New study has shown that a higher body fat mass increases the mortality in heart bypass surgery patients by over four times, while body mass index (BMI) by itself was not associated with the increase in deaths.

The study is being presented at the 'Euroanaesthesia' congress in Vienna, Austria (1-3 June). "Overall, our study showed that unlike BMI, lower lean body weight and higher fat mass in patients were independently associated with increased mortality after heart bypass surgery," said Dr Xavier Leroy, one of the researchers.

The team performed a retrospective study of 3373 patients who had undergone elective cardiac surgery with cardiopulmonary bypass from January 2013 until December 2016. Patient BMI (measured using the WHO definition) and body composition were calculated from clinical and administrative records and compared to patient mortality within 30 days of the operation. A further analysis was performed to investigate the association of BMI and body composition with a prolonged stay in the intensive care unit (ICU), with prolonged defined as being in the uppermost quartile (patients in the highest 25 per cent) of the length of stay (LOS).

Across the entire sample of patients, mortality within 30 days occurred in 2.1 per cent of cases and significant differences were observed among BMI, FM, and lean body weight (LBW0 categories. Unlike BMI however, FM and LBW were found to be independently associated with mortality.

The 25 per cent (quartile) of patients with the highest fat mass (FM) were 4.1 times more likely to die than 25 per cent with the lowest fat mass; and the 25 per cent of patients with the lowest lean body weight (muscle) were 2.8 times more likely to die than the 25 per cent of patients with the highest lean body weight.

There was no observed association between BMI and 30-day mortality but the authors did find an independent association between BMI and length of stay in the ICU. "BMI was independently associated with a prolonged ICU length of stay, as were being in the highest fat mass and lowest lean body weight categories," explained Dr Leroy. "The lower the LBW or the higher the FM and BMI were, the longer the length of stay in intensive care," added Leroy.