HOUSTON—In the general population, the risk of death increases when body mass index (BMI) is extremely high or extremely low. Is the same true in critically ill patients? A recent analysis of a large multi-institutional database has found that in patients admitted to the ICU, low BMI is associated with increased mortality, but high BMI is not.[1]

The Project Impact Critical Care Data System was used to obtain information about 41,011 ICU patients. Age, height, weight at ICU admission, sex, type of ICU admission, and ICU and hospital lengths of stay were recorded. Severity of illness was determined using the mortality prediction model score (MPM-0) and the simplified acute physiologic score (SAPS-II) at time of ICU admission; results were sorted into quartiles based on the estimated likelihood of survival (eg, patients in the lowest quartiles had the highest risk of death). Patients were then grouped according to both their BMI and MPM-0/SAPS-II quartiles; hospital survival and functional status were calculated for each group. Multivariate analyses were performed as well.

UNDERWEIGHT PATIENTS HAVE INCREASED MORTALITY

The underweight patients had increased mortality, regardless of severity of illness (odds ratio of mortality, 1.19 according to the SAPS; 1.26 according to the MPM). However, being overweight, obese, or severely obese had no adverse effect on survival compared to those with normal BMI.

Severe obesity was clearly associated with prolonged ICU and hospital length of stay. Low BMI also prolonged ICU and hospital length of stay.

In a logistic regression analysis, patients with low BMI had poorer functional status at discharge. High BMI was not found to worsen functional status; in fact, there was a small improvement in risk of dependence at discharge in patients who were overweight or obese.

QUESTIONS FOR FUTURE RESEARCH

Alain Tremblay, MDCM, now a Clinical Assistant Professor of Medicine at the University of Calgary in Alberta, admitted that he had expected to find worse outcomes among obese and severely obese patients. “The assumption in the critical care community … is that these patients do worse, although there have not been any data to back this up,” he explained. “Our data suggest that although length of stay may be increased, functional outcome and survival are not affected by high BMI.”

Dr. Tremblay and coauthor Venkata Bandi, MBBS, noted that the study’s findings should be interpreted cautiously, because their analysis did not control for underlying illnesses that could result in weight loss. Nevertheless, their findings have important implications.

“The results of our study bring up several important questions,” commented Dr. Tremblay. “In general, research should focus on further defining reasons behind the poor outcomes in the low-BMI patients and the longer length of stay in the obese patients, in the hopes of coming up with worthwhile specific interventions [for] these adverse outcomes.”

—Gale Jurasek

Reference
1. Tremblay A, Bandi V. Impact of body mass index on outcomes following critical care. Chest. 2003;123:1202-1207.

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