Impact of Body Mass Index on mortality in Swiss hospital patients with ST-elevation myocardial infarction: does an obesity paradox exist?

BACKGROUND
The obesity paradox refers to the phenomenon that obese patients seem to have a better outcome than normal weight patients in a variety of disease conditions. The aim of this study was to investigate the impact of Body Mass Index (BMI) on mortality in patients with ST-elevation myocardial infarction (STEMI) who underwent percutaneous coronary intervention (PCI).

METHODS
Between January 2005 and July 2012, the Swiss AMIS Plus registry enrolled 6,938 patients with acute STEMI who underwent PCI. These patients were stratified into 5 BMI groups according to the classification system of the World Health Organisation. The odds for in-hospital mortality according to BMI groups were analysed using logistic regression with normal weight patients as the reference.

RESULTS
Crude in-hospital mortality rates showed a U-shaped distribution between BMI groups, with the lowest mortality in obese class I patients (2.0%) and the highest mortality in underweight patients (9.0%). The odds for in-hospital mortality were significantly lower for obese class I (OR 0.56; 95% CI 0.35-0.91) and significantly higher for underweight patients (OR 2.72; 95% CI 1.14-6.48) compared to the normal weight group and odds ratios showed a U-shaped distribution. After adjustment for covariates, the odds ratios maintained a U-shape distribution albeit the differences between BMI groups were no longer significant.

CONCLUSION
This study showed that the lower crude in-hospital mortality of obese class I patients can be partly explained by lower age and lower co-morbidity rates. However, further studies are needed to investigate favourable factors associated with class I obesity.