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J Thorac Cardiovasc Surg 2004;128:457-462
© 2004 The American Association for Thoracic Surgery

Evolving technology

Small aortic annulus: The hydrodynamic performances of 5 commercially available bileaflet mechanical valves

Tomaso Bottio, MDa,*, Luca Caprili, MDa, Dino Casarotto, MDa, Gino Gerosa, MDa

a Department of Cardiovascular Surgery, University of Padua Medical School, Padua, Italy

Received for publication September 5, 2003; revisions received February 29, 2004; accepted for publication March 4, 2004.

* Address for reprints: Gino Gerosa, MD, Chief, Department of Cardiovascular Surgery, Via Giustiniani, 2, 35100, Padova, Italy
gino.gerosa{at}unipd.it


   Abstract
Top
 Abstract
Material and methods
 Results
 Discussion
 References
 
OBJECTIVE: Hemodynamic performances of mechanical valve prostheses in patients with aortic valve stenosis and a small aortic annulus are crucial. We analyzed the in vitro hydrodynamics of 5 currently available bileaflet mechanical prostheses that fitted a 21-mm-diameter valve holder of a Sheffield pulse duplicator.

METHODS: Three samples of 5 high-performance production-quality prostheses, including the sewing ring cuffs, were tested in the aortic chamber of a Sheffield pulse duplicator. Sizes of the prostheses fitting the 21-mm valve holder were as follows: 18-mm ATS, 19-mm SJM Regent, 19-mm Sorin Bicarbon Slimline, 19-mm On-X, and 21-mm Carbomedics Top Hat. The tests were carried out at a fixed pulse rate (70 beats/min) and at increasing cardiac outputs of 2, 4, 5, and 7 L/min. Each valve was tested 10 times for each different cardiac output. This resulted in a total of 40 tests for each valve and 120 tests for each valve model. The aortic pressure was set at 120/80 mm Hg (mean pressure, 100 mm Hg) throughout the experiment for all cardiac outputs. Forward flow pressure decrease, total regurgitant volume, closing and leakage volumes, effective orifice area, and stroke work loss were recorded while the valve operated under each cardiac output.

RESULTS: The SJM Regent valve and the Sorin Bicarbon Slimline prosthesis showed the lowest mean and peak gradients at increasing cardiac outputs. The closure volume was higher for the SJM Regent and Sorin Bicarbon Slimline prostheses, unlike with the ATS prosthesis at 7 L/min of cardiac output. The ATS and SJM Regent prostheses showed the largest regurgitant volume, whereas the Sorin Bicarbon Slimline prosthesis showed the lowest regurgitant volume. The calculated effective orifice area and stroke work loss were significantly better with the SJM Regent and Sorin Bicarbon Slimline prostheses.

CONCLUSION: Assuming that the 21-mm valve holder in which all the tested prostheses were accommodated is comparable with a defined aortic valve size, this hydrodynamic evaluation model allowed us to compare the efficiency of currently available valve prostheses, and among these, the SJM Regent and the Sorin Bicarbon Slimline exhibited the best performances.

The presence of a small aortic annulus might force cardiac surgeons to enlarge the annulus or to implant a small prosthesis. The latter strategy might lead to unsatisfactory pressure gradients across the prosthesis, with consequent incomplete left ventricular hypertrophy regression.1 Nevertheless, aortic root enlargement procedures might have double the operative mortality compared with that of standard aortic valve replacement.2,3 Very recently, excellent results with normalization of left ventricular mass in elderly women who received a small aortic prosthesis have been reported,4 suggesting the need for care in applying more complex procedures.

Refinement in hemodynamic performances of mechanical valve prostheses is crucial and might be a continuously challenging task. However, the ability to develop and release new heart valve prosthetic models might clash with the possibility that new valve models could face structural failure.5 Careful in vitro hydrodynamic evaluation and continuous monitoring of patients with new prostheses are therefore mandatory. Comparison of hydrodynamic performances of currently available prostheses is somehow misleading because of differences in reporting nominal valve size, which is not uniform across manufacturers.6

In this report we analyze the hydrodynamic performances of 5 high-performance bileaflet mechanical prostheses (SJM Regent, Sorin Bicarbon Slimline, Carbomedics Top Hat, On-X, and ATS), which, regardless of the manufacturer's nominal size, were fitted on a 21-mm-diameter valve holder of a Sheffield pulse duplicator (SPD).


   Material and methods
Top
 Abstract
 Material and methods
Results
 Discussion
 References
 
The SPD is a system designed to perform pulsatile hydrodynamic testing of prosthetic heart valves by means of continuous measurement of flow and transvalvular pressure gradients. The system has been previously described in detail.7

Production-quality prostheses, including the sewing ring cuffs, were tested in the aortic chamber of the SPD. Prosthetic valve characteristics, geometric profiles, geometric orifice areas, and clear areas of tested valves8 are reported in Table 1. To allow a meaningful comparison regardless of nominal size, we tested valve prostheses that could be accommodated in a 21-mm valve holder. The valves and the holder were not sealed before testing because the holder and the sewing ring fitted perfectly. Therefore paravalvular leakage was not allowed in any test, and the sealing was considered unnecessary. The sizes of the tested valves were as follows: 18-mm ATS, 19-mm SJM Regent, 19-mm Sorin Bicarbon Slimline, 19-mm On-X, and 21-mm Carbomedics Top Hat. The valves were tested according to previously reported standard protocols.8,9


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  		TABLE 1. Geometrics of the tested valves and mean value of EOA calculated over all tests performed for each valve

 
The system was filled with saline solution (0.9%), as recommended by the manufacturer, to optimize measurements (Instruction Manual, Sheffield Pulse Duplicator, Department of Medical Physics and Clinical Engineering, The University of Sheffield). Three production-quality samples of each bileaflet model were tested. The tests were carried out at a single pulse rate (70 beats/min) with a cardiac output (CO) of 2, 4, 5, and 7 L/min according to ISO584010 and US Food and Drug Administration Replacement Heart Valve Guidance protocols.11 Each valve was tested 10 times at each different CO. This resulted in a total of 40 tests for each valve and 120 tests for each valve model. The mean and SD of each measurement parameter for each test condition was calculated from the 10 repeated tests on each valve. Because the diastolic interval was 555 ms, whereas the systolic interval was 300 ms in every case, mean systolic flow rate was only related to CO. Stroke volume of the pump and systemic resistance and compliance were adjusted to obtain the target CO and characteristic aortic pressure waveforms. The aortic pressure was kept constant at 120/80 mm Hg (mean pressure, 100 mm Hg) throughout the experiment for all COs. Simultaneous pressure measurements were recorded by using electromagnetic flowmeters and pressure transducers located 30 mm upstream and 100 mm downstream of the aortic valve. The following parameters were determined for each cardiac cycle: forward flow pressure decrease ({Delta}P), closing volume (in milliliters), leakage volume (in milliliters), total regurgitant volume (in milliliters), effective orifice area (EOA; in square centimeters), stroke work loss (in percentages), and valvular resistance (in (dynes · s · cm–5).

Forward flow pressure decrease, closing volume, leakage volume, total regurgitant volume, and EOA were calculated as previously published by Walker and coworkers.8 Valvular resistance and stroke work loss were calculated as the ratio of the mean transprosthetic gradient to the mean systolic ventricular pressure, as previously published by Voelker and colleagues.9 All data were expressed as means ± SD. The {chi}2 test was used for statistical comparison.


   Results
Top
 Abstract
 Material and methods
 Results
Discussion
 References
 
The obtained mean values and SDs are expressed according to the 4 COs adopted and reported in Table 2.


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  		TABLE 2. Hydrodynamic performances of the tested valves (mean value ± SD) at COs of 2, 4, 5, and 7 L/min

 
Pressure differences
The mean and peak pressure differences for each CO are reported in Figure 1. The SJM Regent valve and the Sorin Bicarbon Slimline prosthesis at COs of 5 and 7 L/min showed the lowest gradients. The results with such valves were significantly lower than those observed with the Carbomedics Top Hat, On-X, and ATS valves (P < .0001). The difference was more evident at a CO of 5 L/min and increased to a maximal level at 7 L/min.



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  		Figure 1. A, Mean systolic pressure difference. The SJM Regent valve and the Sorin Bicarbon Slimline prosthesis at a CO of 5 and 7 L/min showed the lowest gradients. B, Peak systolic pressure difference. The SJM Regent valve and the Sorin Bicarbon Slimline prosthesis at COs of 5 and 7 L/min showed the lowest gradients.

 
Regurgitant volumes
The observed total regurgitant volumes are reported in Figure 2. The Sorin Bicarbon Slimline valve showed the lowest regurgitant volume (<5 mL), which was significantly smaller for each CO when compared with that obtained with the other valves (P < .0001). The ATS and SJM Regent valves showed the highest regurgitant volume (9 mL). The observed trend was toward a reduction of regurgitant volume at increasing CO, unlike the ATS prosthesis, which exhibited an increased regurgitant volume at 7 L/min of CO.



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  		Figure 2. Total regurgitant volume. The Sorin Bicarbon Slimline valve showed the lowest total regurgitant volume, whereas the ATS and SJM Regent valves showed the highest total regurgitant volume.

 
Closure volume
The recorded closure volumes are reported in Figure 3. All the tested valves showed a decreasing closure volume as CO increased. The highest value was observed with the SJM Regent prosthesis. At lower COs, the SJM Regent valve had a significantly higher closure volume when compared with those of the other prostheses. On the other hand, when increasing the CO between 5 and 7 L/min, the closure volumes obtained with the SJM Regent and the Sorin Bicarbon Slimline prostheses were comparable, and such results were significantly higher than those obtained with the other prostheses (P < .0001). The ATS valve showed a peculiar trend: as expected, with increasing CO, the closure volume decreased, but at greater than 5 L/min of CO, a curve inversion was detected.



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  		Figure 3. Closure volumes. All the tested valves showed a decreasing closure volume as CO increased. The highest value was observed with the SJM Regent prosthesis.

 
Leakage volume
The grade of the leakage volume for each tested valve is reported in Figure 4. The Sorin Bicarbon Slimline valve showed the lowest leakage volume, which was significantly smaller for each CO when compared with that obtained with the other valves (P < .0001). All the tested prostheses but the SJM Regent showed a stable leakage volume at increased CO. The highest leakage volume was observed with the ATS valve. The SJM Regent prosthesis had a peculiar trend, with an increasing leakage volume of between 2 and 4 L/min, which remaining stable thereafter. Unlike the Sorin Bicarbon Slimline valve, all the other models showed a leakage volume greater than the closure volume.



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  		Figure 4. Leakage volumes. The Sorin Bicarbon Slimline valve showed the lowest leakage volume. The highest leakage volume was observed with the ATS valve. Unlike the Sorin Bicarbon Slimline prosthesis, all the other models showed a leakage volume greater than the closure volume.

 
Effective orifice area
The calculated EOAs for all valves are reported in Figure 5. The On-X prosthesis showed a different trend when compared with the other valves. It had the highest EOA at the lowest CO, with a decreasing area at increasing CO. On the other hand, with the ATS and Carbomedics Top Hat prostheses, the calculated EOA was stable at increasing CO. SJM Regent and Sorin Bicarbon Slimline prostheses had a similar trend, showing the lowest EOA at 2 L/min of CO. Increasing the CO the EOA improved, becoming significantly higher than that of the ATS, On-X, and Carbomedics Top Hat prostheses (P < .005).



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  		Figure 5. Effective orifice area. The On-X prosthesis showed the highest EOA at the lowest CO. The ATS and Carbomedics Top Hat prostheses showed a stable EOA at increasing CO. The SJM Regent and Sorin Bicarbon Slimline valves had the largest EOA at increasing CO.

 
Stroke work loss
The calculated stroke work loss values for each tested valve are listed in Figure 6. The stroke work loss profile for each valve was similar, increasing concurrently with the increase of CO. The results obtained at 2 and 4 L/min were comparable for all the prostheses. At 5 and 7 L/min, the SJM Regent and Sorin Bicarbon Slimline prostheses showed the best performances, which were significantly better than those obtained with the ATS, Carbomedics, and On-X prostheses (P < .00001).



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  		Figure 6. Stroke work loss. At 5 and 7 L/min, the SJM Regent and Sorin Bicarbon Slimline prostheses showed the best performances.

 

   Discussion
Top
 Abstract
 Material and methods
 Results
 Discussion
References
 
Continuous refinement in hemodynamic performances of mechanical valve prostheses is a challenging task. Comparison of hydrodynamics of different valve prostheses is difficult because of the lack of standardization in labeling by manufacturers.6 To allow a meaningful comparison that is relevant for clinical application, we designed an innovative study in concept to analyze the hydrodynamics of 5 different high-performance bileaflet mechanical prostheses (SJM Regent, Sorin Bicarbon Slimline, Carbomedics Top Hat, On-X, and ATS). Assuming that a defined valve holder is comparable with a defined aortic size annulus in which several different nominally sized valves could be allocated, we analyzed, at the SPD, 5 high-performance prostheses that were possible to be allocated in a 21-mm valve holder. This is the first article that strictly transfers the hydrodynamic inferences in a setting of clinical utility and applicability.

In patients with a small aortic annulus, insertion of small prostheses might lead to higher residual pressure gradients and incomplete left ventricular hypertrophy regression. The clinical effect of such a condition is still controversial.1 Either different surgical procedures or hemodynamically improved prosthetic valve devices have been proposed to reduce transprosthetic gradients in such cases. Nevertheless, when dealing with aortic root enlargement procedures, Sommers and David3 and Carrier and colleagues2 observed that operative mortality is twice that reported during standard aortic valve replacement. On the other hand, Freed and associates4 reported excellent results with normalization of left ventricular mass in elderly women who received a small aortic valve, including in their analysis woman who received both mechanical and biologic tissue valves. The authors' final suggestion was to be careful with the more complex procedures. In several institutional studies small prosthetic size was not a risk factor for intermediate and long-term survival.12-14

Hence the input to the research programs to maximize the prostheses hydrodynamics and improve their clinical performances was born. SJM Regent and Sorin Bicarbon Slimline prostheses, according to our experimental results, exhibited the smallest transprosthetic mean and peak gradients and the lowest stroke work loss.

These results are well explained for the SJM Regent valve by using the evidence of the larger EOA, greater geometric orifice area, and valve clear area when compared with those values in the others prostheses. As previously published, at 5 L/min of CO, the SJM Regent valve showed the greatest closing volume, becoming comparable with the others at 7 L/min. Nevertheless, among all tested valves, we observed the lowest stroke work loss with the SJM Regent valve. With the small valve size, the cause of energy loss was largely the result of the forward flow transvalve pressure difference, with less than 10% being the result of the regurgitant flow,15 and these satisfactory results with the SJM Regent depend mostly on the improved design and increased geometric orifice area.

The Sorin Bicarbon Slimline prosthesis has the smallest opening angle (70°) among the 5 tested valves, and because of this peculiar design, it showed the lowest regurgitant volume. Thereby the underlying cause for the hydrodynamic success with the Sorin Bicarbon Slimline prosthesis should be found either not on the geometric orifice area, which is smaller than that with the SJM Regent and comparable with that of On-X, or on the EOA, which was smaller than that of both the SJM Regent and On-X prostheses, but mostly on the concave-convex leaflets design, which seems to be a rather winning solution.

The worst hydrodynamic results, namely higher stroke work loss obtained with the Carbomedics Top Hat and ATS valves when compared with that of the SJM Regent and Sorin Bicarbon Slimline prostheses, are well explained by either the smaller geometric orifice area or the smaller calculated EOA.

On the other hand, the results obtained with the On-X valve are controversial and difficult to explain. In fact, the On-X valve, which was designed to improve hydrodynamic performance, unlike accredited of the largest clear orifice area, the largest calculated EOA at lower CO, and the greater opening angle (90°), showed results in terms of regurgitant volumes, transprosthetic gradients, and stroke work loss comparable with those of the Carbomedics Top Hat and ATS valves and significantly inferior to those obtained with the Sorin Bicarbon Slimline and SJM Regent prostheses. Therefore further explanations, probably related to the peculiar On-X prosthesis design (higher profile) or to an incomplete valve opening, might be inferred.

In conclusion, according to our experimental model, each of the tested valves could be accommodated on a 21-mm aortic annulus, but in the light of these in vitro results, the best hemodynamic performances should be obtained mostly with the SJM Regent and Sorin Bicarbon Slimline prostheses.

Moreover, the clinical inference that we can draw from the study is as follow: on the basis of either our experience or the experience of other authors,16 a growing problem is the surgical management of small women with a calcified annulus. This hydrodynamic evaluation model, which is strictly related to the surgical aspect, might be helpful in a surgeon's choice when dealing with such insidious cases.


   Acknowledgments
 
We thank Giulio Rizzoli, MD, for his assistance in preparing the statistical analysis of the manuscript.


   References
Top
 Abstract
 Material and methods
 Results
 Discussion
 References
 

  1. Medalion B, Blackstone EH, Lytle BW, White J, Arnold JH, Cosgrove DM. Aortic valve replacement: is valve size important? J Thorac Cardiovasc Surg. 2000;119:963–974[Abstract/Free Full Text]
  2. Carrier M, Pellerin M, Perrault LP, Hebert Y, Page P, Cartier R, et al. Experience with the 19-mm Carpentier-Edwards pericardial bioprosthesis in the elderly. Ann Thorac Surg. 2001;71(suppl):S249–252[Abstract/Free Full Text]
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  4. Freed DH, Tam JW, Moon MC, Harding GEJ, Ahmad E, Pascoe EA. Nineteen-millimeter prosthetic aortic valves allow normalization of left ventricular mass in elderly women. Ann Thorac Surg. 2002;74:2022–2025[Abstract/Free Full Text]
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  6. Chambers JB, Lionel OO, Narracott A, Lawford PM, Blauth CI. Nominal size in six bileaflet mechanical aortic valves: a comparison of orifice size and biologic equivalence. J Thorac Cardiovasc Surg. 2003;125:1388–1393[Abstract/Free Full Text]
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  9. Voelker W, Reul H, Nienhaus G, Stelzer T, Schmitz B, Steegers A, et al. Comparison of valvular resistance, stroke work loss, and Gorlin valve area for quantification of aortic stenosis. An in vitro study in a pulsatile aortic flow model. Circulation. 1995;91:1196–1204[Abstract/Free Full Text]
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