA. More recently, the On-X bileaflet valve has been introduced by the Medical Carbon Research Institute (MCRI, Austin, TX, USA), which incorporates a high-profile valve conduit to achieve an optimal length to annulus ratio for improved hydro-dynamic efficiency.
The in-vitro evaluation of an artificial valve is usually conducted in the aortic position. However, recent in-vivo and in-vitro studies have suggested that the shape of the downstream conduit after a bileaflet valve may strongly affect the hydrodynamic performance of the valve.4–6 Although the downstream shape after the mitral valve is different from that after the aortic valve, prosthetic valves with the same design are implanted in both aortic and mitral positions. Thus, it is necessary to investigate the hydrodynamic performance of prosthetic valves in the mitral position. To this aim, a test chamber was developed to simulate the dimensional geometry of the mitral position and test the valves therein. This study compared the hydrodynamic behavior of the aforementioned valves as mitral prostheses.
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Materials and Methods
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Geometric profiles and crosssectional drawings of the valves are shown in Table 1
and Figure 1. Two samples of each valve were studied. The structures and the major design features of the tested valves are described below.
St. Jude Medical 25 mm (Hemodynamics Plus)
The SJM valve is a bileaflet prosthesis consisting of two flat leaflets retained in an orifice ring by a hinge mechanism. The orifice ring and the leaflets are made of pyrolytic carbon deposited on a graphite core. Each leaflet has two ears inserted into the "butterfly-shaped" pivot depressions on the inner orifice ring. The opening angle of each leaflet is 85° with a travel angle of 60°. There are two series of SJM valves: SJM Masters Series standard cuffed valve and SJM Masters Series Hemodynamic Plus (HP) cuffed valve. In this study, the 25-mm SJM HP was employed. The SJM HP valve has a larger orifice than those with standard cuffs; it is reported to have superior hemodynamic performance to the same size of SJM standard cuffed valve and considered to be equivalent to the next largest size of SJM standard cuffed valve.7
Carbomedics 25 mm
The Carbomedics valve is also a bileaflet prosthesis consisting of two flat leaflets retained in an orifice ring by a similar hinge mechanism to that of the SJM valve. The orifice of the Carbomedics valve is made entirely of pyrolytic carbon. The Dacron sewing ring is Biolite carbonated and held in position by two titanium bands, preventing leaflet escape. This stiffening ring also controls the pivot geometry. Each leaflet opens to 78° with a travel angle of 53°. This limited excursion design was intended to reduce regurgitation and aid rapid closing when it is implanted at any angle.8
Advancing The Standard 25 mm
The ATS bileaflet heart valve is made of pyrolytic carbon. The orifice ring is pure pyrolytic carbon, while the leaflets incorporate a graphite substrate containing 20% tungsten for radiopacity. The sewing ring is made of double-velour Dacron material on a titanium rotation ring, which surrounds the orifice ring. A specific feature of the ATS valve is its open pivot design in which the leaflet pivot point and stoppers are all convex and located on the orifice inner circumference. The opening angle of the ATS valve is 85° and the travel angle is 60°. However, investigations in in vivo and in vitro have demonstrated that the ATS valve does not fully open in situ, although this phenomenon does not produce any functional dis-advantage.4–6
On-X 25 mm
With a newly developed process of carbon coating, the On-X carbon valve is 20% stronger, 50% tougher, and possesses 25% higher fracture strain than the former silicone-alloyed pyrolytic carbon.9 Based on this advan-tage, the On-X valve was designed with an emphasis on improved hydrodynamic efficacy by introducing a flared inlet and a long body with parallel leaflets. The sewing ring is fixed supraannularly, allowing an increase in the internal orifice diameter of the valve as shown in Table 1
. The flared inlet was designed with the aim of reducing the inlet pressure loss, and the elongated body can straighten and organize the inlet flow, thereby reducing the total energy loss, while the long body reduces the leaflet travel angle, which may result in a smaller closing volume. Thus, the travel angle is only 47°, owing to its axial elongation, although the On-X valve opens to 90°.
Steady forward flow tests were conducted using the conventional set-up with 2 overflow tanks. The valve was inserted in its vertical position in a horizontal tube with an internal diameter of 25 mm that coincided with the tissue annular diameter of the valves tested. The pressure drop across the valve was determined by a manometer and measured between 2 points located 1.5 x diameter upstream and 3.5 x diameter downstream.
Pulsatile tests were performed using a pneumatically driven left heart simulator developed in our laboratory (Figure 2). In this system, the preload was adjusted by a reservoir with constant pressure and the afterload was produced by a 3-element Windkessel model. A pneu-matically driven pump was developed, in which the mitral valve and the aortic valve are placed at opposite ends of the ventricular chamber; consequently, the ventricle is tube-shaped. Figure 3 shows the pulsatile pump system. One of the advantages of this tube-pump design is that a downstream conduit can be incorporated below the mitral position in which the valve is tested. The down-stream conduit has an abrupt enlargement after the test valve so that the leaflets stretch into the expanded space during opening. This configuration is valid for simulating a valve implanted in the mitral position because the transvalvular flow in the test chamber is much closer to that of an implanted valve (Figure 4).
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Figure 4. Comparison of the geometric dimensions of (A) mitral valve replacement and (B) the test chamber.
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The valves were mounted in the mitral position and the leaflets were oriented vertically to diminish the effect of gravity. The structure of the valve test chamber and the position of the test valve are shown in Figures 5A and 5B. The inlet pressure was measured at 50 mm (approximately 2 x diameter) upstream of the mitral valve, as the atrial pressure. The ventricular pressure was measured at two perpendicular locations (A and B) at the same downstream distance of 85 mm (approximately 3.5 x diameter) from the test valve. These two perpendicular positions indicated that the pressure measurement was beyond the recircu-lation area when they displayed the same pressure.
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Figure 5. Test chamber in the pulsatile duplicator with (A) St. Jude Medical valve in the mitral position and (B) On-X valve in the mitral position.
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Each valve was tested under pulsatile conditions at 2 pulse rates (70 and 100 beats•min–1). For each pulse rate, 3 different cardiac outputs were chosen for testing; 3, 4, and 5 L•min–1 with a pulse rate of 70 beats•min–1 and 5, 6, and 7 L•min–1 with a pulse rate of 100 beats•min–1. The systolic fraction in one cardiac cycle was set at 35% for both pulse rates. The average aortic pressure was maintained at 90 to 110 mm Hg, depending on the different cardiac outputs. The test fluid was normal saline (0.9% w/v). Flow rates were measured with electromagnetic flowmeters (MFV-2100; Nihon Kohden, Tokyo, Japan) and pressure was determined with strain-gauge-type pressure transducers (UK801; Baxter, Irvine, CA, USA). During the test, 6 signals corresponding to mitral flow rate, aortic flow rate, atrial pressure, 2 ventricular pressures, and aortic pressure were acquired by a computer at a sampling time of 6 msec. These signals were used to analyze parameters of the pulsatile test. The parameters evaluated in this study were mean positive pressure gradient and closing volume; the mean and standard deviation for each parameter were calculated from 10 successive pulse cycles.
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Results
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Figure 6 shows the typical waveforms of flow rate and pressure at a cardiac output of 4 L•min–1 and pulse rate of 70 beats•min–1 in the pulsatile flow duplicator. As shown in Figure 6A, the system can produce mitral and aortic flow waveforms similar to those of physiological conditions. In Figure 6B, a severe fluctuation in atrial pressure (the waterhammer phenomenon of the mechanical valve prosthesis) can be seen when the valve closes. Figures 6C and 6D represent the ventricular pressures obtained at the perpendicular positions A and B (see Figure 5), respectively. It can be seen that the pressures at these 2 positions are identical and therefore, that the pressure measurement is outside the recirculation region at the corner of the enlargement of the downstream conduit. Figure 6E shows the compliance pressure; because the connector between the aortic valve and the compliance is rigid (i.e., it lacks the elasticity of the aortic arch), the compliance pressure shown in Figure 6E can be regarded as closer to the aortic pressure.
Figure 7 shows the pressure drop across the tested valves versus flow rate under steady flow conditions. It can be seen that the On-X valve produced the smallest pressure drop. The pressure drop of the SJM valve was slightly higher than that of the On-X valve. The ATS and CM valves exhibited almost the same high pressure drops. With a flow rate of 15 L•min–1, the pressure gradients for the ATS, CM, SJM, and On-X valves were 1.67, 1.77, 0.55, and 0.39 mm Hg, respectively.
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Figure 7. Pressure drop of the 4 valves under steady forward flow. ATS = Advancing The Standard, CM = Carbomedics, SJM = St. Jude Medical.
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The mean positive pressure gradients in the pulsatile flow tests are listed in Table 2 and summarized in Figure 8. Figure 9 shows the transvalvular flow of each of the 4 valve types under conditions of 4 L•min–1 cardiac output and 70 beats•min–1 pulse rate. It can be seen that flow through all 4 valves was similar. The main differences occurred during valve closure. The SJM valve displayed a markedly larger closing volume compared to the other 3 valves. The On-X valve sometimes produced a large closing volume but the average was consistently low. Comparisons of the closing volumes at different pulse rates can be seen in Figures 10A and 10B.
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Figure 8. Pressure gradients of the 4 valves in the mitral position (standard deviation is less than 5%) at pulse rates of (A) 70 beats•min–1 and (B) 100 beats•min–1. ATS = Advancing The Standard, CM = Carbomedics, SJM = St. Jude Medical.
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Figure 10. Closing volumes of the valves in mitral position (standard deviation approximately 10%) with pulse rates of (A) 70 beats•min–1 and (B) 100 beats•min–1. ATS = Advancing The Standard, CM = Carbomedics, SJM = St. Jude Medical.
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Discussion
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The differences in pressure drop (Figure 7) were related to the valve structures, particularly to the geometric orifice diameter and the opening angle (Table 1). The On-X valve produced the smallest pressure drop, mainly due to its larger internal orifice diameter and the parallel opening of the leaflets. The SJM valve also benefited from its larger orifice diameter and greater opening angle. Because of the smaller orifice diameters of the ATS and CM valves, they were expected to produce larger pressure gradients than the SJM and On-X valves. However, the ATS valve with an 85°-opening angle, might be expected to produce a markedly lower pressure drop than the CM valve with a 78°-opening angle. In fact, the ATS valve did not open fully under the steady flow test conditions, thus, it produced a pressure drop close to that of the CM valve (Figure 7). The reason why the ATS valve does not open fully has been explained previously.5
In the pulsatile flow test, the SJM valve and the On-X valve produced the lowest mean positive pressure gradients and the CM valve had the highest, under each experimental condition. The results also demonstrated that a larger orifice diameter and greater opening angle benefited the hydrodynamic performance of the valve. The ATS valve produced a lower mean positive pressure gradient than the CM valve in pulsatile flow, although it had nearly the same pressure drop as the CM valve under steady flow because it was not fully open. The mean positive pressure gradients for all valves (Table 2 and Figure 8) were in the range of 3.7 to 8.2 mm Hg, which was higher than expected from the steady flow data or the velocity-derived data in patients. The reason for the difference is that positive pressure gradients occur only during the accelerating flow period of the ventricle. In this period, the pressure gradient comprises 3 components: valve resistance; conduit re-sistance; and the pressure gradient caused by the inertia of the accelerated fluid. The contribution of fluid inertia evidently increased the pressure gradient compared to that under steady flow conditions.
Consistent with another report, the closing volume (% of the forward flow volume) increased with decreasing cardiac output.1 From Figure 10, it can be concluded that the SJM valve had the largest closing volume, and the other 3 valves had similar low closing volumes. However, with a low cardiac output of 3 L•min–1 at 70 beats•min–1, the On-X valve had the highest closing volume. Under low cardiac output, some bileaflet valves tend to produce a larger closing volume.1 A certain closing volume is required to wash the leaflets from the open position to the closed position. The high-profile design of the On-X valve decreases the travel angle and reduces the closing volume. On the other hand, this design gives rise to a large leaflet area and consequently, a large leaflet mass that needs more wash to close the leaflets. With very low cardiac output, it was deduced that the factor of large leaflet mass is more predominant so as to require a greater closing volume. Nevertheless, the On-X valve generally produced a low closing volume similar to that of the CM valve. In principle, it is assumed that a large opening angle results in a large closing volume. This is true for the SJM valve; the larger closing volume of the SJM valve might be deduced from its large opening angle. Correspondingly, valves with small opening angles such as the CM valve produced small closing volumes. In the case of the ATS valve, the opening angle was designed to be 85° but the in situ opening angle is much smaller, therefore, this valve behaved similarly to the CM valve. However, the On-X valve produced the smallest closing volumes, in spite of the parallel leaflets design. It demonstrated that the axial elongation of the On-X valve that can achieve a smaller travel angle would effectively reduce the closing volume.
The pulsatile flow simulator developed in our laboratory was considered to replicate physiological conditions of flow and pressure patterns. The tube-pump design and employment of the mitral downstream conduit enabled the system to produce a more physiologically similar mitral transvalvular flow, making these valve tests in the mitral position more valid. It was concluded from the results of this study that the SJM HP 25-mm valve and the On-X 25-mm valve produced smaller pressure gradients than CM and ATS valves with the same annular diameter. This was mainly due to their larger orifice diameters and greater opening angles. The On-X valve was superior to the SJM HP valve because of its smaller closing volume. The design of the On-X valve, based on improved hydrodynamic efficacy, has been effective in producing a small pressure gradient while keeping a low closing volume.
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Acknowledgments
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The authors would like to thank: Century Medical, Inc., Tokyo, Japan, for providing the ATS valves; Japan Lifeline, Inc., Tokyo, for providing the CM valves; Paramedic, Inc., Tokyo, for providing the On-X valves; and Getz Bros. Co. Ltd., Tokyo, for providing the SJM valves. They also express their hearty thanks to Mr. T. Tanaka of Yasuhisa Biomechanics, Inc., Tokyo, for his sound support in developing the pulsatile system. This investigation was supported by the following research funds: the Project Fund (99P13) of Advanced Research Institute for Science and Engineering, Waseda University; the program for promotion of fundamental studies in health science of the organization for drug ADR relief, R&D Promotion and Product Review of Japan (No. 96-12); and Grant-in-aid for scientific research of Japan (No. 09557112, No. 09470288).
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References
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Abstract
Introduction
