Obesity and Stroke After Cardiac Surgery: The Impact of Grouping Body Mass Index
2007; Elsevier BV; Volume: 84; Issue: 3 Linguagem: Inglês
10.1016/j.athoracsur.2007.04.068
ISSN1552-6259
AutoresGiovanni Filardo, Cody Hamilton, Baron L. Hamman, Paul Grayburn,
Tópico(s)Cardiac Valve Diseases and Treatments
ResumoFactors associated with morbidity and mortality after isolated coronary artery bypass graft surgery (CABG) have been extensively investigated, yet uncertainty regarding the risk associated with obesity (or cachexia) and a number of postoperative adverse outcomes remains [1Hamman B.L. Filardo G. Hamilton C. Grayburn P.A. Effect of body mass index on risk of long-term mortality following coronary artery bypass grafting.Am J Cardiol. 2006; 98: 734-738Abstract Full Text Full Text PDF PubMed Scopus (11) Google Scholar, 2Reeves B.C. Ascione R. Chamberlain M.H. Angelini G.D. Effect of body mass index on early outcomes in patients undergoing coronary artery bypass surgery.J Am Coll Cardiol. 2003; 42: 668-676Abstract Full Text Full Text PDF PubMed Scopus (205) Google Scholar, 3Engelman D.T. Adams D.H. Byrne J.G. et al.Impact of body mass index and albumin on morbidity and mortality after cardiac surgery.J Thorac Cardiovasc Surg. 1999; 118: 866-873Abstract Full Text Full Text PDF PubMed Scopus (382) Google Scholar, 4Prabhakar G. Haan C.K. Peterson E.D. Coombs L.P. Cruzzavala J.L. Murray G.F. The risks of moderate and extreme obesity for coronary artery bypass grafting outcomes: a study from The Society of Thoracic Surgeons' database.Ann Thorac Surg. 2002; 74 (discussion 1130–1): 1125-1130Abstract Full Text Full Text PDF PubMed Scopus (154) Google Scholar, 5Habib R.H. Zacharias A. Schwann T.A. Riordan C.J. Durham S.J. 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: 1976-1986Abstract Full Text Full Text PDF PubMed Scopus (114) Google Scholar, 6Kuduvalli M. Grayson A.D. Oo A.Y. Fabri B.M. Rashid A. Risk of morbidity and in-hospital mortality in obese patients undergoing coronary artery bypass surgery.Eur J Cardiothorac Surg. 2002; 22: 787-793Crossref PubMed Scopus (117) Google Scholar, 7Birkmeyer N.J. Charlesworth D.C. Hernandez F. et al.Obesity and risk of adverse outcomes associated with coronary artery bypass surgery Northern New England Cardiovascular Disease Study Group.Circulation. 1998; 97: 1689-1694Crossref PubMed Scopus (217) Google Scholar, 8Koshal A. Hendry P. Raman S.V. Keon W.J. Should obese patients not undergo coronary artery surgery?.Can J Surg. 1985; 28: 331-334PubMed Google Scholar, 9Lindhout A.H. Wouters C.W. Noyez L. Influence of obesity on in-hospital and early mortality and morbidity after myocardial revascularization.Eur J Cardiothorac Surg. 2004; 26: 535-541Crossref PubMed Scopus (35) Google Scholar, 10Moulton M.J. Creswell L.L. Mackey M.E. Cox J.L. Rosenbloom M. Obesity is not a risk factor for significant adverse outcomes after cardiac surgery.Circulation. 1996; 94: II87-II92PubMed Google Scholar, 11Rockx M.A. Fox S.A. Stitt L.W. et al.Is obesity a predictor of mortality, morbidity and readmission after cardiac surgery?.Can J Surg. 2004; 47: 34-38PubMed Google Scholar, 12Ranucci M. Cazzaniga A. Soro G. Morricone L. Enrini R. Caviezel F. Obesity and coronary artery surgery.J Cardiothorac Vasc Anesth. 1999; 13: 280-284Abstract Full Text PDF PubMed Scopus (26) Google Scholar, 13Brandt M. Harder K. Walluscheck K.P. et al.Severe obesity does not adversely affect perioperative mortality and morbidity in coronary artery bypass surgery.Eur J Cardiothorac Surg. 2001; 19: 662-666Crossref PubMed Scopus (96) Google Scholar, 14Romero-Corral A. Montori V.M. Somers V.K. et al.Association of bodyweight with total mortality and with cardiovascular events in coronary artery disease: a systematic review of cohort studies.Lancet. 2006; 368: 666-678Abstract Full Text Full Text PDF PubMed Scopus (1247) Google Scholar, 15Wigfield C.H. Lindsey J.D. Munoz A. Chopra P.S. Edwards N.M. Love R.B. Is extreme obesity a risk factor for cardiac surgery? An analysis of patients with a BMI > or = 40.Eur J Cardiothorac Surg. 2006; 29: 434-440Crossref PubMed Scopus (135) Google Scholar]. For the most part, research focused on describing the relationship between body mass index (BMI), as a proxy for body fatness, and post-CABG adverse events using a wide variation of pre-defined categorizations (eg, the World Health Organization or the American Heart Association) or using arbitrary BMI categorizations [2Reeves B.C. Ascione R. Chamberlain M.H. Angelini G.D. Effect of body mass index on early outcomes in patients undergoing coronary artery bypass surgery.J Am Coll Cardiol. 2003; 42: 668-676Abstract Full Text Full Text PDF PubMed Scopus (205) Google Scholar, 3Engelman D.T. Adams D.H. Byrne J.G. et al.Impact of body mass index and albumin on morbidity and mortality after cardiac surgery.J Thorac Cardiovasc Surg. 1999; 118: 866-873Abstract Full Text Full Text PDF PubMed Scopus (382) Google Scholar, 4Prabhakar G. Haan C.K. Peterson E.D. Coombs L.P. Cruzzavala J.L. Murray G.F. The risks of moderate and extreme obesity for coronary artery bypass grafting outcomes: a study from The Society of Thoracic Surgeons' database.Ann Thorac Surg. 2002; 74 (discussion 1130–1): 1125-1130Abstract Full Text Full Text PDF PubMed Scopus (154) Google Scholar, 5Habib R.H. Zacharias A. Schwann T.A. Riordan C.J. Durham S.J. 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: 1976-1986Abstract Full Text Full Text PDF PubMed Scopus (114) Google Scholar, 6Kuduvalli M. Grayson A.D. Oo A.Y. Fabri B.M. Rashid A. Risk of morbidity and in-hospital mortality in obese patients undergoing coronary artery bypass surgery.Eur J Cardiothorac Surg. 2002; 22: 787-793Crossref PubMed Scopus (117) Google Scholar, 7Birkmeyer N.J. Charlesworth D.C. Hernandez F. et al.Obesity and risk of adverse outcomes associated with coronary artery bypass surgery Northern New England Cardiovascular Disease Study Group.Circulation. 1998; 97: 1689-1694Crossref PubMed Scopus (217) Google Scholar, 8Koshal A. Hendry P. Raman S.V. Keon W.J. Should obese patients not undergo coronary artery surgery?.Can J Surg. 1985; 28: 331-334PubMed Google Scholar, 9Lindhout A.H. Wouters C.W. Noyez L. Influence of obesity on in-hospital and early mortality and morbidity after myocardial revascularization.Eur J Cardiothorac Surg. 2004; 26: 535-541Crossref PubMed Scopus (35) Google Scholar, 10Moulton M.J. Creswell L.L. Mackey M.E. Cox J.L. Rosenbloom M. Obesity is not a risk factor for significant adverse outcomes after cardiac surgery.Circulation. 1996; 94: II87-II92PubMed Google Scholar, 11Rockx M.A. Fox S.A. Stitt L.W. et al.Is obesity a predictor of mortality, morbidity and readmission after cardiac surgery?.Can J Surg. 2004; 47: 34-38PubMed Google Scholar, 12Ranucci M. Cazzaniga A. Soro G. Morricone L. Enrini R. Caviezel F. Obesity and coronary artery surgery.J Cardiothorac Vasc Anesth. 1999; 13: 280-284Abstract Full Text PDF PubMed Scopus (26) Google Scholar, 13Brandt M. Harder K. Walluscheck K.P. et al.Severe obesity does not adversely affect perioperative mortality and morbidity in coronary artery bypass surgery.Eur J Cardiothorac Surg. 2001; 19: 662-666Crossref PubMed Scopus (96) Google Scholar, 14Romero-Corral A. Montori V.M. Somers V.K. et al.Association of bodyweight with total mortality and with cardiovascular events in coronary artery disease: a systematic review of cohort studies.Lancet. 2006; 368: 666-678Abstract Full Text Full Text PDF PubMed Scopus (1247) Google Scholar, 15Wigfield C.H. Lindsey J.D. Munoz A. Chopra P.S. Edwards N.M. Love R.B. Is extreme obesity a risk factor for cardiac surgery? An analysis of patients with a BMI > or = 40.Eur J Cardiothorac Surg. 2006; 29: 434-440Crossref PubMed Scopus (135) Google Scholar]. Typically such studies described the relationship between BMI and a broad spectrum of postoperative adverse outcomes (eg, stroke, operative mortality) using the same BMI categorization for all the outcomes [2Reeves B.C. Ascione R. Chamberlain M.H. Angelini G.D. Effect of body mass index on early outcomes in patients undergoing coronary artery bypass surgery.J Am Coll Cardiol. 2003; 42: 668-676Abstract Full Text Full Text PDF PubMed Scopus (205) Google Scholar, 3Engelman D.T. Adams D.H. Byrne J.G. et al.Impact of body mass index and albumin on morbidity and mortality after cardiac surgery.J Thorac Cardiovasc Surg. 1999; 118: 866-873Abstract Full Text Full Text PDF PubMed Scopus (382) Google Scholar, 4Prabhakar G. Haan C.K. Peterson E.D. Coombs L.P. Cruzzavala J.L. Murray G.F. The risks of moderate and extreme obesity for coronary artery bypass grafting outcomes: a study from The Society of Thoracic Surgeons' database.Ann Thorac Surg. 2002; 74 (discussion 1130–1): 1125-1130Abstract Full Text Full Text PDF PubMed Scopus (154) Google Scholar, 5Habib R.H. Zacharias A. Schwann T.A. Riordan C.J. Durham S.J. 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: 1976-1986Abstract Full Text Full Text PDF PubMed Scopus (114) Google Scholar, 6Kuduvalli M. Grayson A.D. Oo A.Y. Fabri B.M. Rashid A. Risk of morbidity and in-hospital mortality in obese patients undergoing coronary artery bypass surgery.Eur J Cardiothorac Surg. 2002; 22: 787-793Crossref PubMed Scopus (117) Google Scholar, 7Birkmeyer N.J. Charlesworth D.C. Hernandez F. et al.Obesity and risk of adverse outcomes associated with coronary artery bypass surgery Northern New England Cardiovascular Disease Study Group.Circulation. 1998; 97: 1689-1694Crossref PubMed Scopus (217) Google Scholar, 8Koshal A. Hendry P. Raman S.V. Keon W.J. Should obese patients not undergo coronary artery surgery?.Can J Surg. 1985; 28: 331-334PubMed Google Scholar, 9Lindhout A.H. Wouters C.W. Noyez L. Influence of obesity on in-hospital and early mortality and morbidity after myocardial revascularization.Eur J Cardiothorac Surg. 2004; 26: 535-541Crossref PubMed Scopus (35) Google Scholar, 10Moulton M.J. Creswell L.L. Mackey M.E. Cox J.L. Rosenbloom M. Obesity is not a risk factor for significant adverse outcomes after cardiac surgery.Circulation. 1996; 94: II87-II92PubMed Google Scholar, 11Rockx M.A. Fox S.A. Stitt L.W. et al.Is obesity a predictor of mortality, morbidity and readmission after cardiac surgery?.Can J Surg. 2004; 47: 34-38PubMed Google Scholar, 12Ranucci M. Cazzaniga A. Soro G. Morricone L. Enrini R. Caviezel F. Obesity and coronary artery surgery.J Cardiothorac Vasc Anesth. 1999; 13: 280-284Abstract Full Text PDF PubMed Scopus (26) Google Scholar, 13Brandt M. Harder K. Walluscheck K.P. et al.Severe obesity does not adversely affect perioperative mortality and morbidity in coronary artery bypass surgery.Eur J Cardiothorac Surg. 2001; 19: 662-666Crossref PubMed Scopus (96) Google Scholar, 14Romero-Corral A. Montori V.M. Somers V.K. et al.Association of bodyweight with total mortality and with cardiovascular events in coronary artery disease: a systematic review of cohort studies.Lancet. 2006; 368: 666-678Abstract Full Text Full Text PDF PubMed Scopus (1247) Google Scholar, 15Wigfield C.H. Lindsey J.D. Munoz A. Chopra P.S. Edwards N.M. Love R.B. Is extreme obesity a risk factor for cardiac surgery? An analysis of patients with a BMI > or = 40.Eur J Cardiothorac Surg. 2006; 29: 434-440Crossref PubMed Scopus (135) Google Scholar]. However, the shape of the association between BMI and each of the postoperative adverse outcomes is unique, and ignoring this during data analysis (ie, using a single BMI categorization to investigate all adverse outcomes) can critically affect study results [16Harrell Jr, F.E. Lee K.L. Mark D.B. Multivariable prognostic models: issues in developing models, evaluating assumptions and adequacy, and measuring and reducing errors.Stat Med. 1996; 15: 361-387Crossref PubMed Scopus (7526) Google Scholar, 17Harrell Jr, F.E. Regression modeling strategies: with application to linear models, logistic regression, and survival analysis. Springer-Verlag, New York2001Crossref Google Scholar, 18Lee D.S. Austin P.C. Rouleau J.L. Liu P.P. Naimark D. Tu J.V. Predicting mortality among patients hospitalized for heart failure: derivation and validation of a clinical model.JAMA. 2003; 290: 2581-2587Crossref PubMed Scopus (1151) Google Scholar, 19Hamilton C. Filardo G. The dangers of categorizing body mass index.Eur Heart J. 2006; 27: 2903-2904Crossref PubMed Scopus (8) Google Scholar]. Moreover, grouping BMI into classes carries serious dangers in itself, as categorization can bias inference regarding BMI and post-CABG morbidity and mortality [16Harrell Jr, F.E. Lee K.L. Mark D.B. Multivariable prognostic models: issues in developing models, evaluating assumptions and adequacy, and measuring and reducing errors.Stat Med. 1996; 15: 361-387Crossref PubMed Scopus (7526) Google Scholar, 17Harrell Jr, F.E. Regression modeling strategies: with application to linear models, logistic regression, and survival analysis. Springer-Verlag, New York2001Crossref Google Scholar, 19Hamilton C. Filardo G. The dangers of categorizing body mass index.Eur Heart J. 2006; 27: 2903-2904Crossref PubMed Scopus (8) Google Scholar, 20Royston P. Altman D.G. Sauerbrei W. Dichotomizing continuous predictors in multiple regression: a bad idea.Stat Med. 2006; 25: 127-141Crossref PubMed Scopus (1512) Google Scholar, 21Filardo G. Hamilton C. Hamman B.L. Ng H.K.T. Grayburn P. Categorizing BMI may lead to biased results in studies investigating in-hospital mortality following isolated CABG.J Clin Epidemiol. 2007; (in press)PubMed Google Scholar]. We hypothesize that BMI categorization may be one cause of the inconsistent findings regarding the association between obesity and cachexia and adverse outcomes after cardiac surgery. To investigate this hypothesis, we considered the relationship between BMI and the risk of stroke after CABG. We conducted literature searches using PubMed to gather studies that have reported on this topic. Search strategies were formulated to retrieve records published in English that combined terms related to BMI, isolated CABG, and stroke. Articles listed in the references of the identified reports were also considered for a review of the statistical methods and results (see Table 1). All the identified studies [2Reeves B.C. Ascione R. Chamberlain M.H. Angelini G.D. Effect of body mass index on early outcomes in patients undergoing coronary artery bypass surgery.J Am Coll Cardiol. 2003; 42: 668-676Abstract Full Text Full Text PDF PubMed Scopus (205) Google Scholar, 3Engelman D.T. Adams D.H. Byrne J.G. et al.Impact of body mass index and albumin on morbidity and mortality after cardiac surgery.J Thorac Cardiovasc Surg. 1999; 118: 866-873Abstract Full Text Full Text PDF PubMed Scopus (382) Google Scholar, 4Prabhakar G. Haan C.K. Peterson E.D. Coombs L.P. Cruzzavala J.L. Murray G.F. The risks of moderate and extreme obesity for coronary artery bypass grafting outcomes: a study from The Society of Thoracic Surgeons' database.Ann Thorac Surg. 2002; 74 (discussion 1130–1): 1125-1130Abstract Full Text Full Text PDF PubMed Scopus (154) Google Scholar, 5Habib R.H. Zacharias A. Schwann T.A. Riordan C.J. Durham S.J. 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: 1976-1986Abstract Full Text Full Text PDF PubMed Scopus (114) Google Scholar, 6Kuduvalli M. Grayson A.D. Oo A.Y. Fabri B.M. Rashid A. Risk of morbidity and in-hospital mortality in obese patients undergoing coronary artery bypass surgery.Eur J Cardiothorac Surg. 2002; 22: 787-793Crossref PubMed Scopus (117) Google Scholar, 7Birkmeyer N.J. Charlesworth D.C. Hernandez F. et al.Obesity and risk of adverse outcomes associated with coronary artery bypass surgery Northern New England Cardiovascular Disease Study Group.Circulation. 1998; 97: 1689-1694Crossref PubMed Scopus (217) Google Scholar, 8Koshal A. Hendry P. Raman S.V. Keon W.J. Should obese patients not undergo coronary artery surgery?.Can J Surg. 1985; 28: 331-334PubMed Google Scholar, 9Lindhout A.H. Wouters C.W. Noyez L. Influence of obesity on in-hospital and early mortality and morbidity after myocardial revascularization.Eur J Cardiothorac Surg. 2004; 26: 535-541Crossref PubMed Scopus (35) Google Scholar, 10Moulton M.J. Creswell L.L. Mackey M.E. Cox J.L. Rosenbloom M. Obesity is not a risk factor for significant adverse outcomes after cardiac surgery.Circulation. 1996; 94: II87-II92PubMed Google Scholar, 11Rockx M.A. Fox S.A. Stitt L.W. et al.Is obesity a predictor of mortality, morbidity and readmission after cardiac surgery?.Can J Surg. 2004; 47: 34-38PubMed Google Scholar, 12Ranucci M. Cazzaniga A. Soro G. Morricone L. Enrini R. Caviezel F. Obesity and coronary artery surgery.J Cardiothorac Vasc Anesth. 1999; 13: 280-284Abstract Full Text PDF PubMed Scopus (26) Google Scholar, 13Brandt M. Harder K. Walluscheck K.P. et al.Severe obesity does not adversely affect perioperative mortality and morbidity in coronary artery bypass surgery.Eur J Cardiothorac Surg. 2001; 19: 662-666Crossref PubMed Scopus (96) Google Scholar, 15Wigfield C.H. Lindsey J.D. Munoz A. Chopra P.S. Edwards N.M. Love R.B. Is extreme obesity a risk factor for cardiac surgery? An analysis of patients with a BMI > or = 40.Eur J Cardiothorac Surg. 2006; 29: 434-440Crossref PubMed Scopus (135) Google Scholar] investigated the effect of BMI on multiple adverse outcomes in addition to stroke (including mortality, renal failure, deep sternal infection, and extended length of stay), yet reported results were typically based on a single BMI categorization. Categorization schemes used to analyze BMI were found to be very inconsistent across the 13 studies reviewed (Table 1). This inconsistency was especially pronounced in the categorizations used to define BMI classes for extreme values. Results regarding the effect of BMI on the risk of stroke were inconclusive, and in some instances they seemed to conflict across the studies (Table 1).Table 1Adjusted Association Between BMI (kg/m2) and Stroke, and BMI Categorization Schemes for Studies Investigating Adverse Operative Outcomes After Coronary Artery Bypass Graft SurgeryAuthorsCohort SizeStudies' Weight Classes Cutoff Points and LabelsStudy Findings for Stroke (95% CI)UnderweightNormalOverweightObeseSeverely ObeseReeves and colleagues (2Reeves B.C. Ascione R. Chamberlain M.H. Angelini G.D. Effect of body mass index on early outcomes in patients undergoing coronary artery bypass surgery.J Am Coll Cardiol. 2003; 42: 668-676Abstract Full Text Full Text PDF PubMed Scopus (205) Google Scholar)4,372< 2020–24.925–29.930–34.9≥ 35OW vs N = 1.94 (1.17, 3.21)Engelman and colleagues (3Engelman D.T. Adams D.H. Byrne J.G. et al.Impact of body mass index and albumin on morbidity and mortality after cardiac surgery.J Thorac Cardiovasc Surg. 1999; 118: 866-873Abstract Full Text Full Text PDF PubMed Scopus (382) Google Scholar)5,168< 2020–30> 30UW vs N = 1.70 (1.00, 2.90)Prabhakar and colleagues (4Prabhakar G. Haan C.K. Peterson E.D. Coombs L.P. Cruzzavala J.L. Murray G.F. The risks of moderate and extreme obesity for coronary artery bypass grafting outcomes: a study from The Society of Thoracic Surgeons' database.Ann Thorac Surg. 2002; 74 (discussion 1130–1): 1125-1130Abstract Full Text Full Text PDF PubMed Scopus (154) Google Scholar)559,00418.5–24.925–34.9aDefined by investigators as "mildly obese";35–39.9≥ 40O vs OW N = 0.91 (0.83, 0.99)Wigfield and colleagues (15Wigfield C.H. Lindsey J.D. Munoz A. Chopra P.S. Edwards N.M. Love R.B. Is extreme obesity a risk factor for cardiac surgery? An analysis of patients with a BMI > or = 40.Eur J Cardiothorac Surg. 2006; 29: 434-440Crossref PubMed Scopus (135) Google Scholar)1,78020–3030–39.9≥ 40NSHabib and colleagues (5Habib R.H. Zacharias A. Schwann T.A. Riordan C.J. Durham S.J. 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: 1976-1986Abstract Full Text Full Text PDF PubMed Scopus (114) Google Scholar)6,06822–32balso required a body surface area > 1.85;32.1–35.9calso required a body surface area >1.85—this category is defined by investigators as "moderately obese."≥ 36NSKuduvalli and colleagues (6Kuduvalli M. Grayson A.D. Oo A.Y. Fabri B.M. Rashid A. Risk of morbidity and in-hospital mortality in obese patients undergoing coronary artery bypass surgery.Eur J Cardiothorac Surg. 2002; 22: 787-793Crossref PubMed Scopus (117) Google Scholar)4,713< 3030–34.9≥ 35NSBirkmeyer and colleagues (7Birkmeyer N.J. Charlesworth D.C. Hernandez F. et al.Obesity and risk of adverse outcomes associated with coronary artery bypass surgery Northern New England Cardiovascular Disease Study Group.Circulation. 1998; 97: 1689-1694Crossref PubMed Scopus (217) Google Scholar)11,101≤ 3030.1–36> 36NSKoshal and colleagues (8Koshal A. Hendry P. Raman S.V. Keon W.J. Should obese patients not undergo coronary artery surgery?.Can J Surg. 1985; 28: 331-334PubMed Google Scholar)200≤ 27> 27NSBrandt and colleagues (13Brandt M. Harder K. Walluscheck K.P. et al.Severe obesity does not adversely affect perioperative mortality and morbidity in coronary artery bypass surgery.Eur J Cardiothorac Surg. 2001; 19: 662-666Crossref PubMed Scopus (96) Google Scholar)500< 30≥ 30NSLindhout and colleagues (9Lindhout A.H. Wouters C.W. Noyez L. Influence of obesity on in-hospital and early mortality and morbidity after myocardial revascularization.Eur J Cardiothorac Surg. 2004; 26: 535-541Crossref PubMed Scopus (35) Google Scholar)1,130NSMoulton and colleagues (10Moulton M.J. Creswell L.L. Mackey M.E. Cox J.L. Rosenbloom M. Obesity is not a risk factor for significant adverse outcomes after cardiac surgery.Circulation. 1996; 94: II87-II92PubMed Google Scholar)2,299≤ 30> 30NSRockx and colleagues (11Rockx M.A. Fox S.A. Stitt L.W. et al.Is obesity a predictor of mortality, morbidity and readmission after cardiac surgery?.Can J Surg. 2004; 47: 34-38PubMed Google Scholar)1,310NSRanucci and colleagues (12Ranucci M. Cazzaniga A. Soro G. Morricone L. Enrini R. Caviezel F. Obesity and coronary artery surgery.J Cardiothorac Vasc Anesth. 1999; 13: 280-284Abstract Full Text PDF PubMed Scopus (26) Google Scholar)345Men: ≤ 30Men: > 30NSWomen: ≤ 28.6Women: > 28.6NSBMI = body mass index; CI = confidence interval; N = normal; NS = not significant at alpha (ie, 0.05); O = obese; OR = odds ratio; OW = overweight; SO = severely obese; UW = underweight.a Defined by investigators as "mildly obese";b also required a body surface area > 1.85;c also required a body surface area >1.85—this category is defined by investigators as "moderately obese." Open table in a new tab BMI = body mass index; CI = confidence interval; N = normal; NS = not significant at alpha (ie, 0.05); O = obese; OR = odds ratio; OW = overweight; SO = severely obese; UW = underweight. We used data on all consecutive patients who underwent isolated CABG surgery at Baylor University Medical Center (Dallas, TX) between January 1, 1997 and November 30, 2003 to investigate the association between BMI and risk of stroke. These data are described elsewhere [21Filardo G. Hamilton C. Hamman B.L. Ng H.K.T. Grayburn P. Categorizing BMI may lead to biased results in studies investigating in-hospital mortality following isolated CABG.J Clin Epidemiol. 2007; (in press)PubMed Google Scholar]. Preoperative endocarditis, previous valve surgery, minimally invasive procedure, ventricular assist device, or missing BMI (n = 41) resulted in the exclusion of patients. Those patients with missing variables were considered for the present investigation, and multiple imputation was performed for continuous and ordinal missing variables by predictive mean matching [22Little R. An H. Robust likelihood-based analysis of multivariate data with missing values.Statistica Sinica. 2004; 14: 933-952Google Scholar], as recommended by The Society of Thoracic Surgeons' workforce evidence-based surgery report [23Shahian D.M. Blackstone E.H. Edwards F.H. et al.Cardiac surgery risk models: a position article.Ann Thorac Surg. 2004; 78: 1868-1877Abstract Full Text Full Text PDF PubMed Scopus (132) Google Scholar]. The final study cohort included 5,762 patients (98.11% of the initial population). Stroke was defined as a central neurologic deficit persisting postoperatively for greater than 72 hours as in The Society of Thoracic Surgeons' adult cardiac database (see http://www.sts.org) [24Edwards F.H. Evolution of The Society of Thoracic Surgeons National Cardiac Surgery Database.J Invasive Cardiol. 1998; : 485-488PubMed Google Scholar, 25Ferguson Jr, T.B. Dziuban Jr, S.W. Edwards F.H. et al.The STS National Database: current changes and challenges for the new millennium Committee to Establish a National Database in Cardiothoracic Surgery, The Society of Thoracic Surgeons.Ann Thorac Surg. 2000; 69: 680-691Abstract Full Text Full Text PDF PubMed Scopus (220) Google Scholar]. The BMI was investigated without being forced into categories; a smoothing technique (restricted cubic splining [17Harrell Jr, F.E. Regression modeling strategies: with application to linear models, logistic regression, and survival analysis. Springer-Verlag, New York2001Crossref Google Scholar]) was used instead. A generalized propensity score [26Imai K. van Dyk D. Causal inference with general treatment regimes: generalizing the propensity score.JASA. 2004; 99: 854-866Crossref Scopus (537) Google Scholar] approach was used to account for possible confounding of the relationship between BMI and stroke in this cohort of patients. The score was created by a linear regression of BMI onto clinical and nonclinical factors considered by The Society of Thoracic Surgeons as risk factors for stroke following CABG (see http://www.sts.org). The complete list of variables included in the propensity model is presented in Figure 1. Among the BMI categorization schemes identified from the literature, the one used by Reeves and colleagues [2Reeves B.C. Ascione R. Chamberlain M.H. Angelini G.D. Effect of body mass index on early outcomes in patients undergoing coronary artery bypass surgery.J Am Coll Cardiol. 2003; 42: 668-676Abstract Full Text Full Text PDF PubMed Scopus (205) Google Scholar] used the largest number of BMI categories, and hence was ostensibly the most flexible (ie, certainly more flexible than simply dichotomizing BMI [eg, at 30 kg/m2]). We contrasted our findings from the smoothed fit with those obtained using the Reeves and colleagues [2Reeves B.C. Ascione R. Chamberlain M.H. Angelini G.D. Effect of body mass index on early outcomes in patients undergoing coronary artery bypass surgery.J Am Coll Cardiol. 2003; 42: 668-676Abstract Full Text Full Text PDF PubMed Scopus (205) Google Scholar] categorization, which we adjusted using the same generalized propensity score. The median BMI in this cohort of patients was 27.9 kg/m2, and postoperative stroke occurred in 1.9% of the patients (n = 111). The propensity-adjusted model for the smoothed association between BMI and postoperative stroke revealed a moderate effect of BMI on the risk of stroke (p = 0.056). Based on the model using the categorization scheme followed by Reeves and colleagues [2Reeves B.C. Ascione R. Chamberlain M.H. Angelini G.D. Effect of body mass index on early outcomes in patients undergoing coronary artery bypass surgery.J Am Coll Cardiol. 2003; 42: 668-676Abstract Full Text Full Text PDF PubMed Scopus (205) Google Scholar], patients within the ranges of 25 kg/m2 to 29.9 kg/m2 (odds ratio = 0.54; 95% confidence interval: 0.34, 0.85) and 30 kg/m2 to 34.9 kg/m2 (odds ratio = 0.53; 95% confidence interval: 0.30, 0.95) were less likely to experience postoperative stroke compared with patients whose BMIs were within the 20 kg/m2 to 24.9 kg/m2 range. The elevated risk associated with cachexia was not found to be significant likely due to the small number of cachectic patients. The plot of the smoothed association between BMI and the adjusted predicted stroke risk (Fig 1) showed a U-shaped relationship between BMI and the risk of stroke, with the lowest estimated risk of stroke for subjects with a BMI in the low 30s and sharply increasing stroke risk for subjects with BMI values lower than 30 or higher than the mid-40s. Figure 1 also indicates that the Reeves and colleagues [2Reeves B.C. Ascione R. Chamberlain M.H. Angelini G.D. Effect of body mass index on early outcomes in patients undergoing coronary artery bypass surgery.J Am Coll Cardiol. 2003; 42: 668-676Abstract Full Text Full Text PDF PubMed Scopus (205) Google Scholar] categorization provides a very poor fit to the smoothed risk as the estimated hazard for certain BMI categories (BMI, 30 kg/m2 to 34.9 kg/m2) deviate from the curve representing the BMI and stroke association while forcing entire ranges of BMI to receive the same risk estimate. As a result of the poor fit, inferences regarding risk differences between patients in these BMI groupings will be inaccurate. Furthermore, the categorization produces steep changes in the predicted risk, which would result in conclusions that are clinically illogical. For example, consider 2 patients with BMI equaling 24.99 kg/m2 and 25.01 kg/m2, respectively, which is a difference in BMI of 0.02 kg/m2. Based on the results of the model following the Reeves and colleagues [2Reeves B.C. Ascione R. Chamberlain M.H. Angelini G.D. Effect of body mass index on early outcomes in patients undergoing coronary artery bypass surgery.J Am Coll Cardiol. 2003; 42: 668-676Abstract Full Text Full Text PDF PubMed Scopus (205) Google Scholar] categorization, these 2 patients received quite different risk estimates, despite the almost negligible differences in their BMI values. On the other hand, a patient with a BMI of 24.99 kg/m2 and a patient with a BMI of 20.01 kg/m2 (equaling a difference in BMI of 4.98 kg/m2) receive exactly the same risk estimate, despite having a much larger difference in BMI values. The problem with assigning all patients within each BMI category a single risk estimate is particularly notable in the BMI > 35 kg/m2 category, in which it masks the increasing risk associated with a BMI > 40 kg/m2. As there are more patients with BMI closer to 35 kg/m2 than there are with a BMI > 40 kg/m2, the odds ratio comparing the >35 kg/m2 BMI category to the reference group (BMI 20 kg/m2 to 24.9 kg/m2) is heavily weighted toward the patients with a BMI close to 35 kg/m2. Because these patients have a reduced risk of stroke (as shown in Fig 1), the entire >35 kg/m2 BMI category has a biased (low) estimated risk. Furthermore, conclusions based on the risk estimates associated with the categorization will erroneously indicate that the risk of stroke will decrease with increasing BMI. Taken together, these results demonstrate how categorizations can "miss-specify" the relationship between BMI and the risk of stroke. Conceivably, this consideration can be extended to any study investigating the effect of BMI through categorization and any adverse cardiac surgical outcome in the literature. In addition to the poor fit that BMI groupings provide, even categorizations that by chance mimic the true association between BMI and surgical risk may miss significant findings due to small patient counts in some of the BMI groups, especially at the extremities of the BMI range. Future research should avoid grouping BMI and other continuous risk factors, such as age or ejection fraction, into categories to allow for a more flexible and unbiased association with surgical risk. We would like to acknowledge the use of software from Prof Frank Harrell (Hmisc and Design Libraries) and thank Briget da Graca for her writing and editorial assistance.
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