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Ann Thorac Surg 2000;70:1212-1218
© 2000 The Society of Thoracic Surgeons

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Original articles: cardiovascular

Predictors of mortality for patients undergoing cardiac valve replacements in New York State

^a State University of New York, University at Albany, Albany, New York, USA

Address reprint requests to Dr Hannan, Department of Health Policy, Management, and Behavior, School of Public Health, State University of New York, University at Albany, One University Pl, Rensselaer, NY 12144-3456
e-mail: elh03{at}health.state.ny.us

	Abstract

Top Abstract Introduction Patients and methods Results Comment Acknowledgments References

 
Background. The objective of this study was to identify the mortality rates and significant independent risk factors for mortality for each of six valve replacement groups.

Methods. A total of 14,190 patients who underwent valve replacement in New York State from 1995 to 1997 were classified into six major groups and significant independent risk factors for inpatient mortality were identified for each of the groups using stepwise logistic regression.

Results. Mortality rates ranged from 3.33% for isolated aortic valve surgical procedures to 18.72% for multiple valve replacements with coronary artery bypass graft operations. The number of years in excess of age 55 was a significant multivariate predictor of mortality for all six groups of patients. Shock was a significant predictor for five of the six groups, and in each of those groups it was the risk factor with the highest odds ratio.

Conclusions. Significant patient risk factors are relatively consistent across different types of valve replacement procedures. The probability of survival from valve surgical procedures is highly dependent on the patient’s preoperative profile and the type of valve operation.

	Introduction

Top Abstract Introduction Patients and methods Results Comment Acknowledgments References

 
Although patients undergoing many types of valve operations have higher mortality rates than patients who undergo coronary artery bypass graft (CABG) operations, the independent risk factors associated with valve operations have not been studied as widely as the risk factors for CABG surgical procedures [1–23]. Most studies in the literature are not based on patients who

have undergone operations within the last 5 years. This is an important limitation because recent advances in surgical and anesthesia techniques are expected to have yielded improved long- and short-term outcomes for valve operation patients.

One reason for the paucity of recent valve operation studies is that valve operations are performed less frequently than CABG surgical procedures, which means it takes more time or more centers to accumulate enough cases from which to draw conclusions. Undoubtedly another reason is that there is a wide range of valve procedures performed, and the risk of perioperative mortality varies widely with the type of procedure performed. Consequently, what began as a small sample problem becomes substantially worse because it is not appropriate to combine all valve procedures into a single analysis.

In January 1989, the New York State Department of Health initiated a cardiac surgery registry, the CardiacSurgery Reporting System (CSRS), which was developed for the purpose of assessing and improving the quality of care of cardiac surgery in the state by identifying significant risk factors associated with short-term mortality and then using those risk factors to develop risk-adjusted outcomes to be given to hospitals, surgeons, and the public. Most of the work to date in identifying significant risk factors and assessing hospital and physician performance has been focused on CABG surgical procedures and on percutaneous coronary interventions (PCIs) using a similar PCI registry founded in 1991. Another study from this database explored the volume-outcome relationship for pediatric cardiac surgery.

The purpose of this study was to classify valve procedures into relatively homogeneous groups and then to identify the significant independent risk factors for each of those groups. The study then contrasts both the mortality rates and the significant risk factors for each of those groups. The database used consists of data from 1995 to 1997 in the CSRS from New York State.

	Patients and methods

Top Abstract Introduction Patients and methods Results Comment Acknowledgments References

 
Study population
Patients in the study were a subset of those patients in the CSRS ages 18 years and older who were discharged between January 1, 1995, and December 31, 1997, after undergoing valve operations in New York. Patients were excluded from the study if they underwent major surgical procedures in the same admission other than valve operation with or without CABG operation (586 patients), or if their valve procedure could not be classified into one of six major groups: (1) isolated aortic valve replacement (AVR), (2) AVR or aortic valvuloplasty plus CABG operation, (3) isolated mitral valve replacement (MVR), (4) MVR with CABG operation, (5) multiple valvuloplasty or valve replacement without CABG operation, or (6) multiple valvuloplasty or valve replacement with CABG operation. Note that for the second category and for the last two categories, valvuloplasties were not coded separately from valve replacements. The reason for including valvuloplasties in these groups is that they were lumped with valve replacements in a single surgical code in the registry. This was not the case for MVRs. We believe that the large majority of procedures in the three combined groups mentioned above were valve replacements.

The total number of patients in the study was 14,190. These patients underwent operation in one of the 33 hospitals in New York State credentialed to perform valve surgical procedures.

Data collection process
The director of cardiac surgery in each of the hospitals in New York in which cardiac procedures are performed was asked to take responsibility for the data collection process and the quality of the data reported. Data are entered on a data entry form that contains information relating to patient risk factors; complications; admission, operation, and discharge dates; discharge status; and hospital and surgeon identifiers. Then data are transferred from the forms to computer disks and forwarded to the Department of Health for data quality auditing and data analysis. When data fields are found to be missing, hospitals are contacted and asked to complete the missing information. Comprehensive on-site audits of approximately half of the hospitals in the registry are conducted each year, and several hospitals have been asked to recode all or part of their data as a result of the audits.

Data analysis
The first analyses consisted of examining the prevalence and mortality rates associated with all important determinants of in-hospital mortality for each of the six groups of valve procedure patients. The mortality rate for each categorical risk factor was tested for statistical significance using the two-tailed Fisher’s exact test with p = 0.05. Similarly, continuous variables (age, ejection fraction, pulmonary artery systolic pressure) were split into categories and tested using the ² test. It should be noted that numerous variables were tested for significance without using any multiple comparison adjustment, so the nominal levels of significance are optimistic.

Next, for each of the six groups of patients, a stepwise logistic regression model (with p = 0.05) was developed to identify the significant independent predictors of in-hospital mortality. The binary dependent variable was patient disposition, with death coded as a 1 and live discharge coded as a 0. Independent variables included all patient risk factors in the CSRS, including demographics; comorbidities; and measures of ventricular function, hemodynamic state, and severity of atherosclerotic process. Other variables measured the extent of valvular stenosis and insufficiency (none, mild, moderate, severe) for each valve, as well as typical risk factors for valve patients such as cardiac index, endocarditis, and cardiomegaly. Also, for isolated AVRs and isolated MVRs, the types of procedure (mechanical, heterograft, and homograft) were tested for significant mortality differences by making one of the types (eg, mechanical valve replacement) a reference category and coding the others as dummy variables in the logistic regression model. It is notable that none of the dummy variables for different individual procedures proved to be significant in the multivariate analyses despite the fact that their crude mortality rates were quite different. This serves as a confirmation that combining them into the same broad group was legitimate.

All logistic models were validated by splitting the data into two subsets, fitting a model to one subset, and then determining which of the risk factors significant in the first half were also significant in the second half. The C statistic was used to measure the discrimination of the model [24] and the Hosmer-Lemeshow test [25] was used to check for goodness of fit by comparing actual and expected adverse outcome rates for groups of patients that had been separated into deciles on the basis of expected adverse outcome rate.

	Results

Top Abstract Introduction Patients and methods Results Comment Acknowledgments References

 
There was no statistically significant difference in overall mortality rate or in the mortality rates of any of the procedures by year. The total number of procedures performed rose by 120 (2.7%) from 1995 to 1996, and by 498 (10.8%) from 1996 to 1997. Table 1 presents the groupings of valve procedures into each of the six groups used to develop statistical models. The volumes and mortality rates for aortic and mitral valve repairs are also presented. As indicated, from 1995 to 1997 in New York there were slightly more than 4,000 isolated AVRs and about the same number of aortic valvuloplasties or replacements with concomitant CABG operations. However, the in-hospital mortality rates were quite different: 3.33% for isolated AVRs and 7.12% when CABG operations were performed in the same admission. MVRs were less common, with 2,258 isolated MVRs and 1,715 MVRs with CABG procedures. Again, there was a large difference in the mortality rate as a function of whether CABG surgical procedures were performed, with a 6.16% mortality rate for isolated MVRs and a 12.77% mortality rate for MVRs with CABG. It is also notable, as has been reported elsewhere, that the mortality rates for MVRs are considerably higher than they are for AVRs. Multiple valve replacements were performed less frequently, with 1,418 multiple valvuloplasties or replacements, and only 577 multiple valvuloplasties or replacements plus CABG. The mortality rates were considerably higher, with a 9.59% rate for multiple valve operations and an 18.72% rate when CABG operations were also performed.

All risk factors in Table 2 for patients undergoing isolated MVRs were associated with significantly higher mortality rates (p < 0.05) except female gender. The same was true for the combined procedure group except for endocarditis, pulmonary artery pressure greater than or equal to 50, and hepatic failure.

Table 2 demonstrates that multiple valvuloplasty or valve replacement patients without CABG operation tended to be younger, have better ejection fractions, be more likely to have undergone previous open heart operation, and to be one third to one fifth as likely to have carotid/cerebrovascular disease, aortoiliac disease, shock, or an extensively calcified ascending aorta as their counterparts with CABG operation. Also, patients without concomitant CABG were only half as likely to have diabetes, but were more than twice as likely to have endocarditis.

All risk factors in Table 2 for multiple valvuloplasties or valve replacements were significantly associated with mortality except female gender, ejection fraction, carotid/cerebrovascular disease, and shock. For patients with CABG procedures combined with multiple valvuloplasties or valve replacements, several risk factors were not significantly associated with mortality. This was mostly a result of the low statistical power to detect significant differences because of the small sample sizes of patients with the risk factors (only 577 patients underwent multiple valvuloplasty or valve replacement plus CABG operation).

Table 3 presents the significant independent risk factors for isolated AVR and for aortic valvuloplasty or valve replacement plus CABG operation. The significant risk factors for isolated valve replacement were age 55 years and older, hemodynamic instability (requiring pharmacologic or mechanical support to maintain blood pressure or output), shock (systolic blood pressure < 80 mm Hg or cardiac index < 2.0 L · min^-1 · m^-2), congestive heart failure (at NYHA level III or IV) in the same admission, extensively calcified ascending aorta, diabetes, dialysis-dependent renal failure, and pulmonary artery systolic pressure of at least 50. The term "age 55 years and older" means that the risk is flat until age 55, and then rises with an odds ratio (OR) of 1.06 for an additional year of age. Risk factors with the highest ORs were shock (OR = 8.68), dialysis-dependent renal failure (OR = 5.51), and hemodynamic instability (OR = 3.97). Thus, eg, patients in shock before operation had odds of dying in the hospital during or after isolated AVR which were 8.68 times the odds of patients who were not in shock before operation, assuming all other risk factors in Table 3 were identical. The C statistic was very good (0.809).

Table 4 presents the significant independent risk factors for isolated MVR and for MVR plus CABG operation. There were six significant risk factors for isolated MVRs: age of at least 55 years, carotid disease, shock, congestive heart failure in the same admission, dialysis-dependent renal failure, and endocarditis. The most significant risk factors were shock (OR = 9.17), dialysis-dependent renal failure (OR = 5.07), and endocarditis (OR = 4.28). For MVR with CABG operation, three of the risk factors (congestive heart failure in the same admission, dialysis-dependent renal failure, and endocarditis) were no longer independent significant predictors of mortality, and three new risk factors (ejection fraction < 30%, hemodynamic instability, and extensively calcified ascending aorta) were significant. The most significant risk factors for MVR with CABG procedure were shock (OR = 5.29) and hemodynamic instability (OR = 3.40). The respective C statistics were 0.823 and 0.718.

	Comment

Top Abstract Introduction Patients and methods Results Comment Acknowledgments References

 
The New York State Department of Health initiated a clinical cardiac surgery registry in 1989. Although the CSRS has been used extensively to identify significant risk factors for CABG surgical procedures and to employ multivariate statistical models containing these risk factors to assess risk-adjusted outcomes for CABG operations and to disseminate the information to providers and the public, little has been done with the valve procedures in the registry. This study, which is based on valve procedures performed between 1995 and 1997, is a formal effort to conduct a population-based study of the significant risk factors for all patients who undergo each of six different types of valve operations (comprising more than 90% of all valve operations performed in New York). As demonstrated in other studies, the data show that the operative (inpatient) mortality rates vary widely according to the specific valve procedure performed, ranging from 3.33% for isolated AVR to 18.72% for multiple valvuloplasty or valve replacement with CABG operation.

The study to which this study is most comparable is the recent national study by Jamieson and colleagues [1] based on The Society of Thoracic Surgeons (STS) database. It is most similar to the present study by virtue of being relatively recent (1986 to 1995) and because it is based on a very large sample size (roughly 75,000 patients in the six categories investigated here). The in-hospital mortality rates reported by the STS study and this study for each of the six valve procedure groups are also quite similar. For isolated AVR and isolated MVR, the mortality rates were 3.3% and 6.2% in this study, compared with 4.3% and 6.4% in the study by Jamieson and colleagues [1]. When CABG operation was combined with each of these procedures, the respective rates were 7.1% and 12.8% in this study versus 8.0% and 15.3% reported by Jamieson and coworkers. For multiple valve replacement with and without CABG operation, the respective rates were 9.6% and 18.7% in this study versus 9.6% and 18.8% in the study by Jamieson and coworkers.

Thus, in general, the mortality rates reported in the two studies are quite similar, although they tend to be lower in this study. This is due in part to the fact that this study is more recent. Another possible reason is that we were unable to separate AVR plus CABG from aortic valvuloplasty plus CABG, and to separate multiple valvuloplasty plus CABG from multiple replacement plus CABG (because of the available procedure codes in the registry). To the extent that the valvuloplasties are associated with lower mortality rates, this could introduce a bias. However, it is unlikely that numerous valvuloplasties were performed in conjunction with CABG operations.

When comparing the mortality rates reported by the two studies, it is also important to recall that the STS database is both voluntary and unaudited. The similarities in mortality rates and distributions of procedures in the two studies suggest that both studies are reasonably representative of the experience of all patients who undergo valve replacements in the United States.

In total, there were 18 independent preoperative risk factors that were identified in one or more of the six statistical models that were developed to predict mortality for the six groups of valve patients. Odds ratios for those risk factors ranged from 1.05 to 50.12.

The most significant risk factors for AVR, with or without CABG operation, were shock and dialysis-dependent renal failure. All other significant factors being equal, patients in shock had odds of dying in the hospital which were approximately nine times as high as patients not in shock regardless of whether CABG procedure was undertaken. The OR for renal failure was 5.51 for AVR patients without CABG and 3.17 for patients with CABG. Other risk factors that were significant for both of the AVR models were age of at least 55 years, extensively calcified ascending aorta, and pulmonary artery systolic pressure of at least 50.

For MVR with or without CABG operation, shock was again the most significant risk factor, although it had a much higher OR for isolated replacement (9.17) than for replacement with CABG operation (5.29). For patients without CABG operation, dialysis-dependent renal failure was again the second most important risk factor, but it was not significant for patients undergoing MVR with CABG. Instead, hemodynamic instability had the second highest OR (3.40). Common risk factors for MVR patients with and without CABG operation were shock, age of at least 55 years, and carotid disease.

For patients undergoing multiple valvuloplasty or multiple valve replacement with or without CABG operation, there were three common risk factors: age of at least 55 years, aortoiliac disease, and diabetes. The most significant risk factor for multiple valve patients without CABG procedure was dialysis-dependent renal failure (OR = 9.37), followed by renal failure without dialysis and aortoiliac disease (both with OR = 3.55). Shock was not a significant risk factor. The most significant risk factor for multiple valve patients with CABG operation was shock, with an enormous OR of 50.19, followed by hepatic failure (OR = 8.21). It should be noted that only ten of the patients undergoing multiple valvuloplasty or valve replacement with CABG operation experienced preoperative shock, and nine of them died.

In summary, age of at least 55 years was the only risk factor that was a significant multivariate predictor of mortality for all six groups of valve operation patients. As mentioned earlier, this means that the risk increases with each year after age 55, but is relatively constant for persons 55 and younger, all other significant factors being equal. It is important to note that although the ORs for age were fairly close to 1, this is merely an artifact of reporting them for an increment of 1 year of age. For instance, an OR of 1.05 for odds of a person at age n + 1 dying versus a person of age n dying is equivalent to an OR of 1.63 for two people whose ages differ by 10 years.

Shock was a significant predictor for five of the six groups, and in each of those groups it was the risk factor with the highest OR. Dialysis-dependent renal failure was the most significant risk factor in the other group (multiple valvuloplasty or valve replacement) and was the second most significant risk factor in three of the other groups. Extensively calcified ascending aorta, hemodynamic instability, diabetes, and congestive heart failure were all significant for at least three of the groups, and all but diabetes were significant for a procedure in each of the three major subdivisions (AVR, MVR, and multiple valve operations).

Other studies have also found many of these risk factors to be significant independent risk factors for valve operations. Numerous studies reported on significant risk factors for AVR [1–11] and many of these studies found renal failure, age, NYHA class, emergent procedures, and peripheral vascular disease to be significant predictors for AVRs. Of the many studies that reported significant risk factors for MVRs [19–23], commonly reported risk factors were NYHA class, age, shock, ejection fraction, and coronary artery disease. Jamieson and colleagues [1], whose grouping of procedures was used in this study, found that salvage status had the highest OR for five of the six groups, and that dialysis-dependent renal failure had the highest OR for the other group and the second highest OR for four of the other groups. Other highly significant risk factors found by these authors were emergency status, nondialysis-dependent renal failure, reoperations, and NYHA class IV.

Comparing the risk factors reported in this study with the ones from other studies, it should be noted that salvage status, emergency status, and NYHA class were not available in our study, but salvage status and emergency status are correlated with shock and hemodynamic instability. Also, congestive heart failure is defined in our study as NYHA class III or IV.

It is notable that several patient risk factors were added to New York’s cardiac surgery registry specifically for valve operation patients: endocarditis, cardiac index, pulmonary artery systolic pressure, cardiomegaly, and measures of valvular stenosis and insufficiency (none, mild, moderate, severe) for each valve. Of these measures, only endocarditis (for isolated MVR) and pulmonary artery systolic pressure (for isolated AVR with or without CABG operation) were significant risk factors in any of the six statistical models. Thus, eg, although valvular stenosis and insufficiency tend to be associated with very different crude mortality rates for valve patients, there was no significant mortality difference between patients with valvular stenosis and valvular insufficiency after adjusting for other important risk factors.

	Acknowledgments

Top Abstract Introduction Patients and methods Results Comment Acknowledgments References

 
We thank Kenneth Shine, MD, Chair of New York State’s Cardiac Advisory Committee (CAC), and the remainder of the CAC for their encouragement and support of this study. We also thank Donna Doran, Rhonda O’Brien, MA, and the cardiac surgery departments of the 33 participating hospitals for their tireless efforts to ensure the timeliness, completeness, and accuracy of the registry data.

	References

Top Abstract Introduction Patients and methods Results Comment Acknowledgments References

 

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