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Eur J Cardiothorac Surg 2006;29:434-440
© 2006 Elsevier Science NL
40
University of Wisconsin Hospitals and Clinics, Department of Cardiothoracic Surgery, Madison, WI, United States
Received 28 September 2005; received in revised form 8 January 2006; accepted 10 January 2006.
* Corresponding author. Address: H4/352 Clinical Science Center, 600 Highland Avenue, Madison, WI 53792-3236, United States. Tel.: +1 608 263 6311; fax: +1 608 263 0547. (Email: wigfield{at}surgery.wisc.edu).
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Abstract |
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 Top Abstract 1. Introduction2. Methods3. Results4. DiscussionReferences |
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40, respectively. Results
: In our institutional review of 1920 patients, 1780 met the inclusion criteria with BMI < 20 (n
= 53), 20–30 (n
= 1056), 30–40 (n
= 592), and 40 (n
= 79) based on National Health and Nutrition Examination Survey (NHANES) criteria. Significant increase in complications (STS database guideline definitions) were observed with a BMI 40, 58% versus 47% (p
= 0.04). Extremely obese patients (ExtOb) had increased length of stay (LOS) (11.4 days vs 9.6 days; p
0.01), rate of renal failure (14.3% vs 5%; p
0.01) and prolonged ventilation (39%; p
= 0.01) compared to non-obese patients. Extremely obese had no significant increase in hemodialysis (7.3% vs 3.2%; p
= 0.11) or stroke (5.2% vs 2.9%; p
= 0.29). Obese patients (Ob) had increased LOS (10 days vs 9.6 days; p
= 0.04) and prolonged ventilation (28.3% vs 23.5%; p
= 0.03). Conclusions: Cardiac surgery can be performed without significant increase in perioperative and 30-day mortality in obese and extremely obese patients. Overall complication rates and LOS in patients with BMI 40 is increased and demands attention. We provide evidence that rates of few specific complications increase with extreme obesity. For risk stratification in the setting of an obesity epidemic, we advocate an interdisciplinary approach in obese patients undergoing elective cardiac surgery.
Key Words: Risk assessment • Obesity • Cardiac • CABG • Valve
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1. Introduction |
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TopAbstract1. Introduction2. Methods3. Results4. DiscussionReferences |
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Patients undergoing cardiac surgery with excessive body mass index are likely to have other comorbid factors including the five obesity-related illnesses: type II diabetes mellitus, hypertension, hyperlipidemia, stroke, and coronary artery disease [3]. These illnesses account for approximately 85% of the increased health problems and economic burden of obesity [3,4]. Long-term follow-up of the original Framingham cohort revealed that obesity, measured by Metropolitan Relative Weight, was a significant independent predictor of cardiovascular disease (CVD) [5]. In addition to CVD, the risk of ischemic stroke in the obese is twice as high as the normal population. Obesity has also been positively linked to myocardial dysfunction in patients even without coronary artery disease. A direct relationship between diastolic dysfunction and systolic impairment has been described in the context of excessive BMI.
There is currently no systematic assessment of the evidence available regarding extremely obese patients and their perioperative risks. Specifically, the scarcity of data regarding the extremely obese patient population in cardiac surgery, as defined by the NHANES criteria, is addressed. Risk stratification and informed consent are the primary motivation to establish patient-associated factors leading to potentially increased morbidity and mortality.
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2. Methods |
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TopAbstract1. Introduction2. Methods3. Results4. DiscussionReferences |
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Preoperative data were collected using the Society of Thoracic Surgery (STS) data criteria; these included: age, gender, height, weight, date of surgery, current smoking habit, diabetes mellitus, hyperlipidemia, hypertension, and preoperative creatinine level in serum. The operative information reviewed included: cross-clamp time, cardio-pulmonary bypass time, the use of an intraaortic balloon pump, blood product use, and postoperative ventilation time. Postoperative information included the overall complication incidence as well as specific adverse events associated with cardiac surgery, including: deep sternal infection, septicemia, presence of a transient ischemic attach (TIA) or cerebral vascular accident (CVA), prolonged ventilation (>24 h), pulmonary embolism (PE), pneumonia, renal failure (RF) with or without dialysis, heart block, cardiac arrest (CA), adverse events related to anticoagulation (DIC and HIT), tamponade, bleeding requiring reoperation, graft occlusion, valvular dysfunction, atrial fibrillation (AF), gastrointestinal complications, information related to total ventilation time, and reoperation for bleeding or non-cardiac reoperations. Furthermore, outcomes of operative death, 30-day mortality, length of stay (LOS), and intensive care unit stay (ICU) were analyzed.
Body mass index which serves as an indirect measure of total body fat, was calculated using the patients height and weight [BMI = weight (pounds)/height (inches)2
x 703)] or [BMI = kg/m2] at the time of surgery in accordance to NHANES 1999–2000 criteria. The patients with a normal or overweight BMI (20–30) were defined as the non-obese reference group (NonOb). Obesity was defined as BMI 30–40 and extreme obesity as a BMI 40 [1]. To define the BMI categories, the range intervals are closed on the left. A patient with a BMI of 30 but less than 40 is included in the obese group. Patients whose BMI was lower than 20 (n
= 53) were excluded from the analysis.
2.1 Statistical methods
Continuous variables are presented as mean ± one standard deviation. Categorical variables are shown as the percentage of the sample in a given category. A Kruskal–Wallis test was used for comparing medians across groups. For categorical variables, Fisher's exact test was used. For pair-wise contrasts between groups, the Wilcoxon rank sum was used for continuous variables and Fisher's exact test for categorical variables. All computed p values are two-sided. In every instance a value of p
< 0.05 was considered significant. There was no adjustment to the significance level for the number of hypotheses being tested. All computations were performed in R version 1.9.1 for Windows XP (R Development Core Team, 2003).
For the graphic representation of BMI complication rate, the range in BMI (13.14–58.0) was divided into 10 equal-length intervals, closed on the left, as discussed above. The plot excludes the two highest intervals ([48.8, 53.9), [53.9, 59]), since their respective frequencies (n = 7 and n = 2) were deemed to be too low to produce reliable complication estimates.
The review was selective for publications assessing high BMI (BMI > 27) in context of complications after cardiac surgery. We critically appraised the literature available after systematic searches of the terms: obesity, body mass index, cardiac surgery, CABG, complication rate, valve surgery, mortality, and morbidity. We reviewed all literature available after the following terms searched, but excluded publications pre-1985 to prevent excessive historical bias. Furthermore, studies that did not provide control group or reviews of less than 50 obese patients were excluded. Databases searched for these terms included: Medline, Up to Date, and University of Wisconsin electronic library systems.
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3. Results |
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TopAbstract1. Introduction2. Methods3. Results4. DiscussionReferences |
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The observed perioperative and 30-day mortality was not shown to be increased in the obese group (BMI 30); nor was it shown to be elevated in the extremely obese patients (BMI 40) in this retrospective analysis. This corresponds with previous reports of patients undergoing cardiac surgery with elevated BMI (see Table 4). However, the BMI 40 has not been previously investigated in this context and presents a novel evaluation of clinical data. Although the low BMI group (<20) showed a trend towards increased overall complication rates, it did not prove to be statistically significant when compared to the normal BMI index (p
= 0.78).
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4. Discussion |
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TopAbstract1. Introduction2. Methods3. Results4. DiscussionReferences |
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A national health consensus conference on obesity surgery described an increase in demand for bariatric surgery, with potential candidates having a BMI 40 (representing approximately 100 lb excess body weight for a male and 80-lb excess for female patients). The Society for Bariatric Surgery noted an exponential increase of such operations performed over the last five years [6], with >100,000 of these major gastrointestinal procedures were performed in 2004 in the US alone. The operative risks of gastric resections or bypass procedures are both procedure- and patient-related. The risk stratification for other major surgical procedures have not been well defined.
In the era of risk stratification and informed consent, commonly applied estimation of open heart surgical risks may not adequately aim to assess obesity as a risk factor in various scoring systems [7,8]. Additionally, the highly predictive EuroSCORE does not include BMI stratification for cardiac surgical risk [9].
There are surprisingly few studies assessing complications in obese patients undergoing open heart surgery. Most authors provide retrospective studies with limited insight into selective adverse events, associated with coronary artery bypass grafting. Outcomes have not been shown to be adversely affected in patients below BMI of 40 in these studies. This study primarily assessed mortality and morbidity associated with BMI 40 and provides confirmation of low mortality in this patient group. Similarly, our evidence shows that obese patients can have an acceptable perioperative and 30-day mortality rate. In fact, our outcome data for the patients defined as extremely obese is similar to the non-obese patient group. A tertiary medical center with surgical experience in this patient group provides expertise that compensates for certain potential complications (Table 2).
Kuduvalli et al. reported an increased frequency of prolonged ventilation in patients with a BMI > 35 [10]. We confirmed that prolonged ventilation, based upon the definition of postoperative intubation greater than 24 h, is likely in obese and extremely obese patients undergoing major cardiac surgery. The use of bronchodilators following cardiac surgery was found to be increased in one prospective study [10]. The incidence of postoperative pneumonia did not differ across our BMI categories. Others observed frequent respiratory impairment during the postoperative phase, but this has neither been well defined nor established in other reviews [11]. Obese patients are more likely to have a low respiratory reserve with a ventilation/perfusion mismatch and decreased functional residual capacity. This, however, has not led to adverse outcomes in intensive care medicine in the obese patient.
Our finding of increased acute renal failure, defined by creatinine rise two times from baseline and greater than 2.0 mM/ml, has not been previously reported in association with cardiac surgery in obese patients. However, this was not associated with a statistically significant increase in patients requiring renal dialysis for either obese or extremely obese patients. A previous prospective study did not show an association between acute renal failure and body mass index [10].
The previous reported finding that obese patients suffer a higher incidence of postoperative superficial or deep wound infections (INF) could not be verified in this retrospective analysis. The low risk observed in both the obese and extremely obese patient groups of sternal wound infections, probably reflect the use of the STS definition applied and the expertise developed at this tertiary center with high prevalence of morbid obesity. Gaynes et al. [12] reported a similar experience. In other studies, the expected incidence of postoperative mediastinitis has been linked to obesity but maybe mostly associated with diabetes mellitus and perioperative hyperglycemia with the presence of other comorbid conditions prevalent in this patient group [13–15]. Birkmeyer et al. [16] describe the rate of wound infections as up to three times higher in obese patients after adjustment of other predictors of this outcome. Moulton et al. [17] studied patients with a BMI > 30 and found increased superficial sternotomy wound infections and conduit harvest site infections. Similarly, a large retrospective study conducted by Engelmann et al. concluded that wound infections are more frequent in obese patients and observed a high incidence of deep sternal wound infections. Sternal wound infections were a major finding in five of the eight series reviewed. Loop et al. [13] established an odds ratio of 2.9 associated with obesity for sternal wound complications in a multivariate analysis of 6504 patients. Our population group did not show increased incision-related superficial or deep sternotomy wound infections (Table 2).
Findings regarding bleeding complications observed in this setting are controversial. A prospective cohort described by Birkmeyer et al. [16] showed decreased postoperative bleeding in obese patients (BMI > 31). This was not confirmed by a multivariate analysis of extremely obese patients with BMI > 36 [16]. Similarly, our obese patient group had a reduced risk of postoperative bleeding (Table 3 ). This may appear counterintuitive on first assessment but little information regarding bleeding propensity and hemostatic properties after major surgery in obese patients is available.
Only two studies with small patient cohorts studied product transfusion rates in obese patients after cardiac surgery. Uva et al. [18] found an increased transfusion rate while Ranucci et al. [11] found a decreased transfusion rate. The indications for transfusions and parameters used for blood products were poorly defined and no conclusive evidence is available to date [11,18]. The obese and extremely obese patients in our cohort did not show increased hemorrhage tendencies and required no more frequent blood product transfusions than non-obese cardiac surgical patients. In fact, a marked decrease for the need in reoperation for bleeding was observed in both obese patient groups in our retrospective analysis. Blood product use, defined as transfusion intraoperatively or postoperatively according to STS guidelines, was similar across all three groups and does not explain the decreased reoperation rate for obese patients.
A higher frequency of postoperative hypertension requiring pharmacological intervention was not observed in our obese patient group. This association was previously found in a single prospective study published in 1985 by Koshal et al. [10] but not confirmed by other authors. The high preoperative prevalence of hypertension as a co-morbid factor in this group does not pose a clinically prominent problem postoperatively. Similarly, diabetes mellitus present preoperatively in 54.5% of the extremely obese patients and 37.9% of the obese patients has not been shown to result in clinically relevant postoperative morbidity. Little information is available regarding the metabolic syndrome and glucose metabolism derangements in the obese cardiac patient in the postoperative phase. Contrary to bariatric surgery, where significant weight loss, remission of diabetes mellitus and improvement of lipid serum levels are observed, cardiac surgery in the extremely obese patient does not provide therapeutic interventions for these pre-existing comorbidities [19]. Continued long-term medical management is essential in improving life span.
In a relatively small retrospective study, Brandt et al. [20] have reported no statistically significant complication increases in obese patients. They report a discrepancy between total procedure time of CABG and discuss technical considerations in their obese patient group [20]. Procedural difficulty or operative hazards encountered in obese patients cannot be analyzed in this context. As with reoperations, for example, a certain level of expertise is required to avoid operative pitfalls as well as postoperative complications. It is apparent that with relatively minor adjustments intraoperative problems can be avoided.
Methodological differences alone may not conclusively explain the range of findings of the studies compared in Table 4 . Most evidence provided has been extrapolated from relatively small prospective and retrospective studies. Two prospective multi-center studies were conducted adjusting for confounding factors including preoperative comorbidities [10,16].
Although a consensus definition for obesity and extreme obesity exists according to World Health Organization (WHO) and NHANES, authors have used variable BMI cutoffs to define their patient groups (Table 4). Other authors have defined obesity by percentage change from ideal body weight calculations [5]. The comparability of the studies available is therefore limited and does not allow for meta-analysis.
The majority of previous studies available have been conducted with the primary focus on coronary bypass surgery outcomes. Our patient population consisted of both valve and coronary surgical patients combined similar to the studies by Engelman et al. and Moulton et al. [17]. Our decision to exclude transplant and VAD surgeries is based upon previous studies demonstrating an increased mortality in obese transplant recipients. Although patients with cardiac surgery have similar perioperative complications, we assessed a more heterogeneous population of patients and therefore possibly introduced confounding variables. Our reference population is representative for a large tertiary surgical care center in the United States. The referral patterns reflect the demographics of this region and include the largest patient cohort described of extremely obese patients.
Risk stratification is the ultimate motivation for establishing complication rates in obese patients undergoing cardiac surgery. Preoperative evaluation of obese patient undergoing cardiac surgery provides an opportunity for an interdisciplinary approach to allow accurate risk stratification and informed patient choices. The approach for consenting patients, and the medico legal issues for consenting patients, has become a focus of daily clinical practice. Our data provides evidence that cardiac surgery is safe even for the extremely obese patient population.
Since obesity is a chronic condition, a potential risk reduction based on weight loss alone has to take into account the recidivism of weight loss programs. Behavior modifications, exercise, and diet are considered the mainstay of treatment for obesity. Few patients are successful in reducing their weight preoperatively, although some evidence suggests that patients generally have better recovery and increased mobility with major surgical procedures after preoperative weight reduction [21,22]. Genetic factors may be responsible for up to 70% of the variation of patients weights [23]. It is critical to appreciate that patient's weights will fluctuate within 10% of a physiological set value and the regulation of BMI involves maintenance of energy balance as well as nutrient composition and physiological feedback mechanisms. Chronic imbalance between energy intake and energy expenditure seen in a majority of severely obese patients results in adipose tissue mass increases.
Leptin, a circulating protein of lipid homeostasis, may provide novel approaches to regulate adipose tissue proliferation and could theoretically reduce body mass indices in short term prior to cardiac surgery [24]. However, any effect on perioperative outcomes at present is speculative and this requires clinical investigation. Given the polygenic nature of severe obesity, the current constraints of weight loss programs, and the acute nature of cardiac diseases amenable to surgical intervention, deferring such procedures in obese patients may not be justifiable.
The limitations to this study are mostly linked to the retrospective nature of the analysis and to the lack of the ability to adjust for confounding variables. BMI was used to define obesity in this study population. The concept of normal weight and obesity are relative to the environmental and cultural factors, with every population presenting as a continuum of height and weight. WHO criteria for morbid obesity in the Asian population, for example, differ compared to the applied NHANES population criteria. The clinical guidelines employed in this study are applied to the general population and are not specific to the cardiac surgical patients. This allows for a selection bias.
The clinical guidelines established by NHANES were employed in this study. They refer to the general population and are not specific to the cardiac surgical patients. This introduces a potential bias. The relatively small denominator for patients at the extreme end of the BMI spectrum of this study limits the statistical power and did not yield sufficient observations for a multivariate analysis of relatively infrequent complications. The occurrence of deep vein thrombosis, for example, would require a prospective assessment, as this is not currently part of the STS complication data set. A prospective evaluation of obesity as a risk factor for specific complications in cardiac surgery would, therefore, require pooled data from multiple centers in order to provide satisfactory power calculations to detect increased risk ratios with certainty.
In summary, extreme obesity according to our study and other data available is not associated with an increased mortality after cardiac surgery. Extremely obese patients have a statistically significant increase in frequency of specific complications, including prolonged ventilation and acute renal failure not requiring hemodialysis (HD). This may result in prolonged length of stay and may have economic health care consequences. Although it has been established that obese patients are not at an excessive adverse risk while undergoing CABG [10,11,16,20,25] or cardiac surgery utilizing cardiopulmonary bypass [17], further clinical investigation is required focusing on specific cardiac surgical procedures. To exclude patients from cardiac surgery due to high body mass index or quote excessive operative risks may not accurately reflect the evidence currently available.
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References |
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TopAbstract1. Introduction2. Methods3. Results4. DiscussionReferences |
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