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Ann Thorac Surg 2002;74:1098-1106
© 2002 The Society of Thoracic Surgeons

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Original article: cardiovascular

Results of bioprosthetic versus mechanical aortic valve replacement performed with concomitant coronary artery bypass grafting

^a Cardiac Surgical Unit, Department of Surgery, Massachusetts General Hospital, Boston, Massachusetts, USA

Accepted for publication May 29, 2002.

* Address reprint requests to Dr Akins, Department of Surgery, White 503, Massachusetts General Hospital, 55 Fruit St, Boston, MA 02114 USA
e-mail: cakins{at}partners.org

	Abstract

Top Footnotes Abstract Introduction Patients and methods Results Comment Acknowledgments Appendix 1. Factors entered... Appendix 2. Method of... References

 
Background. Concomitant coronary artery disease with aortic valve disease is an established risk factor for diminished late survival. This study evaluated the results of bioprosthetic (BAVR) or mechanical aortic valve replacement (MAVR) performed with coronary artery bypass grafting (CABG).

Methods. From January 1984 through July 1997, combined AVR + CABG was performed in 750 consecutive patients; 469 received BAVR and 281 received MAVR. BAVR recipients were significantly older (mean age, 75 vs 65 years), and had more nonelective operations, congestive heart failure, peripheral vascular disease, preoperative intraaortic balloons, lower cardiac indices, more severe aortic stenosis, less aortic regurgitation, and more extensive coronary artery disease.

Results. Early complications included operative mortality, 32 patients (4.3% total: 3.8% BAVR and 5.0% MAVR); perioperative infarction, 10 (1.3%); and perioperative stroke, 22 (2.9%). Significant multivariable predictors of early mortality were age, perioperative infarction or stroke, nonelective operation, operative year, ventricular hypertrophy, and need for intraaortic balloon. Ten-year actuarial survival was 41.7% for all patients. Predicted survival for age- and gender-matched cohorts from the general population versus observed survival were BAVR, 45% versus 36%; MAVR, 71% versus 48% (survival differences BAVR 9% vs MAVR 23%, p < 0.007). Significant multivariable predictors of late mortality included age, congestive failure, perioperative stroke, extent of coronary disease, peripheral vascular disease, and diabetes. Valve type was not significant. Ten-year actuarial freedom from valve-related complications were (BAVR vs MAVR) structural deterioration, 95% versus 100%, p = NS; thromboembolism, 86% versus 84%, p = NS; anticoagulant bleeding, 93% versus 88%, p < 0.005; reoperation, 98% versus 98%, p = NS.

Conclusions. AVR + CABG has diminished late survival despite the type of prosthesis inserted. Although valve type did not predict late mortality, mechanical AVR was associated with worse survival compared with predicted and more valve-related complications due to anticoagulation requirements.

	Introduction

Top Footnotes Abstract Introduction Patients and methods Results Comment Acknowledgments Appendix 1. Factors entered... Appendix 2. Method of... References

 
Several studies have compared the long-term results of bioprosthetic versus mechanical valves for all patients requiring aortic valve replacement [1–6]. Additional studies have documented that concomitant coronary artery bypass grafting is an incremental risk factor for late mortality when it is performed in association with aortic valve replacement [7–10]. Some investigators have therefore advocated lowering the age for using bioprostheses for aortic valve replacement in patients who require concomitant myocardial revascularization [7].

The current study was performed to determine the early and late results of the selection of a bioprosthetic or mechanical aortic valve for implantation in patients who also required concomitant coronary artery bypass grafting.

	Patients and methods

Top Footnotes Abstract Introduction Patients and methods Results Comment Acknowledgments Appendix 1. Factors entered... Appendix 2. Method of... References

 
Patients
A computerized registry of cardiac surgical patients at the Massachusetts General Hospital was used to identify 750 consecutive patients having combined aortic valve replacement and coronary artery bypass grafting from January 1984 through July 1997. Because the principal focus of the study was the long-term consequences of prosthesis selection, the study was closed at that time to allow a follow-up period of at least 2 postoperative years for every patient in the study. Records of 750 consecutive patients were retrospectively reviewed by trained research personnel for demographic information, clinical and catheterization findings, operative characteristics, and results.

Definitions
Operative mortality was defined as any death occurring within 30 days of the operation if the patient had been discharged from the hospital, or any death occurring during the hospitalization for the operation.

Unstable angina pectoris was defined as either: (1) new onset of angina that is rapidly progressive in frequency and severity; (2) rapid acceleration of an anginal pattern that was previously stable and related to exertion; or (3) severe recurrent chest pain or a bout of intense pain resembling an acute myocardial infarction but without evolution of infarction by electrocardiogram or cardiac enzymes.

Urgent operations were defined as operative procedures performed in patients whose accelerated symptoms prompted urgent hospital admission for evaluation and who were judged to be too unstable to discharge before operative intervention. True emergency operations were defined as procedures performed in patients whose cardiovascular instability required an operative intervention that was outside of normal operating hours or displaced another patient on the normal surgical schedule. The predominant causes of nonelective operations were congestive heart failure that could be managed only with intravenous medications and unstable angina that required intravenous heparin and nitroglycerin.

Perioperative myocardial infarction was defined as either a new Q wave or the elevation of the myocardial fraction of creatine kinase in association with persistent ST segment changes or a new conduction abnormality.

Transient ischemic attack was defined as a neurologic deficit lasting less than 1 hour. Reversible ischemic neurologic deficit was defined as a neurologic deficit lasting more than 1 hour but that resolved by the time the patient was discharged from the hospital. Stroke was defined as a neurologic deficit lasting through the time of hospital discharge.

Postoperative gastrointestinal complications included gastrointestinal bleeding requiring transfusion, pancreatitis, cholecystitis, or other gastrointestinal problems that required operative intervention. Sternal wound infections were defined as infections that required operative intervention. Postoperative renal failure was defined as new renal failure requiring dialysis. Pulmonary complications were defined as pneumonia, empyema, or adult respiratory distress syndrome.

Valve-related events and complications were defined according to the published "Guidelines for Reporting Morbidity and Mortality After Cardiac Valvular Operations" [11].

Follow-up
Follow-up clinical information about survival and subsequent cardiac events was obtained between February 1999 and February 2000 through direct communication with the patients. If subsequent hospitalization, death, or cardiac events had occurred, the patient’s physician or appropriate hospital record department was contacted to document the events.

Of the 750 patients in the study, 718 (95.7%) survived hospitalization. Of those 718 patients, 2 were partially lost and 6 completely lost to follow-up; thus, follow-up was 99% complete. Mean follow-up was 5.7 years (bioprosthetic valve patients, 5.1 years; mechanical valve patients, 6.8 years). Total follow-up was 4,290 patient-years (2,377 patient-years for bioprosthetic recipients and 1,913 patient-years for mechanical valve recipients).

Statistical analysis
To separate the impact of prosthetic valve choice, we divided the patient population into two groups: (1) bioprosthetic valve replacement = 469 patients, and (2) mechanical valve replacement = 281 patients.

To assess the predictors of hospital death, multiple risk factors were inserted into a stepwise logistic regression algorithm, BMDP program PLR [12]. Forward stepping was used with the Hosmer-Lemeshow test for goodness of fit of the logistic model checked at each step. Candidate predictor variables were entered into the logistic model when the associated F-to-enter statistic was significant at the p = 0.05 level. Factors assessed as predictors of events are listed in Appendix 1.

To describe the long-term results, actuarial curves for mortality were obtained by the life-table method [13]. Predictors of late events were estimated using the Cox Proportional Hazards model as a stepwise regression in program P2L. Means are stated ± SD, and differences are considered statistically significant only when the two-tail probability of the test statistic is <0.05.

Actuarial survival rates for age- and gender-matched cohorts from the general population were calculated utilizing projected mortality rates from the Statistical Abstract of the United States [14]. (Appendix 2)

	Results

Top Footnotes Abstract Introduction Patients and methods Results Comment Acknowledgments Appendix 1. Factors entered... Appendix 2. Method of... References

 
Some important demographic and clinical risk factors are listed in Table 1. Bioprosthetic valve recipients were significantly older, had smaller body surface areas, and more peripheral vascular disease.

The distribution of prosthetic types inserted according to patient age is demonstrated graphically in Figure 1. Although relatively more mechanical valves were inserted in younger patients and relatively more bioprostheses in older patients, there was overlap of both prosthetic types in all age groups.

View larger version (34K): [in this window] [in a new window]   Fig 1. Type of aortic valve prosthesis implanted according to patient age at the time of operation.

View larger version (15K): [in this window] [in a new window]   Fig 2. Actuarial survival with 95% confidence intervals of patients after (A) bioprosthetic or (B) mechanical aortic valve replacement with coronary artery bypass grafting (dotted line) compared with predicted survival for age- and gender-matched controls from the general United States population (solid line).

	Comment

Top Footnotes Abstract Introduction Patients and methods Results Comment Acknowledgments Appendix 1. Factors entered... Appendix 2. Method of... References

 
Several published studies have compared the long-term results of the decision to insert a bioprosthetic or mechanical valve in patients who require aortic valve replacement [1–6], but most of the patients in those studies had isolated valve replacement. Other studies have documented the deleterious impact on late survival of concomitant coronary artery bypass grafting when required in conjunction with aortic valve replacement [7–10]. In 1994, Jones and colleagues documented the serious impact of associated coronary artery disease on mortality. They also advocated lowering the age at which insertion of a bioprosthesis seemed appropriate because the diminished long-term survival suggested that most patients with both aortic valve replacement and coronary artery bypass grafting would not live long enough to experience structural deterioration. The purpose of our study was, therefore, to assess the outcome of the choice of a bioprosthetic versus a mechanical aortic valve for patients who required concomitent myocardial revascularization.

In addition to the documentation of the deleterious impact of comcomitant coronary disease on patients requiring aortic valve replacement, a growing body of information suggests that patients with aortic valve disease severe enough to warrant replacement do not have a life expectancy that is equal to that for age- and gender-matched controls from the general population [15]. Aortic valve disease either predisposes a patient to early mortality despite valve replacement, or aortic valve replacement merely exchanges native aortic valve disease for prosthetic valve disease, or both.

That the mere addition of coronary artery bypass grafting to aortic valve replacement should have such an additional adverse impact on late survival is at first glance hard to understand. We have previously demonstrated that patients who have coronary artery bypass grafting have a survival that is essentially equivalent to age- and gender-matched cohorts from the general population up to 5 years after operation [16, 17]. Sergeant and associates demonstrated that the actuarial survival of elderly coronary artery bypass patients can actually be better than that predicted for the general population out to 10 years, whereas the survival of young coronary bypass patients is poorer than a comparable cohort from the general population [18]. That being the case, the addition of concomitant myocardial revascularization in this generally older population might be expected to minimally influence late mortality, but it obviously exerts a greater effect than supposed. The causes for this accelerated mortality rate are not clear, but may include the unfortunate combination of coronary artery disease with the left ventricular hypertrophy that often accompanies aortic valve disease, possibly resulting in increased complications associated with subendocardial ischemia.

By multivariable analysis, in this study valve type was not a significant predictor of late mortality. However, we did find a significant advantage between projected and observed survival for bioprosthetic recipients versus mechanical valve recipients. The verity of this observation is difficult to determine. Because significantly more mechanical valve recipients died of valve-related causes than bioprosthetic recipients, this survival difference may be a function of the prostheses. However, this result could just be a casualty of intense statistical scrutiny of two disparate populations. In fact, this observed difference could be influenced by the 10-year mean age difference between the two prosthetic groups, which could influence the results in two ways. First, given that the mean age of the bioprosthetic recipients was significantly higher at 75 years, the elevated overall mortality rate for these older patients may make the difference between observed and predicted survival less prominent than it would be for younger patients with a lower overall projected mortality rate. Second, the poorer survival of the younger mechanical valve cohort may be due in part to the documented poorer survival of younger coronary artery bypass patients compared with the general population, a finding confirmed by Sergeant and associates [18] and Lytle and colleagues [19]. We must conclude that some evidence in our study suggests a survival advantage for bioprostheses that is not substantiated by multivariable analysis.

In this study, the patients who received a mechanical prosthesis and who died during follow-up had significantly more cardiac- and valve-related causes for death than those patients who received bioprostheses. One explanation for this discrepancy is possibly that older patients, who more typically received bioprosthetic valves, developed more potentially fatal diseases of other organ systems. Conversely, one might wonder if mechanical valves might somehow contribute to more cardiac failure or myocardial ischemia. No mechanism for this is, however, obvious. One possibility is that mechanical valves might be associated with repeated coronary artery microthromboembolism, but there has never been documentation of this. An alternative explanation is that younger patients who require coronary artery bypass grafting have a more accelerated form of the coronary disease. This opinion is supported by the fact that the patients in this study who required coronary reintervention were younger than those who did not. Younger age at coronary artery bypass grafting is a documented predictor of need for repeat myocardial revascularization [20]. However, the mean interval between operation and coronary reintervention was actually longer for the younger mechanical valve recipients than it was for the older bioprosthetic recipients.

Not only did mechanical valve recipients have more cardiac causes of late death, they also required significantly more reinterventions for their coronary disease than did the bioprosthetic recipients. Additionally, reinterventions to treat coronary disease in both groups were much more common than reoperations for failure of the study prosthesis, emphasizing that in these patients the prime determinant of their outcome is their ischemic heart disease, not their aortic valve disease.

The valve-related complications reported in this study are consistent with identifying the principal shortcoming of mechanical valvular prostheses, namely the inherent thrombogencitiy and requirement for anticoagulation [21] versus the increased loss of structural integrity of bioprostheses [9]. Whereas freedom from thromboembolism was not significantly different (despite the fact that most bioprosthetic recipients were not on warfarin), mechanical valve recipients had statistically significantly more anticoagulant-related bleeding. Although bioprosthetic recipients had a statistically insignificant trend toward more structural dysfunction, freedom from reoperation was actually not different than that for mechanical valve patients. One possibility that structural deterioration of bioprostheses was not as commonly seen in this study could be that the mean age of the bioprosthetic recipients was over 75 years of age, and structural deterioration is uncommon out to 10 years in these patients [9]. However, enough bioprosthetic recipients were of younger ages at operation, suggesting that the increased late mortality rate in this study could have acted as a competing risk factor for demonstrating structural dysfunction.

Of the 15 patients who came to reoperation, that is, replacement of the study prosthesis, 8 were bioprosthetic recipients (reoperation rate, 1.8%), and 7 were mechanical valve recipients (reoperation rate, 2.6%). Thus, the anticipated increased risk of reoperation for bioprosthetic recipients was not demonstrated in this study. The absence of a significant difference in reoperation rates out to 10 years also suggests that fear of reoperation as a determinant of prosthesis selection is not warranted in all but the youngest patients.

The high mortality rate in the small number of reoperations in this series contrasts sharply with the much better results from our previous report concerning the risks associated with reoperation to replace a failed bioprosthetic valve [22]. In that series, the total mortality rate for replacing a failed aortic bioprosthesis in 153 patients was only 7.8% (4.6% for elective, isolated replacement of a failed aortic bioprosthesis). The difference is largely due to the fact that two-thirds of the reoperations in this study were caused by acute prosthetic endocarditis. Also of note is that in our previous study most of the patients who came to reoperation for a failed aortic bioprosthesis did not have associated coronary artery disease.

Limitations of the study
The results of this study are potentially compromised by the fact that the choice of bioprosthetic or mechanical valves was not randomized, but rather left to the preference of the surgeon. Ameliorating this effect to some degree is that there were not many significant differences between the two valve groups, except for patient age, number of bypass grafts, and priority of operation. The differences can also be in part accounted for with the logistic regression analysis.

The study also enrolls patients during 13 years, creating the possibility of an impact of changing surgical practices and patient management. Although operative year was a predictor of hospital death, both prosthetic types were used throughout the time of the study. Because this study focuses more on the late implications of prosthetic choice and because the in-hospital mortality rate is quite low, these time-related differences are not as important. Also, operative year was not a predictor of late mortality.

Because the study encompasses 13 years, there were several brands of both mechanical and bioprosthetic valves inserted. However, both mechanical and bioprosthetic groups contain comparable proportions of early and later generation prostheses, which should help to balance this difference.

Because the entrance criteria into the study is the operation performed, this study does not differentiate the patient whose indication for operation was significant aortic valvular disease with some associated coronary artery disease from the patient whose indication for operation was primarily advanced coronary artery disease with some associated aortic valvular disease. Yet this distinction can be difficult to make clinically because both lesions can produce angina pectoris or symptoms of congestive heart failure. Of more importance is the finding that the combination of the two lesions yields disappointing results.

The statistical power of this study goes only out to about 10 years. Longer follow-up might demonstrate different results. However, the late mortality rate is so high that most patients who require concomitant coronary artery bypass grafting with aortic valve replacement will probably not survive to demonstrate other significant nonfatal late differences between the prostheses.

Some investigators may feel that we erred by not including in this study the impact of propensity calculations related to valve choice by the surgeon. Because valve choice did not emerge as a predictor of late events, there is little chance that propensity scores would be of value. Actually, to be complete and in an attempt to see if propensity score could still affect results, we did perform propensity calculations after we knew that valve choice was not a predictor. (The factors that influenced a surgeon to select a bioprosthesis were age >70 years, manufacturer’s valve size greater than 20, operative year after 1990, larger aortic valve area at catheterization, and presence of peripheral vascular disease.) When propensity score to choose a bioprosthesis was inserted into the model for predicting late survival, valve type also entered the model as a significant predictor. However, propensity to choose a bioprosthesis was a significant predictor of better late survival but so was mechanical valve type (ie, the two factors cancelled each other out as predictors). This confirmed our initial belief that when valve type was not a significant predictor of survival, propensity score would not be of value.

Inferences
The results of this study confirm the deleterious impact of concomitant coronary artery disease on the long-term outcomes for patients who require aortic valve replacement. Although mechanical valve selection did not significantly influence late mortality by logistic regression analysis, patients who received a mechanical prosthesis had significantly worse survival compared with that predicted from normal population data. In addition, mechanical recipients had significantly more anticoagulant-related bleeding. The anticipated increase in structural dysfunction for bioprosthetic valves and the subsequent need for reoperation was not borne out by the results of this study.

These results seem to validate the suggestion of Jones and colleagues [7], that bioprosthetic valves are more appropriate for younger patients who require concomitant coronary artery bypass grafting along with aortic valve replacement. Most patients will not survive long enough to have to worry about structural deterioration, and in addition, will not have the increased risk of anticoagulant-related complications.

	Acknowledgments

Top Footnotes Abstract Introduction Patients and methods Results Comment Acknowledgments Appendix 1. Factors entered... Appendix 2. Method of... References

 
This study was supported by a grant from the John F. Welch/GE Fund for Cardiac Surgical Research. The authors wish to acknowledge the assistance of Barbara J. Akins, BSN, and Annetta L. Boisselle, BSN, for their help in data acquisition and management; John B. Newell, former director of the Cardiac Computer Center, Massachusetts General Hospital, for his assistance in the statistical evaluations; and Jerene M. Bitondo, PA-C, for her help with manuscript preparation.

	Footnotes

Top Footnotes Abstract Introduction Patients and methods Results Comment Acknowledgments Appendix 1. Factors entered... Appendix 2. Method of... References

 
Dr Akins discloses that he has a financial relationship with Medtronic, Inc.

	Appendix 1. Factors entered into univariate analysis of predictors of hospital mortality, perioperative infarction, perioperative stroke, and long-term survival

Top Footnotes Abstract Introduction Patients and methods Results Comment Acknowledgments Appendix 1. Factors entered... Appendix 2. Method of... References

 

Age Gender Body surface area Congestive heart failure New York Heart Association functional class Preoperative myocardial infarction Canadian Heart Association anginal class Peripheral vascular disease Hypertension Left ventricular hypertrophy (ECG criteria) Preoperative stroke Preoperative intraaortic balloon Diabetes mellitus Valvular disease (stenosis or regurgitation) Aortic valve gradient Extent of coronary disease Cardiac index Left ventricular ejection fraction Left ventricular end diastolic pressure Pulmonary artery systolic pressure Operative priority (elective or nonelective) Year of operation Type of prosthesis inserted (bioprosthetic or mechanical) Number of bypass grafts Use of internal mammary artery Intraoperative intraaortic balloon Perioperative myocardial infarction Perioperative stroke

	Appendix 2. Method of calculating the projected survival of age- and gender-matched cohorts from the general United States population

Top Footnotes Abstract Introduction Patients and methods Results Comment Acknowledgments Appendix 1. Factors entered... Appendix 2. Method of... References

 
A program was written that propagated the expected probability of surviving n years forward from age x to x + n by multiplying the n relevant probabilities from the Statistical Abstract of the United States [14], which was inferred from 1 - (expected deaths per 1,000)/1,000. This function of initial age, gender, and number of years, n, is C (initial age, gender, n). Given the initial average age, Ai, and the initial proportion of males, Pi, in each subpopulation (mechanical versus bioprosthetic recipients), the projected survival at n years for that subpopulation is:

S(n) was computed for n = 1 to 10 years for each subpopulation, then linearly interpolated to find survival at half-year (6-month) intervals. S(n) then is the matched American Projected Survival for each group.

	References

Top Footnotes Abstract Introduction Patients and methods Results Comment Acknowledgments Appendix 1. Factors entered... Appendix 2. Method of... 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 Author home page(s): Cary W. Akins Gus J. Vlahakes Thomas E. MacGillivray David F. Torchiana Joren C. Madsen Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Akins, C. W. Articles by Madsen, J. C. Search for Related Content PubMed PubMed Citation Articles by Akins, C. W. Articles by Madsen, J. C. Related Collections Valve disease