Ann Thorac Surg 2009;88:1224-1231. doi:10.1016/j.athoracsur.2009.05.085
© 2009 The Society of Thoracic Surgeons
Original Articles: Adult Cardiac
Management of Mild Aortic Stenosis at the Time of Coronary Artery Bypass Surgery: Should the Valve Be Replaced?
Basar Sareyyupoglu, MDa,
Thoralf M. Sundt, III, MDa,*,
Hartzell V. Schaff, MDa,
Maurice Enriquez-Sarano, MDb,
Kevin L. Greason, MDa,
Rakesh M. Suri, MD, DPhil (Oxon)a,
Harold M. Burkhart, MDa,
Soon J. Park, MDa,
Joseph A. Dearani, MDa,
Richard C. Daly, MDa,
Thomas A. Orszulak, MDa
a Division of Cardiovascular Surgery, Mayo Clinic, Rochester, Minnesota
b Division of Cardiology and Cardiovascular Diseases, Mayo Clinic, Rochester, Minnesota
Accepted for publication May 27, 2009.
* Address correspondence to Dr Sundt, Mayo Clinic, 200 First St SW, Joseph 5th Floor, Cardiovascular Surgery, Rochester, MN 55905 (Email: sundt.thoralf{at}mayo.edu).
Presented at the Poster Session of the Forty-fifth Annual Meeting of The Society of Thoracic Surgeons, San Francisco, CA, Jan 26–28, 2009. Winner of the Blue Ribbon as the top Adult Cardiac Poster.
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Abstract
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Background: General consensus favors aortic valve replacement (AVR) for patients with moderate aortic stenosis (AS) undergoing coronary artery bypass graft surgery (CABG); however, the management of similar patients with mild AS is controversial. We therefore investigated such patients to determine incremental risk of concomitant AVR, progression of AS among those undergoing CABG alone, and operative risk of AVR after prior CABG.
Methods: Between January 1993 and December 2003, 316 consecutive patients with mild AS (mean gradient >15, <30 mm Hg) underwent CABG only (107) or CABG plus AVR (209). Follow-up was obtained by review of the medical record, the Social Security Death Index, and postal questionnaire.
Results: The operative mortality was 3.7% for CABG only and 4.3% for CABG plus AVR (p = 1). Survival at a mean of 5.4 ± 3.6 years was similar. Multivariate predictors of late mortality included comorbid illnesses (Charlson comorbidity score and age-weighted summary of diseases; p = 0.001), small body surface area (p = 0.001), low ejection fraction (p = 0.007), preoperative permanent pacemaker (p = 0.04), and congestive heart failure (p = 0.046), but not AVR. Twenty-three CABG-only patients (21%) underwent subsequent AVR (mean 5.6 ± 1.8 years) without mortality. Aortic valve replacement at the time of initial CABG (p < 0.001) and older age (p = 0.02) were multivariate predictors of freedom from reoperation.
Conclusions: Prophylactic AVR for mild AS at CABG does not confer a survival benefit, and the likelihood of requiring AVR after CABG alone is low in the first 5 years. The decision to intervene on the valve is critically dependent upon the incremental operative risk imposed by concomitant AVR and late survival.
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Introduction
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As the age at which patients coming to coronary revascularization increases, the question whether to intervene on incidental aortic valve disease presents itself more frequently. While there is little debate about concomitant aortic valve replacement (AVR) when aortic stenosis (AS) is severe (mean gradient >50 mm Hg), the management of patients with mild or moderate AS is controversial. Initial debate over the management of moderate AS focused on the risk of reoperation, with early reports of high operative risk for AVR after prior CABG marshaled as support for an aggressive approach to AVR at initial CABG [1–3]. More recent studies suggesting that the risk of reoperation for AVR approaches that of primary AVR plus CABG have shifted the debate to the incremental risk of AVR and the likelihood of gradient progression [4]. Current American College of Cardiology/American Heart Association (ACC/AHA) guidelines take a middle ground, stating that AVR is "reasonable" (class IIa indication) for patients undergoing CABG with moderate AS (mean gradient 30 to 50 mm Hg) [5]. For patients with less severe degrees of AS, however, data are scant [1, 6, 7], and the ACC/AHA guidelines give a class IIb indication for concomitant AVR.
The critical issues in deciding for or against concomitant AVR at the time of CABG include the following: the incremental risk of concomitant AVR at the time of primary CABG, the likelihood that AS left untreated will progress requiring reoperative AVR during the patient's lifetime, the mortality of reoperation, and the burden of prosthetic valve disease including risks of endocarditis, early degeneration, and anticoagulant-related complications. We sought to investigate these risks among patients with coronary artery disease and mild AS in our practice over a 10-year interval.
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Material and Methods
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Patient Selection
Approval was obtained from the Mayo Clinic Institutional Review Board; only patients giving research authorization were included. Patients were identified using the computerized database for the division of cardiothoracic surgery at Mayo Clinic, Rochester, Minnesota. In the interest of distinguishing between patients coming to surgery principally for coronary revascularization considered for incidental AVR and patients coming to AVR having incidental CABG, we included only patients with involvement of the left anterior descending artery. While there is debate over the optimal measure of valvular stenosis, we used mean gradient rather than valve area to be consistent with ACC/AHA guidelines. Patients with more than moderate aortic regurgitation were excluded.
We reviewed 316 consecutive patients between January 1993 and December 2003 with low gradient AS (
15, 
30 mm Hg), of whom 209 underwent CABG plus AVR, and 107 CABG only. The age-weighted Charlson index of comorbidities [8] was calculated based on 17 disease states (older age, acquired immunodeficiency syndrome, cerebrovascular disease, chronic pulmonary disease, congestive heart failure, connective tissue disease, dementia, hemiplegia, leukemia, malignant lymphoma, myocardial infarction, peripheral vascular disease, ulcer disease, diabetes mellitus, liver disease, renal disease, and malignant solid tumor). The primary study endpoints were death and reoperation for AVR.
Follow-Up
Follow-up was obtained by reviewing clinical records and postal survey questionnaires. Social security indexes were used to confirm deaths. The mean follow-up was 5.4 ± 3.6 years and maximum was 14.9 years. Seven patients (2%) were lost to follow-up.
Statistical Methods
Statistical analysis was performed with the SAS program (SAS Institute, Cary, NC); descriptive statistics for categorical variables are reported as frequency and percentage while continuous variables are reported as mean (standard deviation) or median (range) as appropriate. Categorical variables were compared using the 
2 test, and continuous variables were compared using the two-sample t test or Wilcoxon rank sum test where appropriate. The Kaplan-Meier method was used to create survival curves and calculate 5-year and 10-year survival statistics. Cox regression models were used to deternime univariate and multivariate predictors of survival, time to AVR, and survival free from reoperation. The multivariable model considered variables with p less than 0.10 in univariate analysis, with model selection using the stepwise method (backward and forward methods resulted in the same model). All statistical tests were two-sided with the alpha level set at 0.05 for statistical significance.
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Results
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Comparison of Surgical Groups
Demographics were similar between groups apart from history of myocardial infarction, which was more common among patients undergoing the combined procedure (Table 1). Patients with bicuspid valves were also more likely to undergo AVR. Those undergoing concomitant AVR also had somewhat higher transvalvular gradients and lower valve areas.
Cross-clamp and bypass times were longer among those having concomitant AVR (Table 2). Associated procedures were more common among the CABG plus AVR group, including 17 mitral valve repairs, 13 mitral valve replacements, 10 patent foramen ovale closures, 9 septal myectomies, 6 composite root replacements, 5 ascending aortic aneurysm repairs, 3 hemiarch repairs, and 2 tricuspid valve repairs. Conversely, in the CABG-only group the mean number of bypass grafts was greater. In the CABG-only group, associated procedures included 6 mitral valve repairs, 2 mitral valve replacements, 1 ascending aortic aneurysm repair, 1 tricuspid valve repair, and 1 pericardiectomy. Among 10 patients who underwent CABG alone, the aortic valve was inspected visually; any in whom the valve was instrumented were excluded.
Morbidity was greater for AVR plus CABG. Reexploration and blood product usage were greater and the length of hospital stay was longer for the CABG plus AVR patients. Despite this, operative mortality was similar (3.7% for CABG only and 4.3% for CABG plus AVR, p = 1).
Late Survival and Reoperation
Survival for all patients was 88% at 1 year, 63% at 5 years, and 37% at 10 years. Importantly, there was no difference in late survival between those who had CABG only and those who had concomitant AVR (Fig 1). Multivariate predictors of late death included greater Charlson index, smaller body surface area, lower ejection fraction (EF), preoperative pacemaker, and preoperative congestive heart failure (Table 3); AVR at CABG was not a predictor of survival.
In a subset analysis to examine the relationship between survival and aortic valve hemodynamics in patients having CABG only, neither valve gradient or aortic valve area was predictive of mortality in the multivariate model. Older age (p < 0.001, hazard ratio [HR] = 2.03), preoperative congestive heart failure (p = 0.004, HR = 2.32), higher Charlson index, (p < 0.007, HR = 1.14), preoperative pacemaker (p = 0.007, HR = 4.46), and elevated preoperative creatinine (p = 0.028, HR = 1.19) predicted late death.
Reoperation for AVR was more frequent in the CABG-only group (Fig 2). By multivariate analysis, the risk of requiring reoperation was inversely related to older age and previous AVR (Table 4). Reoperation for AVR was time related, with freedom from reoperation 100% in the first year, declining to 91% at 5 years and to 48% at 10 years (Fig 3). Survival free of reoperation for any cause was 90% at 1 year, declining to 58% at 5 years and to 11% at 10 years. The hazard function for reoperation peaked at 5 to 6 years (Fig 4).
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Fig 2. Freedom from aortic valve reoperation, coronary artery bypass graft (CABG)-only group (solid line) versus aortic valve replacement (AVR) plus CABG group (dashed line).
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Fig 3. Reoperation-free survival, coronary artery bypass graft (CABG)-only group (solid line) versus aortic valve replacement (AVR) plus CABG group (dashed line).
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Fig 4. Reoperation/hazard rate per time period. (Aortic valve replacement = dotted line; coronary artery bypass graft surgery = solid line.)
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Twenty-three patients (21%) in the CABG-only group underwent subsequent AVR with no early mortality. The mean interval between CABG and reoperation for AVR was 5.6 ± 1.8 years. At reoperation, the mean aortic valve gradient was 51.4 ± 12.2 mm Hg, and average AVA was 0.80 ± 0.19 cm2. Importantly, there was no difference in transvalvular gradient at the index operation between patients requiring late AVR (mean 19.5 ± 3.4 mm Hg and AVA was 1.40 ± 0.22 cm2) and patients who did not (mean gradient of 18.8 ± 3.9 mm Hg and AVA 1.36 ± 0.32 cm2). The mean gradient increased by 5.5 mm Hg per year in patients coming to reoperation. Only older age (p = 0.033, HR = 0.69) and aortic valve inspection at time of CABG (p = 0.038, HR = 3.9) were significant variables related to reoperation for AVR; initial aortic valve mean gradient and AVR were not predictive of late reoperation.
Aortic valve replacement at the time of CABG was not entirely protective of subsequent reoperation. Among patients having AVR, 7 (3%) required reoperation for prosthetic valve related complications including prosthetic valve endocarditis in 5, acute thrombus formation on valve in 1, and leaflet perforation in another. Three of these 7 died perioperatively. No patient undergoing CABG plus AVR had subsequent redo CABG.
Postoperative Functional Status and Valve-Related Complications
Data concerning functional status were available through postal survey in 74 patients (80%) in the CABG plus AVR group and 31 patients (66%) in the CABG-only group, with a mean follow-up of 7.1 ± 2.8 years after operation. Freedom from valve-related complications (stroke, thrombus, major bleeding, and endocarditis) are presented in Figure 5. There were no statistically significant differences (p = 0.56), with freedom from valve-related complications collectively 99% at 1 year, 93% at 5 years, and 81% at 10 years. Of 13 CABG plus AVR patients with valve-related complications, 6 had stroke, 6 had major bleeding, 1 had endocarditis, and 1 had a clot. In the CABG group, 2 patients had major bleeding events. There was also no difference in late congestive heart failure. In the CABG-only group, 90% were in class III–IV preoperatively, improving to 35% postoperatively (p = 0.001); in the CABG plus AVR group, 88% were class III-IV preoperatively, improving to 40% postoperatively (p < 0.001). There was no significant difference in the number of patients remaining in class III or IV (p = 0.62) nor was there a difference in echocardiographic EF as assessed preoperatively versus 1 year postoperatively (p = 0.28); (CABG-only mean preoperative EF 55.1 ± 14.5, first year EF 54.6 ± 13.4; in CABG plus AVR group mean preoperative EF 49.1 ± 16.5, first year EF 51.5 ± 13.5). Quality of life data were not collected preoperatively, rendering late postoperative quality of life information uninterpretable.
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Fig 5. Freedom from valve-related complications (presented as composite endpoints of stroke, valve thrombosis, bleeding, and endocarditis). (Aortic valve replacement = dotted line; no aortic valve replacement = solid line.)
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Selection of Valve Type
Seventy-two percent of patients undergoing concomitant AVR had tissue prosthesis. Survival at 10 years was 53% for recipients of mechanical and 32% for tissue valves, although this did not achieve statistical significance in the univariate (p = 0.06) or multivariate model (p = 0.35). In our practice, patients receiving tissue valves receive aspirin unless warfarin is otherwise indicated. Patients with mechanical valves or atrial fibrillation receive anticoagulation therapy to an international normalized ratio of 2.5 to 3.5.
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Comment
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The cumulative lifetime mortality risk for patients with incidental aortic valve disease coming to coronary revascularization depends on the incremental risk of AVR plus CABG versus CABG only, the risk associated with reoperation if valve replacement is required, the likelihood of progression of AS if the valve is left alone, and the burden of "prosthetic valve disease." The results of this study indicate that, while the incremental risk associated with concomitant AVR in our practice is low, the current risk associated with reoperation for AVR after CABG is likewise low. Furthermore, patients coming to surgical coronary revascularization with mild AS (mean gradient <30 mm Hg) have a low likelihood of reoperation for AVR during the first 5 years postoperatively. Younger patients and those in whom the surgeon was tempted to replace the valve as reflected by their choice to visually inspection the valve are more likely to come to late AVR after CABG despite similar transvalvular gradients. The significance of valve exploration is likely a manifestation of prescient clinical judgment. Finally, the morbidity associated with prosthetic valve disease, including reoperation for repeat AVR, is significant. Overall, concomitant AVR had no impact on late survival. Taken together, these data support a selective approach to the management of mild aortic valve stenosis at the time of CABG, particularly if life expectancy is limited.
The low incremental risk of incidental concomitant AVR in our series is consistent with prior studies [1]; however, the addition of AVR to CABG should not be trivialized. If off-pump CABG is planned, AVR transforms the surgical plan. Furthermore, despite individual reports, the mortality rate reported to the Society for Thoracic Surgeons database for CABG is 2% to 3%, whereas that for AVR plus CABG is 6% to 7% [9, 10]. While the latter includes patients with more severe aortic stenosis as well as those with active endocarditis, it is likely that the addition of AVR adds some risk. This may be particularly the case in cardiac surgical practices performing fewer AVR procedures [11, 12]. Beyond mortality, morbidity in our study was greater among the CABG plus AVR patients, as was the length of hospital stay. This may be of particular significance for the very elderly or those with ascending aortic calcification, impaired renal function, or severe chronic obstructive pulmonary disease for whom an off-pump strategy is favored [13].
The risk of AVR after prior CABG was felt prohibitively high in the seminal study by Collins and coworkers [3]. In their study of 44 patients undergoing AVR after prior CABG between 1975 and 1992, the operative mortality rate was 18.2%. These findings were supported by a study from our own institution in which the operative mortality rate was 12% [2]. In contrast, among patients operated on between 1985 and 1996 at Washington University [4], the operative mortality was 7.4% for AVR after prior CABG as compared with 6.3% for primary AVR plus CABG (p = not significant). These differences may be due to surgical era, as similar improvements have been reported from the Brigham and Women's Hospital between 1992 and 2001 [14]. Reoperative risk would no longer appear an argument for concomitant AVR.
Perhaps the most difficult challenge is predicting the progression of AS among patients for whom valve replacement is withheld. Although progression is more rapid in older patients and in those with calcific disease as compared with rheumatic aortic or congenital aortic stenosis [15], competing causes of death diminish the likelihood that AS will progress sufficiently to become symptomatic [9]. Smith and associates [9] addressed these competing risks using a Markov decision model. They suggest that, for patients with progression of AS at the "average rate" of 5 mm Hg per year, age and gradient are the critical determinants. For example, while AVR is recommended for patients under the age of 70 if the peak (not mean) gradient is greater than 25 mm Hg, it is recommended for those over age 85 only if the peak gradient exceeds 50 mm Hg. A heuristic such as this is, however, is useful only if the rate of progression is known; unfortunately, the rate of progression of AS is notoriously variable. It is more rapid, however, when leaflet calcification is more marked [16], suggesting that noninvasive measures of the degree of calcification may prove a useful tool [17–21]. Conversely, the progression of AS may slowed by medical treatment in hyperlipidemic patients [20, 22–24]. Others have reported predictors of more rapid progression of AS to include left ventricular hypertrophy, greater than 30 mm Hg mean transvalvular gradient, moderate valve calcification with moderate to severely decreased valve motion, and systemic atherosclerotic vascular disease [6, 9]. Aortic valve disease may also progress rapidly in patients with chronic renal failure on hemodialysis [25]. Unfortunately, the early mortality of combined CABG plus valve replacement in dialysis patients has been reported to be as high as 39%, with survival at 1 year only 59% and that at 5 years, 21% [26].
Finally, AVR imposes the burden of prosthetic-valve–related complications [27, 28], including a risk of anticoagulant-related hemorrhage or thromboembolism of 2% to 4% a year if a mechanical prosthesis is chosen [29]. Bioprostheses risk valve deterioration, and both options impose the risk of prosthetic endocarditis. Indeed in our series, 7 patients in the combined group required reoperation for valve-related complications with a high operative mortality.
Limitations of Study
Although both study groups appeared comparable as assessed by most clinical characteristics, the aortic valve disease in the CABG plus AVR group was somewhat greater and EF lower. We did not perform a matched comparison as numbers would be small. There also remains potential for systematic bias with AVR performed in patients thought clinically to have more severe AS or other unmeasured covariates. Furthermore, transvalvular gradient alone is an imperfect gauge of the degree of obstruction, particularly in the presence of impaired left ventricular function. Finally, in the interest of maximizing useful data, we included patients who had associated procedures. While including these patients is a potential confounder, fewer than 25% of patients in each group had associated procedures, and the observed mortality rates were very similar between groups.
Conclusions
The decision to replace the aortic valve in patients with mild aortic stenosis undergoing CABG is critically dependent upon the incremental institutional operative risk imposed by prophylactic AVR as well as on the expected rate of progression of valve stenosis and late survival. It is our practice to perform AVR for mild aortic stenosis at the time of CABG selectively in patients judged to have low operative risk and for whom life expectancy is more than 5 years. We are inclined to replace valves with significant calcification and, if tempted to inspect the valve now, have a low threshold to proceed to replacement.
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Abstract
Introduction
