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(Circulation. 1998;97:1689-1694.)
© 1998 American Heart Association, Inc.

Clinical Investigation and Reports

Obesity and Risk of Adverse Outcomes Associated With Coronary Artery Bypass Surgery

Nancy J. O. Birkmeyer, PhD; David C. Charlesworth, MD; Felix Hernandez, MD; Bruce J. Leavitt, MD; Charles A. S. Marrin, MB, BS; Jeremy R. Morton, MD; Elaine M. Olmstead, BA; Gerald T. O'Connor, PhD, DSc; ; for the Northern New England Cardiovascular Disease Study Group

From the Departments of Surgery (N.J.O.B., C.A.S.M.) and Medicine (E.M.O., G.T.O'C.), Dartmouth Medical School, Hanover, NH; Department of Surgery, Optima Health Care, Manchester, NH (D.C.C.); Department of Surgery, Eastern Maine Medical Center, Bangor (F.H.); Department of Surgery, Fletcher Allen Health Care, Burlington, Vt (B.J.L.); and Department of Surgery, Maine Medical Center, Portland (J.R.M.).

Correspondence to Nancy J.O. Birkmeyer, PhD, Surgical Outcomes Assessment Program, Department of Surgery, Dartmouth-Hitchcock Medical Center, 1 Medical Center Dr, Lebanon, NH 03756. E-mail Nancy.J.Birkmeyer{at}dartmouth.edu

	Abstract

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Background—Obesity is frequently cited as a risk factor for adverse outcomes of major surgery. The results of prior studies of the relationship between obesity and risk of adverse outcomes of coronary artery bypass grafting (CABG) have been contradictory because of insufficient power to assess relatively infrequent outcomes or data to adjust for confounding factors.

Methods and Results—Data on patient age, sex, height, weight, medical history, current clinical status, and treatment factors were assessed prospectively among 11 101 consecutive patients undergoing CABG. Body mass index (BMI) was used as the measure of obesity and was categorized as nonobese (1st to 74th percentiles), obese (75th to 94th percentiles), or severely obese (95th to 100th percentiles). Adverse outcomes occurring in-hospital, including mortality, intraoperative/postoperative cerebrovascular accident (CVA), postoperative bleeding, and sternal wound infection, were defined prospectively. Associations between obesity and postoperative outcomes were assessed by use of logistic regression to adjust for potentially confounding variables. Although obesity was not associated with increased mortality (adjusted odds ratio [OR], 1.16; P=.261) or postoperative CVA (adjusted OR, 1.06; P=.765), risks of sternal wound infection were substantially increased in the obese (adjusted OR, 2.10; confidence interval [CI], 1.45 to 3.06; P<.001) and severely obese (adjusted OR, 2.74; CI, 1.49 to 5.02; P=.001). On the other hand, rates of postoperative bleeding were significantly lower in the obese (adjusted OR, 0.66; CI, 0.49 to 0.90; P=.009) and severely obese (adjusted OR, 0.40; CI, 0.20 to 0.81; P=.011).

Conclusions—With the exception of sternal wound infection, the perception among clinicians that obesity predisposes to various postoperative complications with CABG is not supported by these data. Further work is needed to understand the apparent protective effect of obesity on risks of postoperative bleeding.

Key Words: surgery • risk factors • obesity • morbidity • mortality

	Introduction

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Although there is little evidence in the literature, obesity is often thought to be a risk factor for perioperative morbidity and mortality with cardiac surgery and other major surgical procedures.^{1 2} Factors predisposing and contributing to severity of coronary disease, such as hypertension, hypercholesterolemia, and diabetes,^{3 4 5 6 7} as well as the technical difficulties in surgical and postsurgical care of the obese, likely contribute to these perceptions.

Many of the prior attempts to study the association between obesity and outcomes with cardiac surgery have suffered from limitations caused by sample size and a lack of data about potentially confounding factors.^{8 9 10 11} Although relatively large, a recently published study¹² found no statistically significant associations between obesity and serious complications with CABG.

The NNECDSG is a voluntary research consortium that includes the five medical centers that perform CABG surgery in Maine, New Hampshire, and Vermont. The group maintains prospective data registries that contain information on all cardiac surgeries performed in the region. This large, prospectively collected database on consecutive patients undergoing cardiac procedures provided the opportunity to assess the independent contribution of obesity to risks of in-hospital mortality, intraoperative/postoperative CVA, postoperative bleeding, and sternal wound infection associated with coronary artery bypass grafting.

	Methods

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These analyses are based on 11 101 consecutive patients undergoing CABG between 1992 and 1996 at medical centers in Maine, New Hampshire, and Vermont. Patients undergoing CABG that was incidental to heart valve repair or replacement, resection of a ventricular aneurysm, or other surgical procedure were not included in this analysis.

The NNECDSG CABG registry includes information on the following variables: patient age, sex, height, weight, cardiac catheterization results (degree of left main coronary artery stenosis, total number of significantly diseased coronary arteries, left ventricular end-diastolic pressure, and ejection fraction), prior myocardial infarction, prior CABG, prior PTCA, prior valve procedure (yes or no), comorbidities (diabetes, peripheral vascular disease, renal failure, chronic obstructive pulmonary disease, congestive heart failure, cancers, liver disease, and peptic ulcer disease), treatment factors (time on bypass, use of an intra-aortic balloon bump, use of internal mammary artery grafts, preoperative use of intravenous nitroglycerin or thrombolytic therapy, and in-hospital outcomes (status at hospital discharge [dead or alive], intraoperative or postoperative CVA [yes or no], reoperation for bleeding [yes or no], and sternal wound infection [yes or no]).

Cardiac catheterizations were performed by use of standard methods during the course of regular clinical care. Ejection fractions were scored with the method described by Pierpont et al.¹³ Angiography reports were reviewed to assess the severity of coronary artery disease, expressed as the number of diseased vessels scored by use of methods adapted from the National Heart, Lung, and Blood Institute Coronary Artery Surgery Study.¹⁴ Priority of surgery was assessed by the cardiothoracic surgeons and is defined as follows: "emergency" means that medical factors relating to the patient's cardiac disease dictate that surgery should be performed within hours to prevent morbidity or death; "urgent" means that medical factors require the patient to stay in the hospital for an operation before discharge; and "elective" means that medical factors indicate the need for operation, but the clinical situation allows discharge from the hospital with readmission at a later date.

The number of patients in the data set and their discharge status were verified with hospital discharge data. In addition, hospital discharge information was used to compile a comorbidity index by use of the method described by Charlson et al¹⁵ as modified for hospital discharge data. The advantages of this index are twofold: it allows a single variable to represent the comorbidity burden of the patient, and it allows accounting for the comorbidity contribution of a relatively rare process that may be a substantial contributor to the risk for an individual but occurs too infrequently to be used as a single indicator variable for an entire patient population. This index has been demonstrated to predict in-hospital mortality among patients undergoing cardiac surgery.

The degree of obesity was assessed by the BMI.^{16 17} BMI, derived from Quetelet's formula, equals weight (in kilograms) divided by the square of height (in meters).^{16 17} BMI was the measure chosen because of the obesity indexes that are based on combinations of weight and height; BMI correlates least with height and most with more direct measures of percent body fat, such as underwater weighing and measurement of skin-fold thicknesses.^{6 17} The frequency distribution of BMI among these patients is given in Figure 1. This distribution is skewed to the right, with a mean of 28.2 and a median of 27.6. We classified anyone below the 75th percentile of BMI (BMI <31) as nonobese. Those in the 75th to 94th percentiles (BMI, 31 to 36) were classified as obese, and those above the 95th percentile (BMI >36) were classified as severely obese. These categories are consistent with definitions obesity used in other studies.^{8 9 10 11 12 18 19}

View larger version (23K): [in this window] [in a new window]   Figure 1. Frequency distribution of BMI among 11 101 CABG patients.

Intraoperative or postoperative CVA was defined as a physician-documented new focal neurological deficit that appears and is still at least partially evident more than 24 hours after its onset, occurring during or after the CABG procedure and established before discharge. Postoperative bleeding was coded as present if the patient required repeated sternotomy to assess bleeding after initial departure from the operating room. Sternal wound infection was recorded if two of the following were present with no other recognized cause: organisms and white blood cells seen on gram-stain aspirated fluid, positive deep culture, radiographic evidence of infection, or sternal dehiscence requiring reoperation.

Standard statistical methods were used to calculate the ORs and 95% CIs.²⁰ Logistic regression analysis was used to assess the relationship between BMI category and each outcome and to adjust crude outcome rates for potentially confounding variables.²¹ All probability values were two tailed. All analyses were conducted with STATA release 5.0.²²

	Results

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Table 1 lists patient clinical and treatment factors by BMI category. Those classified as obese or severely obese were on average younger, more likely to be female, and more likely to be diabetic than those in the nonobese BMI category. The severely obese were more likely to have had a prior PTCA. Obesity was associated with higher left ventricular end diastolic pressure, lower mean percent left main stenosis, and longer time on cardiopulmonary bypass.

View this table: [in this window] [in a new window]   Table 1. Patient and Treatment Characteristics Stratified by Obesity Category

The incidences of adverse events among these patients are shown in Figure 2. Among the 11 101 patients, there were 373 in-hospital deaths (3.4%), 204 intraoperative/postoperative CVAs (1.8%), 353 patients with postoperative bleeding events (3.2%), and 142 patients with sternal wound infections (1.3%). The incidence of postoperative bleeding and sternal wound infection, but not mortality or postoperative CVA, differed significantly across obesity categories.

View larger version (33K): [in this window] [in a new window]   Figure 2. Incidence of in-hospital adverse outcomes (mortality, CVA), bleeding, and sternal wound infection (SWI) of CABG. *2 test for association.

The results from univariate and multivariate logistic models are presented in Table 2. There was no association between obesity and in-hospital mortality in either univariate (OR, 1.00; P=.997 for the obese; OR, 0.90; P=.686 for the severely obese) or multivariate (OR, 1.16; P=.261 for the obese; OR, 1.04; P=.873 for the severely obese) analyses. Obesity was also not associated with risk of intraoperative/postoperative stroke in univariate (OR, 0.88; P=.497 for the obese; OR, 0.95; P=.885 for the severely obese) or multivariate (OR, 1.06; P=.765 for the obese; OR, 1.29; P=.456 for the severely obese) analysis.

View this table: [in this window] [in a new window]   Table 2. Association Between Obesity and In-Hospital Adverse Events

Obesity was associated with statistically significant reductions in risk of re-exploration for bleeding in both univariate and multivariate analyses. Compared with those in the nonobese BMI category, univariate odds of re-exploration for bleeding were 0.61 (P=.002) and 0.35 (P=.006) for the obese and severely obese, respectively. These odds were not substantially altered after adjustment for age, number of distal anastomoses, and time on cardiopulmonary bypass for the obese (OR, 0.66; P=.008) or severely obese (OR, 0.39; P=.016).

In univariate analysis, the obese and severely obese were 1.96 and 2.32 times more likely to develop sternal wound infection, respectively. Other predictors of sternal wound infection included age (55 to 64 years compared with <55 years: OR, 2.83; P=.001; 65+ years compared with <55 years: OR, 3.28; P<.001), renal failure (OR, 3.32; P<.001), diabetes (OR, 1.20; P=.312), chronic obstructive pulmonary disease (OR, 2.10; P<.001), bilateral internal mammary artery graft (OR, 2.90; P=.023), and time on bypass >120 minutes (OR, 1.90; P=.001). After adjustment for these factors, obesity (OR, 2.10; 95% CI, 1.45 to 3.06; P<.001) and severe obesity (OR, 2.74; 95% CI, 1.49 to 5.02; P<.001) remained significant predictors of sternal wound infection.

	Discussion

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This study found that obesity does not increase the risk of in-hospital mortality or intraoperative/postoperative CVA among patients undergoing CABG. Obesity decreased risks of postoperative bleeding. The risk of sternal wound infection, however, was more than twice as high among the obese and nearly three times higher among the severely obese after adjustment for other predictors of this outcome.

Except for the finding with regard to sternal wound infection, our results are similar to those of another recent study. This study by Moulton et al¹² analyzed data from 2299 cardiac surgery patients. Obesity was identified as a risk factor for superficial sternal wound infection (OR, 2.3), leg infections (OR, 1.8), and atrial dysrhythmias (OR, 1.2) but not operative mortality, mediastinitis, stroke, renal failure, acquired respiratory distress syndrome, prolonged mechanical ventilation, pneumonia, sepsis, pulmonary embolism, or ventricular arrhythmias. The Moulton et al¹² study distinguished between superficial sternal wound infection and mediastinitis. In our study, there was a single variable for sternal wound infection; however, we do not believe that this underlies the difference in findings between the studies. In our study, sternal wound infection was a serious outcome, with an associated 20% in-hospital mortality rate and a 28-day median postoperative length of stay. Rather than differing case definitions, we believe that the disparate findings are the result of sample size differences between the studies. With >11 000 patients and 142 sternal wound infections, our study had 97% power to detect the observed difference in rates of sternal wound infection between the obesity categories.

A study of 6504 consecutive CABG patients by Loop et al²³ identified obesity (OR, 2.9) as a risk factor for sternal wound complications in multivariate analysis. Other risk factors for sternal wound infection that have been identified by prior studies include bilateral internal mammary artery grafting, diabetes mellitus, postoperative hemorrhage, prolonged operative time, blood transfusion, and low output failure.^{8 9 10 11 23 24 25 26} Our study confirms bilateral internal mammary artery bypass grafting, time on bypass, and postoperative hemorrhage as risk factors for sternal wound infection. In addition, age, renal failure, and chronic obstructive pulmonary disease were independent predictors of sternal wound infection in our study.

Other studies have reported the lack of association between obesity and operative mortality with CABG.^{2 10 11 12} However, many of these studies had limited statistical power to detect such a difference if it existed because of very small numbers of deaths. In a comparison of 250 obese CABG patients and 250 age- and sex-matched control CABG patients who were not obese, Prasad et al¹¹ found a greater risk of any postoperative morbidity among the obese. In a retrospective analysis of 502 patients, Fasol et al⁸ found the obese had greater risk of perioperative and postoperative myocardial infarction, arrhythmias, respiratory infection, leg wound infection, and sternal dehiscence. A study by Koshal et al¹⁰ found greater risk of postoperative hypertension and bronchoconstriction and possibly wound infection (the numbers of wound infections were very small, six in the obese group compared with three in the nonobese group) among the obese group. In a study comparing 56 obese and 56 age-, sex-, and height-matched control subjects, Gadaleta et al⁹ found increased risk of complications and a longer length of stay among obese diabetics.

Prior studies that investigated the relationship between obesity and postoperative bleeding with CABG have not found that obesity increased risk; however, neither did they document any significant protective effect of obesity.^{8 10 11 12} Similar to our results, none of the studies that examined obesity as a risk factor for stroke found an association.^{9 10 12}

There are several potential limitations that should be considered in the interpretation of the results of this study. BMI was chosen as the measure of obesity in this study because, of the indexes that are based on combinations of weight and height, BMI correlates least with height and most with more direct measures of percent body fat.¹⁶ However, there is still some error in measurement of obesity with BMI because there is substantial variation in lean body mass among people of the same height.^{6 16} This potential problem with the use of BMI to measure obesity has been shown to be of greater concern in studies of younger people who have much greater variation in lean body mass than older people.⁶ Another potential limitation to consider is that this study had access only to data on postoperative complications occurring in the hospital. This limitation may underestimate the true effect of obesity on these outcomes to the extent that they occur after discharge from the index hospitalization. A prior study from one institution in our regional collaboration found that 14% of patients undergoing CABG are rehospitalized within 30 days after discharge and that the most common reason for rehospitalization is wound infection (19%).²⁷ Additionally, our study did not collect information for all of the outcomes, such as atrial arrhythmia and leg wound infection, that others have found to be associated with obesity.

In these data, even substantial obesity does not confer additional risk of in-hospital: mortality or intraoperative/postoperative CVA with CABG surgery. However, obesity was associated with substantially increased risk of sternal wound infection. In addition, obesity was associated with substantially decreased risks of postoperative bleeding. Although CABG does not seem contraindicated among obese patients, this group could especially benefit from the development of interventions designed to minimize postoperative sternal wound infections. Further research is needed to understand the protective effect of obesity on postoperative bleeding with CABG.

	Selected Abbreviations and Acronyms

 

BMI = body mass index CABG = coronary artery bypass grafting CI = confidence interval CVA = cerebrovascular accident NNECDSG = Northern New England Cardiovascular Disease Study Group OR = odds ratio PTCA = percutaneous transluminal coronary angioplasty

	Acknowledgments

 
This study was supported in part by grants from the Agency for Health Care Policy and Research (HS-06503 and HS-05745).

Received November 12, 1997; accepted January 1, 1998.

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