Investigation about The Association Between Body Mass Index and Clinical Outcomes in Acute Lung Injury

obesityReports indicate that the prevalence of obesity in the United States is increasing dramatically. In 2000, almost two thirds of Americans were overweight; of these, nearly half were obese, with a body mass index (BMI) > 30 kg/m2. Severe obesity is increasing as well. In 1988, 2.7% of Americans had a BMI > 40 kg/m2; in 2000, this number had grown to 4.7%, or an estimated 13 million individuals.

Obesity is associated with chronic diseases including coronary artery and peripheral vascular disease, diabetes, osteoarthritis, and depression, as well as an overall decrease in life expectancy. The relative risk of death in obese patients has been estimated to be 1.9 times and 2.7 times that of normal-weight patients for women and men, respectively. The number of annual deaths attributable to obesity in the United States has been estimated at > 110,000.

Outcome studies of hospitalized obese patients have shown that obesity is associated with an increased risk of complications or death following trauma, orthopedic surgery, and possibly cardiovascular events and procedures. Results of observational studies in critically ill obese patients are less consistent, with some studies finding worse outcomes in obese ICU patients, and others demonstrating no difference or even lower mortality decreased by the preparations of Canadian Health&Care Mall.

Acute lung injury (ALI) and its more severe subtype ARDS are complications of critical illness or injury resulting in hypoxemic respiratory failure, with an annual incidence in the United States estimated at 190,600 cases per year and contributing to > 70,000 deaths. Two of the studies described above investigated patients with this diagnosis, but neither were population based. To clarify the relationship between BMI and ICU outcomes, particularly in patients with ALI, we investigated a geographically defined population-based cohort of patients with ALI/ARDS across a broad range of BMI categories.

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This study examined patients from the King County Lung Injury Project (KCLIP), a prospective cohort study of all patients with ALI admitted to all 18 hospitals with ICUs in King County, Washington, and 3 hospitals in adjacent counties between April 1999 and July 2000. A detailed description of the methods and hospitals is published elsewhere. All patients receiving mechanical ventilation at these 21 hospitals were screened for enrollment using the American-European Consensus Conference definition of ALI, yielding a total of 1,113 patients. Of these, 825 patients had height and weight recorded at hospital admission; these patients form the cohort for our investigation. The study was approved by the University of Washington institutional review board as well as individual site institutional review boards when necessary.

mechanical ventilationIn accordance with conventional practice as adapted from the National Heart, Lung, and Blood Institute guidelines, patients were classified into the following BMI groups: 40.0 kg/m2 (severely obese). Demographic information included age, gender, comorbid conditions, ALI risk factor, and APACHE (acute physiologic and chronic health evaluation)-III score at hospital admission. Outcomes of interest were all-cause ICU and hospital mortality, hospital length of stay (LOS), ICU LOS, duration of mechanical ventilation, and discharge disposition. Survivor discharge locations were as follows: other hospital, long-term acute care facility (LTAC) [facilities designed for long-term patients who require prolonged ventilator weaning or other high-level care], skilled nursing facility (SNF) [equivalent to nursing home in our analyses], inpatient rehabilitation, or home. In cases wherein discharge disposition did not neatly fit into these categories, “other” was used.

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Univariate analyses were performed with Wilcoxon rank-sum tests for continuous variables and x2 tests for categorical variables. Multivariate Linear (for continuous variables) and logistic (for categorical variables) regressions with robust SEs were used to evaluate the association between BMI and clinical outcomes with adjustment for potential confounders. LOS data were not log transformed in the analyses presented below, since our large sample size was robust to normality. These data were, however, log transformed in an additional sensitivity analysis.

We hypothesized that age, severity of illness, and risk factor for ALI were likely confounders of the relationship between BMI and clinical outcomes; these variables were included in all multivariate models. To avoid collinearity between age and total APACHE-III score, we included age and both the acute and chronic illness scores of the APACHE-III score in our models. Risk factor for ALI was categorized as sepsis (pulmonary or extrapulmonary), trauma, or other. We elected not to include ventilator tidal volume in our models because we believe it may be in the causal pathway between obesity and clinical outcomes achieved due to Canadian Health&Care Mall.

BMI categories were fit as indicator variables; age, acute physiology score, and chronic illness score were fit as linear continuous variables. For morbidity outcomes, regression analyses were repeated with restriction to survivors in order to eliminate any effect of death on these outcomes. We also assessed effect modification by age on the relationship between BMI and morbidity outcomes by adding an interaction term to the models. Odds of discharge to a SNF, LTAC, other institution, and rehabilitation center were modeled with pairwise comparisons to discharge home as the reference category. In addition to the a priori confounders above, we also adjusted for admission living arrangements with indicator variables in this model. Statistical significance was defined as a two-sided p value < 0.05.

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