Is the Obesity Paradox in Cardiac Surgery Really a Myth? Effect of Body Mass Index on Early and Late Clinical Outcomes

Abstract

Objective: The present study was conducted to investigate the obesity paradox and assess the effect of body mass index (BMI) on early and late clinical outcomes after cardiac surgery.

Design: Cohort study with a retrospective analysis of prospectively collected data.

Design: Single-institution cardiology medical center.

Participants: The study comprised consecutive patients undergoing cardiac surgery from January 2009 to January 2019. Patients were divided into the following 4 groups defined by BMI: underweight (UW) (≤18.5 kg/m²): 0.5%, n = 27; normal weight (18.5-25 kg/m²): 25.7%, n = 1,393; overweight (OW) (>25-30 kg/m²): 44.7%, n = 2,423; and obese (OB) (≥30 kg/m²): 29.1%, n = 1,576.

Interventions: No interventions.

Measurements and main results: A multivariate analysis was used to compare clinical outcomes among the different BMI groups. Overall 1-year survival of patients in the BMI categories was determined by the Kaplan-Meier method and compared using the log rank test. The study included 5,419 patients. The BMI groups were significantly different regarding presurgical variables. Mortality according to BMI exhibited a reverse J-shaped relationship: 7.4% in the UW group, 5.2% in the normal weight group, 3.2% in the OW group, and 4.3% in the OB group (p = 0.016). Low- cardiac- output syndrome and bleeding were more frequent in the UW group, whereas mediastinitis and hyperglycemia were more common in the OB group. After adjusting for other risk factors, BMI was not an independent predictor of in-hospital mortality. One-year follow-up was completed in 95% of the patients, and the analysis of long-term mortality did not show a difference among the BMI categories (p log rank = 0.16).

Conclusion: OW patients had a lower mortality and better outcomes after cardiac surgery. However, when other preoperative variables were taken into account, BMI did not have independent effect on in-hospital and 1-year mortality.

Keywords: body mass index; cardiac surgery; mortality; obesity; risk factors.