Underweight or Obesity Paradox: Do BMI Classifications Predict Long-Term Survival after Cardiac Surgery?

Objectives: Numerous studies have analyzed the impact of different body mass index (BMI) classes on the early outcome after cardiac surgery. These studies reported that obese patients may have higher rates of wound healing disorders and perioperative ventilation disturbances but that their survival was not different from that of patients with normal weight. However, little is known about the effects of different BMI classes on the long-term outcome after cardiac surgery.

Methods: We analyzed 18,072 patients who underwent cardiac surgery using cardiopulmonary bypass between 2001 and 2017. Data were obtained from an ongoing quality assessment program. BMI classifications were used according to the WHO definition: underweight < 18.5, normal 18.5 to 25, overweight 25 to 30, and obese >30. Primary end point was all-cause mortality. Risk stratifications were analyzed by Cox multiple regression analysis. Each BMI class was tested as an independent risk factor against age, kind of surgery, LV function, bypass time, and diabetes.

Results: Kaplan–Meier analysis showed a mean survival of 11.8 ± 0.1 years. Multiple regression analysis revealed underweight as the only significant risk factor of the different BMI classes (p < 0.001). Underweight was a risk factor for long-term mortality, independent from age, kind of surgery, LV function, bypass time, and diabetes (as shown in table). Underweight showed the highest hazard ratio for mortality in the long term, despite highly significant results for the other known risk factors.

Multiple regression analyses regarding possible effects on long-term mortality revealed no significant difference between patients with normal weight (p = 0.187), overweight (p = 0.932), or obese (p = 0.528).

	Hazard ratio (CI)	p-Value
Abbreviations: BMI, body mass index; CABG, coronary artery bypass grafting; CI, confidence interval; LV, left ventricular.
Note: Multiple regression analyses on long-term mortality.
Underweight (BMI < 18.5)	2.332 (1.614–3.370)	<0.0001
CABG	0.510 (0.434–0.600)	<0.0001
Valve	0.679 (0.577–0.799)	<0.0001
CABG plus valve	0.707 (0.596–0.839)	<0.0001
Aortic surgery	0.816 (0.577–1.050)	0.114
Cardiopulmonary bypass time	1.002 (1.002–1.002)	<0.0001
LV function	0.988 (0.986–0.991)	<0.0001
Age	1.069 (1.063–1.075)	<0.0001
Diabetes	1.218 (1.166–1.272)	<0.0001

Conclusion: Underweight was identified as an independent risk factor for long-term mortality after cardiac surgery. Furthermore, obese or overweight patients do not have an increased risk for long-term mortality after cardiac surgery.