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Ann Thorac Surg 2002;73:785-792
© 2002 The Society of Thoracic Surgeons

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Original article: cardiovascular

Paravalvular leak and other events in silzone-coated mechanical heart valves: a report from AVERT

^a Division of Cardiovascular Surgery, Mayo Clinic, Rochester, Minnesota, USA
^b the AVERT Coordinating Center, University of Pittsburgh, Pittsburgh, Pennsylvania, USA

* Address reprint requests to Dr Schaff, Mayo Clinic, 200 First Street SW, Rochester, MN 55905, USA
e-mail: schaff{at}mayo.edu

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 Addendum Appendix 1. Principal... Appendix 2. Complications for... Discussion References

 
Background. The Artificial Valve Endocarditis Reduction Trial (AVERT) was designed to compare endocarditis rates in Silzone versus conventional valves. Recruitment ended January 21, 2000, because of higher rates of paravalvular leakage in patients receiving the Silzone prosthesis. The present analysis determined late event rates that might be used in the management of approximately 36,000 patients who have received the Silzone prosthesis.

Methods. A total of 807 patients in 19 centers in North America and Europe were randomized. Mean age was 61 ± 11 years; 41% were women. Operations included aortic valve replacement in 59%, mitral valve replacement in 32%, and aortic and mitral valve replacements in 9%; 41% had concomitant operations (26% coronary artery bypass grafting).

Results. Major paravalvular leakage (followed by repair, explant, or mortality) occurred in 18 of 403 patients receiving Silzone valves and 4 of 404 patients without Silzone valves (2-year event-free rates: 91.1% versus 98.9% conventional, p < 0.003). Similarly, 2-year freedom from any explant was lower in the Silzone arm (19 versus 2 events; 90.1% versus 99.4%, p = 0.0002). Rates of mortality and stroke were similar during follow-up.

Conclusions. Continued follow-up of AVERT supports the conclusion that the Silzone prosthesis has increased risk of paravalvular leakage requiring reoperation. Overall survival is similar in the two groups.

	Introduction

Top Abstract Introduction Material and methods Results Comment Addendum Appendix 1. Principal... Appendix 2. Complications for... Discussion References

 
The Artificial Valve Endocarditis Reduction Trial (AVERT) was designed to evaluate the efficacy of the Silzone (St. Jude Medical, Inc, Minneapolis, MN) silver-coated sewing ring to reduce prosthetic valve endocarditis (PVE), based on studies documenting the safety and efficacy of silver for antimicrobial protection [1–3]. This randomized clinical trial, the protocol for which has been published previously [4], began recruitment of patients in July 1998, with the original goal of randomizing 4,400 patients at the 12 North American and 7 European centers (Appendix 1).

A Data and Safety Monitoring Board (DSMB) of independent experts was convened to review thromboembolic events (TE) in AVERT patients, after a report from a single, non-AVERT center of increased incidence of TE among patients receiving Silzone-coated valves [5]. The DSMB met in April and November 1999 and, after each meeting, unanimously recommended continuation of AVERT based on a finding of no significant differences in TE rates between the Silzone and conventional treatment arms.

Late in 1999 and during the first two weeks of January 2000, the AVERT Coordinating Center received reports of several study patients in the Silzone arm of the trial who developed paravalvular leak leading to valve explant. The DSMB reviewed supporting documentation, convened on January 21, 2000, and advised suspension of patient enrollment because of a higher incidence of explant due to paravalvular leak in the Silzone study arm. The manufacturer (St. Jude Medical, Inc) voluntarily withdrew all Silzone products from the market on the same day.

The AVERT study continues to follow the 807 randomized patients to assess long-term outcomes and quantify any increased risk of adverse events of Silzone prostheses relative to conventional valves. This information is critical for the surgical community, as an estimated 36,000 patients worldwide have received Silzone mechanical valves. The analysis of the trial presented in this report includes all events reported to the AVERT Coordinating Center as of November 30, 2000.

	Material and methods

Top Abstract Introduction Material and methods Results Comment Addendum Appendix 1. Principal... Appendix 2. Complications for... Discussion References

 
Study protocol
Details of the hypothesis, study design, and sample size determination have been presented previously [4]. Briefly, the AVERT trial was developed to determine whether Silzone coating of prosthetic valve sewing rings reduces the risk of PVE. The study assumed that for uncoated prostheses, the rate of early PVE (less than 60 days) is 1%, and the subsequent risk of late PVE is 0.5% per year. To detect a 50% reduction in risk of PVE (80% power, p < 0.05), the trial would have to randomize approximately 4,400 patients. Diagnosis of PVE was to be determined by the Duke criteria [6] using a computer algorithm. Table 1 shows inclusion and exclusion criteria for AVERT. Patients requiring replacement of the aortic or the mitral valve were eligible for the study, and the major exclusion criteria were patients with other existing prosthetic valves not requiring replacement and patients requiring replacement of valves in the tricuspid position. Concomitant surgical procedures, including coronary artery bypass grafting (CABG) and repair of native valves, were permitted by protocol at the time of study valve implant; also, patients having reoperation for valve replacement were included.

Base line patient information and details of the operative procedure were collected at the time of operation. After dismissal, patients were to be contacted 3 months after the implantation and annually for administration of a brief questionnaire regarding vital status and freedom from events possibly associated with endocarditis. After the initial DSMB meeting in January 1999, a 6-month contact point was added, and the questionnaire was expanded to include additional questions regarding the occurrence of possible TEs. The study coordinators administered these questionnaires during a clinic visit or by telephone if possible, and by mail otherwise. By protocol, the Coordinating Center was to be notified of all known adverse events within 24 hours of their occurrence or of their knowledge by the clinical coordinator.

Patient population
At the time AVERT recruitment was stopped on January 21, 2000, a total of 807 patients had been randomized, including 446 at North American centers and 361 at European sites. Design of the AVERT study recognized that the intraoperative randomization scheme would lead to occasional deviations from the randomization assignment. Of 402 patients randomized to Silzone, 395 (98.2%) received Silzone valves, whereas 6 received other mechanical valves, and 1 received a bioprosthetic valve. Of the 405 patients randomized to conventional valves, 394 (97.3%) received St. Jude Medical (SJM) conventional valves per protocol, whereas 8 received Silzone valves, and 3 received non-SJM valves or bioprostheses. As this report focuses on potential differences in outcome between the Silzone-coated and non-Silzone-coated valves, patients were classified in the primary analyses according to whether at least one Silzone valve was implanted during the index procedure. Therefore, all 404 patients not receiving an SJM Silzone valve were classified as "non-Silzone."

At the time of the database freeze on November 30, 2000, the mean follow-up time for study patients (defined as number of days from valve implant to last study contact, whether because of routine follow-up or reported event) was 364 days, and median follow-up time was 367 days.

Study definitions
Mortality included death due to all causes. Other complications reported include the categories of morbidity identified by the STS/AATS Guidelines for Reporting Morbidity and Mortality after Cardiac Valvular Operations [7]: structural dysfunction, nonstructural dysfunction, thrombosis, embolism, bleeding, and endocarditis.

Structural and nonstructural dysfunction
No cases of structural dysfunction (study valve dysfunction or deterioration exclusive of infection or thrombosis) were reported. Nonstructural dysfunction was defined as entrapment by pannus or suture, paravalvular (PV) leak, inappropriate sizing, hemolytic anemia, as determined by reoperation, autopsy, or clinical investigation. "Major PV leak" was defined in this report as a PV leak that led to valve repair, valve explant, or mortality.

Embolic events and thrombosis
Thromboembolic events included frank stroke, transient ischemic attacks (TIA), reversible ischemic neurologic deficits, arterial embolization, and embolic myocardial infarction. Valve thrombosis, listed separately, was defined as thrombus attached to or near the study valve, occluding blood flow or interfering with function (proved by operation, autopsy, or clinical investigation), exclusive of infection.

All reported thromboembolic events and valve thrombosis were classified as to valve-relatedness [7] after adjudication by a committee of 9 independent physicians who reviewed available supporting documentation. This committee was blinded to the type of valve implanted (Silzone or non-Silzone).

Bleeding
All reported bleeding events (causing death, stroke, operation, hospitalization, or transfusion, whether or not the patient was taking anticoagulant or antiplatelet drugs) are included, regardless of severity.

Endocarditis
Endocarditis was reported based on blood cultures, clinical signs, or histologic evidence at reoperation or autopsy. Thrombosis, embolus, and leak associated with active infection were considered as part of the endocarditis event. All reported incidents of suspected or confirmed endocarditis are included.

Statistical methods
Early and late complications were analyzed with conventional techniques. Both internal and external comparisons were conducted: Silzone valves were compared with the uncoated valves, and Silzone valves were compared with standardized complication rate values. For internal comparisons, all valve replacements (aortic, mitral, and double) contributed information. When external comparisons were made, the isolated aortic valve replacement and mitral valve replacement series were used, because the analysis of heart valve results is traditionally reported for isolated valve replacement. Specifically, the FDA’s objective performance criteria (OPC), used for external comparison in this report (Appendix 2) [8], were developed from historical literature for isolated aortic and mitral valve replacement procedures, excluding double-valve procedures.

Not all analyses were performed for all complications. Emphasis was given to those complications for which enough events happened to support meaningful statistical analysis.

Per the AVERT study protocol, patient follow-up ended if all study valves were explanted. Therefore, rates of other reported events including survival must be interpreted as conditional on a patient being explant-free.

• For internal comparison, the Kaplan-Meier event-free analysis method was used. Time zero was defined as the date of study valve implant, and patients were censored at time of death, explant of all study valves, or last contact (scheduled follow-up or report of an adverse event). The Kaplan-Meier analysis includes all events, early and late, but only the first of each type for each patient. These analyses are done individually by valve position, and as global comparisons between valve types, stratified by position. Comparisons (p values) use the log rank statistic.
  
• For external comparison, linearized rates (%/year) for events beyond 30 days were calculated. Linearized rates are calculated by taking the total number of late events divided by the sum of the late (longer than 30 days) follow-up years. Within the subset of patients receiving single study valves, these rates were then compared with the FDA OPC values.
  
• For general information, simple percentages of patients with reported events during the first 30 days were calculated. This percentage is the number of patients who had an early event divided by the total number of patients.
  

	Results

Top Abstract Introduction Material and methods Results Comment Addendum Appendix 1. Principal... Appendix 2. Complications for... Discussion References

 
Patient characteristics
Patients were on average 61 years old, and 41.1% were women. Five percent of patients had active, treated, or suspected endocarditis at the time of operation. Most study patients had New York Heart Association functional class II (40.9%) or III (42.4%) at the time of implant; 8.7% were class I, and 8% were class IV. History of myocardial infarction was reported in 8.9%, history of diabetes in 14.3%, and congestive heart failure within 2 weeks of operation in 22.6%. Coronary artery disease, defined as stenosis of at least 50% in a native vessel, was present in 33.2%.

Overall, 4.7% of patients had previous valve replacement operation, including 2.4% in the aortic position and 2.6% in the mitral position. Previous CABG was reported in 5.5% of patients. Among all patients, 15% had previously had one or more cardiac surgical procedures requiring cardiopulmonary bypass. Medical and surgical history was comparable between patients in the two treatment groups.

Procedural and valve characteristics
Aortic valve replacement was performed in 59% of study patients, whereas 32% underwent mitral valve replacement, and 9% had double valve replacement. Minimally invasive procedures were used in a small number of patients (3.5%). Concomitant operation at the time of valve implant was common, with CABG performed in 26% of patients (19% of European and 32% of North American patients). Surgical repair of other valves occurred in 7% of patients.

Cardioplegia was used for myocardial protection in almost all patients (99.6%), and cold-blood cardioplegia was used in 63%. Antifibrinolytic drugs (aprotinin or tranexamic acid) were given to 68% of patients. Mean cross-clamp time was 76 minutes, and mean bypass time was 106 minutes. Almost all valves replaced were native valves in both the aortic (97%) and mitral (94%) positions. Procedural and valve characteristics, including etiology of the replaced valve, were comparable in the Silzone and non-Silzone patients for each position.

Complications
Mortality
Twenty-seven patients died early (less than 30 days) after operation for an overall mortality of 3.3%; mortality was 2.1% for patients having aortic valve replacement, 4.7% for patients having mitral valve replacement, and 6.7% for patients having double (aortic and mitral) valve replacement. As seen in Table 2, early mortality rates did not differ between the Silzone and non-Silzone groups. At the time of this analysis, 55 of the 807 patients were known to have died, including 26 Silzone patients and 29 receiving non-Silzone valves. Two-year Kaplan-Meier survival rates were 89.0% for Silzone patients and 91.5% for non-Silzone subjects; all-cause mortality did not differ (p = 0.74 for log rank test comparing survival experience during the first 2 years of follow-up).

View larger version (16K): [in this window] [in a new window]   Fig 1. Kaplan-Meier curves showing freedom from reoperation for valve explant during the first 2 years postoperatively (Postop).

View larger version (13K): [in this window] [in a new window]   Fig 2. Kaplan-Meier curves showing freedom from major paravalvular leak for patients having aortic valve replacement (A) and mitral valve replacement (B). (Postop = postoperative.)

View larger version (16K): [in this window] [in a new window]   Fig 3. Kaplan-Meier curves showing freedom from thromboembolism during the first 2 years postoperatively (Postop).

View larger version (20K): [in this window] [in a new window]   Fig 4. Linearized rates of thromboembolism (A) and stroke (B). Risks of thromboembolism were higher for patients in the Silzone group compared with the non-Silzone valve, but these differences were largely due to transient events because the risks of stroke were low.

Bleeding
Early and late bleeding complications were similar for the two groups of patients.

Endocarditis
A total of 19 cases of suspected PVE were reported, including 9 in Silzone patients and 10 in non-Silzone patients. Reported suspected endocarditis rates do not differ between the two study groups at this time; reported endocarditis events will be adjudicated at a future date.

	Comment

Top Abstract Introduction Material and methods Results Comment Addendum Appendix 1. Principal... Appendix 2. Complications for... Discussion References

 
These results confirm and extend the interim analysis of AVERT that led to discontinuation of enrollment and voluntary withdrawal of the Silzone valve because of an unacceptably high risk of explant due to major paravalvular leak. The increased risk of major PV leak was observed for patients with aortic valve replacement, mitral valve replacement, and double valve replacement.

The mechanism responsible for development of PV leak in patients receiving the Silzone prosthesis is unclear. In previous isolated reports of reoperation in Silzone valves, PV leaks were thought to be due to culture-negative endocarditis (B. Goldman, personal communication, May 1999), but in patients in the present study who had explants of Silzone valves, clinical findings did not suggest infection. In addition, none of the patients having late operation for PV leakage had active or recent endocarditis at the time of entry into the study. Explanted valves in AVERT patients exhibited poor tissue ingrowth and loosening of sutures. Thus, it appears that the Silzone coating inhibits normal fibroblast response and incorporation of the fabric of the sewing ring into host tissue in some patients.

Previous in vitro biocompatibility studies [2] showed no evidence of silver toxicity to cultured fibroblasts until concentrations reached high levels (1,200 ppm), and in animal studies (ovine model), tissue ingrowth was said to be comparable in portions of sewing rings coated with Silzone compared with uncoated portions. Nevertheless, this same study noted that pannus formation was thinner over the Silzone portion of the sewing ring, suggesting that the altered surface of the polyester sewing ring did have some biologic effect.

Although patients with allergy to silver were excluded from the study, it is possible that impaired healing of the Silzone sewing ring is idiosyncratic. Further follow-up of patients in the AVERT trial will be crucial to establish late risks of PV leakage and thromboembolism, and these data may give some information on mechanism. It is unclear whether patients who have normally functioning prostheses without leak and without thromboembolic episodes after 1 year are at higher risk for these events subsequently. As seen in Table 3, 4 patients with Silzone prostheses had reoperation after the first postoperative year, and preliminary hazard analysis suggests that risk of major PV risk is constant at 4.6% through the first 2 years postoperatively (Fig 5).

View larger version (14K): [in this window] [in a new window]   Fig 5. Analysis of cumulative hazard of major paravalvular leak suggests a constant risk of 4.6% during the first 2 years after the operation. (CI = confidence interval; Postop = postoperative.)

A limitation of this report is that at present, follow-up after 1 year is relatively limited. Therefore, whereas the data represent the best possible estimates of 2-year outcome, it is possible that findings after the 1-year time point, including the constant risk of major PV leak, may change when 2-year follow-up is complete.

Continued follow-up of these patients in the AVERT trial will be critically important in guiding management of the 36,000 patients with Silzone prostheses. Paravalvular leakage in patients with Silzone valves does not appear to lead to catastrophic failure, and careful follow-up of these patients with clinical examinations and serial Doppler echocardiograms should give clinicians and patients sufficient information to plan for elective reoperation if severe PV leakage develops. The AVERT trial has been amended to include serial echocardiography of all study patients, in both the Silzone and conventional arms, during follow-up.

Management of thromboembolic complications is more problematic. Although our data do not suggest that risk of thromboembolism in patients with Silzone prostheses is significantly greater than for those with standard sewing rings, it does appear that some patients experience the troublesome occurrence of multiple transient neurologic deficits, perhaps related to platelet emboli. Maintenance of anticoagulation at the higher end of the therapeutic range and use of antiplatelet agents with Coumadin may be helpful in these patients.

	Addendum

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As of December 5, 2001 follow-up on Silzone patients in the study had increased to 686 valve-years and included 277 patients reporting 2-year follow-up. No additional instances of valve repair or explant due to PV leak have been reported.

	Appendix 1. Principal investigators and participating centers in the Artificial Valve Endocarditis Reduction Trial

Top Abstract Introduction Material and methods Results Comment Addendum Appendix 1. Principal... Appendix 2. Complications for... Discussion References

 

Institution Investigators North America Mayo Clinic, Rochester, Minnesota Hartzell Schaff, MD Texas Heart Institute, Houston, Texas Denton Cooley, MD Brigham & Women’s Hospital, Boston, Massachusetts Sari Aranki, MD Providence Health System, Portland, Oregon (St. Vincent’s and Providence Hospitals) Hugh Gately, MD E. Charles Douville, MD LDS Hospital, Salt Lake City, Utah Kent Jones, MD Vancouver General and St. Paul’s Hospital, Vancouver, Canada W. R. Eric Jamieson, MD University of Alberta, Alberta, Canada Arvind Joshal, MD Massachusetts General Hospital, Boston, Massachusetts David Torchiana, MD Florida Hospital, Orlando, Florida Patterson Moseley, MD New York Presbyterian Hospital, New York, New York Leonard Gerardi, MD Europe Hôpital Regional Sion, Sion, Switzerland Frank Stumpe, MD Deutsches Herzzentrum, Berlin, Germany Roland Hetzer, MD Inselspital Bern, Bern, Switzerland Thierry P. Carrel, MD, Lars Englberger, MD CHU Nord, Nantes, France Daniel Duveau, MD St. Antonius ZKH, Nieuwegein, The Netherlands Wim Jan van Boven, MD San Raffaele Hospital, Milan, Italy Ottavio Alfieri, MD Hospital Doce de Octobre, Madrid, Spain Juán José Rufilanchas, MD

	Appendix 2. Complications for evaluating mechanical heart valves, and objective performance criteria (OPC) values for event rates (%/year)

Top Abstract Introduction Material and methods Results Comment Addendum Appendix 1. Principal... Appendix 2. Complications for... Discussion References

 

Definitions of Morbidity From Guidelines for Reportinga OPCb Nonstructural Dysfunction: entrapment by pannus or suture, paravalvular leak, inappropriate sizing, hemolytic anemia; as determined by reoperation, autopsy, or clinical investigation. 1.2 (all paravalvular leak); 0.6 (major paravalvular leak) Valve Thrombosis: thrombosis proved by operation, autopsy, or clinical investigation. Excludes infection. 0.8 Thromboembolism: neurological deficit or peripheral arterial embolus that occurs after the immediate perioperative period. Excludes septic and hemorrhagic emboli. 3.0 Bleeding that causes death, stroke, operation, hospitalization, or transfusion, whether or not the patient is taking anticoagulant or antiplatelet drugs. 3.5 Valvular Endocarditis: based on blood cultures, clinical signs, and/or histological evidence at reoperation or autopsy. Includes thrombosis, embolus, or leak associated with active infection. 1.2

^a See Edmunds and associates [7].

^b See Reference 8.

	Discussion

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DR IRVING L. KRON (Charlottesville, VA): Doctor Schaff, let’s say a perivalvular leak is detected in a Silzone valve. Will it act differently? Does one replace it right away? Does it worsen rapidly? How do you follow these patients?

DR SCHAFF: At the present time we cannot answer that, but the AVERT trial has been modified to answer that question by introducing regular Doppler echocardiograms at 6-month and yearly intervals. We do not have any evidence of catastrophic failures. So I think that identifying a perivalvular leak should be managed much the same way that we do other non-Silzone valves.

DR PAUL KURLANSKY (Miami Beach, FL): You demonstrated a relatively constant risk hazard over time for the incidence of major perivalvular leak. I was wondering if you could indicate what was the earliest appearance of paravalvular leak in the AVERT study? Also, is there any report on the pathology of what was actually discovered at the time of surgery? Were there broken sutures, was there a torn cuff? Was there tissue disruption? Was there some other finding, or was there a whole variety of different findings?

DR SCHAFF: We are less secure in the conclusion that there is a constant hazard, although that is what it looks like at the present time. The earliest explant was about 1 month. The pathology findings are interesting, because when anecdotal reports of perivalvular leak came to the company, the surgeons who reported those thought it was culture-negative endocarditis. The valves that had been explanted in AVERT with the limited information that we have do not show any particular characteristic finding. There is not an undue amount of inflammation and not a lot of cellular infiltration that would suggest an allergic reaction. It is more bland perivalvular leakage without vegetations or inflammation.

	References

Top Abstract Introduction Material and methods Results Comment Addendum Appendix 1. Principal... Appendix 2. Complications for... Discussion References

 

1. Tweden K.S., Cameron D.J., Razzouk A.J., et al. Silver modification of polyethylene terephthalate textiles for antimicrobial protection. Am Soc Artif Intern Organs 1997;43:M475-M481.
2. Tweden K.S., Cameron J.D., Rozzouk A.J., et al. Biocompatibility of silver-modified polyester for antimicrobial protection of prosthetic valves. J Heart Valve Dis 1997;6:553-561.[Medline]
3. Illingworth B.L., Tweden K., Schroeder R.F., Cameron J.D. In vivo efficacy of silver-coated (Silzone) infection-resistant polyester fabric against a biofilm-producing bacteria, Staphylococcus epidermidis. J Heart Valve Dis 1998;7:524-530.[Medline]
4. Schaff H., Carrel T., Steckelberg J.M., Grunkemeier G.L., Holubkov R. Artificial Valve Endocarditis Reduction Trial (AVERT): protocol of a multicenter randomized trial. J Heart Valve Dis 1999;8:131-139.[Medline]
5. Ionescu A.A., Fraser A.G., Butchart E.G. High incidence of embolism after St. Jude Silzone prosthetic valve implantation. Circulation 1999;100(Suppl 1):I524.
6. Durack D.T., Lukes A.S., Bright D.K. New criteria for diagnosis of infective endocarditis: utilization of specific echocardiographic findings. Am J Med 1994;96:200-209.[Medline]
7. Edmunds L.H., Jr, Clark R.E., Cohn L.H., Grunkemeier G.L., Miller D.C., Weisel R.D. Guidelines for reporting morbidity and mortality after cardiac valvular operations. Ann Thorac Surg 1996;62:932-935.[Abstract/Free Full Text]
8. Center for Devices, and Radiological Health, Food, and Drug Administration. Draft Replacement Heart Valve Guidance, Version 4.1. 1994.

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