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Ann Thorac Surg 2001;72:1217-1221
© 2001 The Society of Thoracic Surgeons

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Original article: cardiovascular

Exercise hemodynamics of aortic prostheses: comparison between stentless bioprostheses and mechanical valves

^a Departments of Cardiac Surgery and Cardiology, Shaare Zedek Medical Center, Jerusalem, Israel
^b Department of Biostatistics, Hadassah University Hospital, Jerusalem, Israel

Address reprint requests to Dr Silberman, Department of Cardiothoracic Surgery, Shaare Zedek Medical Center, PO Box 3235, Jerusalem, 91031, Israel
e-mail: cts-szmc{at}szmc.org.il

Presented at the Thirty-seventh Annual Meeting of The Society of Thoracic Surgeons, New Orleans, LA, Jan 29–31, 2001.

	Abstract

Top Abstract Introduction Material and methods Results Comment Acknowledgments Discussion References

 
Background. Nonstented bioprostheses have been associated with lower resting gradients than stented bioprostheses or mechanical valves. We compared the hemodynamic performance of nonstented bioprostheses and mechanical valves with normal native aortic valves at rest and exercise.

Methods. Dobutamine echocardiography was used to assess gradients and effective orifice area index at rest and exercise in patients with the Toronto stentless porcine valve (TSPV; n = 13; mean implant size 25.7 mm), Medtronic Freestyle (FR; n = 11; mean implant size 23.9 mm), Sorin Bicarbon (SOR; n = 11; mean implant size 24.5 mm), St. Jude Medical (SJM; n = 10; mean implant size 21.3 mm), and normal native aortic valves (NOR; n = 10).

Results. All groups demonstrated a major rise in cardiac output at maximal dobutamine infusion. At rest and exercise, respectively, mean gradients were 5.48 ± 1.1 mm Hg and 5.83 ± 0.9 mm Hg for TSPV, 5.68 ± 1.2 mm Hg and 7.50 ± 1.7 mm Hg for FR, 10.29 ± 1.4 mm Hg and 20.78 ± 2.7 mm Hg for SJM, 5.26 ± 0.8 mm Hg and 11.1 ± 1.8 mm Hg for SOR, and 1.54 ± 0.4 mm Hg and 2.18 ± 0.7 mm Hg for NOR. In comparison with normal valves, both stentless groups showed no change in mean gradient at exercise, whereas both mechanical groups showed an increase in gradient at exercise (p < 0.04).

Conclusions. Stentless valves behave similarly to normal aortic valves in that there is almost no increase in gradient at exercise. Both mechanical valve groups showed increased gradients at exercise, suggesting that these valves obstruct blood flow. Our data add further evidence that stentless valves are hemodynamically superior to mechanical valves in the aortic position.

	Introduction

Top Abstract Introduction Material and methods Results Comment Acknowledgments Discussion References

 
Nonstented aortic bioprostheses have been the focus of numerous studies in the last decade. Compared with stented valves, these valves are thought to offer superior hemodynamics [1–3] as well as a survival advantage [4, 5]. Their durability is as yet undetermined, although their design may overcome some of the disadvantages stemming from the presence of a rigid stent and the resulting forces on the valve leaflets [6, 7].

Most studies to date have examined valve hemodynamics at rest. On exercise, or even with normal daily activities, flow through the left ventricular (LV) outflow tract is increased, and there may be a gradient through the aortic valve, which was not apparent at rest (a "relative" patient-prosthesis mismatch). The purpose of this study was to compare the hemodynamic performance of nonstented aortic bioprostheses and mechanical valves with that of normal native aortic valves at rest and exercise conditions.

	Material and methods

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Patients
Dobutamine echocardiography was used to assess aortic valve function in 55 patients divided into five groups. Group I (n = 13) included patients having a Toronto Stentless Porcine Valve (TSPV; St. Jude Medical, St. Paul, MN); group II (n = 11) included patients having a Freestyle (FR) stentless bioprosthesis (Medtronic Inc, Minneapolis, MN); group III (n = 11) included patients having a Sorin Bicarbon (SOR) mechanical valve (Sorin Biomedica Cardio, Saluggia, Italy); and group IV (n = 10) included patients having a St. Jude Medical (SJM) mechanical valve (St. Jude Medical, St. Paul, MN). Group V (n = 10) included patients with normal native aortic valves undergoing dobutamine echocardiography for other reasons, and served as control. Patients with aortic valve prostheses were examined at least 6 months after their operation. The study was retrospective and was approved by the hospital and health ministry ethics committees (Declaration of Helsinki), and all patients signed an informed consent.

Patients with mitral insufficiency, conduction disturbance, arrhythmia, or a known contraindication for dobutamine stress testing were not eligible for examination. Patients not reaching their target heart rate were excluded from analysis.

Surgical technique
Patients were connected to cardiopulmonary bypass through ascending aortic and right atrial cannulation. Active cooling was not routinely used. Myocardial protection was achieved by LV venting, and administration of cold blood cardioplegic solution antegradely through the aortic root and by direct coronary cannulation, or retrogradely through the coronary sinus. Stentless bioprostheses were implanted in the subcoronary position using 2-0 or 4-0 interrupted braided polyester sutures for the proximal suture line, and continuous 4-0 polypropylene sutures for the distal suture line. In the FR group, the noncoronary sinus was not scalloped.

Stress protocol
Dobutamine infusion was started at 5 µg · kg^-1 · min^-1, and the dose was increased at 5-minute intervals by increments of 5 µg · kg^-1 · min^-1 until 75% of target heart rate was achieved, or to a maximal dose of 40 µg · kg^-1 · min^-1. If any untoward side effects were encountered, the examination was terminated.

Echocardiographic measurements
Measurements were performed at rest and at maximal cardiac output. Mean and peak gradients were calculated using the modified Bernoulli equation. Effective orifice area and cardiac output were calculated using the continuity equation (see Appendix).

Valves
The type of valve implanted, namely mechanical or biological, was decided taking into account patient age, lifestyle, ability to take anticoagulant agents, and patient preference. Mean implanted valve sizes (manufacturer’s labeled size) were: TSPV 25.7 mm, FR 23.9 mm, SJM 21.3 mm, and SOR 24.5 mm.

Statistical analysis
Dunnett t test was used to compare the change in gradient between rest and maximal exercise in implanted mechanical valves and bioprostheses, in comparison with the change in normal native aortic valves. Values are expressed as mean ± standard error.

	Results

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Table 1 shows the annular diameter as measured by echocardiography preoperatively. Groups II and III both had larger annuli than group I. Otherwise there was no significant difference in annular diameter between the groups.

View larger version (11K): [in this window] [in a new window]   Fig 1. Mean gradient at rest and maximal exercise. The change in gradient from rest to exercise is compared with the change in the normal group. *p = 0.035 versus NOR; **p < 0.001 versus NOR. (NOR = normal native aortic valves; TSPV = Toronto stentless porcine valve; FR = Medtronic Freestyle; SOR = Sorin Bicarbon; SJM = St. Jude Medical.)

View larger version (14K): [in this window] [in a new window]   Fig 2. Peak gradient at rest and maximal exercise. The change in gradient from rest to exercise is compared with the change in the normal group. *p < 0.001 versus NOR. (NOR = normal native aortic valves; TSPV = Toronto stentless porcine valve; FR = Medtronic Freestyle; SOR = Sorin Bicarbon; SJM = St. Jude Medical.)

	Comment

Top Abstract Introduction Material and methods Results Comment Acknowledgments Discussion References

Collinson and coworkers [13], in a retrospective study comparing patients with poor LV function who received stentless valves (homografts or TSPV) with those receiving mechanical valves or stented bioprostheses, found the stentless group had a prompt improvement in LV function, which was attributed to valve performance rather than remodeling. Although it is not certain at which gradient a stimulus remains for hypertrophy, they suggested that a gradient of 30 mm Hg developed across normally functioning mechanical or stented valves may be enough to delay recovery of the left ventricle.

Valve size
It has been reported that for a given annulus size it is possible to implant a nonstented valve of one size larger than a stented or a mechanical valve [14]. This has been our experience as well. The significance of this finding is uncertain, because there is a discrepancy between the labeled valve sizes and the true physical dimensions of the orifice [15]. To prevent any bias regarding implanted valve size, we chose to compare patients with relatively larger mechanical valves, whereas in reality patients with a "25-mm" stentless valve would have received only a "23-mm" or a "21-mm" mechanical valve. Indeed, patients receiving a TSPV on the average had a smaller annulus compared with the other groups, as shown by the preoperative measurements. Even so, the larger mechanical valves in our study, while demonstrating a larger effective orifice area as well as low resting gradients, did have a larger rise in gradient at exercise than the stentless valves. The small mechanical valves had even higher gradients, possibly indicative of patient-prosthesis mismatch.

Gradient
Most valve substitute devices are relatively unobstructive to flow at resting conditions. With increasing cardiac output, however, the rigid sewing ring and stent may pose some obstruction to blood flow. Izzat and colleagues [16] examined small mechanical valves in the aortic position and found that mean and peak exercise gradients were 9.7 mm Hg and 22.3 mm Hg for the 21-mm St. Jude valves, and 8.8 mm Hg and 23.4 mm Hg for the 21-mm Carbomedics valves. In both brands, the indexed effective orifice area was smaller than 1 cm². In another study, Izzat and colleagues [17] examined 21-mm stented bioprostheses and found them to have still higher resting and exercise gradients, and lower effective orifice area index. Their results are similar to ours in the small valve group, and these values indicate residual obstruction.

We have previously shown that even large mechanical valves, despite having a larger calculated effective orifice area, are more obstructive to flow [18]. The present study compared two stentless valve types and mechanical prostheses of large (25 mm) and small (21 mm) sizes with normal native valves. Although the absolute gradients measured were not high, our results showed an advantage in favor of the stentless valves over both mechanical groups once the cardiac output was increased. In effect, the nonstented valves behaved similar to the normal aortic valve at high flow.

Others have also shown that nonstented bioprostheses have lower transvalvular gradients, at rest and exercise, than mechanical or stented valves. Basarir and coworkers [19] demonstrated lower exercise gradients, and a greater decrease in LV mass index in patients receiving homografts compared with those receiving mechanical valves. Fries and coworkers [20] compared gradients in 23-mm stentless and 23-mm stented bioprostheses. They found that the nonstented valves behaved similar to normal native valves, in that there was no increase in gradient at exercise, whereas in the stented group the gradient clearly increased with increasing cardiac output. This finding is in accordance with our results, and adds support to the assumption that stentless valves are indeed less obstructive to flow than various mechanical or stented valves.

Significance
Although no long-term studies are yet available, numerous reports suggest the advantages of stentless valves. Walther and coworkers [21], in a prospective randomized trial, showed that stentless bioprostheses allowed for more complete LV mass regression than stented valves. In another study, Thomson and colleagues [22] showed lower gradients in stentless valves compared with stented bioprostheses or mechanical valves. The same study showed better LV mass regression in patients receiving stentless valves compared with stented bioprostheses, although LV mass regression did not differ between the stentless and the mechanical groups.

In a retrospective, case-matched study patients receiving a TSPV had better survival than those receiving the Hancock II bioprosthesis [4]. Another study showed that patients having re-replacement of their aortic valve with a homograft had better long-term survival if at the initial operation they had received a homograft [23]. Del Rizzo and colleagues [24] showed a survival advantage for patients receiving stentless valves over those receiving stented valves, and this advantage was more pronounced in younger patients. The accumulating data in the literature show a survival advantage in favor of stentless valves, including homografts and stentless xenografts, over mechanical and stented bioprostheses. Although the durability of stentless bioprostheses has not yet been established, they may be an adequate and more available alternative to homografts.

Limitations
One limitation of our study is that it was retrospective. The question of valve size remains a controversy among surgeons despite the data available regarding the inaccuracy of commercial valve sizers. Although the preoperative annulus diameters were presented, it might have been advantageous to have prospectively sized each annulus for each valve type. A second limitation is that the study group was not homogenous in terms of preoperative aortic valve pathology. Thirdly, the adequacy of dobutamine echocardiography to assess valve function at exercise has been questioned. Zussa and colleagues [25] found that echocardiography overestimates valve gradients with dobutamine infusion compared with exercise in the presence of LVH. To overcome this overestimation, we examined patients at least 6 months after their operations, so that a greater degree of LV mass regression would have occurred, and LVH would not be a confounding factor. Because many of our patients were elderly and were not able to perform treadmill or bicycle stress testing, dobutamine echocardiography was considered to be the preferred method.

In conclusion, stentless bioprostheses are hemodynamically superior to mechanical valves because they are less obstructive to flow at high cardiac outputs. This finding was also true when comparing larger mechanical valves, indicating that this advantage is related to valve design and not to valve size. The residual gradient at increased cardiac output in the presence of mechanical valves and stented bioprostheses does not allow for full recovery of the left ventricle, and for maximal LV mass regression. Thus the advantage of stentless valves in active patients may be even more pronounced in terms of long-term survival. The risk of the anticipated reoperation for valve degeneration, if performed electively, may very well be less than the cumulative risk of long-term anticoagulation therapy.

	Acknowledgments

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We thank Ms Esther Adler and Ms Pnina Miriam Avraham for performing the echocardiograms.

	Appendix

 
Pressure drop (gradient) = 4(Vcw² - Vpw²)

Vcw = velocity across the valve by continuous wave Doppler echocardiography;

Vpw = velocity in the left ventricular outflow tract (LVOT) by continuous wave Doppler echocardiography;

VTI(pw) = velocity time integral in the LVOT by pulse wave Doppler;

VTI(cw) = velocity time integral in the LVOT by continuous wave Doppler;

CSA = cross sectional area in the LVOT;

HR = heart rate.

	Discussion

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DR GEORGE T. CHRISTAKIS (Toronto, Ontario, Canada): Congratulations, Shuli, that was an excellent paper, very well presented, and an extremely difficult study to organize, I am sure. The question I have for you is related to the clinical relevance of exercise gradients in prosthetic valves. The indications for aortic valve replacement go back to 1968 with Ross and Braunwald, and essentially we operated on patients merely to prolong life. Now, we exercise patients, and if their gradients increase, that is considered to be a bad sign. On the other hand, we do not exercise patients and replace the aortic valves if they have high resting gradients. So surgical decisions are made based solely on symptoms and rest gradients. What do you think is the clinical relevance of exercise gradients?

DR SILBERMAN: Thank you, Dr Christakis, for your comments and your question. As you saw on the slides, all the valves we examined had low gradients at rest and they were probably all equal hemodynamically. In addition, the larger mechanical valves also had good performance at high outputs, but these were especially large valves. We are not certain at what gradient there is stimulus for hypertrophy, but younger or active patients who exercise will have high cardiac outputs at times, therefore the increase in gradient may be important in these patients, especially if we can implant only small valves. The significance of our findings cannot be interpreted in terms of long-term survival, and I think long-term survival will have to be learned from larger, prospectively randomized studies.

DR DAVID H. ADAMS (Boston, Massachusetts): If I could just ask you to be more specific, because the typical patient that is too young for a more efficient stentless valve is going to get a mechanical valve, so based on your data, is your group going to put in a 21-mm St. Jude valve in an active patient or is this decision more science and less practical in terms of taking care of patients at this point?

DR SILBERMAN: We are using the valves that we can adequately implant, and I think trying to oversize a mechanical valve or a stented tissue valve may have bad results in the long run. In a given annulus you can only put in a given size valve. Whether or not younger patients will do better off in the long term with nonstented bioprostheses I think is beyond the scope of our study.

DR ADAMS: Actually, I was thinking more about root enlargement, which would allow you to put in a size 23 or 25 St. Jude valve.

DR SILBERMAN: Again, I am not sure whether that will eventually give better survival in the long run.

DR IRVING KRON (Charlottesville, Virginia): What is the accuracy of the echo gradient in the St. Jude valve with and without exercise? As you know, this is an issue with echocardiographers.

DR SILBERMAN: Well, we did have our cardiographers do the exercise testing, and there are controversies as to whether these tests are accurate. There are not many good possibilities to measure gradients, especially across mechanical valves. Echo is the best we have at the present.

	References

Top Abstract Introduction Material and methods Results Comment Acknowledgments Discussion References

 

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