The current study assessed the association of body mass index and mortality rate among 502 patients admitted to the ICU. In this study, the highest mortality rate was observed in patients with a BMI between 35 and 40 (obesity class II), and the lowest rate was observed in patients with a BMI between 18.5 and 25 (normal-weight patients).
Obesity is known to play a role in the development and progression of numerous disorders, many of which could easily lead to fatal outcomes . Notwithstanding, there is a point of conflict whether BMI is a proper representation of obesity in predicting mortality-related outcomes. The evidence on the connection between BMI – as a phenomenon of obesity – and mortality rate are extremely conflicting, as some studies have shown no significant relation, while some others have shown an increase or decrease in patients’ mortality [19–21]. In a prospective study conducted to evaluate the effect of obesity on the mortality of patients admitted to ICU in Saudi Arabia, the mortality rate in overweight critically ill patients was lower than patients with normal-weight, despite identical severity of the illness . A retrospective study conducted in the United States determined that patients with BMI > 40 (obesity class III) and patients with BMI < 20 have a higher length of hospital stay . They concluded that lower BMI is associated with an increased mortality rate and worsened functional status at the time of discharge. A cohort of 699 patients also demonstrated that the obese patients have lower in-hospital mortality, though, the findings were not witnessed among the older group of the obese patients, which suggests further studies to elaborate the possible connection between age, obesity, and fatal outcomes in intensive care units . Nasraway et al. showed during a cohort study in the UK that BMI > 40 was an independent cause of death in ICU-admitted surgery patients. It was concluded that severe obesity is a direct and effective risk factor in the mortality of ICU patients in similar conditions regarding age, sex and severity of the disease . Although our study lacked the group of BMIs > 40, the highest mortality rate was still observed in most obese patients.
On the other hand, many studies have claimed that there is no correlation between the BMI and mortality outcomes of critically ill patients. Supporting this claim, a recent study showed although obesity has a direct impact on reducing the need of intubation and inotropic support, there is no significant evidence of an increase or decreasing in patients’ mortality . Another study on 312 patients with sepsis and acute respiratory failure showed that even though overweight and obese patients have an increased length of stay in both hospital and ICU, BMI is independent of mortality rate . Likewise, a study conducted by Lewis et al., on the patients admitted to an adult medical ICU with a more than 24 hours stay, presented that the overweight and obesity were not related to ICU mortality. Still, obesity was significantly associated with longer length of stay and increased comorbid illness . Recent studies are intended to evaluate the issue in larger study populations, among different subgroups, and according to the etiology context [26, 29, 30]. A recent dose-response meta‐analysis regarding the effect of BMI on the mortality of ICU-admitted patients showed that for each unit of increase in BMI (kg/m2), a 0.6% decrease in mortality rate is expected . This study also discussed that while a BMI > 35 is a dangerous feature in ICU-admitted patients, a BMI < 35 can play a protective role against mortality. The reason behind conflicting results of different studies could be explained according to the differences in study designs, ethnicity, classification of BMI, comorbidities, differences in the type of ICU admission, and physiological severity of the illness.
The present study also showed that per each unit of increase in APACHE II score and waist circumference, the risk of mortality increases by 2.79 and 1.15, respectively. These findings display that waist circumference could be a better predictive value than BMI. There are several available studies supporting the current finding. An observational study conducted by Paolini et al., the mortality of 403 patients admitted to the intensive care unit were evaluated. The results showed that unlike BMI, higher waist circumference is a risk factor of mortality in critically ill patients . Also, a recent pooled analysis of eleven prospective cohort studies with a total of 650,000 participants with a median of nine-years follow-up, discussing the relation of waist circumference and mortality concluded that the higher waist circumference is significantly related to higher mortality, and even for patients with a normal BMI, waist circumference could still be a prognostic factor for risk assessment .
Regarding the association of BMI and hospital infection, the present study demonstrated that the patients with BMI < 18.5 had the highest rate of hospital infection, while the normal-BMI patients had the lowest rate. A retrospective cohort study by Papadimitriou-Olivgeris et al. supports the current results . The study, focused on the role of obesity in the prognosis of sepsis patients, revealed that obesity had a direct impact on some morbidities, including bloodstream infection and Klebsiella pneumonia colonization, concluding that obesity affects sepsis upon ICU admission. Regarding the association of high BMI and mortality in ARDS patients, studies showed interesting results which is known as obesity paradox. It means that morbid obese ARDS patients has a lower mortality compared to normal patients. In obesity, the high chest wall elastance could redistribute regional transpulmonary pressure, possibly reducing the potential negative effects of mechanical ventilation in an inhomogeneous lung . However, any positive relationship between obesity and survival may be outweighed by the volume of data linking obesity with a great number of severe illnesses. Any residual uncertainty should not mean that physicians overlook the clear risk-lowering effects of weight reduction in obese individuals who are at a higher risk of different disorders and complication. In spite of the conflicting result for correlation of BMI and mortality, recent studies showed that waist circumferences is independent risk factor for mortality in critically ill patients and showed a significant negative association with mortality . The results of this study also indicated that BMI could be associated with mortality, without considering waist circumference and APACHE score, but once these are considered as confounding factors, BMI does not affect mortality and only affects the morbidity of patients.
While the current study benefited from an adequate study duration and proper sample size, it had a limited follow-up duration. Therefore, further studies with more participants, longer follow-up duration, and with the evaluation of long-term outcomes among different BMI groups are suggested. Moreover, conducting studies based on specific types of admission (medical, surgical, or trauma) and on specific subgroups of age, sex and ethnicities may result in more explicit and clear findings.