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Ann Thorac Surg 2007;83:938-945
© 2007 The Society of Thoracic Surgeons

Original Articles: Cardiovascular

Effect of Prior Valve Type on Mortality in Reoperative Valve Surgery

Gilbert H.L. Tang, MD, Manjula Maganti, MS, Tirone E. David, MD, Christopher M. Feindel, MD, Hugh E. Scully, MD, Michael A. Borger, MD, PhD*

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

Accepted for publication July 21, 2006.

* Address correspondence to Dr Borger, Toronto General Hospital, 4N-467, 200 Elizabeth St, Toronto, ON, Canada M5G 2C4, Canada (Email: michael.borger{at}med.uni-leipzig.de).


   Abstract
Top
 Abstract
Introduction
 Patients and Methods
 Results
 Comment
 References
 
Background: The purpose of this study is to examine the effect of previously implanted valve type on hospital mortality in reoperative valve surgery.

Methods: From 1990 to 2005, 743 patients underwent redo valve surgery at our institution. Patients were divided into those who received a mechanical valve during their prior operation (n = 236) and those who received a biological (porcine or pericardial) valve (n = 507). Perioperative data were analyzed retrospectively.

Results: Patients with failing mechanical valves were younger, more likely to be female, and had a higher prevalence of preoperative stroke and atrial arrhythmias, whereas patients with failing biological valves had more hypertension, diabetes mellitus, hyperlipidemia, and chronic obstructive pulmonary disease (all p < 0.05). Mechanical valve explant patients had fewer prior coronary bypass and aortic valve operations, but more mitral surgeries (all p < 0.05). Mechanical patients also had a higher prevalence of endocarditis (p = 0.001) and urgent or emergent surgery (p < 0.001). Hospital mortality was higher among patients undergoing mechanical valve explant by univariate analysis (13% versus 8%, p = 0.04), but not by multivariate analysis (odds ratio 1.6, 95% confidence interval 0.9 to 2.7, p = 0.1). Logistic regression revealed chronic obstructive pulmonary disease, peripheral vascular disease, endocarditis, and urgency of reoperation as independent predictors of hospital mortality.

Conclusions: Explanting mechanical valves are associated with higher perioperative mortality than are tissue valves, likely because of different modes of failure and presentation. A lower risk of mortality for tissue valve reoperation may be an additional factor to consider when patients and surgeons choose a valve prosthesis type.


   Introduction
Top
 Abstract
 Introduction
Patients and Methods
 Results
 Comment
 References
 
Reoperative (redo) valve surgery is an increasingly common problem. Cardiac surgeons frequently face the challenge of deciding whether a mechanical or bioprosthetic valve is more appropriate for the individual patient. Mechanical valves offer the advantage of longevity but have a higher thrombogenic potential and require life-long systemic anticoagulation. Bioprostheses do not require permanent anticoagulation, but have a limited life-span and may necessitate re-replacement due to structural deterioration [1].

Patients undergoing valve reoperations have a diverse and complex clinical profile. Thrombi may form on mechanical valves and cause orifice obstruction, leaflet malcoaptation, and acute valvular dysfunction. Calcification may make bioprostheses progressively stenotic, or leaflet tears may occur that cause the valves to become acutely or chronically regurgitant. Endocarditis can occur with both types of valves and is associated with high reoperative mortality rates. However, conservative (nonsurgical) therapy can be used more frequently in bioprosthetic versus mechanical valve endocarditis [2].

Repeat valve surgery is associated with a higher operative mortality rate than first time valve surgery [3–5], and certain risk factors may be preventable. In particular, the type of valve implanted at the initial operation is an imminently modifiable variable. Previous evidence suggests that patients with prior mechanical valves have a higher reoperative mortality than those with tissue valves [6–9]. However, there have been few reports in the literature that have focused on this issue. The purpose of our study was, therefore, to examine the risk factors for hospital mortality among patients undergoing reoperative aortic and mitral valve surgery, with a particular focus on the effect of prior implanted valve type (namely, tissue versus mechanical).


   Patients and Methods
Top
 Abstract
 Introduction
 Patients and Methods
Results
 Comment
 References
 
The study was approved by our institution’s Research Ethics Board, and individual patient consent was waived with ethics approval. From 1990 to 2005, 743 patients underwent reoperative mitral valve (MV) or aortic valve (AV), or both, replacement surgery at the Toronto General Hospital. Preoperative, perioperative, and postoperative data were entered prospectively into a computerized database. Only patients who underwent reoperative AV or MV surgery for a dysfunctional valve prosthesis were included in the current study. Patients who underwent reoperation for native AV or MV disease were excluded.

Operations
Of the 743 patients, 335 had prior MV and 522 had prior AV replacement surgery. (The numbers are not additive because 72 patients underwent prior double valve replacement surgery). During the preceding operation, 236 patients (32%) received a mechanical valve, and 507 patients (68%) received a bioprosthesis. All patients who underwent previous double valve replacement received two mechanical valves (n = 35; 15%) or two tissue valves (n = 37; 7%), and therefore were assigned to their respective groups. Patients who received one mechanical valve and one biological valve during the prior operation (n = 2) were excluded.

A total of 814 valves were explanted during reoperation, of which 274 (34%) were mechanical, 390 (48%) were porcine, and 150 (18%) were pericardial (Table 1). The majority of mechanical valves consisted of St. Jude, Bjork-Shiley, Monostrut, or Starr-Edwards prostheses, whereas the majority of explanted bioprosthetic valves were Carpentier-Edwards (CE) porcine, Hancock II, or Ionescu prostheses.


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  		Table 1 Type of Valve Explanted During Aortic and Mitral Valve Reoperation
 
Repeat MV surgery was performed in 389 patients and AV reoperation was performed in 552 patients. Concomitant coronary artery bypass graft surgery (CABG) was performed in 174 patients. Tricuspid valve repair or replacements occurred in 91 patients, and 184 patients had replacement of the ascending aorta. For the redo procedure, 603 of 899 implanted valves (67%) were mechanical, 226 of 899 (25%) were porcine, and 70 of 899 (8%) were pericardial.

Data regarding postoperative morbidity and mortality were tabulated. Early postoperative events were compiled and analyzed according to the "Guidelines for reporting morbidity and mortality after cardiac valvular operations" [10, 11].

Statistical Analysis
Categorical patient variables are expressed as percentages and continuous variables as means ± SD throughout the manuscript. Categorical data were compared using the {chi}2 or Fisher’s exact test, and continuous data were compared using Student t test or Wilcoxon rank-sums test. Stepwise multivariable logistic regression analysis, with backward elimination of nonsignificant determinants, was used to calculate risk-adjusted odds ratios and to determine the independent predictors of hospital mortality, defined as death during the same hospitalization. We used SAS version 9.2 (SAS Institute, Cary, North Carolina) for all statistical analyses.


   Results
Top
 Abstract
 Introduction
 Patients and Methods
 Results
Comment
 References
 
Preoperative and Perioperative Characteristics
Table 2 shows the characteristics of patients undergoing reoperative valve surgery during the study period. Patients who underwent mechanical valve explant were younger, more likely to be female, and had a higher prevalence of preoperative stroke and atrial arrhythmias. They were less likely to have had prior CABG or AV surgery, but more likely to have prior MV surgery. Patients with biological valve explants had more hypertension, diabetes mellitus, hyperlipidemia, and chronic obstructive pulmonary disease.


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  		Table 2 Preoperative and Perioperative Characteristics of the 743 Patients Who Underwent Reoperative Valve Surgery Between 1990 and 2005 at the Toronto General Hospital, According to Prior Implanted Valve Type (Mechanical Versus Biological)
 
Compared with the tissue valve group, a higher proportion of patients in the mechanical valve explant group had infective endocarditis (31% versus 19% respectively, p < 0.001), and underwent urgent or emergent redo surgery (20% versus 12%, p < 0.001). Of the 33 patients who underwent emergent surgery, the most frequent indications were (in decreasing order of frequency and not mutually exclusive) cardiogenic shock (n = 21), acute endocarditis/abscess (n = 17) and acute prosthetic valve dehiscence or failure (n = 10).

The period (mean ± SD) between the primary and repeat operation was not different between the two groups (10.7 ± 8.3 versus 10.5 ± 4.4 years for mechanical versus biological groups respectively, p = 0.8). Biological explant patients were more likely to undergo surgery during the earlier years of the study (Table 2). During reoperation, prior mechanical valve patients underwent fewer concomitant CABG and AV procedures, but more MV operations. Cross-clamp time and cardiopulmonary bypass times were also higher in the mechanical valve explant group (Table 2).

Operative Outcomes
The overall hospital mortality rate was 9.4%. Mortality was 7% in elective, 19% in urgent, and 33% in emergent operations. Patients undergoing mechanical explant had a significantly higher mortality than those undergoing biological explant (Fig 1).


Figure 1
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  		Fig 1. Hospital mortality associated with reoperation for failing mechanical and biological valves.

 
The mean duration of mechanical ventilation was similar among the two groups (49 ± 164 versus 35 ± 79 hours for mechanical and biological explants, respectively; p = 0.4). Length of intensive care unit stay was also similar (3.9 ± 5.1 versus 3.3 ± 4.2 days; p = 0.2). Patients with a prior mechanical valve had a slightly longer length of hospital stay than patients who received a prior biological valve (13 ± 10 versus 12 ± 9 days, respectively; p = 0.049).

Of the 743 patients who underwent reoperative valve surgery, 10 (1%) had perioperative myocardial infarction, 33 (4%) suffered from stroke, 120 (16%) had pulmonary complications including pneumonia, 30 (4%) had renal failure, and 133 (18%) required a permanent pacemaker. Sternal wound infection occurred in 8 patients (1%). There were no differences for any of these perioperative outcomes among the two valve explant groups.

Risk Factors for Hospital Mortality
Hospital mortality was defined as death during the same admission. Table 3 lists all of the risk factors that had a significant univariate association with hospital mortality. The type of valve explanted, specifically a mechanical valve, was a significant predictor of hospital death. Other univariate predictors were chronic obstructive pulmonary disease, peripheral vascular disease, renal failure, congestive heart failure, cardiogenic shock, New York Heart Association (NYHA) class IV symptoms, infective endocarditis, prior and redo mitral valve surgery, and urgent and emergent operations. The period of reoperation had no significant effect on hospital mortality (12% in 1990 to 1994, 9% in 1995 to 1999, and 7% in 2000 to 2005; p = 0.2).


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  		Table 3 Univariate Predictors of Hospital Death in 743 Reoperative Valve Surgery Patients
 
Variables assessed as possible multivariate predictors of hospital mortality are listed in the Appendix. Logistic regression analysis (Fig 2) revealed that urgency of the redo operation, shock, endocarditis, peripheral vascular disease, and chronic obstructive pulmonary disease were significant independent predictors of hospital death. The type of valve explanted trended toward being an independent predictor of hospital mortality (p = 0.1).


Figure 2
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  		Fig 2. Independent predictors of hospital mortality associated with reoperative valve surgery from 1990 to 2005 at Toronto General Hospital. (CI = confidence interval; COPD = chronic obstructive pulmonary disease; OR = odds ratio.)

 

   Comment
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
References
 
With advances in medical therapy and life expectancy, reoperation to replace a dysfunctional heart valve prosthesis is an increasingly common procedure. There has been a gradual decrease in perioperative risk for redo valve surgery over the past 2 decades (Table 4), likely due to increased surgical experience, better myocardial protection, and improved patient management. However, mortality rates remain higher than first-time valve replacement surgery [3–5]. Several studies have identified predictors of mortality during reoperative valve surgery, but the majority of such risk factors are not modifiable. One risk factor that is definitely modifiable is the type of prosthesis inserted during the prior operation. We therefore undertook the current study to assess the effect of prior valve implantation on hospital mortality among patients undergoing reoperative aortic and mitral valve surgery.


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  		Table 4 Results of Reoperative Valve Surgery in Series With More Than 100 Patients Reported Over the Past 20 Years
 
Prior studies have identified age [1, 12, 13], NYHA class [12–17], endocarditis [8, 12, 14, 15, 18, 19], and urgency of the redo surgery [1, 5, 12, 14–16, 20, 21] as significant predictors of operative mortality. In our series, chronic obstructive pulmonary disease, peripheral vascular disease, endocarditis, and urgency status were independent predictors of hospital death. The overall hospital mortality rate of 9.4% in our study was well within the range of those reported in the literature (Table 4) [1, 4–9, 12–18, 20, 22–28].

Urgency of surgery has been reported as a multivariate predictor of hospital mortality in a number of studies on both mechanical and biological valve reoperations [1, 5, 16, 20, 21]. Rizzoli and colleagues [21] examined 863 patients who had redo valve surgery between 1970 and 1995 and found urgent and emergent operation were significant risk factors of early mortality. Akins and coworkers [1] reported a 2.5-fold in increased risk for patients undergoing nonelective redo tissue AV and MV surgery. Our group previously reported that urgency of the operation was an independent predictor of hospital death in reoperative MVR patients [5]. In the current series, having an urgent or emergent reoperation carried a 3.8-fold increased risk of hospital death, with 19% mortality in urgent and 33% mortality in emergent operations.

The current study revealed that patients who required mechanical valve explant were more likely to undergo urgent or emergent surgery and more likely to present with endocarditis, two independent risk factors for hospital mortality. The increased risk of reoperation on a mechanical valve over a bioprosthetic valve has been observed in some studies [6–9], although other series have shown no significant differences [12, 24]. Bortolotti and associates [6] found a hospital mortality of 21% in the mechanical explant group versus 10% in the bioprosthetic group among 549 patients requiring a first-time valve reoperation over a 26-year period [6]. Jones and coworkers [7] recently reported a 26.1% operative mortality for the replacement of a failing mechanical valve compared with 8.6% in the tissue valve replacement group (p < 0.001). The same investigators also found that replacing a mechanical valve was an independent predictor of operative death, with a 2.25-fold increase in mortality.

Patients who receive a prior mechanical valve may present more frequently for urgent or emergent surgery because of a different mode of failure of these prostheses. Tyers and coworkers [9] found endocarditis and thromboembolic complications causing acute prosthetic failure as leading causes of reoperation in mechanical valve patients. In contrast, structural deterioration was the leading cause of redo surgery in bioprosthetic valve patients. This finding was confirmed in a series of 45 explants of Hancock II porcine bioprosthesis at our institution, where tissue degeneration, calcification, cusp tears, and increased stiffness accounted for most valve failures [29]. Differences in the management of prosthetic valve endocarditis may also explain part of our observed differences in redo valve mortality. Bioprosthetic valve endocarditis is much more likely to respond to antibiotic therapy, resulting in surgery under more elective conditions or even avoidance of surgery altogether [2]. In contrast, mechanical valve endocarditis nearly always requires surgical intervention, frequently under urgent conditions.

There is increasing evidence that the mechanism of failure may also be different between porcine and pericardial tissue valves. Gao and colleagues [30] compared long-term durability of 518 CE porcine and 1,021 CE pericardial valves in the aortic valve position and found a lower 10-year freedom from explantation in porcine valves (90% ± 2%) than in pericardial valves (97% ± 1%, p = 0.04). Compared with 4 of 9 pericardial explants, a higher number of porcine explants (25 of 31) were due to structural deterioration. Similarly, Jamieson and colleagues [31] compared modes of failure in 1,266 CE porcine and 429 CE pericardial valves in the mitral position. The porcine valve was an independent risk factor for structural valve deterioration, with 57% of valve failures due to combined calcification and leaflet tears, while 70% of the CE pericardial valves failures were due to calcification. Because porcine valves are more likely to suffer from leaflet tears, causing acute valvular insufficiency [29, 31, 32], patients may present with acute heart failure and hemodynamic instability, increasing the surgical urgency. In contrast, pericardial valves tend to fail owing to progressive calcification and stenosis [31]. The heart may have increased time to adapt to the progressive aortic or mitral stenosis, thereby delaying the acuity of the clinical presentation. This explanation may account for the higher rate of surgical urgency in porcine versus pericardial valves observed in our series (15% versus 8%, respectively, p < 0.05). Hospital mortality, however, was not significantly different between these two groups (8% versus 6% for porcine versus pericardial, p = 0.08).

Study Limitations
The main limitation of the current study is our inability to more completely characterize patients during their prior valve operation. Most patients had prior valve surgery performed at other institutions, and therefore we do not have information on the indication for their primary valve operation, the rationale for the choice of prosthesis, and their clinical presentations at that time. We are unable to comment on the long-term results of the prior operations for this same reason. However, the long-term results of tissue versus mechanical valves are already well characterized in the literature. It is possible that higher-risk patients received a mechanical valve at the prior operation, resulting in worse outcomes during the redo procedure. That is unlikely, however, given that mechanical valve recipients were younger and had fewer comorbidities during reoperation. Another limitation is that we do not have pathology reports of all the explanted valve prostheses to determine the exact mechanism of failure for each valve type in our study. However, previous studies of the mode of mechanical and tissue valve deterioration have been reported by our institution and others [29, 31, 33]. We believe that our study contains important information on the risks of reoperative surgery after prior mechanical versus tissue valve implantation, and that this information may be important for patients and cardiac surgeons to consider when choosing a valve prosthesis.

In conclusion, reoperation on a failing mechanical valve is associated with a higher risk of mortality than a bioprosthetic valve, likely because mechanical valve patients are more likely to present with urgent or emergent indications for surgery. The different modes of failure between mechanical and biological valves likely account for these observed differences. Unlike most risk factors for mortality during reoperative surgery, the type of valve implanted during the previous operation is an imminently modifiable variable. Lower risk of mortality for tissue valves during future reoperative surgery may be another factor for patients and surgeons to consider when choosing a heart valve prosthesis.


   Appendix
 
Variables Assessed as Multivariate Predictors of Hospital Mortality Among Patients Who Had Valve Reoperation


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   References
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
 References
 

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