Abstract & Commentary
By Michael H. Crawford, MD, Editor
Synopsis: Body size is not a significant predictor of CABG mortality.
Source: Jin R, et al. Is Obesity a Risk Factor for Mortality in Coronary Artery Bypass Surgery? Circulation. 2005;111: 3359-3365.
Although obese patients are commonly referred for coronary artery bypass surgery (CABG), the data regarding its effect on operative risk are conflicting. Thus, Jin and colleagues from Portland, OR, studied over 16,000 patients who underwent isolated CABG in one institution from 1997 to 2003. They were divided into 6 groups depending on their pre-operative body mass index (BMI) in Kg/M2: underweight < 18.5; normal 18.5-24.9; overweight 25-29.9; mild obesity 30-34.9; moderate obesity 35-39.9; and severe obesity > 40. The in-hospital mortality was correlated to BMI as a continuous and categorical variable by several techniques. Initially, 32% of the patients were obese, but by the end of the study it was 40% (average 37%). Obese patients were more often younger, female, diabetic, and hypertensive. Underweight patients were also more often female, but also had more vascular and pulmonary disease and more heart failure. Obese patients had less mortality (1.9%) and underweight patients more (8%), as compared to the normal BMI group (3%). Underweight patients had more strokes, bleeding, and ICU ventilation. Obese patients had more sternal wound infections, but less renal failure. Cause of death was not different among the BMI groups.
Using BMI as a continuous variable, the lowest mortality was observed in the high-normal and overweight group (BMI 23-30). Jin et al concluded that body size is not a significant predictor of CABG mortality.
Well, we can add CABG surgery to chemotherapy as a rationale for carrying a little extra fat, but not too much, so that our risk of coronary artery disease, hypertension, and cancer doesn’t increase. Now we just need to know what that balance is exactly. Although this is a single institution study with a modest number of patients as compared to the larger Society of Thoracic Surgeons database, it is largely supportive of the results of other trials. Jin et al point out that only the very large databases seem to show a significant effect of obesity, consistent with a small clinical effect.
Their study has the advantage of including data on underweight patients. However, there were only 90 underweight patients (0.6% of the total), of whom, 7 died. Also, they used BMI rather than body surface area as the variable. It can be argued that BMI is imperfect and that a measure, such as waist circumference, might be better because it is more indicative of abdominal fat. Abdominal fat is predictive of coronary artery disease events, but not necessarily mortality with surgery. In addition, the analysis is robust because it takes into account the relationship between obesity, age, and gender.
Finally, morbidity is predicted by obesity, as has been shown in other studies. The incidence of deep sternal infections rises with increasing BMI. The need for re-operation for bleeding and blood transfusion increases progressively as BMI declines. Clearly, body fat is a double-edge sword, and this study adds to the national debate over its importance to health.