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a Division of Cardiac Surgery, University of Maryland Medical Center, Baltimore, Maryland
b Duke Clinical Research Institute, Durham, North Carolina
c Centennial Medical Center, Vanderbilt University, Nashville, Tennesse
Accepted for publication January 23, 2009.
* Address correspondence to Dr Gammie, Division of Cardiac Surgery, University of Maryland Medical Center, N4W94, 22 S Greene St, Baltimore, MD 21201 (Email: jgammie{at}smail.umaryland.edu).
Presented at the Fifty-fifth Annual Meeting of the Southern Thoracic Surgical Association, Austin, TX, Nov 5–8, 2008
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Abstract |
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Methods: The study population included isolated mitral valve operations performed between January 2000 and December 2007 at 910 hospitals participating in the STS ACSD. Patients with endocarditis, prior cardiac operation, shock, emergency operation, and concomitant coronary artery bypass graft or aortic valve surgery were excluded.
Results: During the 8-year study period, 58,370 patients underwent isolated primary MV operations. For patients with isolated mitral regurgitation (n = 47,126), the rate of MV repair (versus replacement) increased from 51% to 69% (p < 0.0001). Among patients having replacement (n = 24,404), there has been a pronounced decline in the use of mechanical valves: 68% to 37% (p < 0.0001). The operative mortality for MV replacement was consistently higher than that for repair (3.8% versus 1.4%), a finding that persisted after risk-adjustment (adjusted odds ratio 0.52, 95% confidence interval: 0.45 to 0.59; p < 0.0001). Among patients having elective isolated MV repair (n = 28,140), the operative mortality was 1.2%. For asymptomatic (class I) patients, operative mortality was 0.6%.
Conclusions: This study documents several important trends in MV surgery, including the progressive adoption of mitral valve repair and increasing use of bioprosthetic replacement valves. Operative risks of MV repair are significantly lower than those for MV replacement. Operative mortality for isolated elective mitral valve repair is 1% in contemporary clinical practice.
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Introduction |
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TopAbstractIntroductionMaterial and MethodsResultsCommentDiscussionAcknowledgmentsReferences |
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Material and Methods |
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TopAbstractIntroductionMaterial and MethodsResultsCommentDiscussionAcknowledgmentsReferences |
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Patient Population
The study population consists of patients undergoing isolated primary mitral valve surgery with or without concomitant tricuspid or atrial fibrillation correction surgery between January 1, 2000, and December 31, 2007. Patients with infective endocarditis, prior cardiac operation, cardiogenic shock, emergency operation, and concomitant coronary artery bypass graft or aortic valve surgery were excluded.
Data Definitions
The primary outcome variable was operative mortality, defined as either in-hospital or 30-day death from any cause. Patients were considered to be undergoing resuscitation if they were receiving cardiopulmonary resuscitation en route to the operating room or before anesthesia induction. Shock was defined as systolic blood pressure less than 80 mm Hg or cardiac index less than 1.8 despite maximal treatment.
Statistical Analysis
The distribution of patient characteristics and outcomes was summarized by the median and interquartile range (IQR [25th percentile to 75th percentile]) for continuous variables and the percentage distribution for categorical variables. Comparisons of subgroups were performed using Wilcoxon rank sum tests (continuous variables) or 
2 tests (categorical variables). Differences by calendar year were assessed using a stratified Mantel-Haenszel trend test with stratification by hospital.
Logistic regression was used to compare mortality rates for mitral repair versus replacement while adjusting for imbalances in measured covariates. Variables in the logistic regression model included age, tricuspid procedure, preoperative atrial fibrillation, body surface area, congestive heart failure, New York Heart Association (NYHA) class, chronic lung disease, creatinine level, cerebrovascular disease/accident, diabetes mellitus, dialysis, ejection fraction, sex, hypertension, preoperative intra-aortic balloon pump or inotropes, immune suppressive treatment, mitral insufficiency, tricuspid insufficiency, left main disease, myocardial infarction, diseased vessel number, peripheral vascular disease, race, and surgery year. Initially, no adjustment was made for for hospital identity. The model was subsequently reestimated with hospital identity included as a set of category indicator variables.
In addition, the comparison of mortality rates for mitral valve repair versus replacement was repeated using the method of subclassification on the estimated propensity score. [10]. For this analysis, the estimated propensity score was defined as the predicted probability of receiving valve repair (instead of replacement) as determined by a logistic regression model that included the same set of covariates that were incuded in the mortality model described above. Patients were divided into 10 equal-sized subclasses based on their propensity for receiving mitral valve repair. Standardized outcome rates were then calculated across the 10 propensity groups by applying direct adjustment with population total weights. A Mantel-Haenszel test was used to test the hypothesis of no association between mitral valve repair compared with replacement and operative mortality while stratifying on the propensity subclass.
Finally, to assess whether the observed mortality difference for repair versus replacement could be explained by an unmeasured confounder variable, we conducted a sensitivity analysis using the method of Lin and colleagues [11]. The goal of the sensitivity analysis was to determine the minimum prevalence ratio (prevalence of unmeasured risk factor among replacement patients divided by prevalence of unmeasured risk factor among repair patients) that would be sufficient to account for the apparent mortality benefit of valve repair.
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Results |
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Functional class of patients referred for mitral valve surgery
We examined trends in functional classification (NYHA class) among all patients having isolated mitral valve surgery between 2000 and 2007. In aggregate, 17% of patients were considered class I, 31% class II, 39% class III, and 12% class IV. There was a clear trend toward earlier referral of patients with better functional status over time (Fig 7).
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Cross-clamp times were somewhat longer for patients undergoing replacement (median 84 minutes [IQR: 65 to 110]) compared with repair (80 [60 to 105]; p < 0.0001). Cardiopulmonary bypass times were longer in the replacement group compared with the repair group (median 116 minutes [IQR: 91 to 149]) versus (109 [84 to 140], p < 0.0001).
Perioperative Outcomes
Perioperative outcomes are listed in Table 2. Unadjusted mortality, major morbidity, and the combination of morbidity and mortality occurred more commonly among patients having mitral valve replacement compared with repair. Unadjusted operative mortality was significantly influenced by preoperative NYHA functional classification for mitral valve repair, replacement, and the combined group (Table 4).
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Adjusted Outcomes
In the multivariable logistic regression model (incorporating 26 preoperative risk factors), risk-adjusted mortality was lower for mitral valve repair compared with replacement (odds ratio 0.52, 95% CI: 0.45 to 0.59, p < 0.0001; Table 3).
Estimates were similar when indicator variables for hospital identity were included in the logistic model (odds ratio 0.54, 95% CI: 0.47 to 0.63, p < 0.0001). Results were also similar when risk adjustment was accomplished by propensity stratification instead of logistic regression: standardized mortality rate was 2.96% (95% CI: 2.72% to 3.20%) for MV replacement compared with 1.65% (95% CI: 1.49% to 1.80%) for MV repair (p < 0.001).
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Comment |
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We believe that mitral valve repair rates should be considered a key indicator of quality in mitral valve surgery. Efforts to define optimal repair rates are critically dependent on the nature of the population examined, including percentages of patients with endocarditis and rheumatic mitral valve disease. One might ask to what extent can mitral valve repair rates continue to improve. In the most recent year of the study (2007), 39% of isolated mitral valves (3,749 of 9,682) were replaced. Approximately one third of these patients had mitral stenosis and were suboptimal candidates for repair, while the remaining two thirds (2,586 patients) had pure mitral regurgitation and might benefit from repair. Many mitral repair institutions and surgeons report repair rates higher than 90% for degenerative and 80% for geometric (functional) mitral regurgitation [12, 13]. If we assume that a 90% repair rate for pure mitral regurgitation is feasible, then in the most recent year of the study, we project an achievable overall repair rate of 61% + (39% x 2/3 x 0.9 = 20.8%) = 81%. This projection suggests a considerable opportunity for further practice improvement.
Among patients undergoing mitral valve replacement, we found a dramatic increase in the use of tissue compared with mechanical replacements. These findings are similar to recently reported findings from the STS ACSD for aortic valve replacement, where the utilization/choice of tissue valves has increased from 20% to 80% during the last decade [14]. Contributors to this fundamental shift in replacement valve choice likely include improved outcomes for reoperative mitral valve surgery. Some contemporary series document operative mortality rates for failed mitral bioprostheses of between 3.4% and 4.7%, with even lower mortality rates of close to 0% for minimally symptomatic patients. Mortality for nonendocarditis elective reoperations has been 2.2%, with no significant difference in mortality for primary and reoperative mitral valve surgery [15, 16]. With long-term follow-up of current generation mitral bioprostheses available, actual freedom from structural valve deterioration of 87% at 14-year follow-up has been reported for patients aged 61 to 70 years [17]. However, it is clear that valve-related complication and reoperation rates are even lower for reparative procedures, so repair should be favored over bioprostheses whenever possible.
Concomitant tricuspid valve operations nearly doubled during the period studied. That may be related to the increase in the prevalence of moderate and severe tricuspid regurgitation during the study. Additionally, surgeons may be paying increased attention to the tricuspid valve at the time of mitral surgery, reflecting a growing recognition of the negative effects of late postoperative tricuspid regurgitation on morbidity [18]. Finally, data are available that associate the performance of tricuspid valve repair for patients with significant tricuspid annular dilation with better late functional status and less tricuspid regurgitation [19].
The functional class of patients undergoing mitral valve surgery progressively improved during the 8 years studied. This trend likely reflects the publication of data demonstrating a survival advantage for surgery compared with medical therapy among asymptomatic patients with severe mitral regurgitation [20], increasing mitral valve repair rates, and professional society (American College of Cardiology/American Heart Association) guidelines supporting mitral valve surgery for asymptomatic patients with severe MR, provided that there is a more than 90% likelihood of successful valve repair without MR [21]. Despite the trend toward earlier operation, it is noteworthy that nearly half of patients undergoing mitral valve surgery in the latest year of the study still were NYHA functional class III or IV, and preoperative functional class remained a powerful independent predictor of operative mortality in this study as well as in others [2, 22].
This study provides a unique opportunity to characterize outcomes of patients having mitral valve repair and replacement. Unlike most single-center reports, which come from high-volume institutions with a documented emphasis on mitral valve repair, the STS ACSD includes a broad cross-section of patients having mitral valve surgery in North America. Mitral valve replacement was more likely to be performed on older female patients and those with more comorbidities. The rate of mitral valve repair declined with increasing age. Both multivariable regression and propensity analyses were used to assess the independent effect of performing mitral valve repair compared with replacement on operative mortality. Mitral valve repair was associated with a significantly lower risk of perioperative death.
While the apparent benefits of mitral valve repair over replacement are impressive, these observations must be interpreted with caution. First, we were unable to do an intention-to-treat analysis, and it is possible that these results are partly explained by intraoperative crossover from repair to replacement. Improved capture of the underlying disease process and mechanism of mitral regurgitation by the STS ACSD in the future will afford additional possibilities for identifying disease-specific repair rates and outcomes assessment. Second, unmeasured confounders, including valve morphology, etiology of mitral valve disease, and surgeon experience also may have influenced this observation. While mitral valve repair is generally believed to be superior to replacement, no randomized trials have been done, and it is unlikely that any will be performed (other than in highly selected populations such as those with cardiomyopathy and geometric mitral regurgitation). Addtionally, our sensitivity analyses makes it clear that there would need to be rather large unmeasured biases to account for the magnitude of differences seen.
Given the proven durability of mitral repair, the excellent long-term freedom from stroke and infection, and avoidance of the complications associated with prosthetic valve replacement, the operative mortality advantage for mitral valve repair compared with replacement seen in this analysis of a large number of unselected patients with isolated mitral valve disease provides additional support for the performance of mitral valve repair when feasible.
Mitral valve repair for isolated mitral valve disease is increasingly being adopted in North America, but more than one third of patients with mitral valve disease continue to receive prosthetic replacement valves. When mitral valve replacement is performed, tissue valves are now most commonly implanted. Although there has been a trend toward improved preoperative functional status among patients having mitral valve surgery, nearly half of contemporary patients are still in NYHA class III or IV. At present, mitral replacement seems to be preferentially performed in older patients with more comorbidities. Mitral repair appears to be associated with half of the risk-adjusted mortality, as compared with replacement, justifying efforts to extend repair into more complex pathologies. Whenever possible, mitral repair should be performed in preference to replacement, and efforts should be made to operate on patients before they develop significant functional limitations. Among patients having elective isolated mitral valve repair for mitral regurgitation, the nationwide operative mortality rate is approximately 1%, a clear benchmark for the surgical treatment of mitral valve disease.
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Discussion |
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There are two variables that weren't looked at that I think could make a significant difference. One is the crossover rate. There is no question that crossover from mitral valve repair to mitral valve replacement at the time of surgery does carry an increased risk that may affect mortality. The experience of the surgeons involved could also have some effect, since less experienced surgeons tend to do more replacements.
The other thing I found interesting in this very highly reparable group was the repair rate of only 69%. I would have thought it to be higher than that by now. In addition, I was concerned that 45% of the patients were in class III and IV at the time of surgery in a very low risk operative group. I feel that it is very important that we get to our medical colleagues and inform them that this is a low-risk group and that they need to be sending them to surgery before they get left ventricular dysfunction which may not be reversible.
My first question is, in this group of patients who have pure mitral regurgitation, are there any that might benefit from going directly to mitral valve replacement given the fact that replacement after a failed repair does carry a twofold to threefold increase in mortality, or should we just try to repair them all and replace only those that fail?
The other question is in asymptomatic patients with severe mitral regurgitation, Dr Enrique Sarano at the Mayo Clinic showed that even with normal ventricular function there was a substantial increase in cardiac mortality and morbidity if they were treated medically, whereas if you treated them surgically, they would revert back to the same risk as the normal cohort. Steve Bolling at the ACC pointed out that asymptomatic patients aren't always asymptomatic, as we also found at Vanderbilt. Many of these patients fail a stress test or have a low SVO2 max, indicating some element of left ventricular dysfunction. They have simply altered their lifestyle to avoid symptoms and once repaired improve significantly. So the second question is, given this very low operative risk in the asymptomatic patient from a large group of hospitals, should we push for earlier intervention on patients with severe mitral regurgitation and normal left ventricular function, and should we revisit the ACC guidelines, which tell us we should wait until the ejection fraction is less than 60% or the left ventricular end-systolic dimension is greater than 40 mm? Again, thank you for allowing me to discuss your paper. It is a very well written manuscript and very interesting.
DR GAMMIE: Thank you, Dr Petracek. I am grateful for your comments. The simple answer to your second question is yes, we should encourage earlier referral to surgery for the patient with asymptomatic, severe mitral regurgitation on the basis of these data. We acknowledge in the manuscript that there are some potential confounders that we haven't examined, for example, the intention-to-treat approach. That is not currently, or until very recently was not captured in the STS database. The statisticians did a sensitivity analysis to see if there was an unmeasured confounder, and they suggested that even if there was one present in 15% of our population that it probably would not have changed the results.
The other question you asked, is there a patient that you should just go ahead and do a replacement on, my answer would probably be no. If you have a patient with isolated degenerative mitral regurgitation, you should really try and fix every single one of those patients, and I think that these data support such an approach. As you know, there is a prospective trial looking at low ejection fraction patients, the geometric patients, comparing repair to replacement and that should be informative.
DR STEVEN F. BOLLING (Ann Arbor, MI): Jim, you are familiar with the large area variation and small area variation in medical practices. This is a look at the overall STS database. Were you able to look closer? Is there regional variation in mitral valve repair rates, the West versus the South or the Northeast versus the Midwest, or even, for example, small area variations, looking at institutions two miles apart in terms of mitral valve repair rates and also what influences that small area and large area variation?
Again, great paper, Jim.
DR GAMMIE: We have not seen much large regional variation, but you are absolutely correct that there is substantial variation from surgeon to surgeon and that there is a relationship to volume. There are surgeons who repair most valves and those that repair few. Quality is the absence of variation, and right now there is great variation in mitral valve repair rates. Given the benefits that we have shown today, it behooves us to decrease that rate of variation and to increase our repair rates.
DR MICHAEL MACK (Dallas, TX): One of the problems with having our outcomes data available as public information is that it is available to everybody and they can interpret it without totally understanding it. Doctor Feldman, the principal investigator of the Evalve trial, has quoted the STS database repair rate being less than 50% because he was including all comers, including endocarditis, mitral stenosis, et cetera. So I need a number to take away from here as to what is the true repair rate for mitral insufficiency; I understand from your presentation that it is 68%. The problem with that is, however, that it also includes annuloplasty only, which is presumably functional mitral regurgitation. So do we know for degenerative disease of the mitral valve what is the true repair rate?
DR GAMMIE: We do not. The problem is the current data elements in the STS database lack granularity. The next time the data elements are upgraded, it is critically important that the underlying etiology is included so we know the difference between a degenerative patient, a rheumatic patient, and a geometric patient. We can infer that for the degenerative patients—what we can say is for pure isolated mitral regurgitation, the repair rates are currently 69% in 2007, with a clear increase over time. But we don't exactly know the answer to the degenerative question.
DR MACK: But it is presumably less than 68%?
DR GAMMIE: I don't know the answer.
DR MACK: Since most of the functional MRs are annuloplasty only.
DR GAMMIE: That is correct.
DR WILLIAM A. BAUMGARTNER (Baltimore, MD): Jim, great presentation. I underscore the value of the STS database, but as we have just heard, it is an evolutionary database. So it should be constantly refined with these additional data elements. Jim, regarding mortality, did you separate out bioprosthetic valves versus repairs as opposed to all valves, because there is a difference, I think, between the two. I just wanted to know if you were able to determine if there was a difference.
DR GAMMIE: That is a great question, and in fact we didn't do that and it probably would be worth looking at that. Thanks.
DR WILLIAM F. NORTHRUP (Kennesaw, GA): Jim, great paper, as usual. Inasmuch as the STS database is a voluntary database, do we know what percentage of all the mitral valve operations in the United States were captured in the database? In other words, how many are we missing inasmuch as it is a voluntary database?
DR GAMMIE: It is generally believed that about 80% of programs contribute data to the STS database, but at present that is not precise. The goal is 100% participation.
DR JEFFREY JACOBS (St. Petersburg, FL): Really quickly, I would first like to congratulate Jim Gammie for his excellent presentation of these important data. In response to these important questions about the penetration of the voluntary STS database, I would like to state that, right now, under the leadership of Fred Edwards and Eric Peterson, the STS and DCRI are involved in a study where we are looking at the concordance between the STS database and the data from the Centers for Medicare and Medicaid Services (CMS). We have done some preliminary analysis and put an abstract together that looks at how many operations that are in the Medicare data of CMS are also in the STS database. We have also looked at the concordance between CMS data and STS data, in an attempt to determine the penetration, completeness, and representativeness of data in the STS database.
This information is quite new, but hopefully, it will be published soon. Then, hopefully we will be able to answer, or at least provide estimates of the answers, to your important questions about the penetration of the STS database, questions that may be stated as "what percentage of all cardiothoracic operations in the United States are captured in the STS database?" and "what percentage of all cardiothoracic programs in the United States contribute data to the STS database?" In the interim, I can say that we have learned that the completeness of the STS ACSD is quite good because we know that, across the STS ACSD, at hospitals that are participants in the STS database, 87.4% of adult cardiac surgeries submitted as Medicare claims in patients greater than or equal to 65 at a given hospital are also submitted to the STS database. We have also studied the representativeness of the STS data by comparing patient characteristics and outcomes of patients who are in the both STS database and Medicare CMS data with patients who are absent from the STS database but are in the Medicare CMS data. Our initial examination of these data provides reassuring information about the representativeness of the STS database. Furthermore, linking STS ACSD to CMS Medicare data clearly provides a unique tool for studying long-term outcomes of cardiothoracic surgery and the economics of cardiothoracic surgery. This linking project should eventually answer your important questions about the penetration of the STS database.
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Acknowledgments |
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