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Ann Thorac Surg 2003;76:1131-1137
© 2003 The Society of Thoracic Surgeons

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Original article: cardiovascular

Do hospitals with low mortality rates in coronary artery bypass also perform well in valve replacement?

^a VA Outcomes Group, Department of Veterans Affairs Medical Center, White River Junction, Vermont, USA
^b Department of Surgery, Dartmouth-Hitchcock Medical Center, Lebanon, New Hampshire, USA
^c Center for the Evaluative Clinical Sciences, Dartmouth Medical School, Hanover, New Hampshire, USA
^d Center for Outcomes Research and Evaluation, Maine Medical Center, Portland, Maine, USA

^* Address reprint requests to Dr Goodney, VA Outcomes Group (111B), Department of Veteran Affairs Medical Center, White River Junction, VT 05009, USA.
e-mail: philip.goodney{at}hitchcock.org

Presented at the Thirty-ninth Annual Meeting of The Society of Thoracic Surgeons, San Diego, CA, Jan 31–Feb 2, 2003.

	Abstract

Top Abstract Introduction Material and methods Results Comment Acknowledgments Discussion References

 
BACKGROUND: While hospital performance in coronary artery bypass graft (CABG) surgery is reported widely, patients may find it difficult to learn about their hospital's performance in heart valve replacement. We sought to determine if a hospital's performance in CABG is correlated to its performance in heart valve replacement.

METHODS: We studied operative mortality after CABG, aortic valve replacement (AVR), and mitral valve replacement (MVR) using the 1994 to 1999 national Medicare database. After excluding any hospital that did not perform at least 50 CABGs and 20 valve replacements per year we examined the correlation between hospital mortality in CABG and hospital mortality in AVR and MVR using least-squares simple linear regression models. Operative mortality was adjusted for patient characteristics using logistic regression models.

RESULTS: A total of 684 hospitals performed 817,606 isolated CABGs, 142,488 AVRs (54% with concomitant CABG), and 61,252 MVRs (45% with concomitant CABG). Hospital mortality rates with AVR ranged from 6.0% to 13.0% between hospitals in the lowest and highest, respectively, 10th percentile of CABG performance. Similarly hospital mortality rates with MVR ranged from 10.1% to 20.5% in the lowest and highest respectively, 10th percentile of CABG performance. Adjusted mortality rates for both AVR and MVR were closely correlated with isolated CABG mortality rates (correlation coefficients 0.592 and 0.538, respectively; p = 0.001 for both correlations). In stratified analyses these correlations persisted regardless of whether valve replacement was performed with or without concomitant CABG or whether valve replacement was performed in a high- or low-volume hospital.

CONCLUSIONS: Hospital mortality rates with CABG are closely correlated with mortality rates with valve replacement. These findings suggest that shared processes and systems of care are important determinants of performance in cardiac surgery.

	Introduction

Top Abstract Introduction Material and methods Results Comment Acknowledgments Discussion References

 
There has been considerable focus on hospital performance in cardiac surgery [1–5]. Most of this attention has been on a single procedure, coronary artery bypass grafting (CABG), documenting wide variation in risk-adjusted mortality rates across hospitals. This variation has served as incentive for the implementation of continuous quality improvement initiatives [6–8]. Additionally several state and regional initiatives have sought to compile hospital and surgeon level mortality rates with CABG and publicly report this information to patients [9–13]. Although debate exists as to how much they actually use these resources patients interested in their local hospital's performance in CABG can likely find a wide array of written and Web-based information detailing that topic.

However patients facing heart valve replacement surgery may have difficulty finding similar information. Although not as common as CABG, heart valve replacement accounts for more than 20% of all cardiac procedures and because of higher baseline risks accounts for more than 30% of all deaths after cardiac surgery [4]. And while state-based initiatives in New York, Pennsylvania, and California readily publish yearly reports on hospital-level CABG mortality rates, relatively little is published on heart valve replacement surgery. Patients interested in their local hospital's performance in valve replacement may only be able to learn that hospital's risk adjusted mortality rates with CABG. Whether hospital performance in CABG can function as a proxy for performance in valve surgery has not been tested empirically. It remains unknown if patients can use information about hospital mortality with CABG to estimate their operative risk with valve replacement.

For this reason we studied how operative mortality with CABG correlated with operative mortality with valve replacement at the hospital level. Using recent data from the national Medicare database we sought to determine if those hospitals with low mortality rates with CABG also had low mortality rates in valve replacement.

	Material and methods

Top Abstract Introduction Material and methods Results Comment Acknowledgments Discussion References

 
Databases and subjects
Using data from the Medicare claims database we obtained 100% national samples from the Health Care Financing Administration's MEDPAR and denominator files for years 1994 to 1999. This file contains hospital discharge abstracts for acute care hospitalizations of all US Medicare recipients under the hospital (Part A) insurance program. Only patients in fee-for-service arrangements are included in the MEDPAR file; thus our sample excludes Medicare patients enrolled in risk-bearing health maintenance organizations (less than 10%) during this time period. We excluded patients under age 65 or over age 99. Further details on the database are available elsewhere [4].

Analysis
To eliminate unstable mortality estimates from hospital performing very small volumes of procedures we excluded hospitals performing fewer than 50 CABGs per year and hospitals performing fewer than 20 valve replacement procedures per year. Therefore any hospital included in our analysis performed a minimum of 300 CABGs and 120 valve replacements (aortic and mitral combined) during the study period. This resulted in exclusion of 384 of the 1,068 hospitals performing CABG during the study period, leaving 684 hospitals in our analysis.

After identifying our hospital cohort we defined three distinct, procedure groups: (1) isolated CABG; (2)AVR with or without concomitant CABG; and (3) MVR with or without concomitant CABG. These groups were mutually exclusive. For example a patient undergoing a combined AVR-CABG would be counted within the AVR cohort not within the CABG cohort.

We then calculated as our main outcome measure operative mortality at the hospital level for each of the three procedure groups. Operative mortality was defined for each procedure as death before hospital discharge or within 30 days of the index procedure. For purposes of risk adjustment using multiple logistic regression we calculated expected mortality rates, adjusting for patient age group (65 to 69 years, 70 to 74 years, 75 to 79 years, 80 to 84 years, or 85 to 99 years), sex, race (black, nonblack), income (mean income from Social Security assessed at the zip code level), admission acuity (routine, urgent, emergent), and comorbidities (using the Charlson comorbidity score [14]. The expected mortality rates were used to calculate adjusted mortality rates using indirect standardization methods [15]. Our regression models had intermediate discriminative ability with C-statistics ranging from 0.60 to 0.71.

Aggregating these results by hospital (using MEDPAR provider numbers to index patients to hospitals) we were able to calculate hospital level risk-adjusted operative mortality rates for isolated CABG, AVR, and MVR. Then using ordinary least-squares regression we then examine relationships between the adjusted operative mortality of CABG and the adjusted operative mortality of AVR and MVR. These regression models weighted the coefficients by the number of patients within each hospital.

While all analyses were done at the hospital level, for presentation purposes we grouped hospitals into 10 groups (deciles) based on their operative mortality with isolated CABG. All analyses were performed using STATA 7 (Stata Corporation, College Park, Texas) and all p values are two tailed. The Institutional Review Board at Dartmouth approved our study protocol.

	Results

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Patient characteristics
Between 1994 and 1999, 817,606 patients underwent isolated CABG in 684 hospitals included in our study (Table 1). While age, sex, admission acuity, or Charlson score did not vary systematically across hospitals the percentage of patients who were black was slightly higher at hospitals with high CABG mortality rates. Adjusted isolated CABG mortality varied from 2.9% in very low CABG mortality hospitals to 8.7% in high CABG mortality hospitals. The characteristics of the patients within the hospitals in deciles 1 to 10 are shown in Table 1.

View larger version (26K): [in this window] [in a new window]   Fig 1. (A) Correlation of adjusted operative mortality between isolated coronary artery bypass graft (CABG) surgery and aortic valve replacement (AVR). Correlation coefficient R = 0.592. (B) Correlation of adjusted operative mortality between isolated CABG and mitral valve replacement (MVR). Correlation coefficient R = 0.538.

View larger version (11K): [in this window] [in a new window]   Fig 2. Correlation between operative mortality of isolated coronary artery bypass graft (CABG) and mitral valve replacement with concomitant CABG (squares [R = 0.551]) or without CABG (triangles [R = 0.498]) and aortic valve replacement with concomitant CABG (circles [R = 0.619]) or without CABG (diamonds [R = 0.565]).

	Comment

Top Abstract Introduction Material and methods Results Comment Acknowledgments Discussion References

 
Hospital mortality rates with isolated CABG are strongly correlated with hospital mortality rates with valve replacement. The risk of death after AVR was approximately 6% when performed in hospitals with the lowest isolated CABG mortality rates but the risk of death doubled to 13% when AVR was performed in hospitals with the highest isolated CABG mortality rates. Similarly the mortality rate for MVR was 10% in hospitals with low isolated CABG mortality rates and nearly 21% in hospitals with high isolated CABG mortality rates. These correlations persisted even when we stratified our analyses to account for the effect of concomitant CABG at the time of valve replacement surgery as well as for the effect of hospital volume in valve replacement.

Our study has limitations. First we used administrative data for risk adjustment. While our study does contain important risk factors such as age, sex, urgency of admission, and comorbidity status it is impossible to completely rule out the possibility that our findings are a result of unmeasured differences in case-mix. However while the limitations of administrative data have been well documented [16–19], data from clinical studies do not suggest a difference large enough to explain our findings [20]. Second our data are restricted to the Medicare population; it is unclear how these findings relate to those patients under age 65. However it is important to note that more than 60% of all patients undergoing valve replacement are over age 65 [4] and would therefore be contained in our analysis. Nonetheless it is unclear if our findings persist in younger patients. Third our database is limited in its ability to evaluate the influence of new technology and operative techniques such as off-pump coronary bypass and mitral valve repair. Because of our coding strategies we only included data on conventional CABG using cardiopulmonary bypass and we only included mitral valve replacements, not mitral valve repairs. An ideal data source in addition to having clinical data for risk adjustment would also permit measurement of the influence of these new technologies and advanced operative techniques. Future analytic efforts using clinical databases such as The Society of Thoracic Surgery database should work toward accomplishing these goals.

What reasons underlie the strong association between a hospital's mortality in CABG and valve replacement? First CABG and valve replacement share personnel and infrastructure. Personnel such as surgeons, anesthesiologists, intensive care nurses, and perfusionists involved in the care of CABG patients are also likely to be involved in the care of valve replacement patients. Patients undergoing CABG and valve replacement also share structural aspects of the hospital. They undergo surgery in the same operating room suites, are supported by the same cardiopulmonary bypass circuits, and convalesce in the same intensive care units and hospital floors.

But in addition to personnel and infrastructure, CABG and valve patients are also likely share several processes of care. For example when hospitals implement evidence-based processes of care shown to improve outcomes in CABG such as routine administration of aspirin [21], ß-blockers [22, 23], early extubation protocols [24–26], strategies for preventing postoperative atrial fibrillation [27], and critical care pathways [25, 26] it seems likely that patients undergoing valve replacement will receive these processes of care as well. However whether or not these processes of care also improve outcomes in valve replacement surgery has not been studied. Our future work hopes to identify processes of care that result in improvements in outcomes in both CABG and valve replacement. Once identified these processes could be disseminated and widely implemented, with the hope of improving cardiac care across a broad range of hospitals.

In summary our findings reflect a strong correlation between a hospital's performance in CABG and its performance in heart valve replacement. We hypothesize that this relationship is not only a result of shared personnel and infrastructure but also a result of processes of care shared between the two procedures. Future efforts in quality improvement should study shared processes of care in CABG and valve replacement. Improving these shared processes of care will likely help to make both CABG surgery and valve replacement surgery safer in the future.

	Acknowledgments

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Doctor Birkmeyer was supported by a grant from the Agency for Healthcare Research and Quality (R01 HS10141-01) and also by a Career Development Award from the VA Health Services Research and Development Program. The views expressed herein do not necessarily represent the views of the Department of Veterans Affairs or the United States Government.

	Discussion

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DR PETER S. DAHLBERG (Minneapolis, MN): I would like to congratulate Dr Goodney on an excellent presentation on a topic of great importance. I am sure there will be many questions, so I will keep my comments brief.

This work extends Dr Birkmeyer and colleagues' analysis of correlation of outcomes to hospital volumes for procedures from CABG to aneurysm repair to oncologic procedures. The simplest interpretation of their work is that surgery should be centralized in centers of high volume. The New York State experience taught us that coronary bypass surgery, when concentrated in high volume centers was associated with a decrease in risk-adjusted mortality rates by 41%. However, the decrease in mortality in low volume centers was even greater.

The simple concept also raises questions about how innovative programs would be allowed to enter into the marketplace, about what bureaucratic structures would be necessary to regulate surgical procedures, about how high quality care will be delivered to rural areas of the country, whether all surgeons will have access to practice at centers of high volume, and how centers will use market share to negotiate contracts with payers and with physicians.

I have three questions for Dr Goodney. First, do your results hold if patients with combined CABG valve procedures are excluded and just AVR or simply MVR are analyzed in correlation with CABG alone? Second, the R-squared values aren't terribly high. What else accounts for the variance? Finally, I find it challenging to assure the accuracy and reliability of the databases that are orders of magnitude smaller than the ones that you used. Are these databases designed, sufficiently audited, and capable of accurately analyzing the questions you pose?

DR JAMES L. MONRO (Southampton, England): I do congratulate you on getting all these data together and it should be a stimulus to others around the world to do the same. We have in fact in England done it for CABG and aortic valve replacement but not mitral valve replacement and it is on our Web site. But I must say some of those results were pretty worrying and I would agree with Dr Dahlberg that there should be some sort of look at it.

DR ROBERT A. GUYTON (Atlanta, GA): I have one process question for you. If an institution has a policy of performing valve operation at the same time they do coronary bypass with a low threshold for operating upon mild aortic stenosis or mild mitral regurgitation, then they are going to lower their coronary mortality by taking the higher risk patients out of the coronary group and they are also going to lower their valve mortality by putting low risk patients into the valve group. Could that have some effect in a causative way on the results that you have presented?

DR ALVAN ATKINSON (Raleigh, NC): I think it is a very complex statistical analysis with many factors. I think the most striking thing to me was the very poor results that correlated both with coronary bypass surgery and mitral valve surgery in the poorly performing hospitals, how that was fairly consistent, which is what you pointed out. It just reiterates to me the discussion by Tommy Thompson yesterday that we really need to push quality as a central factor in performance of our institutions. I don't know if the public could see data like these but they are pretty striking. I surely would not want to go to one of those hospitals at the far end of your curve.

DR GUYTON: I would also point out that the STS database is much better than the administrative database that is used in this study and would encourage everybody to get into the STS database.

DR GOODNEY: I will address Dr Dahlberg's comments first. I showed the correlation coefficients, which did persist across concomitant CABG and valve, and the shape of the curves was exactly the same, so I did not show those data just for brevity.

I think the other important question goes to the reliability of our database. Any death in our database is confirmed using the national death index. So there are certain variables that I believe are one hundred percent bulletproof; however other clinical variables may or may not be as reliable as a clinical database such as the STS that Dr Guyton correctly pointed out is an outstanding current database in use and should be growing.

The other question pertained to whether or not patients who were low risk could potentially be contaminating the results a little bit. With the database used I basically showed that there is not a whole lot of variability in patient risk using the variables that we have. I can not rule out your question as a possible source of our error given the data that we have but obviously use of a better clinically based database such as the STS would further examination of that question.

	References

Top Abstract Introduction Material and methods Results Comment Acknowledgments Discussion References

 

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Personal Folders Download to citation manager Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Goodney, P. P. Articles by Birkmeyer, J. D. Search for Related Content PubMed PubMed Citation Articles by Goodney, P. P. Articles by Birkmeyer, J. D. Related Collections Valve disease