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Ann Thorac Surg 2007;84:467-472
© 2007 The Society of Thoracic Surgeons

Original Articles: Cardiovascular

Stentless Versus Conventional Xenograft Aortic Valve Replacement: Midterm Results of a Prospectively Randomized Trial

Sven Lehmann, MD, Thomas Walther, MD, PhD*, Jörg Kempfert, MD, Sergey Leontjev, MD, Ardawan Rastan, MD, Volkmar Falk, MD, PhD, Friedrich W. Mohr, MD, PhD

Department of Cardiac Surgery, Heartcenter, University of Leipzig, Leipzig, Germany

Accepted for publication February 6, 2007.

* Address correspondence to Dr Walther, Universität Leipzig, Herzzentrum, Klinik für Herzchirurgie, Strümpellstr 39, Leipzig 04289, Germany (Email: walt{at}medizin.uni-leipzig.de).


Adult cardiac surgery: The Annals of Thoracic Surgery CME Program is located online at http://cme.ctsnetjournals.org. To take the CME activity related to this article, you must have either an STS member or an individual non-member subscription to the journal.

 

   Abstract
Top
 Abstract
Introduction
 Material and Methods
 Results
 Comment
 References
 
Background: The purpose of this study was to analyze the 5- to 8-year clinical results after prospectively randomized stentless versus conventional aortic valve replacement.

Methods: Two hundred twenty-three patients received stentless (Freestyle or Toronto, n = 127) or conventional stented (CE porcine, n = 96) xenograft aortic valve replacement between March 1996 and March 1999 using a prospectively randomized protocol and were discharged from the hospital. Patient age at operation was 71.8 ± 6.7 years stentless and 74.9 ± 4 years stented (p < 0.05). There were no significant differences regarding preoperative morphologic and hemodynamic variables. Aortic annulus diameter indices were comparable at 13.5 mm stentless and 13.6 mm stented. Follow-up after 6.9 ± 0.8 years is complete for 95.6% of the patients at a total of 1,533 patient-years. Analysis of variance, unpaired t test, univariate ({chi}2), and survival analysis (log rank) were performed.

Results: Eight-year survival was 78.1% ± 3.8% stentless versus 66% ± 4.9% stented (p = 0.04); mean survival was 97 months stentless versus 88 months stented. At most recent follow-up, New York Heart Association functional class was 1.6 ± 0.6 stentless versus 1.5 ± 0.7 stented (p = not significant); ejection fraction was 61% ± 11% versus 61% ± 8% (p = not significant); transvalvular maximum flow velocity was 23 ± 9 mm Hg versus 37 ± 15 mm Hg (p < 0.01). Thrombembolic events occurred in 2 of 7 patients, including 2 of 6 patients suffering major stroke (p = not significant). Most (93%) of the patients were satisfied with the postoperative course, having a significantly improved quality of life.

Conclusions: Midterm results after xenograft aortic valve replacement are satisfactory. Overall survival is significantly better after stentless aortic valve replacement in this series. However, there was no cause and effect relationship between lower transvalvular pressure gradient. Fewer thrombembolic events and better survival rates were proven.

Aortic stenosis is the most common acquired heart valve lesion in Western societies. It is usually caused by degenerative changes with complex calcification of the native leaflets and the aortic annulus. Left ventricular hypertrophy (LVH) develops as an adaptation to the increased pressure load. Being an independent cardiac risk factor, LVH is associated with a higher incidence of cardiovascular clinical events and death [1]. In symptomatic patients or in the presence of severe stenosis with significant additional LVH, aortic valve replacement is indicated [2].

During the past decades, aortic valve replacement (AVR) using mechanical valves or conventional stented bioprotheses (CSB) has become a routine procedure with low perioperative risk [3–8]. Stentless aortic valves (SAV) have been used increasingly, with good functional and hemodynamic results [8, 9, 14]. In longitudinal studies, early LVH regression after stentless valve implantation has been demonstrated [8, 10, 11]. Our aim was to analyze the midterm clinical results of a prospectively randomized study after stentless versus conventional AVR.


   Material and Methods
Top
 Abstract
 Introduction
 Material and Methods
Results
 Comment
 References
 
From March 1996 through March 1999, 223 patients with aortic valve disease were prospectively evaluated. The study was approved by the local Ethics Committee, and all patients gave informed consent after the study protocol was outlined in detail (Table 1).


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  		Table 1 Inclusion and Exclusion Criteria
 
Patients amenable for bioprosthetic AV implantation were randomly assigned to receive an SAV (either Freestyle [Medtronic Corp, Santa Rosa, CA] or Toronto SPV [St. Jude Medical, Minneapolis, MN]), or a CSB (Carpentier-Edwards porcine xenograft; Edwards Lifesciences, Irvine, CA). Demographic data for the two groups are given in Figure 1. Differences in group sizes resulted from cross over due to calcifications of the aortic root, atypical coronary artery origin, or other reasons not to insert a stentless valve. Further evaluation revealed that the results in the cross-over patients did not differ from the total stented xenograft group.


Figure 1
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  		Fig 1. Flow chart of all patients suffering aortic valve disease during the period of study inclusion.

 
All operations were performed with the use of complete or partial median sternotomy and standard extracorporeal circulation with hypothermic cardioplegic arrest (Custodiol HTK solution; Köhler Chemie, Alsbach-Hähnlein, Germany). For further comparison, aortic annulus diameter was measured intraoperatively by use of a standard set of sizers before the new valve was implanted [13]. This was performed after excision of the diseased aortic valve and after complete decalcification. Annulus diameter was divided by body surface area to obtain the annulus index as baseline measure.

Aortic valve implantation was performed according to standard techniques, as described previously [14]. All SAV were implanted in subcoronary position. For SAV implantation, single 4-0 Tevdek stitches at the annulus without pledgets and a continuous 4-0 polypropyline suture line at the commissures were used. Conventional stented bioprostheses were implanted in a supra-annular position with 2-0 Tevdek Teflon (Impra, subsidiary of L. R. Bard, Tempe, AZ) armed U stitches (horizontal mattress suture).

Follow-up consists of annual examinations at our outpatient clinic and is complete in 95.6%. Mean follow-up is 94.2 ± 9.6 months (range, 73.8 to 114.2). Total follow-up consists of 1,533 patient-years. Patients living more than 150 km from the hospital (n = 9) were followed by telephone interview; in addition, physical and echocardiographic examinations were performed by their family physicians and transferred to the hospital. All patients were instructed to contact the hospital immediately in the event of any unexpected deterioration of health conditions.

Transthoracic echocardiographic examinations were performed preoperatively, before discharge, and at every follow-up visit. Multiplane transesophageal echocardiography was used intraoperatively or whenever additional information besides transthoracic echocardiographic measurements was required. Cardiac morphology and function as well as valve hemodynamics were assessed using standard measurements.

Quality of life (QOL) was assessed using a modification of the Nottingham Health Questionnaire and the Medical Outcomes Study Short-Form 36-Item Questionnaire (SF-36) [15]. These questionnaires were translated into German; further modification consisted of the inclusion of one possible positive answer in each category. Different aspects of QOL were analyzed, namely mobilization, social status, level of activities, emotional state, pain, and sleeping disorders. Changes in QOL were evaluated between preoperative and postoperative interviews as well as in comparison with the follow-up. Data are given as being dimensionless by intention, with the preoperative data serving as baseline. All patients were interviewed preoperatively, before discharge, and at every follow-up.

Valve-related morbidity and mortality were evaluated according to standard guidelines [12]. Absolute and relative frequencies were calculated. Results are given as mean ± SD. After assessing for normal distribution, the Student t test for matched pairs was applied. For measurements within groups over time one-way analysis of variance with Bonferroni correction was used. In addition, univariate ({chi}2) and survival analysis (log rank) were performed. A p value less than 0.05 was considered significant.


   Results
Top
 Abstract
 Introduction
 Material and Methods
 Results
Comment
 References
 
After decalcification, the annulus diameter was 24.4 ± 2.2 mm SAV and 23.5 ± 2 mm CSB. Consequently, the aortic annulus index was 13.5 ± 1.3 mm SAV and 13.6 ± 1.6 mm CSB (p = not significant). The mean implanted valve diameter was 25.1 ± 2 mm SAV versus 23.3 ± 2 mm CSB (p = 0.01). Aortic cross-clamp duration was 73.8 ± 19 minutes SAV versus 53.6 ± 13 minutes CSB (p = 0.01; Table 2).


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  		Table 2 Demographics and Operative Data
 
Rethoracotomy for bleeding had to be performed in a total of 5 patients, 3 after SAV and 2 after CSB implantation (p = not significant). One of these patients required prolonged mechanical ventilation. All others had an uneventful recovery. New-onset AV block was seen in 11 SAV and 7 CSB patients postoperatively (p = not significant). In 7 and 4 patients, regular conduction completely restored after a maximum of 5 days as documented by 24-hour electrocardiography. The remaining 4 SAV and 3 CSB patients required permanent pacemaker implantation before discharge. These patients had heavily calcified aortic annuli requiring extensive decalcification. All in-hospital mortality (n = 4) was nonvalve related. Patients were discharged from the hospital according to the German standards. Their demographic data are summarized in Table 2.

After stentless valve implantation, permanent anticoagulation with warfarin sodium was prescribed only if additional atrial fibrillation was present. Patients in the conventional group received a 3-month course of warfarin. At discharge 84% SAV and 79% CSB patients were in stable sinus rhythm.

At 8-year follow-up, overall survival was 78.1% ± 3.8% SAV and 66.7% ± 4.9% CSB (p = 0.03). During follow-up, 21.9% ± 0.04% SAV and 32.3% ± 0.05% CSB patients died. Death occurred after 1 year in 2.5% ± 0.01% SAV patients versus 5.4% ± 0.02% CSB patients, after 2 to 5 years in 10.9% ± 0.03% SAV versus 20.4% ± 0.05% CSB, and after 6 to 8 years in 8.5% ± 0.04% SAV versus 6.5% ± 0.05% CSB patients (Table 3 and Fig 2). Causes of death were heart failure in 5 SAV and 4 CSB patients, and malignancies in 2 SAV and 3 CSB patients. Four SAV patients and 1 CSB patient died of infections. One CSB patient died of fulminant pulmonary embolism secondary to bone fracture. Neurologic events were the cause of death in 2 SAV and 7 CSB patients (p = 0.01). This has to be judged in relation to 9 SAV patients and 7 CSB patients having atrial fibrillation at follow-up. All of these patients as well as another 4 CSB and 4 SAV patients with no cardiac indication had received systemic anticoagulation therapy using warfarin.


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  		Table 3 Survival Rate After Aortic Valve Replacement
 

Figure 2
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  		Fig 2. Actuarial overall survival after stentless aortic valve replacement (dotted line), stented aortic valve replacement (broken line), and the age-matched German normal population (solid line).

 
Overall, causes of death during follow-up were noncardiac in 21 SAV patients and in 27 CSB patients. When comparing the SAV patients with an age-matched German control population, there was no significant difference in mortality after hospital discharge, as indicated in Figure 2 [20].

Owing to the nonstratified initial randomization protocol, significant differences in the age of the two groups occurred. To eliminate age as a potential factor for poorer outcome, additional further analysis was performed by using an age-matched control group. To reach a similar age at inclusion in the study, younger patients of the SAV group were omitted from this analysis. Sample size for this analysis was 111 SAV and 96 CSB, and patient age was comparable at 74.6 ± 3.1 years SAV (p = ns). Mortality was significantly different between these groups. SAV and CSB, at 8 years (p = 0.04, log rank).

After randomization, there were 21 cross-over patients from SAV to CSB. Further analysis was performed on an intention-to-treat basis. That resulted in 148 SAV and 74 CSB patients with a similarly significant difference in mortality at 8 years of follow-up (p = 0.03, log rank; Fig 3).


Figure 3
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  		Fig 3. Intention-to-treat analysis after stentless aortic valve replacement (dotted line) versus stented aortic valve replacement (broken line).

 
Nine thromboembolic events occurred in both groups. Two of them occurred after SAV and 7 after CSB (p = 0.01); these events resulted in death of the patients in all cases (Fig 4). This adds up to a rate of 0.0059 events per patient year. One patient presented with endocarditis 41 months after SAV implantation; he was successfully treated using antibiotic therapy. Repeat operation was required in 3 patients, 1 after SAV and 2 after CSB. One patient in each group presented with new-onset moderate to severe paravalvular incompetence after 0.5 years; 1 other patient was admitted for prosthetic valve stenosis 8 years after CSB implantation.


Figure 4
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  		Fig 4. Actuarial freedom from neurologic event after stentless aortic valve replacement (dotted line) and stented aortic valve replacement (broken line).

 
At follow-up, all patients were clinically improved and tolerated more physical activities at no or only little dyspnea. New York Heart Association functional class was 1.6 ± 0.4 SAV and 1.5 ± 0.6 CSB (p = not significant). According to the specific activity questionnaire, all patients had improved, to 5.7 ± 1 for SAV and to 5.4 ± 1 for CSB. In all, 86.7% SAV patients and 85.7% CSB patients noted an improved quality of life as compared with their preoperative condition (Table 4).


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  		Table 4 Quality of Life After Aortic Valve Replacement
 
Good hemodynamic function was documented on follow-up echocardiographic measurements. One patient (SAV) presented with significant transvalvular incompetence. Trivial transvalvular refluxes as caused by the closing volume and seen with most conventional heart valve prostheses were accepted.

Postoperative maximum transvalvular pressure gradients were lower after stentless valve implantation without reaching significance at that time (22.8 ± 9 and 26.7 ± 12 mm Hg). At 6 months, maximum transvalvular pressure gradients were 16.7 ± 7.7 mm Hg SAV and 20.1 ± 7.3 mm Hg CSB (p = 0.01). At late follow-up, maximum transvalvular pressure gradient was 23.4 ± 9.1 mm Hg SAV and 36.5 ± 14.7 mm Hg CSB (p < 0.01). In both groups, there were significant differences after 6 months versus the baseline measurements (Table 5).


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  		Table 5 Maximum Transvalvular Pressure Gradient at Follow-Up After SAV and CSB
 
Calculated cardiac indices were 2.6 ± 1 SAV versus 2.8 ± 1 L · min–1 · m–2 CSB postoperatively and 3.2 ± 1 SAV versus 3.0 ± 1 L · min–1 · m–2 CSB at follow-up. Postoperative end-diastolic left ventricular posterior wall diameter was 15.6 ± 2.4 mm SAV versus 14.8 ± 2.4 mm CSB and postoperative left ventricular mass 207 ± 72 g SAV and 199 ± 72 g CSB. After 6 months, there was a significant decrease in wall diameter and in left ventricular mass between both groups. However, at late follow-up, no significant differences regarding left ventricular wall thickness (12.6 ± 1.6 SAV and 12.6 ± 2 CSB) and left ventricular mass (156 ± 47 SAV versus 158 ± 47 CSB) were documented (Tables 6 and 7). Go


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  		Table 6 Left Ventricular Posterior Wall Diameter at Follow-Up After SAV and CSB
 

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  		Table 7 Left Ventricular Mass From Parasternal M-Mode Echocardiography Indexed for Body Surface Area
 

   Comment
Top
 Abstract
 Introduction
 Material and Methods
 Results
 Comment
References
 
The ideal aortic valve substitute would be simple to implant, provide a hemodynamic profile identical to a normal native valve with unlimited durability, and have a low thrombogenic potential so that anticoagulants were not necessary. No such device is available at present. Patients requiring aortic valve surgery are being operated on at continuously increasing ages. That is in parallel with more frequent use of xenografts. With conventional xenografts, despite good clinical results, the obstructive nature of a stent is imminent and may have a negative impact on long-term valve performance.

Several randomized trails have compared SAV with CSB with different results. Cohen and coworkers [16] found no significant differences in the effective orifice area, transvalvular gradient, or LVH regression after 12 months in 99 patients. Ali and associates [17] randomly assigned 161 patients to implant a stented Carpentier-Edwards Perimount or a stentless Prima plus valve (Edwards Life Sciences) and found no difference in transvalvular pressure gradient and LVH regression. Doss and associates [18] randomly assigned 40 patients aged more than 75 years to stented Perimount or a stentless Prima plus valves and found a difference in effective orifice area, transvalvular gradients, and LVH regression 1 year after implantation. Santini and coworkers [19] compared stented (Hancock) to stentless valves (Toronto SPV or Biocor; Biocor Industria Brasil), with no significant differences in maximum transvalvular pressure gradients or LVH at 1 year postoperatively. Each of these studies were randomized; all were limited by relatively small patients sizes that may have resulted in insufficient power to detect clinically meaningful differences. By comparison, the present study includes a larger number of patients with longer term follow-up.

Left ventricular hypertrophy as present in aortic stenosis correlates with overall cardiovascular morbidity and mortality, especially that caused by congestive heart failure, sudden death, myocardial infarction, and stroke [8]. Regression of LVH has been reported after conventional stented AVR and after stentless AVR [20]. Nevertheless, incomplete regression of LVH was shown to be associated with decreased survival [8]. Regression of LVH was seen in all patients after AVR. To delineate the potential benefits of stentless in comparison with conventional xenografts for AVR, we initiated this prospectively randomized trial. During early follow-up, improved hemodynamic function and regression of LVH was documented in both groups.

There are three important hemodynamic findings from this study. At midterm follow-up, the SAV provides, first, lower transvalvular blood flow velocities with lower transvalvular pressure gradients; second, a similar level of LVH regression; and third, similarly good LV function in comparison with conventional stented xenograft implantation.

In general, mechanisms of valve deterioration were related to leaflet tear, with only 3 cases involving calcification. The absence of significant calcification may be due to long-term follow-up not being available at present, but it may also be a reflection of the stentless valve design or the fixation process, or a combination of these factors. Other reports have described aortic regurgitation and structural valve failure caused by progressive dilation of the sinotubular junction occurring 5.8 to 7 years after complete subcoronary implantation of the Toronto SPV [21, 22]. We did not observe such complications in our present experience.

At echocardiographic follow-up, comparable hemodynamic results with no dilatation of the sinotubular junction was documented after either Toronto SPV or Freestyle SAV implantation. In comparison with the series from David, we included older patients in the present study. Maybe patient age, perhaps together with differences in aortic wall calcification or a change in extracellular matrix composition, is a potential factor for the occurrence of late dilatation of the sinotubular junction.

In this study, after SAV implantation, the reoperation rate was 1 after Freestyle AVR and 0 after Toronto SPV implantation. Owing to these low event rates at 8 years, we cannot draw any definitive conclusions at the present time. Longer follow-up is required.

Hemodynamic results were in favor of stentless valves. This finding can be easily explained by the larger effective orifice areas resulting from the lack of an obstructing stent and larger valve size selection at a given annulus diameter. From an echocardiographic perspective, stentless valves resemble native aortic valve function and can be considered close to an ideal artificial heart valve. Paravalvular leakage was not a major issue in this series. There were no relevant differences between the two groups in overall hospital outcome. Intraoperative aortic cross-clamp time was longer in the stentless group, but the overall duration was acceptable because it did not result in any excess morbidity.

The overall mortality was acceptable when considering patient age and comorbidities. During the follow-up period of 6 to 9 years, 21.9% of the patients after SAV and 33.3% of the patients after CSB died of different—mostly nonvalve-related—causes. There was no difference in survival between the SAV group and an age-matched German population [23]. However, there was a significant difference in survival after CSB and the age-matched German population (Fig 2) [23].

Although there was no difference in atrial fibrillation and anticoagulation therapy between the two groups, the incidence of neurologic events was higher in the CSB group. At most recent echocardiographic examinations in those patients, normal valve function had been documented, and there was no evidence for intracardiac thrombus formation.

In summary, a good clinical outcome with acceptable survival was documented at medium-term follow-up after randomized stentless versus conventional xenograft AVR. We did not observe any valve-related mortality as far as the information obtained from the family physicians is concerned. The patients perceived an improved quality of life after AVR; however, no cause and effect relationship was found with lower transvalvular pressure gradient. Fewer thrombembolic events and better survival were proved by this study.


   References
Top
 Abstract
 Introduction
 Material and Methods
 Results
 Comment
 References
 

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