Objective

Several papers have documented an obesity paradox in which overweight and obese patients undergoing cardiac surgery have a better prognosis compared with a patient with normal body mass index (BMI). However, conflicting results exist, possibly related to different risk profiles among populations and BMI categories. We sought to clarify the effect of BMI on clinical outcomes following cardiac surgery.

Methods

Prospectively collected data were extracted from The National Institute for Cardiovascular Outcomes Research National Adult Cardiac Surgery Audit registry, including all cardiac surgery procedures performed in UK and Ireland between 1 April 2002 and 31 March 2013. BMI was categorised according to the World Health Organisation classification as underweight (BMI<18.5 Kg/m2), normal (BMI 18.5-25), overweight (BMI 25-30), obese class I (BMI 30-35), class II (BMI 35-40), and class III (BMI≥40).

Results

A total of 350,286 patients were included (mean age 65.9 years; male 73%), and 1% of patients were underweight, 26% normal, 43% overweight, and 30% obese. Mortality rates differed among underweight (8.5%), normal (4.4%), overweight (2.8%), and obese (2.8%) patients. These differences remained statistically significant after adjustment for confounders in propensity matching and regression analyses, and after exclusion of patients with poor exercise tolerance, severe chronic diseases, and low BMI.

Conclusion

In this large national registry, the existence of an obesity paradox in cardiac surgery was confirmed. Overweight and obese patients revealed significantly lower in-hospital mortality rates compared with normal-weight patients. The present data argued against reverse epidemiology or unmeasured confounders explaining our results, suggesting an underlying mechanistic explanation for the obesity paradox.