Being underweight, and not overweight, has the highest mortality, cost, length of stay, and readmission rate for those undergoing cardiac catheterisation, according to an analysis of more than one million patients presented at ESC Congress.

This study examined the association of BMI with in–hospital mortality, cost of care, length of stay, and rate of readmission within 30 days in patients undergoing cardiac catheterisation in 2013 in a nationally representative cohort.

Researchers used the National Readmission Database and Nationwide Inpatient Sample Database to retrospectively analyse discharge and readmission data. These are the largest all payer USA inpatient databases and include more than 35 million hospitalisations annually.

In 2013, 1 035 727 patients underwent cardiac catheterisation, of which 42% also received PCI with a stent or balloon. When categorised by BMI, 0.4% of patients were underweight (BMI<19 kg/m2), while 11.4% were obese (BMI 30.1–40 kg/m2) and 8.0% were morbidly obese (BMI over 40 kg/m2). Of those undergoing cardiac catheterisation, only 25.8% of the underweight patients went on to receive PCI, while 32.5% of the morbidly obese, 41% of the overweight, 41% of the obese, and 43.2% of the normal weight categories went on to have a balloon or stent (PCI) placed for coronary blockages.

Despite the low percentage of cardiac catheterisations and lower rate of PCI compared to normal and overweight BMI groups, underweight patients were over three times more likely to die after cardiac catheterisation than morbidly obese patients and five times more likely to die than obese patients (6.0% mortality for underweight patients, 2.3% normal weight, 1.7% overweight, 1.2% obese, 1.9% morbidly obese). Interestingly, despite the extreme BMI, morbidly obese patients had a lower mortality rate than normal weight patients and obese patients had the lowest mortality of all groups undergoing cardiac catheterisation.

Length of stay for underweight patients was more than double that of normal weight patients (10.5 days versus 5.1 days) resulting in nearly 50% higher costs for underweight patients ($USD 33 540 versus $USD 22 581). Morbidly obese patients had a slightly longer length of stay and higher costs compared to normal weight patients (6.2 days, and $USD 23 889).

After adjustment for comorbidities, underweight patients were 18% more likely than normal weight patients to be readmitted within 30 days, while morbidly obese patients were 8.2% less likely to be readmitted within 30 days. Overweight and obese patients had the lowest readmission rates, and were over 10% less likely to be readmitted than normal weight patients within 30 days.

Dr Tariq said: “The obesity paradox has flummoxed researchers for some time, and our research also flips the conventional wisdom that a higher BMI should portend a worse outcome. We found that the lower BMI group had worse outcomes across the board, including readmission, length of stay, cost, and mortality.”

“Furthermore, using the largest all payer publicly available database in the USA, we observe that obese and morbidly obese patients receive stents or balloons at a lower rate than normal weight patients, are less likely to be readmitted within 30 days, and have lower mortality than normal weight patients undergoing cardiac catheterisation,” he continued.

Dr Tariq concluded: “This study also reinforces the notion that the frail, those with the lowest BMI, have the worst outcomes – suggesting that when it comes to cardiac catheterisation, the smaller they are, the harder they fall.”