CI: Body Weight and Outcomes: Mixed ICU Patients (2007)


Obrien JM Jr, Phillips GS, Ali NA, Lucarelli M, Marsh CB, Lemeshow S. Body mass index is independently associated with hospital mortality in mechanically ventilated adults with acute lung injury. Crit Care Med. 2006 Mar; 34(3):738-744.

PubMed ID: 16521268
Study Design:
Retrospective Cohort Study
B - Click here for explanation of classification scheme.
Quality Rating:
Positive POSITIVE: See Quality Criteria Checklist below.
Research Purpose:
The purpose of the study was to determine whether or not there is an association between body mass index (BMI) and hospital mortality for critically ill adults who were mechanically ventilated and had been diagnosed with acute lung injury (ALI).  
Inclusion Criteria:
  • Study sample and subjects came from Project IMPACT, a subscription database with data collected from 106 ICUs in 84 U.S. hospitals from December 1995 to September 2001.
  • Subjects were 18 years of age or older
  • Mechanical ventilation within 24 hours of ICU admission
  • Admission diagnosis consistent with ALI defined as pulmonary edema, acute respiratory distress syndrome or pulmonary edema, noncardiogenic fluid overload without congestive heart failure.
Exclusion Criteria:
  • Patients who were not critically ill
  • Critically ill patients not on mechanical ventilation
  • Critically ill patients whose diagnosis did not include ALI
Description of Study Protocol:


  • Study sample and subjects came from Project IMPACT, a subscription database with data collected from 106 ICUs in 84 U.S. hospitals from December 1995 to September 2001.


  •  Nonconcurrent cohort study (database)

Blinding used (not applicable)

Intervention (not applicable)

Statistical Analysis

  •  Kruskal-Wallis test for continuous variables and Pearson X2 or Fisher's exact test for categorical variables in unadjusted analyses
  • Estimated unadjusted association between BMI and hospital mortality:
    • BMI as a continuous variable in logistic regression model of mortality was not linear; therefore, the investigators did a quadratic transformation of BMI
    • for categorical analysis, used NIH 5-level classification scheme of underweight BMI < 18.5 kg/m2; normal BMI, 18.5 kg/m2 to < 25 kg/mto < 30 kg/m2; obese 30 kg/m2 to < 40 kg/m2; severely obese > 40 kg/m2.
  • Multivariate analyses: adjusted for risk by using transformed continuous variable or the 5-level categorical variable and covariate factors such as severity of illness (Simplified Acute Physiology Score).
  • Level of significance defined a priori at 15% for categorical analysis and 10% for continuous BMI model.
  • Censored data to exclude highest BMIs found in the cohort (n = 14; mean 76.5 kg/m2; range 60.8 - 141. kg/m2). This was done due to concern that highest BMIs in the cohort did not represent realistic measures.
  • Intent to treat analysis.  Eleven percent of database records did not include adequate information to calculate BMI. Hospital mortality was not significantly different between the records with calculable BMI and those  records without BMI (p = .764). 
Data Collection Summary:

Timing of Measurements

  •  all measurements took place in the past as this is a nonconcurrent (database) cohort study

Dependent Variables

  • hospital mortality (measured by death)

Independent Variables

  •  BMI

Control Variables


Description of Actual Data Sample:

Initial N: examined records of 1,673 mechanically ventilated adults with ALI

Attrition: 1488 records were actually used; 185 records were excluded because they did not include enough information to calculate BMI;

Age and other relevant demographics:  Differences in potential confounders among BMI categories

Variable Under Wt Normal Wt Over Wt Obese Severe Obese p value
Mean age, yrs







Probability of survival (SAPS II)







Male gender (percent)







Diabetes diagnosis (percent)







Hypertension diagnosis (percent)






        There were no differences in diagnoses of pulmonary disease, cancer, or cardiovascular disease.

Ethnicity: no significant difference between survivors and nonsurvivors

Location: Analysis done at Ohio State University; subjects from a multicenter database that included 106 ICUs in 84 hospitals.


Summary of Results:


Body Mass Index (kg/m2) Adjusted Odds Ratio (95% CI) p Value
Underweight (< 18.5) 1.94 (1.05 - 3.60) .035
Normal (18.5 - 24.9) Reference  
Overweight (25 - 29.9) 0.72 (0.51 - 1.02) .067
Obese (30 - 39.9) 0.67 (0.46 - 0.97) .033
Severe obesity (> 40) 0.78 (0.44 - 1.38) .385


Other Findings


Author Conclusion:
BMI was independently associated with risk-adjusted hospital mortality among mechanically ventilated adults with ALI (p > .0001). Highest odds of death were at the lowest BMIs, risk of death then declined to a minimum between 35 and 40 kg/m2. Risk of death for the severely obese (>40 kg/m2) was also lower than risk at lowest BMIs.
Funding Source:
Reviewer Comments:
Quality Criteria Checklist: Primary Research
Relevance Questions
  1. Would implementing the studied intervention or procedure (if found successful) result in improved outcomes for the patients/clients/population group? (Not Applicable for some epidemiological studies) N/A
  2. Did the authors study an outcome (dependent variable) or topic that the patients/clients/population group would care about? Yes
  3. Is the focus of the intervention or procedure (independent variable) or topic of study a common issue of concern to dieteticspractice? Yes
  4. Is the intervention or procedure feasible? (NA for some epidemiological studies) N/A
Validity Questions
1. Was the research question clearly stated? Yes
  1.1. Was (were) the specific intervention(s) or procedure(s) [independent variable(s)] identified? Yes
  1.2. Was (were) the outcome(s) [dependent variable(s)] clearly indicated? Yes
  1.3. Were the target population and setting specified? Yes
2. Was the selection of study subjects/patients free from bias? Yes
  2.1. Were inclusion/exclusion criteria specified (e.g., risk, point in disease progression, diagnostic or prognosis criteria), and with sufficient detail and without omitting criteria critical to the study? Yes
  2.2. Were criteria applied equally to all study groups? Yes
  2.3. Were health, demographics, and other characteristics of subjects described? Yes
  2.4. Were the subjects/patients a representative sample of the relevant population? Yes
3. Were study groups comparable? Yes
  3.1. Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT) Yes
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? Yes
  3.3. Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.) N/A
  3.4. If cohort study or cross-sectional study, were groups comparable on important confounding factors and/or were preexisting differences accounted for by using appropriate adjustments in statistical analysis? Yes
  3.5. If case control study, were potential confounding factors comparable for cases and controls? (If case series or trial with subjects serving as own control, this criterion is not applicable.) N/A
  3.6. If diagnostic test, was there an independent blind comparison with an appropriate reference standard (e.g., "gold standard")? N/A
4. Was method of handling withdrawals described? Yes
  4.1. Were follow-up methods described and the same for all groups? Yes
  4.2. Was the number, characteristics of withdrawals (i.e., dropouts, lost to follow up, attrition rate) and/or response rate (cross-sectional studies) described for each group? (Follow up goal for a strong study is 80%.) Yes
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? Yes
  4.4. Were reasons for withdrawals similar across groups? Yes
  4.5. If diagnostic test, was decision to perform reference test not dependent on results of test under study? Yes
5. Was blinding used to prevent introduction of bias? Yes
  5.1. In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate? N/A
  5.2. Were data collectors blinded for outcomes assessment? (If outcome is measured using an objective test, such as a lab value, this criterion is assumed to be met.) N/A
  5.3. In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded? Yes
  5.4. In case control study, was case definition explicit and case ascertainment not influenced by exposure status? N/A
  5.5. In diagnostic study, were test results blinded to patient history and other test results? N/A
6. Were intervention/therapeutic regimens/exposure factor or procedure and any comparison(s) described in detail? Were interveningfactors described? Yes
  6.1. In RCT or other intervention trial, were protocols described for all regimens studied? N/A
  6.2. In observational study, were interventions, study settings, and clinicians/provider described? Yes
  6.3. Was the intensity and duration of the intervention or exposure factor sufficient to produce a meaningful effect? Yes
  6.4. Was the amount of exposure and, if relevant, subject/patient compliance measured? Yes
  6.5. Were co-interventions (e.g., ancillary treatments, other therapies) described? Yes
  6.6. Were extra or unplanned treatments described? Yes
  6.7. Was the information for 6.4, 6.5, and 6.6 assessed the same way for all groups? N/A
  6.8. In diagnostic study, were details of test administration and replication sufficient? Yes
7. Were outcomes clearly defined and the measurements valid and reliable? Yes
  7.1. Were primary and secondary endpoints described and relevant to the question? Yes
  7.2. Were nutrition measures appropriate to question and outcomes of concern? N/A
  7.3. Was the period of follow-up long enough for important outcome(s) to occur? Yes
  7.4. Were the observations and measurements based on standard, valid, and reliable data collection instruments/tests/procedures? Yes
  7.5. Was the measurement of effect at an appropriate level of precision? Yes
  7.6. Were other factors accounted for (measured) that could affect outcomes? Yes
  7.7. Were the measurements conducted consistently across groups? Yes
8. Was the statistical analysis appropriate for the study design and type of outcome indicators? Yes
  8.1. Were statistical analyses adequately described and the results reported appropriately? Yes
  8.2. Were correct statistical tests used and assumptions of test not violated? Yes
  8.3. Were statistics reported with levels of significance and/or confidence intervals? Yes
  8.4. Was "intent to treat" analysis of outcomes done (and as appropriate, was there an analysis of outcomes for those maximally exposed or a dose-response analysis)? Yes
  8.5. Were adequate adjustments made for effects of confounding factors that might have affected the outcomes (e.g., multivariate analyses)? Yes
  8.6. Was clinical significance as well as statistical significance reported? Yes
  8.7. If negative findings, was a power calculation reported to address type 2 error? N/A
9. Are conclusions supported by results with biases and limitations taken into consideration? Yes
  9.1. Is there a discussion of findings? Yes
  9.2. Are biases and study limitations identified and discussed? Yes
10. Is bias due to study's funding or sponsorship unlikely? Yes
  10.1. Were sources of funding and investigators' affiliations described? Yes
  10.2. Was the study free from apparent conflict of interest? Yes