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Ann Thorac Surg 2000;70:1939-1945
© 2000 The Society of Thoracic Surgeons

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Original article: cardiovascular

Mortality after aortic valve replacement: results from a nationally representative database

^a Center for Devices and Radiological Health, Food and Drug Administration, Rockville, Maryland, USA

Accepted for publication April 26, 2000.

Address reprint requests to Mr Astor, Welch Center for Prevention, Epidemiology and Clinical Research, The Johns Hopkins University, 2024 East Monument St, Baltimore, MD 21205
e-mail: bastor{at}jhsph.edu

	Abstract

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
Background. Nationally representative estimates of in-hospital mortality after aortic valve replacement are needed to evaluate whether results from The Society of Thoracic Surgeons National Cardiac Surgery Database are applicable to other institutions in the United States performing these procedures.

Methods. Data from the 1994 Nationwide Inpatient Sample were used to estimate the patient characteristics and in-hospital mortality rates associated with aortic valve replacements performed in nonfederal hospitals in the United States. Procedural and hospital characteristics were examined for possible associations with in-hospital mortality.

Results. An estimated 46,397 aortic valve replacements were performed. In-hospital mortality occurred in 4.3% of first-time isolated aortic valve replacements and 6.4% overall. The highest quartile of procedure-specific hospital volume, compared with the lowest quartile, was associated with lower in-hospital mortality (adjusted odds ratio, 0.58; 95% confidence interval, 0.42 to 0.81).

Conclusions. The in-hospital mortality rates observed in this study are very similar to those reported from The Society of Thoracic Surgeons database. These data provide substantial evidence that results from The Society of Thoracic Surgeons database are representative of those achieved at other institutions. However, procedure-specific hospital volume must be considered in applying these results to individual institutions.

	Introduction

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
Replacement of the native aortic valve with a mechanical or bioprosthetic valve has been performed for four decades and is the treatment of choice for most patients with symptomatic aortic valve disease in the United States. As the US population ages, the incidence of aortic valve disease, and thus the number of prosthetic aortic valve implantations, is expected to increase [1]. Concomitantly, changes are expected in the distribution of clinical and demographic characteristics of prosthetic aortic valve recipients, including increases in the mean age of patients and the proportion of patients undergoing concurrent coronary artery bypass graft (CABG) or replacement of a previously implanted prosthetic valve [2, 3]. These changes in patient characteristics may have a significant impact on the rates of perioperative mortality after aortic valve replacements observed by clinical centers in the United States.

Published studies of mortality after aortic valve replacement have important limitations. Many studies report a single large center’s experience over an extended period of time, often more than 10 years [4–9]. Although these reports are necessary to assess long-term valve survival and incidence of valve-related complications, the perioperative results reported may not be representative of current practice or be generalized to other centers. Also, many reports do not include results from operations to replace existing prosthetic valves [4, 10]. In contrast, The Society of Thoracic Surgeons (STS) National Cardiac Surgery Database [11] reports results from multiple institutions and includes data on repeat heart valve replacements and valve procedures combined with CABG. However, these data are also from a self-selected subset of the institutions performing these procedures, and how representative these participating hospitals and patient population are has been questioned [12]. The effects of hospital characteristics on mortality after aortic valve replacements that may bias the results of this database are not known.

The present study characterizes the patient population and examines risk factors for in-hospital mortality in a nationally representative sample of both first-time and repeat aortic valve replacement operations in the United States. These results provide a benchmark useful for tracking future changes in demographic and clinical characteristics of patients and rates of in-hospital mortality. Hospital characteristics that may have an impact on in-hospital mortality after aortic valve replacement are examined, and the patient characteristics and in-hospital mortality rates observed in this sample are compared to those reported from the STS database. These comparisons allow evaluation of whether results from the STS database can be generalized to other institutions in the United States.

	Material and methods

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
Data source
Data were obtained from the 1994 Nationwide Inpatient Sample (NIS), a part of the Healthcare Cost and Utilization Project (HCUP-3) which was sponsored by the Agency for Healthcare Research and Quality (AHRQ) [13]. The NIS is a stratified probability sample of more than 900 nonfederal hospitals in 17 states participating in HCUP-3 and is designed to approximate a 20% national sample. Administrative data were collected from all discharges by each participating hospital, compiled by state data organizations, and edited and checked for invalid entries by the AHRQ. The sampling frame consisted of all nonfederal hospitals in the participating states as identified by the American Hospital Association Annual Survey of Hospitals. Sampling probabilities were proportional to the number of community hospitals in each stratum based on geographical region of the United States (northeast, north central, south, or west), location (urban or rural), teaching status, ownership/control (government controlled, private nonprofit, or private for-profit), and bed size (small, medium, or large, the definitions of which varied by location and teaching status). Due to the inconsistent definition and for computational efficiency, strata were collapsed on bed size. Weights based on the sampling probabilities for each stratum in the dataset were assigned to each sampled hospital to allow projection to the universe of all nonfederal hospitals in the United States [14].

All hospital discharges with a procedure code indicating implantation of a bioprosthetic (code 35.21) or mechanical (code 35.22) aortic valve, as defined in International Classification of Diseases, 9th revision, Clinical Modification (ICD-9) [15], were extracted from the NIS. Selected additional procedures, including replacements of other valve positions (codes 35.23 to 35.28) and coronary artery bypass graft (codes 36.10 to 36.19), occurring during the same hospital stay were also identified. Patients less than 18 years of age were excluded. We divided the patient population into quartiles (ie, low, medium-low, medium-high, and high volume) based on the unweighted volume of aortic valve replacements performed during 1994 at the hospitals from which the patients were discharged.

Diagnoses were identified by ICD-9 diagnosis codes. The following diagnoses were specifically identified, with the ICD-9 codes in parentheses: diabetes mellitus (250), rheumatic valve disease (394 to 397), hypertensive disease (401 to 405), ischemic heart disease (410 to 414), cardiomyopathy (425), cardiac conduction disorders (426), cardiac dysrhythmias (427), heart failure (428), chronic renal failure (581 to 583 and 585 to 589), cerebrovascular disease (430 to 438), and failure of a previously implanted heart valve (996.02, 996.61, and 996.71). Up to 15 procedures and 15 diagnoses were coded for each stay. Hospital admission status was coded as emergent, urgent, or elective. A dummy variable was used to code missing values of hospital admission status (n = 164 records). The results were unchanged in analyses that excluded records with missing data.

Data analyses
All analyses were performed by weighting the response variables by the sampling weights. Age- and sex-specific as well as total estimates of the number of prosthetic aortic valve implantations and in-hospital mortality rates were calculated, as were descriptive statistics of patient and hospital characteristics. Age- and sex-specific population projections for 1994 based on data from the 1990 US Census were used to estimate rates of implantation [1]. Bivariate tests of independence were performed by corrected ² tests [16]. Multivariate logistic regression models were used to identify independent predictors of in-hospital mortality. Those comorbid conditions found to be associated with increased mortality in the bivariate analyses were included in the multivariate models. We constructed separate models that included all procedures, only procedures with concurrent CABG, only procedures without concurrent CABG, and only first-time isolated aortic valve replacements without concurrent CABG. Similar models were constructed after stratification by hospital teaching status. The reported confidence intervals (CI) are based on weighted estimates of variance adjusted for the correlation of responses within each hospital (ie, clustering by hospital). All CIs reported are 95% confidence. Data extraction from the NIS database was performed using SAS (Cary, NC) [17], and analysis was performed using Stata (College Station, TX) [18].

	Results

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
Patient and procedural characteristics
Among the 6,385,011 hospital discharges in the NIS database, 8,741 were identified as including replacement of an aortic valve in an adult (>18 years) patient. These procedures were performed at 176 hospitals. Of these hospitals, 173 (98.3%) were in an urban (as opposed to a rural) location, 103 (58.7%) were teaching hospitals, 140 (79.7%) were private nonprofit, 24 (13.4%) were government-controlled, and 12 (7.0%) were private for-profit. Hospital procedure-specific volume during 1994 was categorized as low (60 or fewer procedures), medium-low (61 to 100 procedures), medium-high (101 to 180 procedures), or high (more than 180 procedures). Weighted projection to all nonfederal hospitals in the United States resulted in an estimated total of 46,397 (95% CI 40,005 to 52,789) aortic valve replacements in adult patients during 1994. All subsequent results are based on weighted estimates. The rate of aortic valve replacement increased significantly with age up to 75 to 79 years, peaking at 0.95 and 1.80/1,000 for women and men, respectively (Fig 1). The rate of replacement decreased significantly for ages 80 years and older. The mean age of aortic valve recipients was 67.3 years (Table 1). This varied significantly by the type of aortic valve received (72.5 years for those receiving a tissue valve versus 66.1 years for those receiving a mechanical valve; p < 0.001). An estimated 5.9% of patients underwent reoperations with replacement of a previously implanted prosthetic heart valve. Mechanical failure was noted in 63% of these reoperations, and infection was noted in 15%. Urban hospitals performed 98.5% of the aortic valve replacements, and one-half were in teaching hospitals. Private nonprofit hospitals accounted for 82.2% of the hospitalizations, whereas 11.6% were in government-controlled hospitals and 6.2% were in private for-profit hospitals. Diagnoses and comorbid conditions coded for these hospitalizations included ischemic heart disease (51.3%), cardiac dysrhythmia (46.0%), heart failure (37.8%), hypertensive disease (32.7%), rheumatic valve disease (21.6%), diabetes mellitus (12.3%), cardiac conduction disorders (10.3%), cerebrovascular disease (6.8%), cardiomyopathy (2.9%), and chronic renal failure (2.0%).

View larger version (23K): [in this window] [in a new window]   Fig 1. Estimated rate of aortic valve replacement (per 1,000) from the 1994 Nationwide Inpatient Sample, by age and sex.

View larger version (23K): [in this window] [in a new window]   Fig 2. Estimated in-hospital mortality after aortic valve replacement in the 1994 Nationwide Inpatient Sample, by age and sex.

In-hospital mortality by procedure
In-hospital mortality for procedures without concurrent CABG varied from 4.3% for isolated replacements of a native aortic valve to 15.2% for reoperations involving multiple valves (Table 2). The rates for procedures involving concurrent CABG were higher for every category, ranging from 7.6% (95% CI 6.7 to 8.6) for first-time aortic valve replacements to 25.3% (95% CI 8.2 to 42.3) for reoperations involving multiple valves.

	Comment

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
These results provide the first nationally representative estimates of the number and characteristics of adult patients undergoing aortic valve replacement in the United States. The aging of the US population is expected to have a significant impact on the number and distribution of demographic, clinical, and procedural characteristics of aortic valve recipients in the future. Some of these changes have already been observed in large registries [3, 19] and specific institutions [20]. The distribution of patient characteristics observed in this study will be useful as a comparison for future data to track expected changes in these characteristics among prosthetic aortic valve recipients in the United States.

This study also provides nationally representative estimates of in-hospital mortality associated with aortic valve replacement in the United States. Direct comparison of these results to previous reports is clouded by the difference in the definition of risk used in the present study (ie, in-hospital mortality) and the definition used elsewhere (ie, operative mortality, which includes both in-hospital deaths and postdischarge deaths occurring within 30 days of the procedure) [21]. This difference would tend to bias the comparisons toward slightly lower rates in the present study. Nonetheless, the mortality rates observed in the present study are comparable to those reported recently by the STS National Cardiac Surgery Database. For example, Jamieson and colleagues [19] report a 30-day mortality rate of 4.3% in patients undergoing an isolated aortic valve replacement from 1986 through 1995 in the STS database, based on 26,317 procedures. The present study estimates the in-hospital mortality rate at 4.5% (95% CI 3.8 to 5.4) for the same procedure. Combined CABG and isolated aortic heart valve replacement increased the 30-day mortality to 8.0% in the STS database (n = 22,713), similar to the 8.0% (95% CI 7.0 to 9.0) in-hospital mortality rate estimated for the same procedure in the present study. For procedures involving multiple valves, the STS data report 9.6% operative mortality for procedures without concurrent CABG (n = 3,840), compared to 8.8% (95% CI 6.6 to 11.5) in-hospital mortality in multiple valve procedures among aortic valve recipients in this report. The present study limits this category to those procedures involving the aortic position in addition to any other position, whereas the STS results do not exclude those procedures involving only other (ie, nonaortic) multiple valve positions. Procedures involving multiple nonaortic valve positions are likely associated with significantly higher mortality.

Patient populations and hospital characteristics may be quite different between institutions that voluntarily participate in a professional organization’s database and the remaining community hospitals performing specific procedures. In addition, as the STS operates the National Cardiac Surgery Database as a voluntary system, there is no systematic assurance that all eligible patients undergoing aortic valve replacement at participating centers are entered. Even with these potential differences, the results reported from the NIS and the STS database are very similar. This suggests that the institutions participating in the STS database are representative of all institutions performing aortic valve replacements in the United States, and that the patients entered into the database are representative of all patients undergoing these procedures. Thus, the results from the STS database can be applied to other institutions in the United States that perform these procedures but do not participate in the STS database. It should be noted, however, that the results referenced from the STS database include procedures from 1986 through 1995, whereas the present study used data from 1994. In the STS database, significant declines in perioperative mortality among patients undergoing CABG procedures were observed in this same time period [12], and declines may also be expected to have occurred in other procedures, including aortic valve replacement. However, with the rapid growth in the number of surgeons and institutions participating in the STS database during this same time, a 12-fold increase occurred in the number of patient records entered between 1990 and 1994 [12], weighting the results heavily toward the later years of this interval. The relevance of results from the STS database to nonparticipating institutions is further evidenced by comparison of the present study population with the 11,987 aortic valve recipients entered into the STS database in 1994 in terms of several demographic and procedural characteristics, including age (67.3 versus 67.6 years), female sex (38.1 versus 39.2), and concurrent CABG (42.9 versus 44.6) [22]. More detailed comparisons of clinical characteristics are encumbered by the present study’s reliance on coded diagnoses to capture comorbidities, whereas the STS data collection forms specifically request this information.

We found a significant association between higher procedure-specific hospital volume and lower in-hospital mortality in adjusted analyses. When limited to first-time isolated aortic valve replacements without concurrent CABG, the highest quartile of hospital volume, as compared to the lowest quartile, was associated with an adjusted odds ratio of 0.39 (95% CI 0.23 to 0.64). This association was found among both teaching and nonteaching hospitals. Although there was a significant trend of decreasing in-hospital mortality across increasing quartiles of hospital volume, the second and third quartiles of hospital volume did not differ significantly from the lowest quartile. Higher hospital volume has been associated with lower mortality in other surgical procedures [23–26]. This association may be due to the experience of the physicians and staff, differences in case-mix based on characteristics not included in our analyses, or referral of patients to centers with better results. Due to the lack of detailed clinical, procedural, and institutional data, the NIS database is not well suited for investigating possible causes for this relation. However, the present study demonstrates the need to explore the association between hospital volume of aortic valve replacements and lower mortality in datasets with more detailed information. Surgical and postoperative procedures practiced by hospitals with more experience may be instructional for other institutions. Policies regarding regional resources may also be significantly affected by results of such studies. The results of the present study also point out that hospital volume must be considered in applying results from multicenter databases to individual institutions. There was a trend toward higher in-hospital mortality in teaching hospital as compared to nonteaching hospitals in the present study. Because of assumed differences in case-mix between teaching and nonteaching hospitals, a more detailed risk stratification algorithm than was available in this database may explain part or all of this association.

In the present study, receipt of a bioprosthesis, as compared to a mechanical valve, was associated with a higher rate of in-hospital mortality in unadjusted analyses. A weaker association was found after adjustment for other factors, and no significant association was found in a model limited to first-time isolated aortic valve procedures without concurrent CABG (adjusted odds ratio, 1.16; 95% CI 0.76 to 1.81). These results suggest that differences in patient characteristics, such as comorbid conditions that we did not include in our models, may explain the nonsignificant increase in odds of in-hospital mortality among recipients of bioprosthetic valves. Most previous studies have found no difference in acute mortality results between patients receiving bioprosthetic and mechanical valves after adjustment for clinical characteristics [9, 27].

Female sex was a significant predictor of in-hospital mortality (adjusted odds ratio, 1.33; 95% CI 1.12 to 1.59). This result is similar to that reported by the STS National Database for operative mortality, which found adjusted odds ratios associated with female sex of 1.25 in isolated aortic valve replacements and 1.61 in combined aortic valve replacement plus CABG [19]. Several other studies have found similar results [2, 3, 28, 29], although some smaller studies found female sex to be associated with reduced mortality [9]. Several studies have examined the higher periprocedural and late mortality in women after CABG and other cardiac procedures [30, 31], and numerous reasons for this disparity have been proposed (eg, referral bias, smaller vessel diameter) with no obvious conclusions. Unfortunately, the current study can only provide additional evidence of the existence of this disparity following aortic valve replacement.

The large size and representative nature of the data are obvious strengths of this study. However, the study is limited by the lack of detailed clinical information in the NIS. This precludes adjustment for measures of disease severity (eg, cardiac functional class) and some significant comorbid conditions. As all of the diagnoses are abstracted from discharge summaries, it cannot be determined whether a specific condition was present before the valve replacement or was a result of such operations. Thus, conditions that may be the result of cardiac operations cannot be analyzed as possible preoperative predictors of in-hospital mortality.

	Acknowledgments

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
We thank Josef Coresh, MD, PhD, for his insight and advice on the preparation of this manuscript.

	References

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 

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