Obesity is associated with increased burden of cardiovascular risk factors, morbidity and mortality. However, several studies have counterintuitively shown better outcome after cardiac surgery in obese than in normal weight patients, a phenomenon known as obesity paradox. Furthermore, obesity has been linked with decreased need of red blood cell (RBC) transfusions. The purpose of this study was to evaluate the impact of body mass index (BMI) on 30-day mortality and RBC transfusions in patients undergoing cardiac surgery, a clinically important topic with conflicting previous data.

We retrospectively investigated 1691 patients who underwent coronary and/or valve or aortic root surgery using cardiopulmonary bypass between 2013 and 2016. The patients were categorized by BMI based on the World Health Organization classification. For analysis, logistic regression was used with adjustment for potential confounding factors.

Of the patients, 28.7% were normal weight, 43.3% overweight, 20.5% mildly obese and 7.5% severely obese. Thirty-day mortality was 1.9% without significant differences between the BMI groups. 41.0% of patients received RBC transfusion. Overweight [odds ratio (OR) 0.75, 95% confidence interval (CI) 0.56–0.99, P = 0.045], mildly (OR 0.65, 95% CI 0.46–0.92, P = 0.016) and severely obese (OR 0.41, 95% CI 0.24–0.70, P = 0.001) patients needed less frequently RBC transfusions than patients with normal weight.

Obesity was not associated with 30-day mortality but was associated with lower use of RBC transfusions in cardiac surgery.