Ann Thorac Surg 2000;69:1408-1413
© 2000 The Society of Thoracic Surgeons
Original articles: Cardiovascular
The effect of sizing on the hydrodynamic parameters of the medtronic freestyle valve in vitro
Zsolt L. Nagy, MDa,
John Fisher, PhDb,
Peter G. Walker, PhDb,
Kevin G. Watterson, FRACSa
a Yorkshire Heart Centre, Leeds, United Kingdom
b Department of Mechanical Engineering, University of Leeds, Leeds, United Kingdom
Address reprint requests to Dr Watterson, Yorkshire Heart Centre, Leeds General Infirmary, Calverley St, Leeds LS1 3EX, England
e-mail: kevingw{at}ulth.northy.nhs.uk
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Abstract
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Background. An in vitro model has been established to investigate the effect of sizing on the hydrodynamic characteristics and leaflet motion of the Medtronic Freestyle valve.
Methods. The valves were tested in fresh porcine aortic roots. Two or three different sizes of valves were implanted in the same aortic root one after the other. The compliance of the fresh aortic and the composite roots was measured in the pressure range of 0 to 120 mm Hg, and the composite roots were then tested in a pulsatile flow simulator. The transvalvular gradient and degree of regurgitation were measured and the effective orifice area and performance index were calculated. Leaflet motion was recorded on video.
Results. The fresh aortic roots dilated by average 39.4% as the hydrostatic pressure rose from 0 to 120 mm Hg. Implantation of the Medtronic Freestyle valve did not change the distensibility of the aortic root significantly. The sizing protocol did not affect significantly the hydrodynamic performance. However, a significantly lower open leaflet bending deformation was found in the undersized valves. Regurgitation was found only at 2-mm undersized valves.
Conclusions. Leaflet motion of the Medtronic Freestyle valve in vitro was best if 1 mm undersized, and this may be beneficial to long-term durability.
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Introduction
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Interest in stentless porcine aortic valves has increased recently due to the superior hemodynamic performance and potentially improved durability. Due to increasing surgical demand, all major heart valve manufacturers have now developed their own version of stentless porcine valve. The Medtronic Freestyle (Medtronic, Irvine, CA) bioprosthesis was introduced in 1992. This is a stentless porcine aortic root crosslinked in buffered glutaraldehyde solution with 40 mm Hg pressure applied to the root and zero pressure differential across the valve leaflets. It has been treated with 
-amino oleic acid to reduce the potential for leaflet calcification. The device is suitable for: (a) root replacement, (b) miniroot inclusion cylinder aortic valve replacement, (c) partial scalloped subcoronary valve implantation, or (d) completely scalloped subcoronary implantation [1]. In clinical practice, the latter two techniques are most commonly used [2]. Although the number of implantations is increasing, debate continues about the best sizing protocol and implantation technique. We have established an in vitro model to investigate the effect of sizing on the hydrodynamic characteristics and leaflet deformation of the Medtronic Freestyle stentless xenograft, using the partially scalloped subcoronary implantation technique.
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Material and methods
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One 19-mm, three 21-mm, and two 23-mm Medtronic Freestyle xenografts were investigated in our series using the partially scalloped subcoronary valve implantation technique. The right and left coronary sinuses were excised from the aortic root cylinder and the remnant of the aortic wall was trimmed to approximately 3 mm above the commissures. The valves were then tested in different sized fresh porcine aortic roots. In order to measure the effect of sizing, two or three different sized valves were implanted in the same aortic root one after the other.
Aortic roots
Aortic roots were dissected out from fresh pig hearts, stored at 4°C in normal saline, and used within 24 hours. The right and left coronaries were ligated. The external diameter of the aortic roots was measured at the sinotubular junction at hydrostatic pressures of 60, 80, 100, and 120 mm Hg. The annulus size was measured by passing an obturator through the annulus from the ventricular side. Two roots of 20-mm, two of 21-mm, two of 22-mm, and one of 23-mm internal annulus diameter were tested. The sizes of valves implanted in the different aortic roots are listed in Table 1. The internal diameter of the xenograft valves is 2 to 4 mm less than the external diameter measured at the sinotubular junction. Altogether, 16 valve implantations were performed.
Technique of valve insertion
The lower rim of the valve was sewn just beneath the host annulus from the ventricular side using 4/0 continuous polypropylene suture in such a way that the valve’s commissures were in alignment with the host’s commissural attachment. The upper row of subcoronary continuous sutures was placed through the previously trimmed upper margin of the valve and through the half thickness of the host aortic wall using 5/0 polypropylene, leaving the coronary ostia clear. The external diameter of the composite roots was measured at the sinotubular junction at hydrostatic pressures of 60, 80, 100, and 120 mm Hg.
Hydrodynamic testing
The composite roots were tested in a pulsatile flow simulator, details of which have been described previously [3]. The flow simulator consisted of two rigid cylindrical test sections for each of the mitral and aortic valves, a compliance chamber, peripheral resistance, and an atrial reservoir. The system was driven by a servo-controlled piston pump. The composite roots were mounted in place of the rigid aortic valve section and fixed on an inflow and an outflow spigot using strong plastic straps and then tested at a rate of 72 cycles/min with a stroke volume of 70 mL for a systemic pressure of 120/80 mm Hg. The pressure difference across the root was measured directly by a differential transducer and the flow was measured with an electromagnetic flow meter positioned downstream of the valve. Pressure, flow pump displacement, and velocity signals were collected digitally for a period of 10 seconds (12 cycles) at a sampling frequency of 200 Hz and stored on a disk for analysis using an IBM PS/2 computer. The data were ensemble averaged to create one cycle, and valve function was analyzed using this averaged waveform. The effective orifice area (EOA) was calculated using the formula: EOA = Q/51.6
p, where Q is the root mean square forward flow (in mL/s) and p is the mean pressure drop during forward flow (in mm Hg) [3]. The performance index (PI) of the valve was derived from the formula: PI = EOA/theoretical orifice area (theoretical orifice area was calculated by the formula 
r2, where r is the radius of the valve annulus). Valve leaflet movements were recorded with video camera positioned axial to the flow through the aorta to determine the configuration of the open valve leaflets. A spigot of the same diameter as that of the aorta in its distended state allowed a video recording of the leaflet motions of the entire valve, including the commissural area. The open leaflet bending deformations were determined from the still image of the fully opened position in mid systole using an image analysis system. The open leaflet deformation at the commissures was quantified by taking three points along the leaflet edge in the region of maximum deformation (Fig 1). The spatial deviation of the center point (BD) from the straight line formed by joining the two end points (AC) was used as a measure of leaflet deformation [4]. The BD/BC was used as a leaflet bending deformation index (BDI). BDI was quantified at all three commissures, and the average of the data was given as a characteristic of the valve.
The mean and standard deviation of the data were calculated. Statistical analysis was performed by comparison of the means using the Student’s t test. Statistical significance was taken at the 5% level (p 
0.05).
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Results
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The distensibility of the native and composite roots is shown in Table 2. The fresh porcine aortic roots were very elastic: the external diameter of the roots at the sinotubular junction dilated 39.4% ± 2.99% as the pressure increased from 0 to 120 mm Hg. Implantation of the Medtronic Freestyle valve as a partially scalloped device did not change significantly the external diameter of the host aortic root at the sinotubular junction at zero pressure (27.8 ± 2.11 mm for the native vs 27.4 ± 1.99 mm for the composite roots; p = 0.33) and no significant difference was observed regarding the compliance of the host root (p = 0.46 at 120 mm Hg). Over- or undersizing the valve did not alter the distensibility of the composite roots.
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Table 2. The Mean Percentage of Dilation of the Native and Composite Aortic Roots at the Sinotubular Junction in the Pressure Range of 0 to 120 mm Hg
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The transvalvular gradients measured on the composite roots and the calculated effective orifice areas and performance indices are listed in Table 3. In most cases, the bigger valve was implanted in the same aortic root, the transvalvular gradient was slightly lower, and the effective orifice area was a little larger, although the differences were not significant (p = 0.32 for the 20-mm roots, p = 0.24 for the 21-mm roots, and p = 0.32 for the 22-mm roots). The only exception was a 23-mm valve implanted in 21-mm aortic root, where the valve showed high transvalvular gradient and smaller orifice area due to poor opening (Fig 2). For each valve size, the measurements showed decreasing gradient and increasing orifice area as they were implanted in different aortic roots of increasing internal diameter. The orifice area for the undersized valves was significantly larger than for the oversized valves (p = 0.004 for the 21-mm valves). The closing volume was similar in all valves. Major regurgitation was not found in any of the valves. Two of the three valves that were undersized by 2 mm showed tiny central coaptation defects in the diastolic phase (Fig 3), but the regurgitant volume was less than 5% of the stroke volume. All the other valves were competent. The sizing protocol did not affect the performance index of the valves; no significant difference was found between the oversized and undersized valves (p = 0.46).
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Table 3. The Transvalvular Gradient, Effective Orifice Area, and Performance Index of the 19-mm, 21-mm, and 23-mm Medtronic Freestyle Stentless Valves Implanted in Different Sized Fresh Porcine Aortic Roots
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Table 4 shows the open leaflet bending deformation indices of the implanted Freestyle valves. The least leaflet deformation was found when the valve was undersized. The opening of the undersized valves approaches the ideal triangular shape of the natural aortic valve (Fig 4). There was no significant difference between the 1-mm and 2-mm undersized valves regarding the open leaflet bending deformation indices. The size for size-implanted valves produced significantly higher leaflet deformation (Fig 5), while the worst results were obtained if the valves were oversized (Fig 6).
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Comment
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Recent clinical studies showed that postoperative complication rate and life quality are superior in adult patients who received bioprostheses in contrast to mechanical valves in the aortic position, despite the risk of a reoperation [5]. Since 1965, when the first experience with heterograft valve replacement was reported [6], continuous effort has been made to improve their performance and durability. The two important determinants of long-term durability are: (a) method of tissue preservation, and (b) valve geometry and stress distribution on the leaflets. The tissue preservation has been much improved since the introduction of glutaraldehyde [7]. Changing the conditions during fixation to low pressure then to zero pressure resulted in better preservation of the structural characteristics of the porcine valve tissue [8]. Development of new antimineralization agents to prevent calcification of the glutaraldehyde-treated tissues may also contribute to improve xenograft durability [9]. Although the first stentless aortic valve heterograft was implanted in 1965 [10], due to the easier implantation technique and favorable early and mid-term results, the glutaraldehyde-treated, frame-mounted porcine valve became the choice of bioprosthesis in the next two decades. However, long-term results are not ideal; late structural failure occurs through calcification or cusp rupture an average of 8 to 10 years after implantation. In vitro studies on the frame-mounted xenografts show that the rigid stent is obstructive, produces flow disturbance, and leads to suboptimal valve geometry and maldistribution of stress on the leaflets [11]. These factors result in early degeneration and calcification of the bioprosthesis. To overcome these problems David and coworkers reexplored the use of stentless xenografts in 1988 [12]. The hemodynamic superiority and improved flow characteristics of the stentless valves has been documented in several reports [13–16]. However, the long-term clinical durability of stentless valves has still to be determined.
In our in vitro study, we have investigated whether the sizing has any effect on the hemodynamic performance and the leaflet motion of the Medtronic Freestyle stentless valve implanted in subcoronary position. The porcine aortic roots, which were comparable in size with the human aorta, were harvested from piglets. Two major differences were found compared with the human clinical practice: the porcine aortic root was a lot thicker and much more distensible than the aortic root of elderly patients with calcific aortic stenosis. However, implantation of the Medtronic Freestyle valve in subcoronary position did not reduce the pressure-related dilation of the porcine aortic root at the sinotubular junction, and this rises the possibility of full valve opening, minimal leaflet deformation, and low stresses on the cusps. In order to study the effect of sizing, smaller and bigger valves have been tested in the same aortic root, and the hemodynamic parameters and leaflet motion were compared with each other. This protocol was similar to clinical conditions, when the surgeon has to decide what valve size should be implanted in a certain size aortic annulus. When our results are compared with in vitro tests on frame-mounted valves [4], they support the findings of other investigators that the hemodynamic performance of the stentless valves is better than the frame-mounted ones, especially in small sizes [17–19]. Following standard practice, the size of the host aortic root was measured at the level of the annulus. Most porcine aortic root’s internal diameters were 1 to 2 mm larger at the sinotubular junction than at the annulus. Furthermore, the aortic wall at the sinotubular junction was more distensible than the fibrous annulus. This phenomen has an important role in normal aortic valve function [20]. The present recommendation for sizing the stentless valves is based on the theory that the difference in dilation between the sinotubular junction and the annulus may cause regurgitation, so it has been advised to oversize the valve 1 to 2 mm on the annulus size. Our measurements did not show any regurgitation in the 1-mm undersized valves and very mild central leakage in two of the three 2-mm undersized valves, even if they were implanted in very compliant piglet aortic roots; however, poor coaptation may have detrimental effects on the long-term durability. We did not find a significant effect of over- or undersizing the valve in the same aortic root regarding the other hydrodynamic parameters. On the other hand, the most important effect of sizing was on the open leaflet bending deformation. The leaflet bending deformation index was significantly less in each root as the valve size was decreased from the 2-mm oversized to the 1-mm undersized implantation. No significant difference was found between the 1-mm and 2-mm undersized valves.
Our in vitro measurements suggest that 1-mm undersizing of the Medtronic Freestyle valve implanted in fresh porcine aortic roots in the subcoronary position resulted in better leaflet motion without the hazard of regurgitation. In clinical practice, many aortic roots in patients with calcific aortic stenosis are already thickened or calcified and lost their ability for dilation, so the risk of valve incompetence would be even less. Moreover, reduced open leaflet bending deformation might have an important effect on long-term durability. However, undersizing a stentless valve might increase the risk of suture dehiscence in the case of poor tissue quality.
Conclusions
In our series, the hydrodynamic performance of the Medtronic Freestyle valve implanted in fresh porcine aortic roots in the subcoronary position was not significantly different if it was 1 to 2 mm over- or undersized. However, minimal open leaflet bending deformation was found if a 1-mm smaller valve was implanted in the host aortic root and the valve still remained competent. This reduced leaflet deformation might be an important determinant of long-term durability.
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Acknowledgments
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This study was supported by the National Heart Research Fund (U.K.) and the National Lottery Charities Board (U.K.). The Medtronic Heart Valve Division, Irvine, California, supplied all the valves. TissueMed (Leeds, U.K.) supplied the aortic roots for the study. We thank Devon Darby and John Moore for their kind technical assistance.
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Accepted for publication November 8, 1999.
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Abstract
Introduction
