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Eur J Cardiothorac Surg 2007;32:108-112. doi:10.1016/j.ejcts.2007.03.031
Copyright © 2007, European Association for Cardio-Thoracic Surgery. Published by Elsevier B.V. All rights reserved

Antiplatelet therapy early after bioprosthetic aortic valve replacement is unnecessary in patients without thromboembolic risk factors

Martin Bruecka,*, Wilfried Kramera, Paul Vogtb, Nicole Steinertb, Peter Rothb, Gerold Görlachb, Markus Schönburgc, Martin C. Heidtb

a Department of Cardiology, Clinic of Wetzlar, Germany
b Department of Cardiovascular Surgery, University of Giessen, Germany
c Department of Cardiovascular Surgery, Kerckhoff-Clinic, Bad Nauheim, Germany

Received 27 December 2006; received in revised form 13 March 2007; accepted 20 March 2007.

* Corresponding author. Address: Clinic of Wetzlar, Department of Cardiology, Forsthausstrasse 1, D-35578 Wetzlar, Germany. Tel.: +49 6441 792327; fax: +49 6441 792328. (Email: Martin.Brueck{at}hkw.med.uni-giessen.de).


   Abstract
Top
 Abstract
1. Introduction
 2. Method
 3. Results
 4. Discussion
 5. Study limitations
 References
 
Background: The use of antithrombotic therapy during the postoperative period after biological aortic valve replacement (AVR) in patients without thromboembolic risk factors remains controversial. Treatment with warfarin is recommended for the first 3 months after biological AVR. The use of antiplatelet therapy – mainly aspirin (ASA) – is suggested as an alternative treatment but its efficacy is still unsettled. Due to the increased risk of bleeding complications even no use of any antithrombotic or antiplatelet therapy was advocated. Given this ongoing dispute, the aim of this retrospective double-institutional study was to evaluate the necessity of antiplatelet treatment by ASA with no postoperative antiplatelet therapy in terms of survival, major bleedings and cerebral thromboembolism of patients undergoing biological AVR without thromboembolic risk factors. Methods: From January 2001 to December 2003, 288 consecutive patients (72.8 ± 7.5 years, 134 males) with sinus rhythm and no other thromboembolic risk factors underwent single biological AVR with porcine or bovine pericardial valves without concurrent coronary artery bypass graft surgery. By surgeons preference, 100 mg ASA was given to 132 patients, and 156 patients received no antiplatelet therapy. Patients were followed for cerebral ischemic events, major bleedings, need for repeat operation, NYHA class and survival at three time intervals postoperatively (30 days, 3 and 12 months). Results: None of all patients died during the operation. Mortality within 30 days was 3.8% in the ASA and 3.9% in the no ASA group (p = 0.777). There were no statistically significant differences for cerebral ischemia within 3 months after AVR (ASA 0.8% vs no ASA 1.3%: p = 0.884) and 3 to 12 months after AVR (ASA 0.8% vs no ASA 0%; p = 0.933). Major bleedings occurred in two ASA-treated patients and in one patient without antiplatelet therapy (p = 0.884). The incidence of NYHA class III–IV after 3 months (1.5% vs 1.9%; p = 0.850) and 12 months (9.0% vs 5.1%; p = 0.278) were similar, as were the need for repeat operative AVR after 12 months (0.8% vs 0.6%; p = 0.553). Survival rates at 12-month follow-up were 95.5% for ASA treatment and 94.9% for no ASA treatment (p = 0.963). Conclusions: In patients without thromboembolic risk factors undergoing biological AVR administration of ASA confers no advantage compared to no antiplatelet therapy. Functional status, thromboembolic events and survival were not adversely affected by withholding any antiplatelet therapy. Guidelines need to be reviewed for the antithrombotic therapy of patients without risk factors undergoing bioprosthetic AVR.

Key Words: Anticoagulation • Valves • Prosthesis • Thromboembolism • Antiplatelets


   1. Introduction
Top
 Abstract
 1. Introduction
2. Method
 3. Results
 4. Discussion
 5. Study limitations
 References
 
Despite improvement in valve designs, patients undergoing prosthetic valve replacement are at risk of developing valve thrombosis and systemic thromboembolism. The need of lifelong anticoagulant therapy is beyond doubt in all patients with mechanical heart valves and in patients with bioprosthetic heart valves in the presence of thromboembolic risk factors like atrial fibrillation, previous thromboembolism, left ventricular dysfunction, left atrial thrombus at surgery or hypercoagulable conditions. However, the use of anticoagulant therapy after biological aortic valve replacement (AVR) in patients with no thromboembolic risk factors remains controversial.

The European Society of Cardiology (ESC) [1] guidelines recommend warfarin for the first 3 months in all patients with bioprosthesis involving the use of a prosthetic annuloplasty ring. The intention of early anticoagulation is providing protection from thromboembolic events during the high-risk period for such events while the cloth sewing ring is endothelialized [2]. Unfortunately, the risk of anticoagulant-related hemorrhage is highest during the first 3 months of therapy. The recommended strategy, therefore, subjects those patients we are most reluctant to anticoagulate to a significant risk of complications. A postal questionnaire among UK cardiothoracic consultant surgeons showed that only 16% followed the ESC guidelines for bioprosthetic AVR [3]. The recently revised guidelines from the American College of Cardiology/American Heart Association (ACC/AHA) [4] imply that aspirin (acetylsalicylic acid (ASA)) is an acceptable alternative to warfarin. ASA inhibits platelet aggregation, thereby conferring some degree of protection against thrombosis-mediated vascular events. The advantages of using ASA are a minor risk of bleeding, lower costs, and a better quality of life. Due to these somewhat inconsistent guidelines, the objection was even raised that there is no need for any antithrombotic or antiplatelet therapy at all in patients without thromboembolic risk factors after bioprosthetic AVR.

Given to this ongoing dispute, the aim of the current study was to evaluate the necessity of antiplatelet treatment by ASA with no postoperative antiplatelet therapy in terms of survival, major bleedings and cerebral ischemia in patients after bioprosthetic AVR without thromboembolic risk factors.


   2. Method
Top
 Abstract
 1. Introduction
 2. Method
3. Results
 4. Discussion
 5. Study limitations
 References
 
From January 2001 to December 2003, all patients undergoing single bioprosthetic AVR at the Department of Cardiovascular Surgery, University of Giessen, Germany and the Department of Cardiovascular Surgery, Kerckhoff-Clinic, Bad Nauheim, Germany, were checked for inclusion in this retrospective, observational and double-institutional study. Cases were identified by a search of our computerized clinical database. Patients with single bioprosthetic AVR and sinus rhythm preoperatively and at discharge were included, those undergoing associated coronary artery bypass or other concomitant procedures like double valve disease were not. Further exclusion criteria were: atrial fibrillation, coagulopathy, concomitant mitral valve disease, previous chronic anticoagulation therapy, severe atherosclerotic carotid disease necessitating thrombendarterectomy and allergies to ASA.

Members of the Department of Cardiovascular Surgery, University of Giessen or Kerckhoff-Clinic, Bad Nauheim, performed all operations. Mild hypothermic or normothermic cardiopulmonary bypass was routine. Cardioplegic arrest was achieved by blood or crystalloid cardioplegia (Brettschneider, Calafiore). The prosthesis type inserted was at the discretion of the operating surgeon.

Postoperative administration of ASA or no ASA was at the discretion of the individual surgeon, as well. Therefore, the assignment of patient to treatment depended on the surgeon. Patients experiencing atrial fibrillation after operation were medically or electrically converted. The administration of ASA was started on day 2 after operation. Low molecular weight heparin was administered for 14 days postoperatively.

2.1 Follow-up
Follow-up was divided into three postoperative time frames: 30 days, 3 and 12 months. Thirty-day follow-up was obtained by the physicians of the patients who determined treatment and performed all the routine evaluations. A holter monitoring ECG within this time period was obligatory to detect patients with atrial fibrillation. Patient follow-up at 3 months was obtained through hospital chart review of their rehabilitation stay. After 12 months, all patients were contacted by telephone. The questions included current medication, drugs discontinued since operation, occurrence of strokes or other thromboembolic events, bleeding complications, need of repeat operative AVR and functional status. When patients could not be reached or the information remained incomplete the treating physician was contacted. Therefore, all questions were answered by either the patient or the treating physician, or both. If a patient was admitted to another hospital, details of that admission were obtained with the patient's consent. Events such as thromboembolism, major bleedings and other valve-related complications were defined according to the guidelines of Edmunds et al. [5].

2.2 Statistical analysis
All data were compiled and stored on a computerized database. Data are expressed as mean ± SD for continuous data and as percentages for categorical data. Univariate analysis ({chi}2 and t-test) was used to compare the characteristics of the groups of patients. Statistical tests were performed using the commercially available software SPSS (Statistical Package for Social Sciences, 12.0, Chicago, Illinois).


   3. Results
Top
 Abstract
 1. Introduction
 2. Method
 3. Results
4. Discussion
 5. Study limitations
 References
 
3.1 Baseline and perioperative parameters
A total of 311 consecutive patients undergoing bioprosthetic AVR were identified. Nine of them (2.9%) were excluded because of atrial fibrillation prior to surgery. Fourteen patients (4.5%) were lost during follow-up or refused further inquiry leaving 288 patients available for the study. One hundred and thirty-two patients received ASA early after the operation and 156 patients received no antiplatelet therapy.

Characteristics of the patients at baseline are shown in Table 1 . The mean age of the ASA group (72.9 ± 7.8) was similar with that of the no ASA group (72.6 ± 7.2; p = 0.684). There were no significant differences in gender distribution, body mass index or in the incidence of hypertension, diabetes and dyslipidemia or smoking. Nearly three quarters of the population suffered from hypertension, approximately 21% were diabetics. Half of the patients revealed a dyslipidemia. Sixteen patients had already undergone thoracotomy due to coronary artery bypass surgery, AVR or Fallot tetralogy.


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  		Table 1 Characteristics of patients at baseline (n = 288)
 
The distributions of aortic pathologies, such as aortic stenosis, aortic mean gradient, aortic valve orifice and aortic insufficiency, were similar in both groups. Those with antiplatelet therapy had a significantly higher incidence of coronary artery disease (26.5% vs 5.1%; p < 0.0001). Patients in both groups were of similar New York Heart Association class and had similar left ventricular ejection fractions.

Operative variables of both study groups are presented in Table 2 . Bypass time of the no ASA group (80.7 ± 30.2 min vs 89.3 ± 34.0 min) was significant (p = 0.046) and crossclamp time (57.0 ± 21.6 min vs 60.3 ± 19.8 min) was slightly longer (p = 0.249) compared with the ASA group, although these differences were not likely of clinical significance. None of all patients died during the operation. There was no difference in distribution of stented or stentless and porcine or bovine prostheses. Bioprostheses used were Hancock (43%), Epic (21%), Mitroflow (13%), Shelhigh (9%), Carpentier–Edwards (3%), Perimount (2%), Medtronic (2%), Saint Jude Medical (1%) and others (6%). Renal failure and postoperative intra-aortic balloon counterpulsation (IABP) insertion were rare events in both groups. Rethoracotomy due to bleeding was necessary in 1.5% in patients with ASA compared to 3.8% in patients without ASA (p = 0.401). No patient suffered from postoperative myocardial infarction (Table 3 ).


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  		Table 2 Operative Results (n = 288)
 

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  		Table 3 Postoperative parameters (n = 288)
 
3.2 Thirty-day follow-up
Within 30 days after bioprosthetic AVR five patients with ASA (3.8%) and six patients without ASA (3.9%) died due to infectious complications or heart failure. This difference was not statistically different between the two groups (p = 0.777). The mean hospital stay for patients treated with ASA and without ASA did not differ significantly (15.3 ± 6.9 days vs 14.8 ± 5.9 days; p = 0.523). The prevalence of atrial fibrillation in the 282 aortic valve recipients during the first 30 days after operation was extremely high: 60 patients in the ASA group (45.5%) and 67 in the no ASA group (42.9%) had at least a single documented period of atrial fibrillation. Most of these patients (86.7% in ASA group and 88.1% in no ASA group) were converted into sinus rhythm by medical or electrical cardioversion, the minority converted spontaneously. All patients were discharged with sinus rhythm.

3.3 Three-month follow-up
One patient of the ASA group and two patients of the no ASA group died between the 30-day and 3-month follow-up due to non-cardiovascular diseases. There was no major hemorrhage within 3 months after AVR. Three cerebral ischemic events in the first 3 months after AVR were observed, one in the ASA group (0.8%) and two in the group without antiplatelet therapy (1.3%; p = 0.884) (Table 4 ).


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  		Table 4 Three-month follow-up
 
3.4 Twelve-month follow-up
During the complete follow-up, 14 patients died: 6 in the ASA group (4.5%) and 8 in the no ASA group (5.1%; p = 0.963). Three episodes of major bleeding were reported for the entire study occurring between 3 and 12 months after AVR: two in the ASA group (1.5%) and one in the no ASA group (0.6%; p = 0.884). All the cases were gastrointestinal bleedings and were resolved with conservative treatment. No blood transfusions were required. All of these patients had a negative history for previous gastrointestinal pathology. Late thromboembolic events occurred in one patient in the ASA group (0.8%) (Table 5 ). He suffered from a permanent neurologic deficit. Of all patients with cerebral ischemic events none was fatal.


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  		Table 5 Twelve-month follow-up (telephone call)
 

   4. Discussion
Top
 Abstract
 1. Introduction
 2. Method
 3. Results
 4. Discussion
5. Study limitations
 References
 
The results of our study demonstrate no apparent benefit to early antiplatelet therapy after bioprosthetic AVR in patients without thromboembolic risk factors compared to no antiplatelet therapy. While there was no apparent increase in bleeding complications, there was also no reduction in cerebral thromboembolism. If a drug is not beneficial, then why give it?

Bioprosthetic aortic valves are commonly used in elderly patients and in other risk groups for bleeding complications because of their freedom from anticoagulation. However, early thromboembolic events, particularly with bioprostheses in the mitral position, have been published [6–8]. Heras et al. [2] reported a thromboembolic rate of 41%, 1–10 days after bioprosthetic replacement of the aortic valve, if no anticoagulation at all was used. In accordance to the guidelines from ESC [1], anticoagulation for the first 3 months after biological AVR is recommended. It is believed that the early administration of warfarin allows time for endothelialization of the sewing ring, suture knots and valve leaflets [9]. However, the optimal postoperative management of biological aortic valves remains under discussion because of the better quality and durability of these prostheses nowadays [10,11].

The majority of studies examining the efficacy of antithrombotic treatment after biological AVR have used warfarin during the initial 6 weeks–3 months [12–17] because thromboembolism seems to occur early after operation with a peak incidence during the first 3 months, probably reflecting the lack of endothelialization of the newly implanted bioprosthetic valve [2]. Oyer et al. [18] observed that 50% of all thromboembolic episodes occurred during the first 6 weeks. However, the efficacy of anticoagulation in reducing the incidence of thromboembolic complications during this period remains unproved. Babin-Ebell et al. [14] and Gherli et al. [19] found no benefit in early warfarin therapy compared with ASA and concluded that standard anticoagulation did not seem to be beneficial. Additionally, the retrospective study by Moinuddeen et al. [13] reported no advantage in the prevention of ischemic cerebral events between patients treated with warfarin for the first 3 postoperative months and patients treated with ASA after bioprosthetic AVR. As a consequence, the revised ACC/AHA [4] guidelines recommend ASA as an acceptable alternative to warfarin in patients with biological AVR and no thromboembolic risk factors.

In the study by Blair et al. [20] of 748 patients, no significant difference was shown in the rate of thromboembolism between patients who underwent anticoagulation using warfarin (n = 103), ASA (n = 509), or no postoperative anticoagulation/antiplatelet therapy (n = 136). The incidence rates were 2.9%, 0.8% and 1.5% per patient-year, respectively (p = 0.07). These data were similar to those shown in our study: 1.5% in the group treated by ASA and 1.3% in the group without any antithrombotic treatment. There was, however, an increased incidence of bleeding seen with warfarin therapy relative to ASA or no anticoagulation/antiplatelet therapy: 16.0%, 3.4% and 3.1% per patient-year, respectively (p = 0.03). As in our study, the incidences of major bleeding complications between patients receiving ASA (1.5%) to those without ASA (0.6%; p = 0.884) were not statistically different.

In contrast, Heras et al. [2] reported an incidence of thromboembolic episodes of 41% per year during the first 10 postoperative days for patients with aortic and mitral bioprosthetic valve replacement when anticoagulation was either not administered or was subtherapeutic, but decreased to 3.6% and 1.9% per year 11–90 days and >90 days after operation, respectively. In this retrospective study ASA did not show a beneficial effect in preventing thromboembolic episodes in patients with bioprostheses. These data suggest that early anticoagulation may be needed for the first 10 days postoperatively.

The decision to anticoagulate a patient is based on several factors such as age, the absence, presence and duration of atrial fibrillation, left ventricular dysfunction, left atrial dimensions at echocardiography, previous thromboembolism and hypercoagulable state. The benefit of anticoagulation in patients with these thromboembolic risk factors is beyond doubt. An unresolved issue is the effectiveness of anticoagulant or antiplatelet agents after bioprosthetic AVR in patients without these thromboembolic risk factors. The necessity of anticoagulant or antiplatelet treatment after biological AVR until the sewing ring, suture knots and valve leaflets are endothelialized is logical but unproven. The results of the present study suggest that routine early administration of ASA after biological AVR in patients without thromboembolic risk factors is unnecessary. This could be of special interest for risk groups with bleeding complications like gastroduodenal ulcers, hematuria or thrombocytopenia. A multicenter randomized study including warfarin, ASA and no anticoagulant or antiplatelet therapy is warranted to corroborate these findings in a larger group of patients in order to update guidelines on anticoagulant or antiplatelet management of biological aortic prosthetic valves.


   5. Study limitations
Top
 Abstract
 1. Introduction
 2. Method
 3. Results
 4. Discussion
 5. Study limitations
References
 
Our current study suffers from significant weakness to be sure. The retrospective design makes ‘intention to treat’ analysis impossible, complicating the interpretation of our data. Moreover, the retrospective nature of the study increases the likelihood that event rates will be underestimated and that important risk factors may not have been taken into account. ASA and no ASA therapy were not randomized, because patients were treated according to the surgeon's preference, which gives the allocation a certain degree of unpredictability. The patient groups were not homogeneous due to the different incidence of coronary artery disease who were more likely to receive ASA. While a phone conversation with the patients at 12 months may allow for the analysis of severe strokes, this does not seem to be a very sensitive method for detecting the more common subtle strokes or change in neurocognitive function. Hence, a more objective method of analysis at 12 months such as a Neurologist re-examination or brain imaging could have detected more cerebral ischemic events. Finally, our sample size remains far too small to definitively answer the questions posed.


   References
Top
 Abstract
 1. Introduction
 2. Method
 3. Results
 4. Discussion
 5. Study limitations
 References
 

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