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Interact CardioVasc Thorac Surg 2008;7:750-754. doi:10.1510/icvts.2008.182469 © 2008 European Association of Cardio-Thoracic Surgery
The changing spectrum of bioprostheses hydrodynamic performance: considerations on in-vitro testsCardiovascular Institute, University of Padova, Via Giustiniani 2, 36100 Padova, Italy Received 24 April 2008; received in revised form 19 June 2008; accepted 23 June 2008
*Corresponding author. Tel.: +39-049-8212408; fax: +39-049-8212409.
The aims of the present study were to compare hydrodynamics of three pericardial and two porcine valves while performing at different stroke volume (SV) and increasing pulse rate (PR). Carpentier-Edwards Magna-21 (CEM), Sorin Soprano-20 (SS), Mitroflow-23 (MF), SJM-Biocor-Epic-Supra-21 (SJME), and Medtronic Mosaic Ultra-23 (MMU) were tested in the aortic chamber (23-mm in diameter) of the Sheffield-Pulse-Duplicator. The tests were carried out at increasing pulse-rate and at each pulse-rate the valve was tested at different SV. CEM and MF showed significantly lower gradients than porcine valves and SS. Transvalvular gradients were unrelated to PR showing a constant value with increasing PR. While SJME valve showed the lowest regurgitant volume, on the contrary CEM showed the highest. At increasing SV, effective-orifice-area (EOA) observed with CEM was significantly larger than with the other tested valves, even though at SV 60 ml MF was comparable and at SV 65 ml significantly better. SS, SJME and MMU showed comparable EOAs with bigger area at increasing PR. The latter relation was reversed for CEM and MF. Our results show that CEM and MF have shown significantly better in-vitro hydrodynamics in comparison with their porcine counterpart and SS. Nevertheless, at increasing pulse rate, porcine tissue valves and SS may guarantee higher EOA values.
Key Words: Sheffield Pulse Duplicator; Supra-annular tissue valves; In-vitro tests; Patient prosthesis mismatch
The ideal prosthesis should be easily implanted, should demonstrate excellent durability and hemodynamic, allowing a complete regression of left ventricular hypertrophy [1, 2]. Unfortunately, up to the present day, the hemodynamics of different prostheses have not yet been sufficiently tested and elucidated, mostly due to the fact that in-vivo studies are frequently limited by confounding postoperative conditions of analysis and by technical echocardiographic pitfalls [3]. Since 2004, we decided to compare hydraulic performances of supra-annular pericardial tissue valves vs. porcine valves by in-vitro studies [4]. To our knowledge, the Sheffield-Pulse-Duplicator (SPD) provides a controlled experimental setting by selecting physiological parameters such as heart rate, stroke volume (SV), and aortic root dimension. We have previously shown that pericardial tissue valves offer a significant and progressive hydrodynamic advantage to the porcine supra-annular counterpart. Nevertheless, our previous study protocol is flawed by some false experimental condition, since it is not very realistic to reach 7 l/min cardiac output at 70 beats/min, even when the patient is well trained [4]. Therefore, the aims of the present study were to compare hydrodynamic performances of three pericardial valves [Carpentier-Edwards Magna (CEM), Sorin Soprano (SS), and Mitroflow (MF)] and two porcine valves [SJM Biocor Epic Supra (SJME) and Medtronic Mosaic Ultra (MMU)], while performing at different SV and increasing pulse rate (PR).
The SPD used in this study has been previously described in detail [4, 5]. We tested the valves with a tissue annulus diameter which could be fitted in a 21-mm pulse duplicator ring in the aortic chamber of the SPD. The size of the tested valves were as follows: 20-mm Sorin Soprano (SS), 21-mm St Jude Medical Biocor Epic Supra (SJME) and Carpentier-Edwards Magna (CEM), 23-mm Mitroflow (MF) and Medtronic Mosaic Ultra (MMU). All valves were tested within a 23-mm aortic root, which is the smallest rigid aortic root available for the SPD. Concerning the manufacturers' sizes, we have already published that often the latter do not bear any relationship with any of the real measurements that one can make and, additionally, that the pulse duplicator does not mimic well the anatomical conditions of the annulus [4]. Nevertheless, we have compared all the supra-annular bioprostheses available on the market with a tissue annulus diameter comparable to a 21-mm pulse duplicator ring and fitting inside a rigid 23-mm aortic root with sinus of Valsalva, excluding by our tests all the other sizes of the same models being much larger or smaller than those we tested. Standard testing protocols were followed [4–7]. The tests were carried out at increasing PR and SV: PR (70, 75, 80, 85, 90, 95, 100, 105, and 110 beats/min) and SV (45, 50, 55, 60, 65 ml). Thus, cardiac output (CO) ranged between 3.1 l/min and 7.2 l/min. Transducers are located as previously described [4, 5]. All the data have been acquired and calculated as previously described [4, 6, 7].
3.1. Valve dimension and labelling These are summarized in Table 1.
3.2. Pressure differences Mean and peak gradients are reported in Figs. 1 and 2 (Table 2). Pericardial valves, with the exception of SS, showed significantly lower gradients than porcine valves. At the lowest SV (45 ml), the CEM showed the lowest mean and peak pressure differences behaving significantly better than all others (P<0.05). At increasing SV (50–55 ml), MF increased its performances showing pressure differences comparable to CEM (P=NS) and significantly better results in the higher SV range (60–65 ml) (P<0.05). Porcine valves showed variable results. At lowest SV, SJME is significantly worse than MMU (P<0.05), although comparable to SS (P=NS), while at increasing SV it performs equally or sometimes significantly better than MMU (P<0.05). Contrary to SV, results of all the tested valves were unrelated to PR so that transvalvular gradients showed a constant value with increasing PR.
3.3. Regurgitant volumes The SJME valve showed the lowest regurgitant volume, which resulted in significantly smaller volume when compared to that obtained with all the other tested valves (P<0.005). On the contrary, the CEM showed the significantly highest regurgitant volume (P<0.0005). For all the tested valves the total regurgitant volume resulted inversely proportional to SV and PR. The highest value was observed with the MF prosthesis (P<0.005). The SJME valve showed the lowest closure volume at each test condition. The results with this valve were significantly different in comparison with all the others (P<0.05). The CEM showed closing volumes lower in comparison to the other pericardial prostheses (P<0.05). The SS valve showed the lowest leakage volume, which resulted in significantly smaller volume when compared to that obtained with CEM and MF and both the porcine valves (P<0.05). The leakage volume observed with SJME was comparable to that of the MMU valve. The largest leakage volume, as high as 18% of the SV, was observed with the CEM valve (P<0.005). All the tested tissue valves showed a leakage volume greater than the closure volume. At 45–50–55 ml SV, EOA observed with CEM was significantly larger than with all the other tested valves (P<0.05). At increasing SV (60 ml), MF showed comparable results to those observed with CEM, and better results at 65 ml (P<0.05). SS, SJME and MMU showed comparable EOAs throughout the SV spectrum with an incremental area directly related to increasing PR. The latter relation was reversed for pericardial tissue valves, mostly CEM and MF. However, due to a larger cross-sectional area for blood flow the superior hemodynamic of MF and CEM in comparison to the porcine counterpart was maintained. 3.7. Stroke work loss and valvular resistance Stroke work loss and valvular resistance profile for each valve were similar and increased concurrently with the SV and PR. CEM showed the lowest value up to 55 ml SV, MF had the best performance at larger SV, although the difference was not significant (P=NS). The SS valve showed results comparable to both porcine prostheses. The SJME showed alternately better and worse results in comparison with MMU, performing significantly better at 55, 60 and 65 ml SV (P<0.05), worse at 45 and 50 ml SV (P<0.05).
This study compares the hydrodynamic performances of the most commonly used pericardial and porcine stented supra-annular valves, according to their response to increasing PR and SV, with the purpose to characterize before clinical application the ideal prosthesis for each patient. 4.1. Patient prosthesis mismatch concept All valve substitutes are responsible of some residual stenosis [1, 8]. This should be minimized by an accurate surgical strategy and preoperative prosthesis selection. As a matter of fact, Pibarot and Dumesnil [2] suggested the use of a fixed value of in-vivo measured EOA [2]. Unfortunately, in-vivo EOA has some inherent variability mainly related to the techniques used for its measurement, as well as to a certain flow dependency.Therefore, contrary to previous studies [4, 5], we tested the prostheses performance at increasing PR and SV, an experimental condition for which few data have been published [9]. We additionally tested a new porcine bioprosthesis: MMU. 4.2. In-vitro prostheses hydrodynamics Kuehnel et al. [10] suggested that patients with a lower SV may take advantage from a porcine valve and patients with a higher SV from a pericardial valve: this is due to higher pliability of porcine tissue. Therefore, at increasing heart rate, such as we observe in elderly patients, this major pliability might modify the performances of porcine versus pericardial models. To verify this hypothesis we modified our in-vitro experimental setting by changing the PR while keeping the SV constant and vice versa. Our results show that at lower SV and at all rates, the pericardial prostheses, most notably CEM and MF, exhibit the smallest trans-prosthetic mean and peak gradients, the largest EOA and the lowest stroke-work-loss and valve-resistance.As a matter of fact, concerning the EOA, we additionally observed that at increasing PR a concomitant increase in EOA with the porcine models is expected, while the contrary was true with CEM and MF. Therefore, contrary to published results [10], we observed that, when larger CO is obtained with faster PR and not with higher SV, the porcine valve shows a cardiac output dependent effective opening area reserve. In summary, we maintain that for patients with a low SV, a porcine prosthesis guarantees acceptable hydrodynamics and additionally it offers an opening area reserve at increasing PR. As an example, according to our results, the SJME 21 mm – the valve with the smallest calculated EOA – showed gradients, even at 7.2 l/min CO, absolutely acceptable and significantly lower than those observed in our previous study at fixed PR [4]. Our observations suggest that the routine use of pericardial bioprostheses over porcine valves in all patients, on the presumption that they confer always hemodynamic advantages, is unlikely to be justified. Concerning the aortic root, its complex anatomy modulates prosthetic fitting and hemodynamic responses, since the larger sewing ring and higher profile may play a crucial role in terms of encumbrance in the sinus portion of the aorta [11]. In this study, we tested all the valves by using the smallest aortic root available for the SPD. We used a rigid aortic root 23 mm large in bore diameter, in which the SJME and MMU fitted more smoothly than all others, and particularly the SS. Since the complex aortic root anatomy is responsible for different prosthesis fitting in the sinus portion of the aorta, the miniaturized dimensions of the porcine prostheses are relevant mostly when we are dealing with patients with a small aortic root. Concerning regurgitant volumes, the SJME valve had the lowest total regurgitant volume and the lowest valve closing volume proving that this prosthesis, with the compositum three-leaflets design, provides an effective opening–closing matching with an adequate central leaflet coaptation. CEM, confirming our previous results [4], showed a regurgitant fraction significantly higher than all other prostheses. We speculate, as well as Bakhtiary and co-authors already discussed [12], that the leakage volume could be related to the new design of the stent, which does not favor a perfect central leaflet coaptation and to the absence of a sealing between the outlet and the inlet on this model with transit of fluid through the fabric. Concerning closing volume, CEM showed data significantly better than the other two pericardial prostheses, probably a reflection of superior pliability of the pericardial tissue of this prosthesis. The greater adaptability of porcine tissue as compared to the pericardial one also explains the significantly better diastolic behavior of both SJME and MMU. On the other hand, recently Bakhtiary et al. [13] by magnetic resonance imaging scanning, showed a relation between PPM (turbulent aortic root flow) and reduction of coronary flow reserve (CFR). The authors suggest that this property should be included in the comparisons between different prostheses in the future. We speculate that the impaired physiological backflow during diastole might have a role on the CFR. Although the blood viscosity is higher than that of the solution used in these tests, further in-vivo evaluations should be recommended not only in terms of PPM during systole but also in terms of regurgitation during the diastolic phase.
The in vitro system that we have used has a virtually rigid arrangement section downstream the aortic valve, and that represents perhaps the single largest distortion from reality. Nevertheless, the pulse duplicator device is not really designed to give an accurate representation of the true anatomy, but it is a system that provides an extraordinary and unquestionable bench-test for comparison of different prostheses exposed to the same conditions. However, we have to keep in mind that the transfer of the in-vitro results to an in-vivo situation is limited by the fact that the in-vivo hemodynamic behavior of a valve may differ from in vitro idealized assumptions. In conclusion, our results show that CEM and MF have shown significantly better in-vitro hydrodynamics in comparison with porcine counterpart and SS. Nevertheless, at increasing pulse rate, porcine tissue valves and SS may guarantee higher EOA values. Therefore, since in elderly patients with left ventricle hypertrophy, the increase in CO during exercise is mediated mostly by increasing heart rate, we speculate that this hydrodynamic evaluation model extends the horizon for bioprostheses use.
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Abstract
1. Introduction
