Ann Thorac Surg 2007;83:1403-1411
© 2007 The Society of Thoracic Surgeons
Original Articles: Cardiovascular
Cardiac Surgery in Patients With Body Mass Index of 50 or Greater
Mauricio A. Villavicencio, MD,
Thoralf M. Sundt, III, MD*,
Richard C. Daly, MD,
Joseph A. Dearani, MD,
Christopher G.A. McGregor, MB, FRCS,
Charles J. Mullany, MB, MS,
Thomas A. Orszulak, MD,
Francisco J. Puga, MD,
Hartzell V. Schaff, MD
Division of Cardiovascular Surgery, Mayo Clinic, Rochester, Minnesota
Accepted for publication October 30, 2006.
* Address correspondence to Dr Sundt, Mayo Clinic, 200 First St SW, Rochester, MN 55905 (Email: sundt.thoralf{at}mayo.edu).
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Abstract
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Background: The seemingly inexorable rise in obesity worldwide is creating a new set of challenges for healthcare providers. Demand for cardiac surgical intervention among patients at extreme levels of obesity (body mass index [BMI] 
50) is increasing; however, the risks, benefits, and resources required to meet this need have not been established.
Methods: Between 1993 and 2004, 57 patients with a BMI of 50 or more underwent cardiac surgical procedures at our institution. The mean BMI was 54 ± 4, weight range was 124 to 226 kg. The mean age of the study group was 55 ± 12 years, and comorbidities included diabetes mellitus in 29 (51%), hypertension in 40 (70%), hyperlipidemia in 22 (39%), and obstructive sleep apnea in 16 (28%).
Results: The operative mortality was 7% (4 patients). Eleven patients (20%) required prolonged intubation (more than 24 hours), and mean intensive care unit stay was 5 ± 9 days. Wound complications requiring surgery occurred in 3 (5%). Survival at 1 and 5 years was 93% ± 4% and 76 ± 8%, respectively. By univariate analysis, age and endocarditis were associated with long-term mortality and major perioperative complications. As a dichotomous variable, BMI greater than 54 was a significant predictor of renal failure and prolonged mechanical ventilation.
Conclusions: Cardiac surgery in the patient with a BMI of 50 or greater is associated with significant resource utilization, including prolonged intensive care unit and hospital stay, with prolonged intubation and wound complications relatively common.
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Introduction
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Obesity has reached near epidemic proportions in both the developed and developing world. The seemingly inexorable increase is observed both in terms of prevalence and degree, with new extremes of obesity continually being defined. Obesity, defined as a body mass index (BMI) of 30 or greater, has been shown to be associated with increases in wound infection and prolonged mechanical ventilation after cardiac surgery [1]. A decrement in long-term survival after coronary artery bypass graft surgery (CABG) has also been shown for patients with BMI greater than 36 at 12 years of follow-up [2]. Clinicians are increasingly confronted, however, with patients at much greater extremes of obesity, including those now with BMI in excess of 50. Given the prevalence of cardiovascular risk factors among the obese, demand for cardiac surgery in this challenging population can only be expected to rise.
The risks of cardiac surgery among the extremely obese and resources consumed in delivering this care remain undefined. It can be anticipated that patients at the extremes of BMI will pose particular logistic as well as medical challenges; however, recent large studies of the impact of obesity on outcome after cardiac surgery included no patients with BMI of 50 or more [3, 4]. We therefore sought to review our clinical experience among these patients undergoing cardiac surgery at our institution with the aim of clarifying the resource utilization and clinical outcomes that might be expected in this important subset of patients.
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Patients and Methods
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After project approval by the Mayo Clinic Foundation Institutional Review Board on April 5, 2005, the computerized Cardiovascular Surgery Division Database at the Mayo Clinic, Rochester, was searched to identify all patients coming to operation between June 1993 and February 2005 with a record of their BMI. Data are collected in this database according to Society of Thoracic Surgeons database guidelines. All Mayo Clinic patients are asked to consent for clinical record review for research. Patients who have not authorized are deleted from the databases given to the investigators. In addition, for those with BMI of 50 or greater, Institutional Review Board–approved questionnaires and HIPPA (Health Insurance Portability and Accountability Act) forms were sent by regular mail for follow-up. A maximum of three mailings were sent in case of no response. Only returned questionnaires with patient-approved HIPPA forms were considered. Follow-up with clinical charts, questionnaires, and death certificates was performed between April and October 2005.
Surgical Technique
Modifications in the operative procedure have evolved over time. There is a program in the surgical treatment of obesity at our institution, and we have learned from our colleagues who perform bariatric procedures. We routinely use reinforced bariatric surgery operative tables, and lateral, metallic, curved-shaped extensions to protect the patient’s upper extremities. Particular care must be given to avoiding nerve compression. Among patients undergoing CABG with internal thoracic artery grafts, we prefer to use the Delacroix-Chevalier retractor (Delacroix-Chevalier, Paris, France) for harvest of the left internal thoracic artery as it provides exposure through both downward pressure on the right hemisternum as elevation of the left hemisternum in contrast to table mounted retractors, which rely solely on elevation of the left hemisternum. As such, the Delacroix-Chevalier is less likely to exert excessive traction on the brachial plexus [5]. An additional benefit is that this retractor does not require fixation to the operating table side rails. Exposure of the side rails can be problematic and can, in itself, cause significant compression of the arm and nerve injury. When saphenous vein is required for coronary bypass grafts, it is preferentially harvested endoscopically in the interest of minimizing lower extremity wounds [6].
All but one CABG was performed with the use of cardiopulmonary bypass. This is consistent with our institutional preference for on-pump surgery in most cases. Given the efficiency of the current generation membrane oxygenator technology, few modifications have been required to accomplish satisfactory perfusion. We routinely used the Terumo-Capiox SX 25 (Terumo Cardiovascular Systems, Ann Arbor, MI). Arterial cannulation with a 22F DLP cannula (Medtronic, Minneapolis, MN) provided adequate arterial inflow. Most cardiac surgical procedures at our institution are performed with only mild permissive hypothermia (34°C). Given the difficulty in rewarming the extremely obese patient, active cooling is only pursued in cases requiring circulatory arrest. Sternal closure was preferentially with Myo/Wires II (A&E Medical, Farmingdale, NJ), which provide double wires for added strength (3 to 6 Myo/Wires II per patient).
Patient transfer after the procedure is a particular challenge in the extremely obese. Like our colleagues performing bariatric surgery, we have used the Airpal system (Patient Transfer Systems, Center Valley, PA), a radiolucent device placed under the patient that is inflated at the time of transfer from the operative table to the intensive care unit bed. In the intensive care unit, the Flexicair Eclipse Ultra mattress (Hill-Rom, Batesville, IN) was used because is a multilayered system with airflow, which is designed to reduce the risk of pressure ulcers. The V-Cue Dynamic Air Therapy Mattress (Hill-Rom) provides full rotation, vibration, and percussion for patients with high ventilatory requirements.
Statistical Analysis
Statistical analysis was performed using JMP statistical software (SAS Institute, Cary, NC). A p value less than 0.05 was considered significant in all cases. Univariate analysis of dichotomous predictors of operative mortality, prolonged ventilation, renal failure, deep wound infection, and survival among patients with BMI 50 or greater was performed with 
2, Fisher’s exact test, and log- rank. Survival analysis was completed using the Kaplan-Meier method.
In the interest of evaluating the impact of obesity on operative risk, all patients with recorded BMI operated at our institution during the study interval were subdivided according to nutritional status using the criteria set by the Expert Panel on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults [7]: BMI less than 20 (underweight), BMI 20 to 29 (normal/overweight), BMI 30 to 39 (obesity class I–II), BMI 40 to 49 (obesity class III), and BMI 50 or greater (obesity class IV, not previously defined). For the subgroup with BMI 20 to 29, demographics and mortality were compared with all other nutritional classes by univariate analysis with 2, Fisher’s exact test, and the t test when appropriate.
To assess the possible influence of obesity in operative mortality, a multivariate risk analysis of predictors (forward, Wald) was performed with binary logistic regression. Four models were created for analysis: (1) No exclusions; (2) urgent/emergent cases excluded; (3) only isolated CABG included; and (4) only isolated CABG included, urgent/emergent cases excluded.
Possible predictors analyzed are listed in the Appendix. The following obesity cut-offs were tested: BMI 30 or greater, BMI 40 or greater, and BMI 50 or greater.
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Results
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During the study interval, 18,810 patients underwent cardiac surgery for acquired heart disease at Mayo Clinic, Rochester, Minnesota, of whom 57 had a BMI of 50 or greater. An additional 495 patients had a BMI of 40 to 49, and 5,338 had a BMI of 30 to 39. The percentage of patients coming to surgery who have a BMI of 40 or greater has increased continuously during the study interval, such that currently 4% of patients undergoing cardiac surgery at our institution have a BMI of 40 to 49, and 0.7% have a BMI greater than 50 (Fig 1). At the same time, the percentage of patients with BMI 30 to 39 has increased from 24% to 30%, and patients who are normal/overweight (BMI 20 to 29) have declined from 71% to 63%. (Fig 2). During this same study interval, 532 patients with a BMI less than 20 underwent cardiac surgical procedures.
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Fig 1. Percentage of patients with body mass index (BMI) 40 to 49 and BMI 50 or greater by year of operation.
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Fig 2. Percentage of patients with body mass index (BMI) less than 20, BMI 20 to 29, and BMI 30 to 39 by year of operation.
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The demographics of the study population with BMI of 50 or greater are presented in Table 1. The maximum weight was 226 kg and the maximum BMI 68. Almost two thirds of these patients were male, and the mean age was only 55 years. As expected, the prevalence of obesity-related risk factors including hyperlipidemia, diabetes mellitus, sleep apnea, and hypertension was high. The majority was in functional class III or IV. Almost 20% had experienced recent myocardial infarction; however, only 7% underwent their procedure as an emergency.
The most common surgical procedure performed was CABG, all but one of which was performed on cardiopulmonary bypass (Table 2). Emergent cases included 1 patient with acute aortic dissection diagnosed by transesophageal echocardiography who underwent aortic root replacement, 1 patient with hepatic cirrhosis who had aortic root replacement/mitral valve repair for active endocarditis, and 2 patients who underwent CABG for refractory angina after a recent myocardial infarction.
There were 4 operative deaths (7%), including a 78-year-old patient after aortic valve replacement and CABG, a 74-year-old individual discharged from hospital after uncomplicated aortic valve replacement dying suddenly three days after dismissal, and two patients with endocarditis including a 69-year-old individual with hepatic cirrhosis and mitral endocarditis dying with hepatic failure and a 46-year-old with aortic valve endocarditis, chronic obstructive pulmonary disease and hepatic cirrhosis dying of heart failure. There were no mortalities among patients undergoing isolated CABG, one after isolated aortic valve replacement (11%) and none after isolated mitral valve replacement.
Perioperative morbidity was considerable, as shown in Table 3. One fifth of patients required intubation for more than 24 hours, and almost 30% required bilevel positive airway pressure after extubation. Although reexploration for bleeding was uncommon, wound complications requiring a second surgical procedure occurred in 5% of patients. Only 1 patient had mediastinitis as a definitive diagnosis; however, 2 had sternal dehiscence. The duration of both hospital and intensive care unit stays was prolonged, at 13 ± 21 and 5 ± 9 days, respectively.
Of the 53 patients surviving hospitalization, 44 were alive at the time of follow-up (closing interval April 2005 to December 2005). Nine patients did not respond to requests for follow-up, and 4 refused to answer the questionnaire sent (HIPPA form). All survivors had a minimum of 6 months of follow-up, however, as described in their clinical records or questionnaires. Mean follow-up was 39 months, with a total of 184 patient-years. At 1, 3, and 5 years, survival was: 93% ± 4%, 88% ± 5%, and 76% ± 8%, respectively (Fig 3).
Both increased operative risk and reduced long-term survival were associated with endocarditis by univariate analysis (Table 4). Two of 4 patients with known liver cirrhosis died postoperatively. By univariate analysis, hypertension was associated with improved survival, although multivariate analysis could not be performed given the small number of patients and events. Prolonged mechanical ventilation was associated with age greater than 55 years and BMI greater than 54, as well as stroke and history of chronic obstructive pulmonary disease. Patients with active endocarditis also experienced prolonged ventilation. Renal failure was more common among those over the age of 60 and with a BMI greater 54.
To more precisely define the risk associated with very high degrees of obesity and to put the results obtained in the subgroup of patients with extreme obesity in the context of our overall clinical practice, we reviewed results of all patients undergoing cardiac surgery for acquired conditions and our institution during the study interval (n = 18,810) stratified by BMI (Table 5). Nonelective operative status was more common among all obesity classes, and the prevalence of diabetes mellitus and hypertension were, as might be expected, higher among the obese. Female sex was predominant among the underweight (BMI < 20). The incidence of endocarditis was significantly higher at the extremes of nutritional status (BMI < 20 and BMI 50). There was a consistent trend toward higher operative mortality with increasing levels of obesity, particularly with BMI 50 or greater (7%), although that is did not achieve statistical significance. The observed mortality rate was highest for the undernourished (7.7%).
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Table 5 Demographics and Mortality by Nutritional Status, Univariate Comparison of Body Mass Index (BMI) 20 to 29 With All Other Nutritional Classes
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Given the significant percentage of obese patients undergoing surgery under nonelective circumstances and the potential for significant distortion of the results on that basis, we examined the subgroup of patients only undergoing surgery under elective circumstances (Table 5). The results did not differ substantially for this subset of patients from those obtained for the whole group in the multivariate analysis. Similar trends were observed among isolated CABG patients. Among those with BMI 50 or greater, CABG procedures were performed under nonelective circumstances twice as often as in the normal/overweight (p = 0.006) or obesity class II patients. Despite this, there were no deaths among the subpopulation of patients with obesity class IV.
Nine hundred and ninety-five patients had missing data. Therefore, multivariate analysis of predictors of operative mortality was performed in 17,815 patients (Table 6). When all patients were considered, risk factors for operative death in included age, female sex, endocarditis, ejection fraction, renal failure, nonelective status, and BMI less than 20. In only 1 case, of elective cases of all types, did BMI 30 or greater achieve statistical significance as a risk factor for operative death. When BMI 40 or greater was entered into the model, it did not achieve statistical significance as a risk factor.
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Comment
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The results of this study demonstrate that cardiac surgical procedures can be performed in selected cases, even among those at the extremes of obesity, provided appropriate equipment and expertise in care of this particular subgroup of patients. That equipment includes efficient membrane oxygenators, specially reinforced bariatric operating tables, curve-shaped lateral metal table extensions, Delacroix-Chevalier internal thoracic artery retractor, endoscopic vein harvesting, Myo/Wires II, Airpal tranfer system, and special postoperative mattresses. Resource utilization as measured by intubation time, intensive care unit stay, and total length of hospital stay, however, can be expected to be prolonged over and above that anticipated in routine practice. That may have important implications for hospital planning and contracting negotiations.
This study suffers important limitations that must be recognized in the interpretation of the data presented. Perhaps most significantly, the study examines only the outcomes of patients accepted for surgical intervention, and not those of all potentially referred for the same. Although a number of these patients were in extremis with quite significant comorbidities such as hepatic cirrhosis, it is likely that the patients with less clear-cut indications for surgery, such as those with coronary artery disease, were highly selected. That may be in part the explanation for the relatively young mean age of the obesity class IV patients, almost a decade below that of the normal BMI group. The observation that almost half of the patients undergoing CABG in obesity class IV underwent the procedure under nonelective circumstances suggests continued reluctance on the part of surgeons to accept these patients for elective procedures and is evidence of this selection process, although we have no direct data to support or refute the hypothesized high threshold for accepting patients such as this for cardiac surgery. Given this question of selection bias, however, it would be erroneous to conclude that cardiac surgery can be undertaken in any and all patients with this level of obesity without incurring increased operative risk. Rather, the data indicate that such procedures can be done despite the challenges of extreme obesity. Obesity itself in poses significant technical limitations to operative exposure of which the surgeon is particularly aware, and which likely discourages acceptance of these patients for complex procedures.
An additional caution is the small number of patients in our study with BMI 50 or greater available for analysis, limiting the statistical methods applicable. Conclusions might differ if applied in a larger group. In our study, ony a weakly statistically significant association of mortality was demonstrable with obesity as defined by BMI 30 or greater (odds ratio of 1.256 and p value of 0.043), and only when the analysis was limited to elective cases. Again, this may be due to a dominant effect of other variables in the model such as emergent status or endocarditis when all patients are included. When we considered BMI 40 or greater and BMI 50 or greater as cutoffs, operative mortality was not associated with obesity in the multivariate risk analysis, despite observed mortality rates of 4.5% and 7% in these subgroups, respectively. Again, that may have been due to insufficient statistical power.
We observed a bimodal impact of nutritional status on operative mortality, as has been previously observed. It is well established that underweight patients have increased operative mortality in cardiac surgery regardless of the threshold utilized to define this subgroup [1–4]. Our results confirm these findings, with poor nutritional status (BMI < 20) strongly associated with perioperative death in three of the four models tested. Conversely, a number of studies have demonstrated increased risk of wound infection or respiratory complications, but not death, with obesity. The explanation for that may reside in part with clinical selection as suggested above, with surgery undertaken only in patients thought either to be ideal candidates or those with extreme and less frequent conditions such as acute dissection or active endocarditis, which themselves likely dominate the models used to assess operative risk. In addition, older studies have included fewer patients at the true extremes of obesity.
Importantly, our data also suggest that even modest changes in weight within this extreme nutritional class may decrease perioperative morbidity. Among patients with a BMI 50 or greater, BMI greater than 54 was a predictor of prolonged mechanical ventilation and renal failure on univariate analysis. Accordingly, we agree with recommendations that a serious effort be made at a 10% weight loss during a 6-month period when elective cardiac surgery is contemplated. That could be accomplished in patients with BMI greater than 40 at a rate of 0.45 to 0.9 kg per week [7]. More rapid weight loss, however, is not recommended as it may increase the risk of QT-interval prolongation, ventricular ectopy, nonsustained ventricular tachycardia, ventricular fibrillation, myocardial fibrosis, myofibrillar fragmentation, and sudden cardiac death [8]. Voluntary and controlled weight loss results, however, may decrease left ventricle dimension, wall stress, ventricular mass, and increase shortening fraction. Obesity cardiomyopathy is a reversible process [9, 10].
We have observed a sevenfold increase of patients with BMI 50 or greater in our practice over the past decade. Mokdad and colleagues [11] have estimated the prevalence of obesity in the general population of the United States to be approximately 20%, with an estimated 97,000,000 adults in the United States overweight or obese. This trend appears to be continuing unabated and at an accelerating rate. As currently 2.3% of the population is classified as "extremely obese" (BMI > 40) [11], the demand for cardiac surgical procedures in this subset can be expected to increase. Given increased resource utilization, it may be important to consider these patients specifically in future for carve-out in financial contract negotiations.
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Appendix
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Acknowledgments
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Special thanks to Judy Lenoch for her help in data recollection.
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References
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- Engelman DT, Adams DH, Byrne JG, et al. Impact of body mass index and albumin on morbidity and mortality after cardiac surgery J Thorac Cardiovasc Surg 1999;118:866-873.[Abstract/Free Full Text]
- Schwann TA, Habib RH, Zacharias A, et al. Effects of body size on operative, intermediate, and long-term outcomes after coronary artery bypass operation Ann Thorac Surg 2001;71:521-531.[Abstract/Free Full Text]
- Potapov EV, Loebe M, Anker S, et al. Impact of body mass index on outcome in patients after coronary artery bypass grafting with and without valve surgery Euro Heart J 2003;24:1933-1941.[Abstract/Free Full Text]
- Habib RH, Zacharias A, Schwann TA, Riordan CJ, Durham SJ, Shah A. Effects of obesity and small body size on operative and long-term outcomes of coronary artery bypass surgery: a propensity-matched analysis Ann Thorac Surg 2005;79:976-986.
- Jellish WS, Blakeman B, Warf P, Slogoff S. Somatosensory evoked potential monitoring used to compare the effect of three asymmetric sternal retractors on brachial plexus function Anesth Analg 1999;88:292-297.[Abstract/Free Full Text]
- Yun KL, Wu Y, Aharonian V, et al. Randomized trial of endoscopic versus open vein harvest for coronary artery bypass grafting: six-month patency rates J Thorac Cardiovasc Surg 2005;129:496-503.[Abstract/Free Full Text]
- Expert Panel on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults Executive summary of the clinical guidelines on the identification, evaluation, and treatment of overweight and obesity in adults Arch Intern Med 1998:1855-186728;158.
- Ahmed W, Flynn M, Alpert M. Cardiovascular complications of weight reduction diets Am J Med Sci 2001;321:280-284.[Medline]
- Alpert MA, Terry BE, Mulekar M, et al. Cardiac morphology and left ventricular function in normotensive morbidly obese patients with and without congestive heart failure, and effect of weight loss Am J Cardiol 1997;80:736-740.[Medline]
- Alpert MA. Management of obesity cardiomyopathy Am J Med Sci 2001;321:237-241.[Medline]
- Mokdad AH, Ford ES, Bowman BA, et al. Prevalence of obesity, diabetes, and obesity-related health risk factors, 2001 JAMA 2003;289:76-79.[Abstract/Free Full Text]
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Abstract
Introduction
