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a Section of Cardiac Surgery, University of Michigan, Ann Arbor, Michigan
b Duke Clinical Research Institute, Durham, North Carolina
c Division of Cardiac Surgery, University of Maryland, Baltimore, Maryland
Accepted for publication July 16, 2010.
* Address correspondence to Dr Bolling, University of Michigan Hospitals, Section of Cardiac Surgery, 5144 Cardiovascular Center; SPC 5864, Ann Arbor, MI 48109 (Email: sbolling{at}umich.edu).
Presented at the Forty-sixth Annual Meeting of The Society of Thoracic Surgeons, Fort Lauderdale, FL, Jan 25–27, 2010.
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Abstract |
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Methods: Between January 2005 and December 2007, 28,507 patients undergoing isolated mitral valve surgery (with or without tricuspid valve surgery, with or without atrial fibrillation surgery) by 1,088 surgeons at 639 hospitals in the STS ACSD were identified. Patient characteristics independently associated with mitral valve repair were identified using a generalized estimating equations logistic regression model. Observed mitral valve repair rates were plotted against surgeon-specific annual mitral volume, and predicted probabilities of mitral repair by surgeon volume were calculated after adjusting for patient baseline covariates.
Results: On average, patients undergoing mitral valve surgery were 62 years old, with 51% female and 82% Caucasian. Among surgeons performing mitral procedures, the mean rate of mitral valve repair was 41% (range, 0% to 100%) and the median number of mitral valve operations per year was 5 (range, 1 to 166). Several patient characteristics were independently associated with a decreased odds of mitral repair (versus replacement), including mitral stenosis (odds ratio 0.09; 95% confidence interval: 0.08 to 0.11) and active endocarditis (odds ratio 0.21; 95% confidence interval: 0.17 to 0.25). While substantial variability in repair rates was observed among low-volume surgeons, increased surgeon-level mitral volume was independently associated with an increased probability of mitral repair.
Conclusions: This analysis demonstrates marked variability in the frequency of mitral valve repair, and the influence of both patient- and surgeon-level factors on the likelihood of mitral valve repair. Increasing surgeon-specific annual mitral valve volume is associated with a higher probability of mitral repair. Identification of these predictors of mitral valve repair creates substantial opportunity for quality improvement in patient outcomes in mitral valve surgery, potentially through education, adoption of best practices, and improved mitral repair enabling technology.
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Introduction |
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The association between procedural volume and mortality outcomes for many cardiac surgical procedures, including mitral surgery, has been studied extensively, with many reports demonstrating improved clinical outcomes in high-volume centers [3–6]. Furthermore, mitral valve repair rates have been correlated with increased hospital mitral surgical volume [7]. However, similar analyses for mitral valve repair predictors have not been performed at the individual surgeon level. Using data from The Society of Thoracic Surgeons (STS) Adult Cardiac Surgery Database (ACSD), a multivariable model was created to predict the relative likelihood of mitral valve repair (versus replacement). The multivariable model included clinical covariate characteristics and individual surgeon mitral volume. A detailed risk-adjustment model was used to assess the association between individual surgeon mitral volume and each outcome while adjusting for differences in patients. Identifying and understanding clinical and surgeon factors that predict mitral repair could lead to higher mitral repair rates and better patient quality outcomes.
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Material and Methods |
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TopAbstractIntroductionMaterial and MethodsResultsCommentDiscussionAcknowledgmentsReferences |
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The present study population consists of patients who underwent an isolated mitral valve operation (with or without concomitant tricuspid valve and/or atrial fibrillation surgery) at an STS-participating institution between January 2005 and December 2007. Patients were included if the operating surgeon participated in the STS ACSD and reported at least one mitral operation in each month throughout the time period of the study. This study involved human subjects. The Institutional Review Board of the University of Michigan approved this study and waived the need for patient consent, as individual patients were not identified. Also because the data used in analysis of the STS ACSD represents a limited data set (no direct patient identifiers) that was originally collected for nonresearch purposes, and the investigators do not know the identity of individual patients, the analysis of these data was declared by the Duke University Health System Institutional Review Board to be research not involving human subjects and is therefore considered exempt (Duke University Health System Protocol 00005876).
Relationship Between Patient Characteristics and Probability of Repair
Characteristics of patients undergoing repair versus replacement were compared using Mantel-Haenszel 
2 tests for categorical variables and the Wilcoxon rank sum test for continuous variables. The multivariable association between valve repair (rather than replacement) and patient clinical characteristics was assessed using logistic regression. In this analysis, the dependent (outcome) variable was a binary indicator of the type of operation (ie, 1 = repair, 0 = replacement). Patient-level covariates consisted of age, sex, race (white, nonwhite), endocarditis (active, treated, none), ejection fraction, mitral stenosis (yes/no), status (emergent, urgent, elective), concomitant tricuspid operation (yes/no), concomitant afibrillation correction procedure (yes/no), and data harvest period (January to June 2007, June to December 2007, January to June 2006, June to December 2006, June to December 2005 versus January to June 2005).
Relationship Between Surgeon Volume and Probability of Repair
Rates of surgical repair of the mitral valve were plotted against surgeon mitral volume for all eligible surgeons performing mitral procedures. To assess the association between valve repair and surgeon volume, the model described above was modified to include surgeon annual volume as an additional (continuous) explanatory variable. The effect of surgeon volume on risk-adjusted repair rates was summarized two ways. First, adjusted odds ratios were calculated using a reference point of 5 cases per year. These results depict the extent to which the "odds" of repair (defined as the probability of repair divided by the probability of replacement) increases as a function of surgeon volume after subtracting the effect of patient characteristics. To enhance interpretation, the results were also summarized by calculating risk-adjusted repair rates. The risk-adjusted repair rate was defined as the predicted probability of repair (calculated as a function of volume) for a hypothetical patient having "average" values for each patient-level covariate. To gain further insight regarding the relationship between volume and repair, the observed (unadjusted) surgeon-specific repair rates were plotted against surgeon annual mitral volume along with exact binomial prediction intervals. Only 4 surgeons performed more than 100 isolated mitral cases per year, making model predictions in this range unstable. Therefore, odds ratios and predicted probabilities are only plotted for surgeons with 100 or fewer annual mitral cases. (The observed repair rates for the 4 surgeons with volume more than 100 are reported in the text). Because of clear differences in the probability of repair among patients with stenosis, endocarditis, and nonelective status, the multivariable analysis described above was subsequently repeated in the subpopulation of patients having none of these risk factors.
Technical Details
Parameters of the logistic regression models were estimated using generalized estimating equations methodology to account for clustering (ie, statistical dependence) of observations from the same surgeon [8]. The generalized estimating equations method was implemented with a compound symmetric working correlation matrix and empirical (sandwich) standard error estimates using SAS PROC GENMOD (SAS version 9.2; SAS Institute, Cary, NC). As a subsequent sensitivity analysis, the model without surgeon volume was reestimated after removing surgeons with repair rates of 0% or 100% and using surgeon-specific intercepts (fixed-effects) to adjust for within-surgeon correlation. Because results of the two models were similar, only the first set is presented. Volume was modeled nonlinearly using a restricted cubic spline function with knots at 10%, 50%, and 90% of the empirical distribution of volume [9]. To assess whether inferences about the effect of surgeon volume were sensitive to the choice of patient-level covariates, the model including surgeon volume was subsequently reestimated with an expanded list of covariates that included all of the variables in the current STS risk model for isolated valve surgery [10]. While we did not seek to develop an absolute predictive model, issues like overfitting and discrimination were tested. Model discrimination was C = 0.746 for the model excluding surgeon volume and C = 0.777 for the model including surgeon volume. When patients with mitral stenosis, endocarditis, or nonelective status were excluded, the discrimination changed from C = 0.777 to C = 0.689. Calibration was assessed for the model including surgeon volume in the overall population (including patients with stenosis). It was assessed graphically by comparing observed versus predicted rates of mitral repair within subgroups based on deciles of the model-predicted probability of repair. As shown in Figure 1A and B, there was close agreement between observed and predicted repair rates with slight under-prediction in the first and tenth deciles.
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Results |
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Overall, isolated mitral patients were 62 years of median age, with 51.3% female and 81.7% Caucasian (Table 1). Endocarditis was present in 9.5% and mitral stenosis in 15.3%. As expected, there were significant differences between mitral repair and replacement patients. Compared with patients who underwent mitral valve replacement, mitral repair patients were younger (median age 61 versus 64 years), with a lower burden of comorbidities and less mitral stenosis (3.7% versus 28.6%) and endocarditis (5.7% versus 13.8%).
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Although there was marked variability in the observed frequency of mitral valve repair (versus replacement), this was especially noted among surgeons with lower annual mitral valve volumes (Fig 1A). On average, repair rates increased with increasing surgeon-specific annual mitral valve volume, with a prominent observed threshold effect. However, only 46 of the 824 surgeons (5.6%) with fewer than 10 cases per year performed mitral repairs on 75% or more of their mitral cases. Four very high mitral volume surgeons (more than 100 isolated mitral cases per year) were followed in the database, with repair rates greater than 80% for three of these surgeons and 52% for one.
After adjustment for patient-level characteristics, the probability of performing a mitral repair (versus replacement) increased from 36.5% to 73.6% with increasing mitral case volume (Fig 3A). The effect of volume appeared to be roughly linear in the lower tail of the volume distribution (Fig 3A) and relatively flat (zero slope) in the extreme upper tail. The predicted probability of repair was substantially higher (range, 49.9% to 82.6%) among the patient subgroup without the negative predictors of mitral stenosis, endocarditis, or urgent/emergent status (Fig 3B).
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Comment |
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The most influential negative factor for repair is mitral stenosis, which in this database is likely a surrogate for rheumatic disease. It is recognized that rheumatic mitral valves are far less easily repaired than degenerative valves, and that the durability of a rheumatic repair may be substantially lower than for other mitral disease processes [11]. There is a declining incidence of mitral stenosis (ie, rheumatic valves) among patients referred for mitral valve surgery [12]. Additionally, infective endocarditis, particularly when active, is associated with markedly lower rates of mitral valve repair [13]. However, single-center reports have reported significantly higher rates of mitral valve repair in infective endocarditis [14], suggesting an area for expanded repair and quality improvement. Interestingly, it was noted that women and non-Caucasians are less likely to receive mitral valve repairs. Previous reports have demonstrated higher mortality for women undergoing mitral valve surgery [15]. Reasons for lower repair rates in women may include underlying higher rates of mitral annular and leaflet calcification and a higher incidence of rheumatic mitral stenosis in women. Race has been shown not to be a risk factor for mortality in mitral valve replacement surgery [16]. Explanations for the lower rates of mitral repair in non-Caucasians remain undetermined.
Interestingly, relative changes in a given patients' age or LV function or concomitant atrial fibrillation surgery were neither negatively or positively associated with mitral repair rates. These factors were noninfluential predictors of mitral repair, with odds ratios near unity.
Conversely, the largest positive predictor and influence upon the utilization of mitral valve repair for patients was an individual surgeon's annual mitral volume. For example, for a surgeon doing 100 mitral cases per year, the risk-adjusted odds ratio for a mitral repair was 3.78 times (95% confidence interval: 1.87 to 7.64) that of a patient operated upon by a surgeon who performed the median number (n = 5) of mitral cases per year.
Previous studies have examined the relationship between hospital cardiac surgical volume and mortality outcomes [17–19]. Birkmeyer and colleagues [3] found an absolute mortality difference of 3.5% between the lowest and highest volumes hospital volume for mitral valve replacement. This study did not consider patients having mitral valve repair. Recently, Gammie and associates [7] also examined mortality outcomes in mitral valve surgery as a function of hospital volume. Again, there was a significant decline in both unadjusted and risk-adjusted mortality as hospital mitral procedural volumes increased. In that study, there was also an effect of hospital volume on mitral valve repair rates, with repair rates of 48% in the lowest to 77% in the highest [7]. However, our study is the first to examine individual surgeon mitral volume as a predictor of mitral repair rates.
This study has identified clinical and surgeon-level predictors of mitral valve repair. Understanding the likelihood of repair may have an important impact on the timing of referral to surgery. Rosenhek and colleagues [20] medically followed 132 mitral patients for 5 years with no mortality, in "the absence of guidelines" for surgery. However, Kang and coworkers [21] followed 286 asymptomatic patients with significant mitral regurgitation and showed a 5% mortality at 7 years. Additionally, Sarano and associates [22] followed 456 patients with severe MR; patients treated medically had a cardiac mortality/event rate of 33% at 5 years, and there was also a significant mortality for moderate mitral regurgitation treated medically at 5 years. In that study, the early surgery mitral repair group had the same survival as the general population, implying that mitral valve repair had suppressed the impact of the mitral disease process [22]. In fact, a recent comparison study from Montant and coworkers [23] demonstrated a long-term survival advantage of early mitral repair over medical therapy in asymptomatic patients with severe mitral regurgitation. In the Montant propensity-score based comparison, the overall 10-year survival was only 50% in the conservative group versus 86% in the repaired cohort. David and coworkers [24] also showed that late outcomes of mitral valve repair were identical to the general population, if before operation, patients were not allowed to experience heart failure or LV dysfunction.
Despite these possible advantages of an early mitral repair strategy, the timing for referring a patient for mitral valve surgery remains delayed. Presently, the ACC/AHA guidelines suggest patients with significant mitral regurgitation be sent to mitral surgery on the basis of symptoms or if asymptomatic, on the basis of left ventricular LV functional deterioration, LV chamber dilation, or the onset of complications including atrial fibrillation and pulmonary hypertension [25]. However, a Canadian survey showed that the rate of adherence to published American College of Cardiology/American Heart Association guidelines for surgical referral of mitral valve disease varied between 2% and 30% of the recommended rate [26]. Furthermore, a recent study from Bach and associates [27] demonstrated only a 50% adherence to the guidelines and "underreferral" for surgical intervention in patients with severe mitral regurgitation.
This underutilization of mitral valve surgery has even been portrayed as a public health issue in the lay press. An editorial in the US News and World Report addressed the low rate of mitral valve surgery by challenging medicine and stating "that too many mitral valves are removed and not repaired, that is unacceptable" [28]. One of the major reasons cited for nonadherence to the guidelines or underreferral for surgery for severe mitral regurgitation was the fear of not obtaining a mitral valve repair for patients and subjecting them to the incremental risks of mitral replacement.
The present analysis demonstrates marked variability in the frequency of mitral valve repair rates, largely dependent upon individual surgeon mitral volume. In Europe, Bridgewater and associates [29] have published proposed mitral repair best practice standards, which would set volume thresholds of 25 mitral repairs per surgeon undertaking mitral repair surgery and 50 mitral repairs for any hospital unit, without any real supporting data for these cutpoints. In the United States, Adams and Anyanwu [30] have suggested that cardiologists be aware of specific mitral lesions that predict or increase the rate of mitral repair and refer to a "mitral subspecialist." He states that the anticipated mitral repair rate should be 90% or higher in degenerative disease. Finally, the concept of the "referent" mitral valve surgeon has been forwarded by McCarthy [31], among others. From these data, it may be appropriate to conclude that the noted inflection point at 40 isolated mitral cases per year, could be a potential minimum annual volume basis to be as a reference mitral valve surgeon. Although mitral volume is not the only factor influencing mitral repair rate, this study shows that the likelihood of mitral valve repair is heavily influenced by individual surgeon mitral valve volume.
Limitations of this study include the retrospective observational nature of the STS ACSD. It is certainly possible that unmeasured confounders (including mitral valve anatomy/pathology issues, namely, anterior versus posterior leaflet, and so forth) might have influenced results. Furthermore, while the database does report repair rates, quality of repair (as judged by predischarge or late echocardiographic assessment) is not reported. Future data specification upgrades of the valvular component of the STS ACSD will address some of these limitations.
In conclusion, both patient and surgeon-level factors influence whether patients will undergo mitral valve repair (versus replacement) surgery. This study also shows that the likelihood of mitral valve repair is heavily influenced by individual surgeon mitral valve volume. These findings create substantial opportunity to enhance quality patient outcomes in mitral valve surgery [32], through education of both the cardiac surgical and the cardiology communities, adoption of best practices and improved mitral repair enabling technology.
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Discussion |
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DR BOLLING: That is a very good question, Pat. I think many people have thought about how to increase the mitral valve repair rate for our patients. Some of it is perhaps regionalization or changes of practice pattern inside a specific group. But also I think you can see from data, and the data are very robust, even a very small increase in annual rate of mitral cases per surgeon increases the predictable mitral repair rate. And there is a shoulder right around 50 or 60 cases per year, if you will, in the data curve that I think somehow through education and perhaps technology, three-dimensional echocardiography, and so forth, we can perhaps shift that curve to the left so that our patients are better served.
DR McCARTHY: Do you think as a Society that we should address this? There have been papers from England and others saying that we should actually monitor repair rates and that there should be a lower limit and that a mitral repair surgeon should, do at least 25 mitral repairs per year and should have a high repair rate and then submit data on follow-up, looking at recurrence of MR. Is that something you think STS should be involved in based on these data?
DR BOLLING: Those guidelines certainly have been published in the literature in Europe of 25 cases per year per surgeon, and an expected 90% repair rate with less than 5% recurrence. I think that those are laudable goals, because I think that serves our patients well. I think we have to educate our cardiologists about the importance of mitral valve repair long term in terms of quantity and quality of survival of life for patients as well.
DR OTTAVIO ALFIERI (Milan Italy): Steve, did you find a difference in repair rate according to the specific lesion, let's say, for instance, a P2 versus anterior leaflet or Barlow's disease?
DR BOLLING: Thank you very much, Dr Alfieri. That is an excellent question. The problem right now with the STS database is we don't have that pathologic data, and there may be clinical confounders in this data in terms of pathoanatomy. An isolated P2 resection may be much easier to repair than some other very complex lesion. We don't have that data from the database. As you know, the STS database is changing next year, and by 2011 we will have a far more complete valvular database, and we will be able to answer that very important question.
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Acknowledgments |
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TopAbstractIntroductionMaterial and MethodsResultsCommentDiscussionAcknowledgmentsReferences |
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