Ann Thorac Surg 2002;74:63-68
© 2002 The Society of Thoracic Surgeons
Original article: cardiovascular
Hydrodynamic function of the second-generation mitroflow pericardial bioprosthesis
Louise M. Jennings, MEngb,
Abdusalam El-Gatit, MDa,
Zsolt L. Nagy, PhDa,
John Fisher, DEngb,
Peter G. Walker, PhDb,
Kevin G. Watterson, FRACS*a
a Yorkshire Heart Centre, University of Leeds, Leeds, United Kingdom
b School of Mechanical Engineering, University of Leeds, Leeds, United Kingdom
Accepted for publication March 19, 2002.
* Address reprint requests to Mr. Watterson, Yorkshire Heart Centre, Leeds Teaching Hospital, Calverley St, Leeds LS1 3EX, UK
e-mail: kevin.watterson{at}leedsth.nhs.uk
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Abstract
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Background. The hydrodynamic function of the smaller size Mitroflow Synergy stented pericardial bioprostheses has been studied in an in vitro fresh tissue aortic root model and compared with previous studies of free-sewn bioprostheses.
Methods. Three valves of each of the sizes 19, 21, and 23 mm were sutured into fresh tissue aortic roots and tested in a pulsatile flow simulator using two different ventricular input impedance conditions. A high-speed camera was used to study the leaflet opening and closing configurations. Mean pressure difference as a function of root mean square forward flow, effective orifice area, regurgitant volumes, and total energy loss across the valves was measured.
Results. Mean pressure difference with respect to root mean square forward flow decreased as the valve size increased. Thus effective orifice area increased as the valve size increased. The open leaflet configuration images showed that all three sizes of Mitroflow valves had a large circular orifice with minimal open leaflet deformation. All valves closed competently with no visible leakage and no closed regurgitant volume. The Mitroflow valves showed better effective orifice areas compared with previously tested frame-mounted porcine bioprostheses but lower effective orifice areas compared with porcine stentless bioprostheses; however, the open leaflet bending deformation was better than for any of the previously tested bioprosthetic valves.
Conclusions. The hydrodynamic function of the Mitroflow Synergy stented pericardial bioprosthesis shows potential for good in vivo hemodynamic performance. The good hemodynamic performance combined with relative ease of implantation technique makes the pericardial valve a good valve in the aortic position, particularly in older patients with small annuli.
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Introduction
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Glutaraldehyde-preserved bovine pericardium has been used for the manufacture of valvular bioprostheses since 1971 [1]. There are currently four pericardial prosthetic heart valves marketed internationally: They are the Mitroflow Synergy PC (Sulzer Mitroflow Corp, Richmond, CA), Carpentier-Edwards Perimount (Baxter Lifesciences LLC, Irvine, CA), Sorin Pericarbon (Sorin Biomedica CardioSpa, Saluggia, Italy), and St. Jude Medical Biocor (St. Jude Medical Inc, St. Paul, MN) pericardial bioprostheses.
The first generation of the Mitroflow pericardial heart valve (model 11) was first introduced in 1982 as a cardiac valvular substitute for aortic and mitral valve replacement. These pericardial heart valves have the leaflets mounted outside the support frame, in common with the Pericarbon and Biocor bioprostheses. In contrast, the Carpentier-Edwards bioprosthesis has the leaflets mounted inside the frame. In 1990, the model 11 series of the Mitroflow pericardial heart valve was identified as having a mode of structural failure related to abrasion of pericardial tissue from contact with the ribbed polyethylene terephthalate fiber (Dacron, CCR Bard, Haverhill, MA) surface. Model 12 of the Mitroflow pericardial heart valve was accordingly developed in 1991, having the Dacron reversed with the smooth side to the exterior [2].
Although the hydrodynamic function of stented bioprostheses has been reported previously, they have not been studied in a fresh tissue root model. This fresh aortic root model has been previously used to study the free-sewn homograft, the pulmonary autograft, and stentless porcine valves [3–6]. This was the first study of stented valves in this model.
The aim of this study was to ascertain the hydrodynamic performance of the three smaller sizes of Mitroflow Synergy pericardial heart valves (model 12) using an in vitro fresh aortic root model.
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Material and methods
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The hydrodynamic function of three valves of each of the sizes 19, 21, and 23 mm was assessed. All the valves were supplied in their sterile packaging and were of clinical quality.
Each valve was implanted into a fresh porcine aortic root. The aortic roots were dissected from fresh pig hearts, stored at 4°C in normal saline, and used within 36 hours. The annular size of the roots was determined by passing a size obturator through the annulus from both the ventricular and aortic sides. Each valve was implanted in a size-matched aortic root. The sewing cuff of the valve was sewn to the host annulus with 3-0 continuous polypropylene suture from the ventricular side. The coronary arteries were ligated with 2-0 Ethibond suture (Ethicon, Somerville, NJ). The composite aortic root was mounted onto the rigid aortic valve section of the pulsatile flow simulator by means of spigots of the same size as that of the aortic root using strong linen ties and plastic straps, as shown in Figure 1.
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Fig 1. Fresh porcine aortic root mounted in the aortic valve section of the pulsatile flow simulator, after implantation of the Mitroflow pericardial heart valve.
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The schematic and technical details of the flow simulator used in this study have been extensively reported elsewhere [7–9]. Essentially, the simulator consisted of two rigid cylindrical test sections for each of the aortic and the mitral valves, an atrial reservoir, and an afterload made up of a compliance chamber, inertial element, and variable peripheral resistance. The system was driven by a servocontrolled piston pump, which was programed to give physiologic flow waveforms and rates. The pressure difference across the valve was measured directly by a differential transducer, and the instantaneous flow through the valve was measured with an electromagnetic flowmeter positioned downstream from the valve. Pressures at the ventricular, aortic, and atrial positions were continuously monitored, and the peripheral resistance was set to represent in vivo systemic pressure at 120/80 mm Hg. All valves were tested in normal physiologic saline.
Pressure, flow, and pump displacement signals were collected digitally for 10 seconds at a sampling frequency of 200 Hz and stored on disk for analysis with an IBM PS/2 computer (International Business Machine Corp, White Plains, NY). The data were averaged for one cycle, and valve function was analyzed using this averaged waveform.
The pulsatile flow simulator had recently been improved with the use of variable ventricular input impedance [9]. With the positive displacement pump having infinite impedance and the simulator chambers being rigid, the inertia of the test fluid resulted in severe pressure oscillations when the valve under test was opening and closing. Such oscillations decayed quickly when mechanical or stented valves were tested in the rigid aortic test section. However, because of the compliance of biological tissue roots, the resultant pressure and flow oscillations were extreme and extended throughout systole. The ventricular input impedance was reduced with the use of a viscoelastic impedance adaptor (Vivitro Systems Incorporated, Victoria, BC, Canada). The viscoelastic impedance adaptor contained two adjacent compliance chambers and a fixed resistive element achieving more physiologic boundary conditions such as maximum rate of increase of left ventricular pressure.
In this study, the Mitroflow Synergy pericardial heart valve model 12 (MF-12) series of valves was tested under this revised condition of reduced input impedance, which corresponded to the viscoelastic impedance adaptor having maximum input compliance. To compare results with previous studies using this simulator, all valves were also tested under the original condition of infinite input impedance, that is, minimum viscoelastic impedance adaptor input compliance.
At infinite impedance, the MF-12 series was tested at a heart rate of 72 beats per minute and a stroke volume of 70 mL, corresponding to a cardiac output of 5 L/min. However, at reduced impedance (maximum viscoelastic impedance adaptor compliance), the net forward flow volume was reduced. Hence the peak flow was used as the input variable to set the test conditions instead.
The mean pressure difference (in millimeters of mercury) during forward flow was assessed as a function of root mean square forward flow (in milliliters per second). The effective orifice area (EOA) was calculated as described by Gabbay and associates [10], according to the formula EOA = Q/51.6
p (where Q is the root mean square forward flow in milliliters per second and p is the mean pressure drop during forward flow in millimeters of mercury). The performance index (PI) of the valve was derived using the formula: PI = EOA/theoretical orifice area (%). The theoretical orifice area was 
r2, with r the radius of the valve annulus as measured from the external diameter [4]. The maximum possible value for the performance index is 100%, that is, when the EOA of the valve is equal to the area of the tissue annulus. In practice, the finite thickness of the structure of a valvular implant makes this difficult to achieve. The regurgitant volume per cycle (in milliliters) was obtained by integrating the flow signal when the valve was closing and when the valve was closed. The total energy loss per cycle (in millijoules) was calculated by integrating numerically the product of the pressure difference across the valve and the volumetric flow through the valve during the periods of forward, closing, and closed flows.
Valve leaflet dynamics were recorded with a high-speed camera recording at a rate of 500 frames/s positioned axially to the flow through the aorta to determine the configuration of the open and closed valve leaflets. The bending deformation was not quantified in this study as owing to the thickness of the leaflets a suitable model to define the deformation of pericardial leaflets could not be found.
The mean and standard deviation of the data were calculated. Statistical analysis was performed by Student’s t test. Statistical significance was taken at 5% level (p < 0.05).
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Results
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The mean transvalvular pressure difference measured with respect to root mean square flow at both compliance conditions is listed in Table 1.
These results are based on the mean of all data sets for each valve size. The mean pressure difference increased as the valve size decreased. In addition, the mean pressure difference was higher at the condition of maximum compliance compared with minimum compliance, statistically significantly so for the size 21-mm and 23-mm valves (p = 0.022 and p = 0.005, respectively). However, the root mean square flow was also statistically significantly higher at maximum compliance for all valve sizes.
The EOA and performance indices (based on the mean EOA) of each valve size, again taking the mean for all the data sets for the same sized valves, are shown in Table 2.
There was no statistically significant difference between the EOAs at maximum and minimum compliance. The larger Mitroflow pericardial heart valve showed larger EOA. At maximum compliance the 19-mm Mitroflow pericardial heart valve had the highest performance index, with the 21-mm and 23-mm Mitroflow pericardial heart valves being comparable to each other.
All the Mitroflow pericardial heart valves were completely competent when they were fully closed. The closing regurgitant volume increased as the valve size increased, but was small compared with the stroke volume at all valve sizes.
The total energy loss for the Mitroflow pericardial heart valves is shown in Figure 2.
The total energy loss decreased at the condition of minimum compliance as the valve size increased as expected. The difference in total energy loss among the three sizes was not statistically significant according to the 95% confidence limits.
From the high-speed images, we observed that all the Mitroflow pericardial heart valves showed synchronous leaflet opening. The majority also showed synchronous leaflet closure. Most of the Mitroflow pericardial heart valves showed maximum circular fully open orifices, as shown in Figure 3.
Occasionally, however, the fully open orifice was not completely circular, being slightly restricted as shown in Figure 4.
A small amount of bending deformation was observed, but this was confined to the commissural region. Three of the nine valves exhibited leaflet fluttering before closure.
The images also showed that all valves closed competently with no visible leakage orifice, as shown in Figure 5,
although coaptation was not always on the same level at the free edge. In the closed valve configuration, all the leaflets were deformed along their free edge, a phenomenon specific to pericardial heart valves and often referred to as "pin-wheeling."
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Comment
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There has been a steady increase in the use of bioprosthetic heart valves in recent years partly because of the increasing age of the patient population. During this period there has also been a steady increase in the number of bioprosthetic products available, ranging from various stented and stentless porcine valves to an array of stented and stentless pericardial valves. Pericardial bioprostheses have been used as a valve substitute for more than 30 years. Their hemodynamic advantages over the standard porcine bioprostheses (lower pressure gradient, greater EOA, better flow profile) have already been confirmed, especially in small valve sizes [11].
Small aortic annuli can cause a dilemma for the surgeon in terms of choosing the optimal valve and implantation technique. The stentless valves have hemodynamic advantages, but at the expense of a more difficult and time-consuming implantation technique as compared with the stented porcine valves [12]. Root replacement either with porcine aortic root [13] or homograft is another option, but is too extensive an operation in elderly patients for simple aortic valve disease. The stented pericardial valve is a hybrid in terms of both hemodynamics and technique, but previous generations of pericardial valve did not have favorable durability owing to poor design and technology failure rather than primary tissue weakness [14]. This appears to have been addressed by the next generation of pericardial valves.
In an effort to make in vitro testing more physiologic and more comparable to the previous results with stentless valves, in the Leeds model the heart valves were implanted into fresh porcine aortic root and then tested. To date, the aortic homograft, the pulmonary autograft, and stentless porcine aortic and pulmonary valves have been tested in this manner. The Mitroflow pericardial valve was the first stented valve to be so tested. This allows for comparison to be made with other tissue heart valves, especially in the smaller sizes of 19 and 21 mm, for these sizes of aortic annulus pose a conundrum for the cardiac surgeon in decision making. The 21-mm stentless valve has a pressure gradient of 3.4 to 8.5 mm Hg with an EOA of 2.0 to 2.66 cm2. The corresponding values for the 21-mm Mitroflow valve are 11 mm Hg and 1.58 cm2 (Table 3).
The 19-mm stentless valve has a gradient range of 6.2 to 8.5 mm Hg with EOA of 1.77 to 1.91cm2 compared with the Mitroflow of 15.7 mm Hg with 1.33 cm2 EOA (Table 4).
The performance index of the 19-mm stentless valve is 60 compared with the Mitroflow of 49.5. The hydrodynamic performance of the 23-mm Mitroflow Synergy valve was not significantly inferior to the previously tested stentless porcine bioprostheses (Table 5).
One of the fundamental design characteristics of the Mitroflow pericardial heart valve is that its pericardial leaflets are made from a single piece of bovine pericardium mounted on the outside of the stent without commissural transfixation sutures. Theoretically the purpose of this is to increase the usable fully open orifice area. Indeed this was apparent in the images taken of the valves in this study, which showed a large circular orifice in most cases, with minimal bending deformation situated solely at the commissural region. This is a significant difference among the Mitroflow pericardial heart valve and the previously tested stentless [5, 6] and stented [15] bioprostheses, in which the porcine bioprostheses showed higher bending deformations along the whole leaflet length. However, the relative fatigue resistance of the porcine and pericardial bioprosthetic tissue to cyclic bending strain is not currently known.
It may be that the significance of the findings is dependent somewhat on the age of the patients. Increasingly, patients of 70 years of age and older, who often have small annuli and calcified aortic roots and sinuses, are referred for aortic valve operation. What is the best valve to implant in this group of patients? Stentless valves have better hemodynamics with a more difficult implantation technique, whereas the Mitroflow stented pericardial valve is relatively easy to implant, although the hemodynamic performance is somewhat inferior. There is a continuous debate about pressure gradient and ventricular regression [16, 17]. Is this of relevance in the elderly patient? Also, should the cardiac surgeon maximize the EOA at the time of valve implantation or just aim to ensure an EOA that gives reasonable quality of life for these older patients?
The overall impression is that the good hemodynamics of the Mitroflow pericardial valve combined with relative ease of implantation technique makes the pericardial prosthesis a suitable valve in the aortic position, particularly in older patients with small annuli.
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Acknowledgments
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This study was supported by the National Heart Research Fund and the National Lottery Charities Board, United Kingdom. The authors thank Sulzer Carbomedics Ltd, United Kingdom, for supplying the Mitroflow Synergy PC pericardial valves and Aortech Europe Ltd, Leeds, United Kingdom, for supplying the fresh aortic roots.
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References
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Abstract
Introduction
