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Ann Thorac Surg 2002;74:1459-1467
© 2002 The Society of Thoracic Surgeons
a Division of Cardiology, University of Michigan, Ann Arbor, Michigan, USA
b Duke Clinical Research Institute, Durham, North Carolina, USA
c Division of Cardiothoracic Surgery, University of Florida Health Science Center, Jacksonville, Florida, USA
d Division of Adult Cardiac Surgery, University of Michigan, Ann Arbor, Michigan, USA
Accepted for publication April 25, 2002.
* Address reprint requests to Dr Mehta, Division of Cardiology, Department of Internal Medicine, University of Michigan, 2215 Fuller Rd, 7E 111A, Ann Arbor, MI, 48105, USA.
e-mail: rmehta{at}umich.edu
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Abstract |
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METHODS: A population of 31,688 patients from The Society of Thoracic Surgeons National Cardiac Database undergoing mitral valve replacement either alone or in combination with coronary artery bypass grafting or tricuspid surgical procedures from 1997 to 2000 was examined to assess age-related variation in clinical features, morbidity, and mortality. Multivariable logistic regression was used to determine the effect of age after adjusting for other known risk factors. A classification tree was used to identify low-risk elderly (
75 years) patients.
RESULTS: Operative mortality increased four-fold from 4.1% in patients aged less than 50 years up to 17.0% in patients aged 80 years or more. Similarly, major operative complications (stroke, prolonged ventilation, reoperation for bleeding, renal failure, and sternal infection) also increased with age, rising from 13.5% (age < 50 years) to 35.5% (age 80 years). Multivariable adjustment attenuated the odds of operative mortality, but age remained a significant risk factor. After adjusting for other patient risk factors, age accounted for 13% and 10% of the explainable risk for mortality and morbidity, respectively. Among the elderly, four variables (hemodynamic instability, New York Heart Association class IV, renal failure, and concomitant coronary artery bypass grafting) were identified to distinguish levels of risk, from operative mortality rates exceeding 31% to those with 7.7% mortality.
CONCLUSIONS: Operative mortality and morbidity rise with increasing age of patients undergoing mitral valve replacement. Although this excess risk is partially a result of increased comorbid burden and other operative factors, age remains an independent powerful risk factor for operative risk for mitral valve replacement. Understanding the relationship of age with other risk factors for mitral valve replacement can help stratify risk, enabling physicians to identify lower risk patients.
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Introduction |
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The purpose of the current study was to provide comprehensive information on the association of age with clinical presentation, care, and outcomes of patients undergoing mitral valve replacement (MVR) across North America. In addition, we sought to examine to what extent the excess mortality with increasing age is explained by age-related trends in preexisting comorbidities, differences in clinical presentations, and variations in the patterns of care. Finally, if age is truly an independent predictor of operative mortality, we wondered whether we could identify the elderly patients at low risk (compared with those at high risk) of operative mortality.
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Material and methods |
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TopAbstractIntroductionMaterial and methodsResultsCommentAcknowledgmentsReferences |
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Patient population
We analyzed data on all patients undergoing MVR either alone or in combination with CABG or tricuspid valve operation from January 1, 1997, through December 31, 2000, who were enrolled in the STS database. We included those patients receiving either mechanical or bioprosthetic valve replacement. Patient records were excluded from the study if they had mitral valve repair instead of replacement, if they had an associated procedure other than CABG or tricuspid valve operation, or if they had missing age or sex information. As tricuspid valve disease requiring concomitant operation along with MVR is almost always a result of mitral valve disease, these patients were not excluded from the analysis. Of the 262,718 patients in the STS database undergoing mitral valve operation during this time, 45 had missing age or sex information, 230,985 received mitral valve repair or an associated procedure other than CABG or tricuspid valve operation, and the remaining 31,688 met the criteria for this study.
Data definitions
We defined operative mortality as (1) all deaths occurring during the hospital period in which the operation was performed; and (2) those deaths occurring after hospital discharge, but within 30 days of the procedure, unless the cause of death is clearly unrelated to the operation. Morbidity was defined as the presence of any one of the following complications: permanent stroke, renal dysfunction or renal failure requiring dialysis, prolonged ventilation (>48 hours), reoperation for bleeding, or deep sternal wound infection. For the purpose of this analysis, we used standard definitions of risk factors as provided in the STS Web site.
Statistical methods
For display purposes we categorized patients undergoing MVR into five age-based strata: less than 50 years (n = 4,315), 50 to 59 years (n = 5,037), 60 to 69 years (n = 8,472), 70 to 79 years (n = 11,144), and 80 years or older (n = 2,720). Patient characteristics, operative details, and unadjusted outcomes for each age category were tabulated as frequencies and percentages for categorical variables, mean and standard deviation, or median and interquartile range for continuous variables. For modeling purposes, we used the median value to replace missing values for continuous variables, whereas the most prevalent value was used to replace missing categorical values. Each model variable was missing less than 10% of the time with the exception of ejection fraction (14% missing) and New York Heart Association (NYHA) class (17.5% missing). Next, we estimated the risk-adjusted effect of age on mortality and morbidity. On the basis of clinical evaluation and the current STS valve models, we identified 24 potential risk factors other than age for predicting mortality and morbidity after MVR. Stepwise logistic regression was then used to determine the final set of risk factors. To this model we added age (as a continuous variable) to determine whether age was an independent predictor of mortality after adjusting for these risk factors. Age is modeled as a two-piece linear spline with a flat slope until age 50 and a linear slope after age 50. A similar method was used to examine the risk-adjusted effect of age on morbidity and the combined outcome of mortality or morbidity. The c-index was calculated to evaluate model discrimination.
To determine the relative influence of age in explaining patient risk, we estimated the ratio of the reduction in variation using all risk factors except age to the reduction in variation when age was included as a risk factor (the total explainable risk) and subtracted this value from one. The value of minus twice the maximized log likelihood was used to determine reduction in variation.
Finally, we examined interactions between age and other risk factors in the models. To determine whether the impact of age varied according to overall risk, we tested for an interaction between risk score and age. In addition, we divided patients into three risk groups of equal size based on their predicted risk from the logistic model that did not contain age and graphed mortality rates by age group for low-risk, medium-risk, and high-risk patients.
Finally, we used a classification tree to identify characteristics among elderly patients (75 years) that allowed for identification of MVR patients at low risk of operative mortality. The classification tree is a method whereby the data are successively split in a manner that maximally distinguishes the response variable in the left and right branches [21]. All analyses were performed using SAS 8.2 (SAS Institute Inc, Cary, NC).
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Results |
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2 statistics along with their corresponding odds ratios, 95% confidence interval, and p values. Age was entered into the initial model (without age) as a continuous variable. After adding age, the c-index increased from 0.76 to 0.78. Figure 1
indicates that the risk of operative mortality after adjustment for other baseline variables still increases with increasing age. In fact, comparison of the models with and without age as a covariate revealed that 13% of the variation in mortality explained by the covariates was related to age, making it the most predictive variable in the model(s). Similarly, 10% of variation in morbidity explained by covariates was related to age alone (model and data not shown).
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75 years) with hemodynamic instability had the highest risk of operative death (31.9%). Patients without any of the four risk factors constituted approximately one third of the elderly population and had a substantially lower mortality rate of 7.7%.
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Comment |
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Age and clinical characteristics
We found significant differences in clinical features with increasing age. The elderly (75 years) population represents a significant proportion of patients that undergo MVR (25%) in North America, a reflection of the growing elderly population and changing patient and physician attitudes to the health care of the elderly in this region. Thus, physicians may be less reluctant to recommend MVR in older subjects and older patients may also be more willing to accept this recommendation in the current era of advances in cardiac surgery. On the other hand, this may also reflect the greater incidence of mitral regurgitation seen with increasing age or a greater proportion of younger patients undergoing mitral valve repair instead of replacement [22]. There was a female preponderance among all age groups with the highest proportion in the youngest group, probably as a result of rheumatic mitral valve disease that predominantly affects women in this age group. More African Americans received MVR at a younger age than at an advanced age. This may be a result of greater frequency of mitral valve disease among younger African Americans secondary to higher incidence of rheumatic fever or infective endocarditis among them than among whites. Alternatively, the greater reluctance to accept surgical procedure, shorter lifespan, and less frequent access to health care may all contribute to fewer older African Americans receiving MVR.
Not surprisingly, most comorbid conditions such as hypertension, diabetes, renal failure, myocardial infarction, and peripheral and cerebrovascular disease increased with aging. The advanced arteriosclerosis noted in other vascular beds (peripheral and cerebral) was also evident in the coronary circulation with a greater proportion of the older patients showing three-vessel or left main coronary artery disease. Despite more cerebrovascular disease in the older cohorts, the proportion of patients with prior history of stroke was similarly distributed between the young and the old patients. A greater risk of atrial fibrillation in patients of all age groups with mitral valve disease may explain the similarity in the risk of stroke among all age groups. Presentation with heart failure, cardiogenic shock, NYHA functional class IV, and prior cardiac resuscitation, all shown to be predictors of increased mortality in patients undergoing MVR, increased with aging. As a result of these adverse hemodynamic features, the older patients were more likely to require support with inotropic agents or intraaortic balloon pump in the perioperative period.
Age and operative factors
Prior studies have not examined the influence of age on the operative factors in patients undergoing MVR. Our study revealed that the proportion of patients requiring emergent or urgent operation increases with aging. This is not surprising as older patients are more likely to present with greater hemodynamic instability (congestive heart failure, cardiogenic shock, or NYHA functional class IV) compared with the younger group. Similarly, because of more diffuse arteriosclerosis that also involves the coronary artery, more older patients require concomitant CABG than the younger cohort. The higher incidence of pulmonary hypertension, resulting either from mitral valve disease or diastolic dysfunction, as well as the higher frequency of degenerative process that involves the tricuspid valve with advancing age may explain the greater need for concomitant tricuspid valve operation. As a result of a greater need for concomitant CABG and tricuspid valve operation with increasing age, the median pump time and cross-clamp time increased with aging, a factor that in itself may add to the increased age-related risk of morbidity and mortality. The greater risk of anticoagulation (necessary for preventing dysfunction of the mechanical prosthetic valves) in the elderly explains the preference for bioprosthetic valves for the elderly undergoing MVR [23]. In contrast, the extremely low rate of structural deterioration of mechanical prosthetic valves and the accelerated degeneration of bioprosthetic valves (particularly in younger patients) account for predominant use of mechanical valves in the younger cohorts [23].
Age and outcomes
Our study is consistent with the findings of prior studies that showed age to be associated with increasing in-hospital complications and mortality [24–28]. Thus, permanent stroke, requirement for prolonged ventilation, reoperation for bleeding, and renal failure as well as mortality increased significantly with aging. The risk of atrial arrhythmias increased 2.2-fold in the oldest patients as compared with the youngest group. The length of hospital stay paralleled the increase in the in-hospital fatal and nonfatal adverse events, such that older patients were more likely to be hospitalized longer than the younger age group. The greater in-hospital adverse event rates and longer hospitalization translate into greater resource utilization and thus have tremendous implications in terms of health-care costs.
Our study also attempted to determine whether the increase in comorbid conditions with age accounted for the higher age-related morbidity and mortality. We found that even after adjustment for differences in patients’ baseline clinical characteristics, age remained a strong independent predictor of increased morbidity and operative mortality. Thus, although all of us would like to think that a vigorous, healthy 80-year-old has the same chance of surviving mitral valve operation as a 50-year-old with similar clinical characteristics, our findings do not support this supposition. In fact, multivariable logistic regression analysis with and without age as a covariate identified age to be the most important predictive variable in the model, accounting for 13% of the variation explained by the covariates in the model. Importantly, however, we have identified four decision rules to help physicians define an elderly patient’s risk of mortality after MVR (Fig 3). Although patients who are hemodynamically unstable had a one in three chance of dying, elderly patients who are hemodynamically stable, do not have renal failure, are not NYHA class IV, and do not require concomitant CABG have a mortality rate that is less than the average for 60-year-old to 69-year-old patients. The group of patients without these four risk factors make up 32% of the elderly population. Thus, our study helps identify a significant portion of the elderly population that has a relatively low risk of mortality after MVR.
It is conceivable that a possible reason for age remaining associated with outcomes after adjustment is that important factors associated with both age and outcomes, known (such as functional status) or unknown, were not included and adjusted for in this study. Similarly, accounting for age-related abnormalities of diastolic compliance, vascular resistance, arterial wall thickness, hemorheologic changes, coagulability, fibrinolysis, endothelial dysfunction, or a host of other factors may eventually eliminate or markedly reduce the relationship between age and mortality. Although adjustment for these factors, which are unavoidable concomitants of aging, may remove association of age with mortality per se, such adjustment would have little meaning in assessing the risk of all patients undergoing MVR.
Clinical implications
Should age be used as a criterion for offering or withholding MVR because of its important prognostic implications in patients undergoing MVR? Not to consider age as an important factor makes little sense, but to consider it alone, in the absence of detailed patient-related data, would be equally unwise. Although increasing age is often accompanied by an increased prevalence of advanced disease states, this study demonstrates that the risk factors affect survival in all age groups. Age alone is only one component of operative risk. Even among the octogenarians operative risk varies by five-fold from the healthy (low-risk) elderly to those who are very sick (high-risk, Fig 2).
Physicians should therefore discipline themselves to look beyond a patient’s age to consider his or her medical condition and the potential for benefit versus the risks. Needless to say, even though older patients have increased morbidity and mortality, these risks are not prohibitive (7 of 8 patients age 80 years or older survived MVR, and 2 of 3 in this age group survived hospitalization after MVR without any complications). Thus, the findings of our study should be used for understanding and estimating patients’ risk based on their clinical factors as well as their age. This information should be used for counseling patients and their families and helping them understand their perceived risk in relation to their age as well as associated comorbid conditions so that they may have a more realistic expectation in terms of outcomes. This should not necessarily be used to deny MVR solely based on a patient’s age. Our study allows for identification of elderly patients at low risk for operative mortality for whom physicians should not hesitate to recommend MVR, if indicated, despite their older age.
Limitations
Our study has some limitations. Data in the STS database were obtained through voluntary participation of different institutions. However, the degree of completeness of the database is high (with less than 5% missing on most model variables). Additionally, a comparison of results from the STS National Database with those in audited databases (New York State and the Northern New England Cardiac Database) revealed similar patient risk factor profiles and similar mortality and morbidity results [29]. Long-term mortality and other outcome domains such as quality of life and functional status after MVR were also not considered in this study, meriting future studies to address these issues. Finally, patients undergoing mitral valve repair were excluded, thus limiting the application of our study findings to this group of patients.
Conclusions
We conclude that age is strongly associated with all factors that adversely affect the prognosis of patients undergoing MVR and as such is associated with increased in-hospital complications and operative mortality in these patients. Although a part of this excess risk is related to increased comorbid burden and other operative factors, age remains an independent powerful predictor of operative risk in patients undergoing MVR. Understanding the relationship of age with other risk factors for MVR can help stratify risk, enabling physicians to identify lower risk patients. Future studies are needed to understand the specific mechanisms underlying this increased risk with aging so as to evaluate specific newer therapeutic strategies that may help attenuate this age-related risk.
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Acknowledgments |
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TopAbstractIntroductionMaterial and methodsResultsCommentAcknowledgmentsReferences |
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