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(Chest. 2005;127:2125-2131.)
© 2005 American College of Chest Physicians

The Effect of Body Mass Index on Patient Outcomes in a Medical ICU*

Daniel E. Ray, MD, MS, FCCP; Stephen C. Matchett, MD; Kathy Baker, RN; Thomas Wasser, PhD and Mark J. Young, MD

* From Medical Critical Care (Drs. Ray and Matchett, and Ms. Baker), Department of Health Studies (Dr. Wasser), and Health Studies Unit (Dr. Young), Lehigh Valley Hospital, Allentown, PA.

Correspondence to: Daniel E. Ray, MD, MS, FCCP, 1210 South Cedar Crest Blvd, Suite 2300, Allentown, PA 18103; e-mail: daniel.ray{at}lvh.com


   Abstract
TOP
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 Conclusion
 References
 
Study objectives: To examine the effect of patient body mass index (BMI) on outcome in intensive care.

Design: In a prospective study, the patients were classified into groups based on the calculated BMI, as follows: BMI < 19.0 (n = 350), ≥ 19.0 and < 25.0 (n = 663), ≥ 25.0 and < 29.9 (n = 585), ≥ 30.0 and < 40.0 (n = 396), and ≥ 40.0 (n = 154). Groups were compared by age, APACHE (acute physiology and chronic health evaluation) II score, mortality, ICU length of stay (LOS), hospital LOS, number receiving ventilation, and ventilator-days. Adverse events including nosocomial pneumonia, ventilator-days per patient, failed extubations, and line-related complications were recorded.

Setting: The study was conducted in a 9-bed medical ICU of a 650-bed tertiary care hospital.

Measurements: Height and weight were prospectively recorded for the first ICU admission during a hospital stay.

Results: Between January 1, 1997, and August 1, 2001, 2,148 of 2,806 patients admitted to the ICU had height and weight recorded. There were no differences in APACHE II score, mortality, ICU LOS, hospital LOS, number receiving ventilation, ventilator-days, average total cost, or average variable cost among the five groups. However, the severely obese patients were more frequently female and younger than those who were overweight and obese (p < 0.001). Adverse events were infrequent, but there were no differences between the obese/very obese compared with others.

Conclusion: BMI has minimal effects on ICU outcome after patients are admitted to a critical care unit.

Key Words: body mass index • ICU • obesity • morbidity • mortality • patient outcomes


   Introduction
TOP
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 Conclusion
 References
 
As more and more Americans combine an increase in food consumption with a decrease in physical activity, it is not surprising that there is a growing trend toward overweight and obesity in the United States. The prevalence of obesity has increased from 14.5 to 22.5% in the past 10 to 15 years.1 In 1998, 97 million American adults, representing 55% of the population, were designated as being overweight or obese.2 With the obese population in the United States continuing to rise, it becomes evident that obesity and obesity-related disorders will be encountered more frequently in the health-care industry.

On the basis of guidelines released by the National Institutes of Health, a person with a body mass index (BMI) of 25 to 29.9 is defined as overweight, whereas obesity is defined as having a BMI of ≥30.13 In addition to psychological and social difficulties faced by people who are categorized as obese, they are more susceptible to physiologic complications and having decreased length of life.4

Many studies concur that as BMI increases, so does the risk of mortality.5678 The overall mortality is approximately twice as high in the severely obese and may be 2 to 25 times higher than normal in disease-specific mortality.5 In addition, obesity increases the risk of cardiovascular disease, noninsulin-dependent diabetes mellitus, hypertension, respiratory dysfunction, and certain types of cancer.5678 Obese patients undergo more frequent hospitalizations because obesity exacerbates the onset and progression of illnesses.5 Although many hospitalized patients pass through the ICU at some time during their stay, it is expected that the number of obese patients requiring intensive care will also increase substantially. There are several studies that do not concur that BMI correlates with mortality; however, this is normally a result of smaller populations.56 In larger populations, the significant association is shown.5 This study examines the effect of BMI on the outcome of patients in a medical ICU (MICU).

Until very recently, no data have been published on the influence of BMI on outcomes after critical care. Surprisingly, Tremblay and Bandi6 found that there was no increased mortality in a study of a national cooperative database. Their study did not have information on complication rates. Therefore, our investigation complements their multicenter study by providing additional analyses on outcomes and specific information on complication rates. Tremblay and Bandi6 did find low discharge functional status. We did not have that data at the time of this study, but we are planning to obtain that data and to examine functional discharge status in a future investigation.

Published trials7 evaluating the influence of BMI in critical illness have demonstrated varying effects of BMI on outcomes. We hypothesized that there would be a relationship between the degree of obesity, as determined by BMI, and the mortality and rate of complication in the ICU.


   Materials and Methods
TOP
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 Conclusion
 References
 
This study was conducted in a 9-bed MICU of a 650-bed tertiary care hospital. Patients ≥ 20 years old who were admitted to the unit between January 1, 1997, and August 1, 2001, and had MICU stays > 24 h were enrolled in the study. The age limit of 20 years was used because most growth in stature is complete, and any increase in weight is generally attributable to excess adipose tissue. If any patient was admitted more than once during a single hospital admission or during the study period, only data from the first encounter were used for comparisons.

Whenever possible, height and weight were measured and recorded, and BMI was calculated on admission for the MICU registry. According to these data, the patients were then separated into five groups based on BMI, as follows: underweight (BMI < 20.0), normal weight (20.0 to 24.9), overweight (25.0 to 29.9), obese (30 to 39.9), and severely obese (≥ 40.0). These groupings were based on BMI categories devised by the World Health Organization and the National Heart, Lung, and Blood Institute of the National Institutes of Health to classify overweight and obesity.9

As part of a formalized critical care database in our institution, data were concurrently collected for each patient, including the following: age, gender, APACHE (acute physiology and chronic health evaluation) II diagnosis, length of stay (LOS) in the MICU and hospital, and the number of days the patient required mechanical ventilation. It was also documented when a patient had certain complications while in the MICU. These complications included central or arterial line infection with bacteremia, pneumothorax, deep venous thrombosis, nosocomial or ventilator-associated pneumonia (VAP), failed extubation, self extubation, GI hemorrhage, or prolonged paralysis from neuromuscular blockade (NMB) [Table 1 ]. Patient deaths that occurred in the MICU or the hospital were recorded. Finally, total hospital costs and variable costs were separately extracted from the administrative database of the hospital for each patient. Total cost comprises both fixed and variable costs incurred while furnishing services to patients. Fixed cost is the combination of fixed direct (administration salaries and supervisory salaries, office supplies depreciation expenses) and fixed indirect (also known as overhead or supporting cost or costs of the finance, information services, and telecommunication department expenses, etc). Variable cost is cost that only occurred when a procedure is performed, for example, labor (registered nurse, license practical nurse, and medical assistant wages) or variable supplies costs (medical supplies, drugs, radiographs, etc).


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  		Table 1.. Definitions for MICU Complications

 
Using analysis of variance for continuous variables and {chi}2 for discrete variables, BMI groups were compared by age, APACHE II diagnosis, MICU LOS, hospital LOS, mortality in the MICU and hospital, standardized mortality rate (SMR) for MICU and hospital, ventilator-days, and hospital costs. Logistic regression was performed to examine BMI and mortality. Overall complication rates were compared using analysis of variance.


   Results
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 Conclusion
 References
 
During the period from January 1, 1997, through August 1, 2001, 2,806 patients > 20 years old were admitted to the MICU. Of these, 2,148 patients had height and weight recorded. The average age (± SD) of the population was 63.4 ± 17.9 years, with a male to female ratio of 1:1. The mean APACHE II score was 18.1 ± 8.9, with a predicted mortality of 31.3 ± 25.3%. The average LOS was 4.5 ± 6.4 days. Forty-nine and a half percent of the patients required mechanical ventilation for an average of 5.15 ± 6.1 days. The MICU mortality rate was 15.0%, with an overall hospital mortality rate of 22.1%. The SMR calculated by dividing the observed death rate by the expected death rate based on APACHE II score was 0.714 (95% confidence interval [CI], 0.633 to 0.804) [Table 2 ]. Total costs for each hospital admission ranged from $279 to $438,491 (median, $12,976), and variable costs ranged from $147 to $193,656 (median, $5,398).


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  		Table 2.. Population Characteristics*

 
Patients were then grouped into different categories based on their calculated BMI (Table 3 ). There was no statistical difference between BMI groups in relation to MICU LOS, hospital LOS, APACHE II score, need for mechanical ventilation or number of days on the ventilator, mortality rate, SMR, and total or variable costs. However, both age and gender showed a significant difference between BMI groups (p < 0.001). Using Scheffe post hoc analysis, it was found that severely obese patients are younger and predominantly female when compared with overweight and obese patients (Table 4 ). This age difference, however, was only present in those patients who survived to hospital discharge.


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  		Table 3.. Patient Characteristics by BMI Categories*

 

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  		Table 4.. Scheffe Post Hoc Testing for Age Comparing the Severely Obese Group With Other Groups by Age

 
To further evaluate the effect of BMI on hospital and MICU mortality, regression analysis was completed using APACHE II and BMI as predictors (Table 5 ). Once again, BMI was not found to be a significant predictor of patient mortality.


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  		Table 5.. Logistic Regression on MICU and Hospital Mortality

 
BMI groups were then stratified to determine whether any BMI group could have an preferential APACHE II score for a particular diagnosis. Because of the infrequency of several diagnoses, patients were classified into five general diagnostic categories: (1) respiratory failure or insufficiency, (2) cardiac failure or insufficiency, (3) infection/sepsis, (4) neurologic disorders, (5) and renal disorders. There was no difference in frequency among or within each diagnostic category or group (p = 0.89).

Finally, the influence of BMI on the risk for complications was examined. Table 6 shows a detailed breakdown of complications for the different categories of BMI. Overall, there was no difference in total complications. There were 73 of 550 complications (13.3%) in individuals who were in the obese/very obese categories, compared with 218 of 1,598 complications (13.6%) in the remaining three categories (p = 0.827) [Table 7 ]. The difference of 0.37% was not statistically significant. Interestingly, there was a significant difference for the specific complication of arterial line infection. The sole infection occurred in a very obese patient. Even though the p value is 0.012, we are reluctant to uncritically accept the result because it represents a single patient with a complication.


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  		Table 6.. Complication Rates by BMI Category*

 

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  		Table 7.. Total Complications*

 

   Discussion
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
Conclusion
 References
 
In multiple epidemiologic studies, obesity has been shown to increase morbidity and mortality.5678 With an increase in number of overweight and obese patients admitted to the ICU, the attributable morbidity and mortality has become a concern for the intensivist. Several physiologic changes in obese patients place them at risk for adverse outcomes in the ICU.

Many patients with severe obesity have an underlying ventilation-perfusion mismatch that causes hypoxia.8 Basilar atelectasis and general anesthesia that further reduce the functional residual capacity amplify this problem and has been implicated as a cause of sudden death in severely obese surgical patients.10 Additionally, abnormalities in control of breathing are common, with 40% of men and 3% of women with severe obesity having sleep apnea.11 Mechanical ventilation is hampered by the reduction in compliance of the lung and chest wall and an increase in airway resistance in morbidly obese patients as compared with nonobese patients.12 Weaning and liberation from the ventilator is believed to be more difficult in the obese patient.13 When compared with nonobese patients, obese patients have a fivefold increase in oxygen uptake when changing from positive pressure ventilation to spontaneous breathing as a result of the increased work of breathing.14

The risk for aspiration and subsequent VAP is increased in severely obese patients.1516 This phenomenon has been attributed to a higher gastric volume, lower normal pH of gastric fluid in fasting obese patients, increased intra-abdominal pressure, and a higher incidence of gastric reflux. Obesity has been implicated as the single most important risk factor for the development of thromboembolic disease.17 Several mechanisms are believed to account for this increased risk, including decreased mobility, venous stasis, low levels of antithrombin III,18 and diminished circulating fibrinolytic activity.1920 Endotracheal intubation is often difficult in the obese patient. In one observational study,21 obesity combined with limited neck mobility and mouth opening accounted for the majority of cases of difficult intubations.

In addition to these physiologic changes, the distribution, metabolism, protein binding, and clearance of many drugs are altered in the obese patient. Despite the unchanged oral absorption of most drugs, the volume of distribution is significantly different for lipophilic drugs. Hepatic metabolism is not reduced, but the renal excretion may increase because of an increase in the glomerular filtration rate in obese patients with normal renal function.2223 Estimation of the creatinine clearance using the standard formula does not correlate to the measured creatinine clearance.24

Despite these physiologic abnormalities and an increase in morbidity and mortality found in large epidemiology studies,5678 no increase in poor outcomes has been identified in the surgical literature.252627282930 Most of the published literature pertains to postoperative risk in the obese patient. There has been no identified increase in mortality or morbidity in obese patients undergoing hysterectomy for endometrial cancer, duodenal ulcer surgery, or cholecystectomy.25 Laproscopic cholecystectomy has also been shown to have similar outcomes in obese and nonobese patients.26 With the exception of superficial wound complications and atrial dysrrhythmias, obesity is not a multivariate risk factor for adverse outcomes after cardiac surgery.2728 Obesity did not predict intraoperative or postoperative complications following liver transplantation including length of ICU and hospital stay.29 Finally, in a review30 of preoperative pulmonary evaluation, obesity was not determined to be a significant risk factor for postoperative complications.

However, in studies of acutely or critically ill patients, the effect of BMI on outcomes has been mixed. In a retrospective review3132 of the Study to Understand Prognoses and Preferences for Outcomes and Risks of Treatment database, a low BMI (≤ 15 percentile), but not high BMI (≥ 85 percentile), was a significant and independent predictor of mortality. The high missing data rate (30%) limits the interpretability of the findings. A second study32 using case-control design showed an increased incidence of prolonged mechanical ventilation, ICU LOS, and hospital mortality for morbidly obese patients (BMI > 40). However, two independent, retrospective studies using a large multi-institutional ICU database (Project IMPACT) found that low BMI was associated with increased mortality and worsened hospital discharge functional status.67 In fact, both of their analyses suggest that overweight and obese patients may have lower mortality. There was no effect on outcomes in the morbidly obese patients. Again, the exclusion rate was high because of missing data (35.6%). The excluded patients did show differences in gender, hospital mortality, and hospital LOS, raising concern that these patients were different than the analyzed cohort.67

These studies have also been limited by the accuracy of height and weight measurements.67 In retrospective studies, these variables are often estimated rather than measured. Estimation of these specific data33 has been shown to be inaccurate for individual observers. Additionally, the studies did not assess the differences in complication rates among the different BMI groups.

In a prospectively designed study by Goulenok et al,34 a high BMI of > 27 was predictive of an increased mortality. Obesity was associated with longer ICU stays, increased severity of illness, and a higher SMR. No difference was observed in frequency of nosocomial infections or duration of mechanical ventilation. The completeness of data (94.4%) and the accuracy of measurements provide strong support for these findings. However, the limitation of this study is the definition of obesity. Instead of using the World Health Organization guidelines, the authors defined obese patients as those with a BMI > 75th percentile of their ICU population. As shown in the larger studies mentioned previously, a BMI of 25 to 40 may be protective in critical illness.3132 The number of patients with BMI > 40 in this study population is not clear. Finally, the sample size is relatively small (n = 813).

Our study complements theirs in several ways. First, our methodology was different in that we used APACHE II scores to adjust for severity of illness, and we used SMRs to analyze the effect of BMI on mortality. It is noteworthy that our findings were identical to their findings: an increased BMI is not associated with poorer outcomes after critical care. Second, our analyses were intended to be of maximal usefulness to critical care clinicians; actual rates of outcomes are easier to interpret than odds ratios based on multivariate analyses. Finally, we were able to provide specific information about complication rates that were not available in the multicenter database.

Our finding that severely obese patients were more likely to be female and younger when compared with obese and overweight patients has not been previously reported for ICU populations. The relative risk of death associated with obesity is higher for younger adults than older adults.35 Additionally, obesity at an early age is more of a predictor for cardiovascular end points and affects intervening risk factors such as diabetes and hypertension much more strongly than later in life.36 In fact, some geriatricians believe that the incidence of obesity in the elderly is low because of the increased mortality rate among the obese middle-age people.37 Obese patients often do not live past the age of 50 years because of cardiovascular problems.4

The limitations of our study include the relatively small sample size (n = 2,148), the limited generalizability of the experience of a single hospital, the missing data rate of 23.4%, and the estimation of height that is inherent in our data collection. However, these limitations are somewhat lessened by other studies showing similar results.3132 Specifically, the finding of no increased mortality with increasing BMI is consistent between our study and the larger ICU studies. The fact that we found small differences in LOS that were not statistically significant is also consistent with a small difference that was statistically significant in the large multicenter study.


   Conclusion
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Conclusion
References
 
Despite significant physiologic changes and subsequent increased risk for complications, obesity has not yet been found to increase morbidity, mortality, or costs in the MICU. BMI, in our study, had no effect on outcomes. Studies using larger populations are needed to confirm these observations.


   Footnotes
 
Abbreviations: APACHE = acute physiology and chronic health evaluation; BMI = body mass index; CI = confidence interval; LOS = length of stay; MICU = medical ICU; NMB = neuromuscular blockade; SMR = standardized mortality rate; VAP = ventilator-associated pneumonia

This study was supported by the Dexter F. and Dorothy H. Baker Fund.

Received for publication February 3, 2004. Accepted for publication December 14, 2004.


   References
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Conclusion
 References
 

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