Is the Obesity Paradox Valid in Pediatric Intensive Care?

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In a recent issue of Critical Care Medicine, Ward et al (1) evaluated the impact of weight extremes on clinical outcomes such as in-hospital mortality considering pediatric acute respiratory distress syndrome (ARDS). In obese patients, the mortality rate was significantly lower when compared with normal weight subjects in the indirect lung injury group. Interestingly, there were no significant changes in the mortality rate of obese children with direct lung injury including when subjected to prolonged mechanical ventilation.
The association between obesity and mortality for critically ill patients is uncertain. Of note, the method used to perform nutritional evaluation may be misleading to obtain a lowest ICU mortality risk in children who are mildly to moderately overweight and should not be taken at face value (2, 3). Ward et al (1) correctly used body mass index (BMI) z score (not only weight-for-age), but he also evaluated children above 2 years old to minimize inaccurate anthropometric measurements; however, the parameters used to classify the nutritional status were not the ones recommended by the World Health Organization and consequently a greater number of patients may have been classified as obese. Furthermore, measurement of BMI may be altered by IV fluid administration before weight is obtained or erroneous assessment of height in supine critically ill patients. Studies that take into account height, it was observed that the lowest ICU mortality risk occurred in normal weight. Risk-adjusted PICU mortality increases as weight for height/BMI go beyond overweight/obese range. The misclassification of patients without using height data alter the nadir of the mortality curve and erroneously indicates a protective benefit for being mildly to moderately overweight (1, 2, 4).
Interestingly, obese patients who happen to have influenza A–associated ARDS have demonstrated an increased risk for mortality although these studies involved small population sample (2, 5). The literature is limited regarding obesity and outcomes in pediatric critical care (6, 7). Evidently, these studies do not reflect the population of United States and the same results could be obtained in developing countries (2). Overweightness or obesity was associated with increased PICU mortality even by considering severity of illness, demographic data, and comorbidities. This pattern of increased mortality with obesity seems to occur as early as 1 year old (7).
Although Ross et al (2) confirmed a U-shaped curve for PICU mortality, his data do not support a protective effect of mild obesity. Important confounders, which may explain this association between obesity and mortality (e.g., socioeconomic status, host immune response, and illness severity), are not identified by severity of illness scores.
Finally, diagnosing ARDS and assessing the degree of critical illness in extremely obese patients can be very difficult (noninvasive blood pressure measurements or interpreting chest radiographs), thus leading to misclassification and incorrect case ascertainment, and further studies using the new definition of ARDS in children can be useful (4, 8). It is worth noting that although there is an association between obesity and mortality, this finding does not imply causality (2). To come up with the best practices to manage respiratory disease in obese critically children even in countries with limited resources and technology are essential, in order to reduce mortality rate. Obesity paradox seems to be not so palpable in pediatric intensive care.

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