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Ann Thorac Surg 2005;80:2180-2185
© 2005 The Society of Thoracic Surgeons

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Original article: Cardiovascular

Stentless Aortic Valves are Hemodynamically Superior to Stented Valves During Mid-Term Follow-Up: A Large Retrospective Study

Division of Cardiovascular Surgery, Toronto General Hospital, University Health Network, and Department of Surgery, University of Toronto, Toronto, Ontario, Canada

Accepted for publication May 17, 2005.

^_* Address correspondence to Dr Borger, Division of Cardiovascular Surgery, Toronto General Hospital, Room 4N-451, 200 Elizabeth Street, Toronto, Ontario, Canada M5G 2C4 (Email: michael.borger{at}uhn.on.ca).

Presented at the Poster Session of the Forty-first Annual Meeting of The Society of Thoracic Surgeons, Tampa, FL, Jan 24–26, 2005.

	Abstract

Top Abstract Introduction Material and Methods Results Comment References

 
BACKGROUND: Several studies have compared left venticular mass (LVM) regression and hemodynamic data for stentless versus stented aortic bioprostheses with conflicting results. The major limitations of these studies are their small sample size and short-term follow-up. We therefore compared midterm LVM regression, hemodynamic data, and survival in a large population of tissue aortic valve replacement (AVR) patients.

METHODS: All patients undergoing tissue AVR at our institution between 1998 and 2001 were included (n = 737). Patients were divided into two groups according to type of bioprosthetic implanted: stentless patients (total n = 310) (Toronto SPV [St Jude Medical, St Paul, MN], n = 146 and Freestyle [Medtronic, Minneapolis, MN], n = 164) and stented patients (total n=427) (Perimount [Edwards Life Sciences Inc, Irvine, CA], n = 291 and Mosaic [Medtronic], n = 136).

RESULTS: The two groups of patients had similar preoperative transvalvular gradients and LVM index (130 ± 47 vs 130 ± 42 g/m² for stentless versus stented valves, respectively). Predischarge echos revealed that stentless patients had significantly lower mean transvalvular gradients (11 ± 5 vs 15 ± 6 mm Hg, p < 0.001) and larger effective orifice areas (1.32 ± 0.52 vs 1.22 ± 0.48 cm², p = 0.01). Follow-up echocardiograms were obtained in 99% of surviving patients 28 ± 22 (range, 0–79) months postoperatively. Stentless patients had significantly lower LVM index during follow-up (100 ± 32 vs 107 ± 32 g/m², p = 0.005) and stentless valves were an independent predictor of LVM regression. Furthermore, a higher proportion of stented patients had residual LV hypertrophy during follow-up (28% vs 18%, p = 0.001). Stentless valves were associated with improved midterm survival by univariate analysis, but not by multivariable analysis.

CONCLUSIONS: Midterm follow-up in a large number of patients reveals that stentless bioprostheses are hemodynamically superior to stented valves.

	Introduction

Top Abstract Introduction Material and Methods Results Comment References

 
Left ventricular mass (LVM) regression occurs after aortic valve replacement (AVR) surgery [1] and is an important therapeutic goal for this procedure. The LVM regression may lead to improved long-term survival post-AVR, since persistent left ventricular hypertrophy is a strong predictor of mortality in other patient populations [2]. Stentless aortic tissue valves were developed with the goal of maximizing hemodynamic performance and improving LVM regression post-AVR [3].

Several studies have compared hemodynamic values for stentless versus stented tissue aortic valves with conflicting results [4–8]. Each of these studies was limited by a relatively small sample size and lack of significant follow-up. The goal of the current study was therefore to compare midterm hemodynamic data, LVM regression, and survival in a large group of consecutive AVR patients receiving stentless versus stented bioprostheses.

	Material and Methods

Top Abstract Introduction Material and Methods Results Comment References

 
Ethics approval was granted by our Institutional Research Ethics Board. Our computerized database was examined to identify all patients undergoing AVR with a bioprosthetic valve between 1998 and 2001 (n = 737). Of these patients, 310 received a stentless bioprosthetic and 427 received a conventional stented valve. The stentless bioprostheses consisted of the Toronto SPV (St Jude, St Paul, MN) (n = 146) and the Freestyle valve (Medtronic Inc, Minneapolis, MN) (n = 164). The stented tissue valves consisted of the Carpentier-Edwards Perimount (Edwards Life Sciences Inc, Irvine, CA) (n = 291) and the Mosaic valve (Medtronic) (n = 136).

Aortic valve replacement was performed with previously described techniques [9, 10]. The decision as to whether the patient received a stented or a stentless bioprosthesis was at the discretion of the attending surgeon and the proportion of patients who received stentless valves varied among surgeons. Severe calcification of the aortic root wall and dilation of the sinotubular junction were contraindications to subcoronary implantation of stentless valves. Aortic root replacement was performed in patients with aortic root disease (ie, aneurysm, annuloaortic ectasia, dissection, or severe atherosclerotic calcification), but not in patients with isolated aortic stenosis and normal aortas.

Annular sizing was performed with standard bioprosthetic sizers. The aortic annulus was enlarged in patients with a small annulus, according to previously described techniques [11]. Patients requiring concomitant replacement of the ascending aorta received a supracoronary Dacron tube graft if a stented valve was implanted, or a stentless porcine root (with or without a Dacron tube graft) if a stentless valve was implanted. We performed a septal myectomy in patients with asymmetric hypertrophy of the septum and an increased gradient across the left ventricular outflow tract, using previously described techniques [12].

Echocardiography
Preoperative, predischarge, and midterm echocardiographic reports were obtained on surviving patients. Preoperative and predischarge echos were performed at our institution. Follow-up echos were performed in our institution and in outside laboratories. Echos from outside laboratories were obtained by contacting the referring cardiologists.

Standard techniques were used to obtain echocardiographic measurements, in accordance with the American Society of Echocardiography guidelines. Pulsed wave Doppler was used to measure mean and maximum systolic blood flow velocities (V_mean and V_max) in the left ventricular outlow tract (LVOT), and continuous wave Doppler was used to measure systolic blood flow velocities across the aortic valve (AV).

Peak and mean transvalvular gradients were obtained using the modified Bernoulli equation:

The effective orifice area (EOA) was calculated with the continuity equation:

where CSA_LVOT = LVOT cross-sectional area (r² / 4) in square centimeters, TVI_LVOT = LVOT time velocity integral of forward blood flow in centimeters, and TVI_AV = transvalvular time velocity integral of blood flow in centimeters.

The LVM was calculated according to previously published guidelines [13]:

where LVID_d = left ventricular internal dimension at end-diastole, IVS_d = interventricular septal thickness at end-diastole, and PW_d = posterior wall thickness at end-diastole (all in centimeters). The left ventricular mass indexed (LVMI) (g/m²) = LVM / body surface area.

Statistical Analysis
Categorical variables are expressed as percentages and continuous variables are expressed as mean ± SD throughout the manuscript. All statistical analyses were performed with the SAS system (SAS version 8.1; Cary, NC). Comparison of categorical variables was performed with ² or Fisher's exact tests, and continuous variables were analyzed with unpaired t tests. Independent predictors of midterm LVM regression were determined by multiple linear regression. Midterm survival was compared univariately with the methods of Kaplan-Meier and multivariately with Cox regression. Statistical significance was defined as p less than 0.05.

	Results

Top Abstract Introduction Material and Methods Results Comment References

 
A total of 737 patients underwent tissue AVR at our institution from 1998 to 2001. Patients were divided into two groups according to the type of bioprosthetic received: those receiving a stentless valve (Freestyle and Toronto SPV, total n = 310) and those receiving a stented valve (Perimount and Mosaic, total n = 427).

Table 1 displays the preoperative demographics for the two groups of patients. Stentless AVR patients were significantly younger, had larger body surface areas, and were more likely to be male. Stented patients had significantly more hypertension and peripheral vascular disease, as well as aortic stenosis as the primary aortic valve pathology.

View larger version (17K): [in this window] [in a new window]   Fig 1. Labeled valve sizes for patients receiving stentless (white bars) and stented (black bars) aortic valve prostheses.

We also compared the proportion of patients with residual LV hypertrophy during follow-up. Residual LV hypertrophy was defined as greater than 134 g/m² for males and greater than 110 g/m² for females, in accordance with previously published recommendations [14]. Figure 2 reveals that stentless AVR patients had a lower prevalence of residual LV hypertrophy during follow-up.

View larger version (46K): [in this window] [in a new window]   Fig 2. Prevalence of residual left ventricular hypertrophy (LVH) during follow-up in stentless versus stented patients. (LVH defined as LV mass index > 134 g/m2 for males and > 110 g/m2 for females). (AVR = aortic valve replacement.)

Midterm survival is displayed in Figure 3. Patients who received a stentless valve had better five-year survival than those who received a stented valve by univariate analysis (p = 0.006), but not by multivariable analysis. Cox regression revealed that the independent predictors of midterm survival were (with hazard ratios and 95% confidence interval in parentheses): age (1.06; 1.03–1.08); peripheral vascular disease (1.93; 1.10–3.34); reoperative surgery (2.05; 1.21–3.47); LV ejection fraction less than 40% (2.26; 1.45–4.51); and preoperative renal failure (4.78; 1.49– 15.40).

View larger version (16K): [in this window] [in a new window]   Fig 3. Kaplan-Meier survival curves for stentless and stented aortic valve patients. Implantation of a stentless valve was associated with improved survival by univariate (log-rank) analysis. (Five year survival: - - -, stentless 87 ± 2%; —, stented 79 ± 3%.)

	Comment

Top Abstract Introduction Material and Methods Results Comment References

Increased LVM is an important predictor of future cardiovascular events in noncardiac surgery patients. Studies have revealed that increased LVM is associated with increased cardiac events and decreased long-term survival in patients without cardiovascular disease [16], in patients with hypertensive heart disease [17], and in patients with medically treated aortic stenosis [18].

Left ventricular mass can remain significantly elevated post-AVR, particularly in patients with patient-prosthesis mismatch (PPM) [19]. We previously demonstrated that PPM is an independent predictor of short- and long-term mortality post-AVR [20]. Patient-prosthesis mismatch is a common phenomenon, occurring in over one-half of patients undergoing stented tissue AVR [21]. However, PPM is less likely to occur after stentless valve implantation [19, 21, 22].

Stentless bioprostheses were developed with the goal of achieving superior hemodynamic performance compared with conventional stented valves. Elimination of the sewing ring and stents was designed to result in less obstruction to blood flow and decreased transvalvular gradients. Improved hemodynamic performance may lead to less PPM, increased LVM regression, and increased long-term survival. We previously performed a case-match study to demonstrate that stentless aortic valves (Toronto SPV) are associated with improved long-term survival and freedom from valve-related complications compared with stented valves (Hancock II, Medtronic) [23]. However, stentless valves are more difficult to implant than stented bioprostheses and may therefore have a higher rate of perioperative complications. Many surgeons have thus been reluctant to use stentless valves.

Several randomized trials have compared stentless with stented bioprostheses with conflicting results. Walther and colleagues [4] randomized a total of 180 patients to receive a stentless (Freestyle or Toronto SPV) or a stented aortic valve (CSB, Edwards Life Sciences). Stentless patients had significantly lower peak gradients and a lower LVM index six months postoperatively. In contrast, Cohen and colleagues [6] randomized 99 patients to receive a stented Perimount or a stentless Toronto SPV and failed to demonstrate any significant differences in the EOA, transvalvular gradients, or LVM regression twelve months postoperatively. Similarly, Doss and colleagues [7] randomized 40 patients over 75 years of age to receive a stented Perimount or a stentless Prima Plus valve (Edwards Life Sciences) and failed to demonstrate differences in gradients, EOA, or LVM regression one year postoperatively. Santini and colleagues [8] also performed a randomized trial in elderly patients. Seventy-seven patients over 75 years of age received a stented (Hancock) or stentless valve (Toronto SPV or Biocor, Biocor Industria LTDA, Belo Horizonte, Brazil). There were no significant differences in peak gradients or LVM one year postoperatively.

Although each of the above studies was a randomized trial, they were all limited by relatively small sample sizes that may have resulted in insufficient power to detect clinically meaningful differences. We therefore examined a large number (n = 737) of consecutive patients undergoing bioprosthetic AVR. To the best of our knowledge, the current study is the largest to date examining postoperative hemodynamic data in stentless versus stented valves. In addition, the maximum length of follow-up in the abovementioned randomized trials was 12 months, compared with 79 months in the current study. Although the majority of LVM regression occurs in the first 12 to 18 months post-AVR, further regression can occur up to 8 years postoperatively [24, 25].

In the current study we found stentless patients had significantly lower transvalvular gradients and larger EOA than patients who received stented valves (Table 4). Stentless patients had several baseline characteristics that may partially explain these findings including increased prevalence of male sex, larger body surface areas, and implantation of valves with larger labeled sizes. However, stentless valves had a significant effect on EOA and gradients even after adjusting for these possible confounders.

We also found marked improvements in EOA and transvalvular gradients over time in the stentless group, with minimal or no improvement in the stented group (Table 4). Similar observations have been reported by other investigators [5, 22, 26, 27]. The reason why stentless valve orifice areas increase over time is unclear. Possible explanations include remodeling of the aortic root and resorption of perivalvular edema or hematoma. Such factors would not have the same effect for stented aortic valves since their support structure is rigid and noncompressible. It should be noted that we cautiously avoid oversizing of stentless valves in order to avoid excessive prosthetic tissue within the left ventricular outflow tract. It has been our experience that this technique results in excessively high postoperative gradients.

Stentless valves were also associated with improved LVM regression (Table 4) and decreased risk of residual LV hypertrophy (Fig 2) during follow-up in the current study. This result is not surprising given the observed larger valve areas and decreased gradients for stentless prostheses. Multivariable analysis revealed that stentless valves, preoperative aortic stenosis, and increasing time since operation were independent predictors of LVM regression. Patients with aortic stenosis have less preoperative LVM compared with patients with aortic insufficiency because of the LV cavitary dilation and increased wall radius that occurs with aortic insufficiency [5]. Postoperative LVM regression may be greater in patients with aortic stenosis, however, because there is less irreversibly injured myocardium [5]. When we eliminated patients with pure aortic insufficiency, we found an even greater benefit for stentless valve implantation. We also found that time since operation was an important predictor of LVM regression, illustrating the need to perform long-term assessment when comparing different aortic prostheses. Multivariable analysis revealed that patient age, sex, body surface area, and history of hypertension were not significantly associated with LVM regression.

Our study also revealed that stentless aortic valves were associated with improved midterm survival (Fig 3). It may be speculated that this difference is due to improved LVM regression in the stentless group, but it is more likely to be related to the younger age. Indeed, Cox regression revealed that stentless valves were not an independent predictor of midterm survival.

Study Limitations
The current study was retrospective in design with all of the inherent limitations of such studies. In addition, there were significant differences in preoperative characteristics between the two groups of patients. However, our statistical analyses were designed to account for such baseline differences when comparing the two groups. Multivariable techniques confirmed the beneficial effects of stentless valves on postoperative hemodynamics and LVM regression.

Another limitation of our study was that all echocardiographic measurements were performed at rest. Exercise echocardiography would result in a more physiologic assessment of valve function. However, such assessments would probably result in an even more beneficial effect of stentless valves, since stentless prostheses have improved hemodynamic performance during exercise stress when compared with stented valves [28].

The final limitation of the current study is that long-term follow-up echocardiograms were performed at multiple laboratories, rather than at a single institution. This may result in increased variance of measured values. However, it is unlikely that there is valve-specific bias at outside laboratories and therefore the differences between groups should be valid.

Summary
We performed a large retrospective study with medium-term follow-up to compare stentless and stented bioprostheses. Stentless patients had improved transvalvular gradients and increased orifice areas early and late postoperatively. Stentless patients also had improved LVM regression, decreased LV hypertrophy, and improved survival during medium-term follow-up. Our findings were confirmed after adjusting for baseline differences between groups. The current study is the largest to date and confirms the beneficial hemodynamic effects of stentless aortic valves.

	References

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Personal Folders Download to citation manager Author home page(s): Michael A. Borger Vivek Rao Hugh E. Scully Christopher M. Feindel Tirone E. David Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Borger, M. A. Articles by David, T. E. Search for Related Content PubMed PubMed Citation Articles by Borger, M. A. Articles by David, T. E. Related Collections Valve disease