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a E. Kenneth Hatton, MD, Institute for Research and Education, Cincinnati, Ohio
b Good Samaritan Hospital, Department of Surgery, Cincinnati, Ohio
c Cardiac, Vascular, and Thoracic Surgery Inc, Cincinnati, Ohio
Accepted for publication July 30, 2009.
* Address correspondence to Ms Engel, Hatton Research Institute, Good Samaritan Hospital, 375 Dixmyth Ave, Cincinnati, OH 45220 (Email: amy_engel{at}trihealth.com).
| CARDIOTHORACIC ANESTHESIOLOGY:
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Abstract |
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Methods: Data in a cardiac surgery database were examined retrospectively. Data selected from the database included CABG surgery from January 2003 to December 2007. The resulting cohort included a total of 10,590 patients. The BMI was grouped into four categories: underweight (BMI 
19), normal weight (BMI 20 to 29), obese (BMI 30 to 39), and morbidly obese (BMI 
40). Regression analysis was conducted to determine whether BMI was an independent predictor of morbidity and mortality after CABG.
Results: Our results indicate that patients with an obese BMI are not at greater risk for morbidity or mortality after CABG. Logistic regression analysis found that CABG patients in the underweight body mass index group had the greatest risk of mortality, prolonged ventilation, reoperation for bleeding, and renal failure. Linear regression indicated length of hospital stay and intensive care unit stay after surgery were the longest for patients with an underweight BMI.
Conclusions: Despite the comorbidities that are often present with obesity, an obese BMI was not found to be an independent predictor of morbidity or mortality after CABG. On the contrary, the underweight patients are at greater risk for mortality and complications after CABG surgery.
In the early 1980s, the Framingham Heart Study identified obesity as a risk factor for coronary artery disease (CAD) [1]. Since this finding, obesity has become a global epidemic and has been recognized as a risk factor for multiple chronic diseases [2]. Based on the 2006 data from the National Health and Nutrition Examination Survey, more than one third of adults in the United States are obese [2]. The American Heart Association and the World Health Organization define obesity as a body mass index (BMI) of 30 or greater and morbid obesity as a BMI of 40 or greater. The BMI is calculated by weight in kilograms divided by height in meters squared (kg/m2).
Coronary artery bypass graft surgery (CABG) is necessary for many obese patients with CAD. The published literature is equivocal regarding the morbidity and mortality of obese patients after CABG. Several studies found that obese patients have a higher incidence of morbidity and mortality after CABG [3–6]. Prabhakar and coworkers [7] documented that obese patients often undergo surgery at a younger age than patients of normal weight. In addition, these patients had a higher frequency of comorbidities such as hypertension and diabetes mellitus. It was also noted that obesity resulted in higher risks of reoperation, renal failure, and prolonged ventilation [8].
Many other studies, however, concluded that obesity does not negatively affect morbidity or mortality of patients after CABG [9–11]. Syrakas and colleagues [12] compared 30-day mortality in obese versus nonobese patients who underwent cardiac surgery and found that normal weight patients had a higher 30-day mortality than the obese patients. In another study, in-hospital mortality was significantly lower in the overweight, obese, and extremely obese groups when compared with the underweight group [13]. The study also associated obesity with lower in-hospital mortality risk compared with healthy weight patients.
With the increasing prevalence of obesity [2], it is imperative that investigators reach a definitive conclusion regarding the postoperative prognosis of obese patients undergoing CABG. Our study evaluates the effect on in-hospital mortality and morbidity of patients undergoing CABG in the midwestern United States.
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Material and Methods |
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TopAbstractIntroductionMaterial and MethodsResultsCommentAcknowledgmentsReferences |
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Based on our inclusion criteria, data selected from the database included any CABG surgery performed by the surgical group from January 2003 to December 2007. For inclusion, the CABG surgery was conducted without any other procedure during the same operation. Patients undergoing any additional procedure in conjunction with the CABG were excluded. Additionally, patients without a BMI reported were also excluded. The resulting cohort included a total of 10,590 patients. Body mass index was grouped into four categories: underweight (BMI 19), normal weight (BMI 20 to 29), obese (BMI 30 to 39), morbidly obese (BMI 40).
Surgical risk factors examined included age, sex, race, family history of CAD, smoking history, diabetes mellitus, hypertension, chronic obstructive pulmonary disease (COPD), and urgency of surgical procedure. Surgical outcomes examined included prolonged ventilation, reoperation for bleeding, sternal wound infection, renal failure, intraoperative complication, length of hospital stay (LOS) after surgery, intensive care unit (ICU) stay, and mortality.
A sample size calculation was performed using n-Query Version 6.0 (Statistical Solutions, Los Angeles, CA). With a total sample size of 10,590 (underweight group 163, normal weight group 6,004, obese group 3,850, and morbidly obese group 573), a 
2 with a 0.05 two-sided significance level has more than 90% power to detect a statistical significant difference between the BMI groups and mortality. Comparisons between the four BMI groups with continuous variables were performed using one-way analysis of variance. The 2 analyses were used to compare categorical risk factors and outcomes between the BMI groups. Logistic regression was used to examine the effect of each BMI group on each significant categorical outcome while adjusting for significant risk factors, and linear regression was used for the continuous outcome variables. The significance level for all analysis was 
= 0.05. Statistical analysis was performed using SPSS 15.0 for Windows (SPSS, Chicago, IL).
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Results |
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Univariate analysis comparing BMI groups and surgical risk factors found a significant difference in seven of the nine variables (Table 1). The mean age of the cohort was 64.5 ± 10.7 years. Age was significantly different among the BMI groups, with the morbidly obese being the youngest (60.2 ± 9.9 years), followed by the obese (62.3 ± 10.4 years) and normal weight (66.2 ± 10.6 years), and underweight being the oldest (67.3 ±10.8 years; p < 0.001). Sex (p < 0.001) and race (p = 0.03) were also significantly different among the groups. The normal weight BMI group was 73% male and 93% Caucasian. Patients in the morbidly obese group (69%) and obese group (66%) were more likely to have a family history of CAD than patients in the normal BMI group (62%) and underweight BMI group (57%; p < 0.001). Prevalence of diabetes (p < 0.001) and hypertension (p < 0.001) both significantly increased as BMI increased. Chronic obstructive pulmonary disease occurred most frequently in the underweight group (31%) and morbidly obese group (24%; p < 0.001). There was no significant difference among the BMI groups with smoking history (p = 0.18) or urgency of procedure (p = 0.89).
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Mortality
The logistic regression model for BMI groups regressed on mortality while controlling for significant risk factors found the underweight group (p = 0.029, adjusted OR 2.21, 95% CI: 1.08 to 4.52), age (p < 0.001, adjusted OR 1.06, 95% CI: 1.04 to 1.07), sex (p = 0.005, adjusted OR 0.67, 95% CI: 0.51 to 0.88), race (p = 0.043, adjusted OR 1.56, 95% CI: 1.01 to 2.41), and COPD (p = 0.001, adjusted OR 1.70, 95% CI: 1.26 to 2.29) all had a significant impact on mortality (Table 3). This model indicated that CABG patients in the underweight group had a greater risk of mortality than normal weight persons (6.7% versus 2.4%, respectively). Additionally, the risk factors of age, sex, race, and COPD also impacted mortality. The risk of mortality increased with age. The average age of patients who died was 70.6 years, and the average age of patients who survived was 64.3 years. Mortality was also higher for females (3.3% versus 1.9%), non-Caucasians (3.3% versus 2.2%), and patients with COPD (3.3% versus 2.1%). There was no significant difference in mortality comparing the obese or the morbidly obese to the normal weight group when controlling for the significant risk factors.
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Comment |
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Several studies [9, 16, 17] support our findings. According to Rahmanian and colleagues [6], the patients with BMIs lower than 20 kg/m2 had the highest mortality rate (4.6%) as compared with patients with BMIs of 20 to 25 kg/m2 (3.1%), 25 to 30 kg/m2 (3.3%), or even greater than 30 kg/m2 (3.8%). In a study comparing underweight and obese, the underweight group was found to have significantly increased mortality [16].
Other research found no association with BMI and mortality after CABG [12, 18, 19]. A Finland study [19] followed more than 500 patients who were grouped into obese and nonobese groups 1 year after CABG. Groups were relatively equal in terms of preoperative characteristics and intraoperative factors. The study found obese patients did not have greater mortality than nonobese patients at 1 year after surgery [19].
Studies have found that obesity was associated with mortality after CABG surgery [3, 7, 8]. Prabhakar and colleagues [7] found obesity results in higher postoperative mortality, with an increase of 50% in the extremely obese group. The increased risk for moderate obesity, however, was small. Another study examined CABG patients in two BMI groups [8]. Extremely obese patients had a BMI greater than 45 and the control group had a BMI of 21 to 35 kg/m2. Results found 30-day postoperative mortality was 2.5 times higher in the extremely obese BMI group than in the control group. For those extremely obese patients who survived surgery, 73.5% survived 5 years after operation [8]. Although these studies found an increase in mortality with an increased BMI, they were limited by their exclusion of underweight patients.
More likely, mortality after CABG is increased for patients with an extreme BMI in either the very low or the very high range. This U shape for mortality risk was supported both in short-term [20] and long-term [4, 21] follow-up studies. Schwann and colleagues [21] found more deaths among patients with very low and very high BMIs than among patients with normal or mild obesity at 5 years after operation. Similar findings were noted at 12 years after operation, indicating that this trend continues [4]. A systematic review of cohort studies found that underweight and severe obesity were both associated with the greatest risk for mortality after CABG [15].
In examining outcomes other than mortality, we found a significant difference in the need for prolonged ventilation among the BMI groups. Prolonged ventilation was necessary most frequently in the underweight group (15%) followed by the morbidly obese group (8%). This is supported by Schwann and colleagues [21], who found longer ventilation times in patients with a very low or a very high BMI. The underweight patients appear to have greater respiratory problems and therefore require longer ventilation after CABG [6, 17]. Other research, however, found that prolonged ventilation occurred most frequently in the morbidly obese [7, 9, 22]. Patients in our study who required prolonged ventilation tended to be female, non-Caucasian, were older, and had COPD.
As in previous studies, we found the need to reoperate for bleeding mostly occurred in the underweight group [4, 6, 16]. In addition to being underweight, these patients were older. Reoperation for bleeding was extremely rare and was required least in the obese and the morbidly obese; similar to the findings of Reeves and colleagues [8] and Tyson and coworkers [17], bleeding and blood transfusions decreased as the patient's BMI increased [21]. It appears that additional weight may actually be protective against excess bleeding.
Similar to other outcomes in our study, renal failure occurred most frequently in the underweight group followed by the morbidly obese group, a finding supported by studies conducted by Rahmanian and associates [6] and Engelman and colleagues [16]. Conversely, other research found renal failure occurred most frequently among the morbidly obese [4, 7, 23]. As with other morbidity outcomes after CABG, patients in our study with renal failure tended to be older, non-Caucasian females with diabetes and COPD.
We found LOS and ICU stay were the longest in the underweight group. Previous research has reported that underweight patients had longer LOS after surgery than did overweight or obese patients [4, 10, 13]. Other research found the longest hospital LOS and ICU stays for the extreme BMIs [9, 21]. Although there was a significant difference in ICU stay among BMI groups in our study, other research found no difference [10, 18]. We found that the morbidly obese had the second longest hospital and ICU LOS. Previous research disagrees and reports that the morbidly obese have the longest LOS and ICU stay [23]. Both Prabhakar and coworkers [7] and Kuduvalli and associates [22] found that the morbidly obese were more likely to have a LOS longer than 14 days after CABG, which has been supported by other research [18, 19, 23]. Our results indicate that underweight patients experienced more severe complications such as prolonged ventilation, reoperation for bleeding, and renal failure after CABG. These complications, therefore, resulted in longer hospital LOS and ICU stay.
Although our study did not find any differences in sternal wound infection rates based on BMI, the majority of previous researchers agree that obese patients have a higher incidence of sternal wound infections, noted to be likely due to poor healing of adipose chest wall tissue [4, 6, 16, 21, 23]. Orhan and colleagues [18] cited the increased operative time in obese patients for the increase in sternal wound infections because the surgical site is open to the external conditions for a longer period of time. Our reason for not finding any significant difference with this outcome may be its extremely low occurrence rate in our cohort.
Limitations of our study include its retrospective nature, although data were collected concurrently. This study only examined postoperative inpatient outcome, which was a relatively short period. Additional research using longer term follow-up is recommended. Although we used the BMI categories that are commonly accepted, using BMI as a measure for obesity has limitations. As BMI is not a direct measure of body composition, it may not truly represent the degree or distribution of body fat. In this study, we controlled for other risk factors that have been found to impact outcomes after CABG such as sex, race, and age; however, other risk factors may have been present that were not controlled and had an impact on the surgery outcome.
Based on the results of this study, it appears that obesity does not lead to an increased risk for mortality or other negative outcomes after CABG surgery. Even morbidly obese patients can be operated on safely. Perhaps that is because of the extra perioperative precautions taken with this group of patients. One might question whether the physicians and nurses believe the morbidly obese patients to be at higher risk and therefore provide more careful attention to them perioperatively. Unfortunately, this information is not available in our database. We do know, however, that it is common practice by many of the surgeons from this database to order duplex scans before surgery for the morbidly obese patients to assist in locating vessels. Postoperatively, duplex scans are more frequently ordered for the morbidly obese to check for blood clots. It is also typical for albumin levels to be checked preoperatively in both the morbidly obese and the underweight before surgery. It is the underweight patients who frequently have lower than optimal albumin levels. Additional nutrition may need to be administered to these patients to increase the albumin levels before surgery. More time and attention may need to be given to the history and physicals of underweight and morbidly obese CABG patients. If additional risk factors are present besides an extreme low or high BMI, additional perioperative care may be needed.
Patients in the underweight BMI group experienced the greatest mortality and other complications after CABG. Our findings, however, imply that the obese and morbidly obese require CABG at an earlier age, suggesting that obesity is a risk factor for cardiovascular disease. It is imperative that the health care system works to emphasize the importance of maintaining a healthy weight to reduce the risk of cardiovascular disease and the need for CABG surgery.
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Acknowledgments |
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This article has been cited by other articles:
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  R. Harvey, B. Haluska, J. Mundy, A. Wood, R. Griffin, and P. Shah Association between body mass index and outcome of coronary artery bypass Asian Cardiovasc Thorac Ann, October 1, 2011; 19(5): 333 - 338. [Abstract] [Full Text] [PDF]
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