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Ann Thorac Surg 2002;74:1125-1131
© 2002 The Society of Thoracic Surgeons
a West Virginia University, Morgantown, West Virginia, USA
* Address reprint requests to Dr Murray, Dept of Surgery, PO Box 9238, HSC North, Morgantown WV 26506, USA.
e-mail: gmurray{at}hsc.wvu.edu
Presented at the Thirty-eighth Annual Meeting of The Society of Thoracic Surgeons, Fort Lauderdale, FL, Jan 28–30, 2002.
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Abstract |
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40) obesity on bypass operation outcomes using the Society of Thoracic Surgeons National Cardiac Database.
METHODS: The study population consisted of 559,004 patients from the Society of Thoracic Surgeons database who underwent first-time, isolated coronary artery bypass grafting between January 1997 and December 2000. We compared 42,060 moderately obese patients (BMI, 35 to 39.9) and 18,735 extremely obese patients (BMI 40) with 498,209 normal or mildly obese patients (BMI, 18.5 to 34.9). Multivariable logistic regression was used to determine whether BMI subgroups were independent predictors of operative risk after adjusting for other preoperative factors.
RESULTS: Compared with normal or mildly obese patients (BMI, 18.5 to 34.9), moderate and severely obese patients were younger and more likely to be diabetic and hypertensive. After adjusting for these and other known preoperative risk factors, moderate obesity slightly elevated patients’ operative risk (adjusted odds ratio, 1.21; confidence interval, 1.13 to 1.29). In contrast, extremely obese patients had marked higher risk for operative mortality (adjusted odds ratio, 1.58; confidence interval, 1.45 to 1.73). Major perioperative complications, particularly deep sternal wound infection, renal failure, and prolonged postoperative hospital stay also increased for extremely obese patients.
CONCLUSIONS: Extreme obesity (body mass index 40) is a significant independent predictor for adverse outcomes and prolonged hospitalization after coronary artery bypass operation.
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Introduction |
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The largest study to date is comprised of 11,101 patients from the Northern New England Cardiovascular Disease Study Group, and found that obesity did not significantly increase postoperative complications apart from sternal wound infection [6]. Our current study re-examines this question of obesity using the Society of Thoracic Surgeons (STS) database. This database’s large sample size provides the ability to examine cardiac surgical risks in several obesity subgroups.
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40 [1]. Mild obesity has become prevalent and formed the majority of patients undergoing coronary artery bypass grafting; it was therefore combined with the normal BMI group to allow comparison with the higher ends of the obesity scale (moderate and extreme obesity). Unadjusted mortality and morbidity rates were calculated and differences between the three BMI groups were tested using 
2 tests. To make risk-adjusted comparisons, the risk factors (excluding body surface area) contained in the multivariable logistic regression models recently developed by the STS for mortality, five major morbidities, prolonged length of stay, and the combined outcome of mortality or major morbidity were used. Variables representing moderate and extreme obesity were then added to each model. Odds ratios and 95% confidence intervals for the BMI groups were calculated to determine whether obesity was an independent predictor of outcome.
Finally we examined interactions between BMI and age and gender. The interaction was examined both graphically and by adding interaction terms to the multivariable logistic regression models.
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Results |
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40). Men formed 66.9% of the normal to mild BMI population, 61.7% of the moderate BMI group, and 52.5% of the BMI 40 group as shown in Table 1. The distribution of other risk factors for the three BMI groups is also provided in the table. Obese patients appeared to undergo operations at a younger age, but had a higher frequency of hypertension, and were more often diabetic. The preoperative incidence of cerebrovascular accident was slightly higher, and chronic lung disease slightly lower, whereas renal failure was similar. The preoperative ejection fraction and myocardial infarction rate in each group were essentially identical. The operative status (elective vs urgent or emergent) was similar for the three groups, as was perioperative use of the intraaortic balloon pump (Table 2). The extremely obese group had a slightly higher frequency of use of single and double graft procedures, whereas at the same time they had comparable cross-clamp and perfusion times. There was little difference in use of the internal mammary artery in the three groups studied; however, bilateral mammary artery use was notably higher in the normal and mildly obese subgroup.
The unadjusted operative mortality rates were highest among patients with extreme obesity 3.1% versus 2.6% for the normal to mild obesity group and 2.5% for the moderate obesity group (Table 3). Unadjusted rates of stroke and reoperation were actually highest among those with normal to mild obesity patients relative to extreme obesity patients. On the other hand, rates of renal failure, prolonged ventilation, and deep sternal wound infection progressively increased as BMI increased, achieving statistical significance.
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40 had adjusted OR for operative mortality of 1.42 (95% CI, 1.30 to 1.56). In patients older than 75 years of age, extreme obesity was somewhat rare, but still resulted in a trend toward higher adjusted OR for operative mortality relative to those with normal or mild obesity, adjusted OR of 1.15 (95% CI, 0.91 to 1.45).
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Comment |
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Obesity is a widely recognized risk factor for coronary artery disease, diabetes, stroke, hypertension, and certain cancers [1]. Obese individuals have been shown to have a 50% to 100% increased risk of death from all causes (mostly cardiovascular) compared with normal individuals [7, 8]. Altered pulmonary functions result from an increase in functional residual capacity and a decrease in vital capacity and maximum voluntary ventilation [5]. These factors have led to the presumption that obese individuals are at a higher risk for operative mortality and morbidity. However, this presumption has not been previously proven, despite a number of studies examining the influence of obesity on cardiac operations.
Whereas low BMI (< 20) has been related to increased mortality, renal failure, pneumonia, and reoperation for bleeding [9, 10], high BMI has not been shown to be at a higher risk in previous publications [6, 9, 11]. For example, the Northern New England Cardiovascular Disease Study Group reported that obesity was not associated with increased operative mortality or cerebrovascular accidents [6].
In our larger study, moderate obesity was associated with a slight but statistically significant increase in risk-adjusted increase in mortality (OR, 1.21; CI, 1.13 to 1.29) and those with extreme obesity had nearly a 50% increase in risk-adjusted mortality (Table 4). This high mortality risk for those with extreme obesity has not been evident in previous studies because of the small number of subjects with BMI of more than 40.
The incidence of cerebrovascular accidents was similar in both the obese and nonobese groups, which mirrored other reported studies [6, 9]. However, in contrast to previous publications, we found the incidence of renal failure increased markedly with increasing size of the patient (relative risk of 1.58 in the moderately obese and 1.92 in the extremely obese). The reason for the increased risk of renal failure is not entirely clear. The higher incidence of hypertension and diabetes in these patients could explain this exaggerated risk, with operations unmasking the renal dysfunction. The cardiopulmonary bypass times were not significantly longer in the obese patients. Further studies evaluating renal function in a prospective manner would be needed to better understand this higher incidence of renal failure.
Prolonged periods of ventilation were also seen in patients of both obesity subgroups (7.1% and 7.9% compared with 5.6% in the normal and mildly obese subgroup). This could contribute to the slightly increased length of postoperative hospital stay in the extremely obese group. Sternal wound complications have been higher for obese patients after cardiac operations according to most studies [2, 6, 12], but not all studies [2]. This higher incidence of wound complications has been seen in both superficial and deep sternal wounds, as well as leg wound incisions. For example, Prasad and colleagues’ [4] study compared 250 obese patients with 250 control patients, which showed an incidence of 9.2% of deep sternal infections for the obsese patients versus 2.8% for the nonobese. In Birkmeyer and colleagues’ [6] study, the incidence of deep sternal wound infection was shown to be higher in obese patients and was associated with age, renal failure, diabetes, chronic obstructive pulmonary disease, bilateral internal mammary artery grafting, and cardiopulmonary bypass time. Kouchoukos and colleagues [13] also identified bilateral internal mammary artery bypass grafting, obesity, and prolonged mechanical ventilation as significant risk factors for sternal infections by multivariate logistic regression analysis. Prolonged operative time in obese patients was also thought to have contributed to the higher incidence of wound complications [14].
In our patients the relative risk of deep sternal infections was 2.22 (95% CI, 2.01 to 2.45) in moderately obese patients and rose to 3.15 (95% CI, 2.79 to 3.55) in extremely obese patients. Of interest, the cardiopulmonary bypass times and cross-clamp times were nearly identical in the three subgroups. The use of internal mammary arteries was identical in the normal and obese groups in our study. Bilateral internal mammary artery harvesting was undertaken slightly less often in the obese groups (2.1% to 3.0%) in comparison with the normal BMI population (3.9%). Leg incision complications were difficult to elucidate from the STS database because most of them occurred after hospital discharge.
Study limitations
Whereas the current study sheds light on the problem of coronary artery bypass grafting in the obese patient, there are limitations. First, calculating BMI as an indicator of obesity is simple, quick, and inexpensive, but it does have its shortcomings. Very muscular people may fall into the overweight category when they are actually fit, and some of the elderly may fall into the normal weight category after losing muscle mass [7].
Second, participation in the STS database is voluntary, which causes possible concern of underreporting of adverse events. The STS assures strict confidentiality, and a comparison of results from the STS database with those in audited databases revealed similar risk factor profiles and mortality and morbidity rates.
Third, this is an observational analysis, so there may be concerns of selection bias. Although we have adjusted for patient level risk factors, we cannot account for possible surgeon bias in referring obese patients to surgical treatment.
Finally, this study does not evaluate the morbidity that occurs after the patients are discharged from the hospital. For example, it is known that 14% of the patients can be re-hospitalized after hospital discharge [6]. There are a paucity of data both on long-term concerns of coronary artery disease and on long-term outcomes after coronary artery bypass operations in obese patients, and these have not been addressed in the present study. Prasad and colleagues [4] has previously shown a 14.9% recurrence of angina at 12 months with a majority of the 250 obese patients in the study with BMIs of 25 to 40. These authors speculate that this was a result of increased myocardial oxygen demand and more associated risk factors. Schwann and colleagues [10] showed that the 5-year survival in obese patients was worse despite them being younger.
Conclusion
In conclusion, the proportion of moderately and extremely obese patients requiring coronary artery bypass grafting is about 11%. The relative risks of coronary artery bypass grafting for patients with moderate obesity was slightly higher than in patients with normal or mildly increased BMI, but the risk increase was quite small in terms of absolute risk. In contrast, extreme obesity significantly increased mortality and morbidity after coronary artery operations by more than 50%. For example, a typical patient with mild obesity may have an expected mortality of 3.0%. A patient with similar risk factors but moderate obesity would have an expected mortality of 3.4%, whereas extreme obesity would result in an expected mortality of 4.7%.
From a health policy perspective, aggressive interventions are needed nationally to alleviate the epidemic of obesity in the United States. This would both diminish progression of coronary artery disease and bypass operative risk when needed. In addition, methods to decrease cardiac surgical morbidity and mortality must be studied. Whether better patient selection, early ambulation, and methods to improve postoperative respiratory mechanics will alter this remains to be demonstrated. Finally, the long-term benefits of myocardial revascularization in obese patients need to be examined in a careful prospective study.
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Discussion |
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Using the statistical power associated with these larger numbers, the authors have shown a significant increase in operative mortality for the extremely obese patient. This represents new and, I think, very useful clinical information. I do have three questions, though, that perhaps can help us better understand the important work that has been presented today.
First, sometimes these mortality differences can be confined to different parts of the risk spectrum. For example, several years ago when the STS took a look at the impact of gender on coronary bypass operative morality, we found that the women had a higher risk of death only in the low and moderate risk categories. When we looked at high-risk patients, there was no difference between men and women undergoing bypass operations. Now these extremely obese patients also cover a spectrum of operative risk, from the very low-risk patient up to the patient at very high risk, and that, of course, can be very easily determined using the STS mortality models. That leads to the first question. Did these extremely obese patients have an increased mortality in all parts of the risk spectrum, or, as with our gender study, only in specific risk categories?
Second, you used arbitrary National Institutes of Health (NIH) guidelines to classify the patients into body mass index (BMI) groups, and that certainly is fair enough, but it would be useful to know if the data defined any natural break points. In other words, did the data identify any BMI values that seemed to separate those patients having complications from those that tended to be free of complications?
Last, I would appreciate your comments on the practical implications of your study. Should we change our approach to patients having a BMI greater than 40? Specifically, was there a particularly high risk subset of obese patients that perhaps should not be offered bypass operations?
Again, I would like to congratulate the authors for a very fine presentation and I thank the Society for the privilege of discussing the article.
DR GIORGIO M. ARU (Jackson, MS): What about the pulmonary embolism? It has been our feeling that a patient with a large body mass, and especially a female, has a much higher incidence of pulmonary embolism; and some of the deaths are actually related to pulmonary embolism.
DR PRABHAKAR: Thank you for those comments. To address the questions that Dr Edwards raised with regards to risk categorization, we did not look further into the BMI categories greater than 40. We divided them purely by the NIH guidelines, because most publications seemed to be based on that. It would certainly be interesting to look and subclassify the BMI greater than 40.
In terms of changing strategy with BMI greater than 40, I think all of us have consciously or subconsciously already changed the strategy over many years in that we seem to be choosing younger patients, as well as those with lower risk factors, and that is what is keeping the mortality low. I think if we blinded ourselves to other risk factors, I suspect the mortality would be considerably more than has been shown today. So I think all of us are already pre-selecting patients.
What is also important noting is that when patients with BMI greater than 40 were followed over the intermediate or long term, almost 20% of them developed angina in the first 1 to 2 years. We do not have any long-term results. So I think it is certainly worth following up these patients over the long term to see, one, whether we should be operating; two, what we should be doing differently; and three, how we should be selecting the patients better. Unless we have more long-term results, I think it will be hard to do any more selection than we are already doing. I think we are fortunate in having the results we have today.
The pulmonary embolic rates in The Society of Thoracic Surgeons’ database are very similar for all BMI categories. I know there are one or two other studies that show that the pulmonary embolic rate is higher, but in the STS database there is no difference in the pulmonary embolic rate based on clinical criteria that are used in the database.
Thank you.
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