Is extreme obesity a risk factor for cardiac surgery?An analysis of patients with a BMI≥40

Abstract

The prevalence of obesity (Ob) has increased in the United States and most European countries during the last decade and is continuing to rise [1,2]. This has also been the experience in other affluent and developing societies [2]. The International Obesity Task Force, a non-government organization that studies the obesity epidemic, estimates one out of every five people worldwide is overweight or obese. Obesity, defined as a body mass index (BMI) higher than 30, has increased 8% in the United States since 1994 to 30% of the population. This was estimated from the most recent National Health and Nutrition Examination Survey (NHANES) of the Nation Health Centers of Disease Control and Prevention. It has also been estimated that age-adjusted extreme obesity (ExtOb), defined as a body mass index greater than 40, has increased from 2.9% [NHANES III (1988–1994)] to 4.7% [NHANES continuous (1999–2000)] during the same time period [1]. The costs of disease prevention and medical management of obese patients are of increasing concern for society [1]. 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. This is a retrospective analysis of 1920 consecutive patients undergoing cardiac surgery at the University of Wisconsin Hospitals and Clinics between January 1, 1999 and May 28, 2004, excluding cardiac transplantation and ventricular assist device (VAD) surgeries. 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 × 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. 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