The Body-Mass Index, Airflow Obstruction, Dyspnea, and Exercise Capacity Index in Chronic Obstructive Pulmonary Disease
Bartolome R. Celli, M.D., Claudia G. Cote, M.D., Jose M. Marin, M.D., Ciro Casanova, M.D., Maria Montes de Oca, M.D., Reina A. Mendez, M.D., Victor Pinto Plata, M.D., and Howard J. Cabral, Ph.D.
Background Chronic obstructive pulmonary disease (COPD) is characterizedby an incompletely reversible limitation in airflow. A physiologicalvariable the forced expiratory volume in one second(FEV1) is often used to grade the severity of COPD.However, patients with COPD have systemic manifestations thatare not reflected by the FEV1. We hypothesized that a multidimensionalgrading system that assessed the respiratory and systemic expressionsof COPD would better categorize and predict outcome in thesepatients.
Methods We first evaluated 207 patients and found that fourfactors predicted the risk of death in this cohort: the body-massindex (B), the degree of airflow obstruction (O) and dyspnea(D), and exercise capacity (E), measured by the six-minutewalktest. We used these variables to construct the BODE index, amultidimensional 10-point scale in which higher scores indicatea higher risk of death. We then prospectively validated theindex in a cohort of 625 patients, with death from any causeand from respiratory causes as the outcome variables.
Results There were 25 deaths among the first 207 patients and162 deaths (26 percent) in the validation cohort. Sixty-onepercent of the deaths in the validation cohort were due to respiratoryinsufficiency, 14 percent to myocardial infarction, 12 percentto lung cancer, and 13 percent to other causes. Patients withhigher BODE scores were at higher risk for death; the hazardratio for death from any cause per one-point increase in theBODE score was 1.34 (95 percent confidence interval, 1.26 to1.42; P<0.001), and the hazard ratio for death from respiratorycauses was 1.62 (95 percent confidence interval, 1.48 to 1.77;P<0.001). The C statistic for the ability of the BODE indexto predict the risk of death was larger than that for the FEV1(0.74 vs. 0.65).
Conclusions The BODE index, a simple multidimensional gradingsystem, is better than the FEV1 at predicting the risk of deathfrom any cause and from respiratory causes among patients withCOPD.
Chronic obstructive pulmonary disease (COPD), a common diseasecharacterized by a poorly reversible limitation in airflow,1is predicted to be the third most frequent cause of death inthe world by 2020.2 The risk of death in patients with COPDis often graded with the use of a single physiological variable,the forced expiratory volume in one second (FEV1).1,3,4 However,other risk factors, such as the presence of hypoxemia or hypercapnia,5,6a short distance walked in a fixed time,7 a high degree of functionalbreathlessness,8 and a low body-mass index (the weight in kilogramsdivided by the square of the height in meters),9,10 are alsoassociated with an increased risk of death. We hypothesizedthat a multidimensional grading system that assessed the respiratory,perceptive, and systemic aspects of COPD would better categorizethe illness and predict the outcome than does the FEV1 alone.We used data from an initial cohort of 207 patients to identifyfour factors that predicted the risk of death: the body-massindex (B), the degree of airflow obstruction (O) and functionaldyspnea (D), and exercise capacity (E) as assessed by the six-minutewalktest. We then integrated these variables into a multidimensionalindex the BODE index and validated the indexin a second cohort of 625 patients, with death from any causeand death from respiratory causes as the outcome variables.
Methods
Between January 1997 and June 2002, a total of 859 outpatientswith a wide range in the severity of COPD were recruited fromclinics in the United States, Spain, and Venezuela. The studywas approved by the human-research review board at each site,and all patients provided written informed consent. COPD wasdefined by a history of smoking that exceeded 20 pack-yearsand a ratio of FEV1 to forced vital capacity (FVC) of less than0.7 measured 20 minutes after the administration of albuterol.1All patients were in clinically stable condition and receivingappropriate therapy. Patients who were receiving inhaled oxygenhad to have been taking a stable dose for at least six monthsbefore study entry. The exclusion criteria were an illness otherthan COPD that was likely to result in death within three years;asthma, defined as an increase in the FEV1 of more than 15 percentabove the base-line value or of 200 ml after the administrationof a bronchodilator; an inability to take the lung-functionand six-minutewalk tests; a myocardial infarction withinthe preceding four months; unstable angina; or congestive heartfailure (New York Heart Association class III or IV).
Variables Selected for the Bode Index
We determined the following variables in the first 207 patientswho were recruited between 1995 and 1997: age; sex; pack-yearsof smoking; FVC; FEV1, measured in liters and as a percentageof the predicted value according to the guidelines of the AmericanThoracic Society11; the best of two six-minutewalk testsperformed at least 30 minutes apart12; the degree of dyspnea,measured with the use of the modified Medical Research Council(MMRC) dyspnea scale13; the body-mass index9,10; the functionalresidual capacity and inspiratory capacity11; the hematocrit;and the albumin level. The validated Charlson index was usedto determine the degree of comorbidity. This index has beenshown to predict mortality.14 The differences in these valuesbetween survivors and nonsurvivors are shown in Table 1.
Table 1. Characteristics of the First 207 Patients, According to Whether They Survived.
Each of these possible explanatory variables was independentlyevaluated to determine its association with one-year mortalityin a stepwise forward logistic-regression analysis. A subgroupof four variables had the strongest association thebody-mass index, FEV1 as a percentage of the predicted value,score on the MMRC dyspnea scale, and the distance walked insix minutes (generalized r2=0.21, P<0.001) and thesewere included in the BODE index (Table 2). All these variablespredict important outcomes, are easily measured, and may changeover time. We chose the post-bronchodilator FEV1 as a percentof the predicted value, classified according to the three stagesidentified by the American Thoracic Society, because it canbe used to predict health status,15 the rate of exacerbationof COPD,16 the pharmacoeconomic costs of the disease,17 andthe risk of death.18,19 We chose the MMRC dyspnea scale becauseit predicts the likelihood of survival among patients with COPD8and correlates well with other scales and health-status scores.20,21We chose the six-minutewalk test because it predictsthe risk of death in patients with COPD,7 patients who haveundergone lung-reduction surgery,22 patients with cardiomyopathy,23and those with pulmonary hypertension.24 In addition, the testhas been standardized,12 the clinically significant thresholdshave been determined,25 and it can be used to predict resourceutilization.26 Finally, there is an inverse relation betweenbody-mass index and survival9,10 that is not linear but thathas an inflection point, which was 21 in our cohort and in anotherstudy.10
Table 2. Variables and Point Values Used for the Computation of the Body-Mass Index, Degree of Airflow Obstruction and Dyspnea, and Exercise Capacity (BODE) Index.
Validation of the Bode Index
The BODE index was validated prospectively in two ways in adifferent cohort of 625 patients who were recruited betweenJanuary 1997 and January 2003. First, we used the empiricalmodel: for each threshold value of FEV1, distance walked insix minutes, and score on the MMRC dyspnea scale shown in Table 2,the patients received points ranging from 0 (lowest value)to 3 (maximal value). For body-mass index the values were 0or 1, because of the unique relation between body-mass indexand survival described above. The points for each variable wereadded, so that the BODE index ranged from 0 to 10 points, withhigher scores indicating a greater risk of death. In an exploratoryanalysis, the various components of the BODE index were assigneddifferent weights, with no corresponding increase in predictivevalue.
Study Protocol
In the cohort, patients were evaluated with the use of the BODEindex within six weeks after enrollment and were seen everythree to six months for at least two years or until death. Thepatient and family were contacted if the patient failed to returnfor appointments. Death from any cause and from specific respiratorycauses was recorded. The cause of death was determined by theinvestigators at each site after reviewing the medical recordand death certificate.
Statistical Analysis
Data for continuous variables are presented as means ±SD.Comparison among the three countries was completed with theuse of one-way analysis of variance. The differences betweensurvivors and nonsurvivors in pulmonary-function variables andother pertinent characteristics were established with the useof t-tests for independent samples. To evaluate the capacityof the BODE index to predict the risk of death, we performedCox proportional-hazards regression analyses.27 We estimatedthe hazard ratio, 95 percent confidence interval, and P valuefor the BODE score, before and after adjustment for coexistingconditions as measured by the Charlson index. We repeated theseanalyses using the BODE index as the predictor of interest indummy-variable form, using the first quartile as the referencegroup. These analyses yielded estimates of risk similar to thoseobtained from analyses using the BODE score as a continuousvariable. Thus, we focus our presentation on the predictivecharacteristics of the BODE index and present only bivariateresults for survival according to quartiles of the BODE indexin a KaplanMeier analysis. The statistical significancewas evaluated with the use of the log-rank test. We also performedbivariate analysis on the stage of COPD according to the validatedstaging system of the American Thoracic Society.3
In the Cox regression analysis, we assessed the reliabilityof the model with the body-mass index, degree of airflow obstructionand dyspnea, and exercise capacity score as the predictor ofthe time to death by computing bootstrap estimates using thefull sample for the hazard ratio and its 95 percent confidenceinterval (according to the percentile method). This approachhas the advantage of not requiring that the data be split intosubgroups and is more precise than alternative methods, suchas cross-validation.28
Finally, in order to determine how much more precise the BODEindex is than the FEV1 alone, we computed the C statistics29for a model containing FEV1 or the BODE score as the sole independentvariable. We compared the survival times and estimated the probabilitiesof death up to 52 months. In these analyses, the C statisticis a mathematical function of the sensitivity and specificityof the BODE index in classifying patients by means of the Coxmodel as either dying or surviving. The null value for the Cstatistic is 0.5, with a maximum of 1.0 (with higher valuesbeing better).29
Results
The validation cohort consisted primarily of elderly patients(Table 3 and Table 4) with all degrees of severity of COPD.The FEV1 was slightly lower among patients in the United Statesthan among those in Venezuela or Spain. The U.S. patients alsohad more functional impairment, more severe dyspnea, and morecoexisting conditions. The 27 patients (4 percent) lost to follow-upwere evenly distributed according to the severity of COPD anddid not differ significantly from the rest of the cohort withrespect to any measured characteristic. There were 162 deaths(26 percent) over a median follow-up of 28 months (range, 4to 68). The majority of patients (61 percent) died of respiratoryinsufficiency, 14 percent died of myocardial infarction, 12percent of lung cancer, and the rest of miscellaneous causes.The BODE score was lower among survivors than among those whodied from any cause (3.7±2.2 vs. 5.9±2.6, P<0.005).The score was also lower among survivors than among those whodied of respiratory causes, and the difference between the scoreswas larger (3.6±2.2 vs. 6.7±2.3, P<0.001).
Table 4. Characteristics of the Validation Cohort According to Country.
Table 5 shows the BODE index as a predictor of death from anycause after correction for coexisting conditions. There weresignificantly more deaths in the United States (32 percent)than in Spain (15 percent) or Venezuela (13 percent) (P<0.001).However, when the analysis was done separately for each country,the predictive power of the BODE index was similar; therefore,the data are presented together. Table 5 shows that the BODEindex was also a predictor of death from respiratory causesafter correction for coexisting conditions (hazard ratio, 1.63;95 percent confidence interval, 1.48 to 1.80; P<0.001). TheKaplanMeier analysis of survival (Figure 1A) shows thateach quartile increase in the BODE score was associated withincreased mortality (P<0.001). Thus, the highest quartile(a BODE score of 7 to 10) was associated with a mortality rateof 80 percent at 52 months. These same data are shown in Figure 1Bin relation to the severity of COPD according to the stagingsystem of the American Thoracic Society. The C statistic forthe ability of the BODE index to predict the risk of death was0.74, as compared with a value of 0.65 with the use of FEV1alone (expressed as a percentage of the predicted value). Thecomputation of 2000 bootstrap samples for these data and estimationof the hazard ratios for death indicated that for each one-pointincrement in the BODE score the hazard ratio for death fromany cause was 1.34 (95 percent confidence interval, 1.26 to1.42) and the hazard ratio for death from a respiratory causewas 1.62 (95 percent confidence interval, 1.48 to 1.77).
Figure 1. KaplanMeier Survival Curves for the Four Quartiles of the Body-Mass Index, Degree of Airflow Obstruction and Dyspnea, and Exercise Capacity Index (Panel A) and the Three Stages of Severity of Chronic Obstructive Pulmonary Disease as Defined by the American Thoracic Society (Panel B).
In Panel A, quartile 1 is a score of 0 to 2, quartile 2 is a score of 3 to 4, quartile 3 a score of 5 to 6, and quartile 4 a score of 7 to 10. Survival differed significantly among the four groups (P<0.001 by the log-rank test). In Panel B, stage I is defined by a forced expiratory volume in one second (FEV1) that is more than 50 percent of the predicted value, stage II by an FEV1 that is 36 to 50 percent of the predicted value, and stage III by an FEV1 that is no more than 35 percent of the predicted value. Survival differed significantly among the three groups (P<0.001 by the log-rank test).
Discussion
We devised a simple grading system for COPD the BODEindex and validated its use by showing that it is abetter predictor of the risk of death from any cause and fromrespiratory causes than is the FEV1 alone. We believe that theBODE index is useful because it includes one domain that quantifiesthe degree of pulmonary impairment (FEV1), one that capturesthe patient's perception of symptoms (the MMRC dyspnea scale),and two independent domains (the distance walked in six minutesand the body-mass index) that express the systemic consequencesof COPD. The FEV1 is essential for the diagnosis and quantificationof the respiratory impairment resulting from COPD.1,3,4 In addition,the rate of decline in FEV1 is a good marker of disease progressionand mortality.18,19 However, the FEV1 does not adequately reflectall the systemic manifestations of the disease. For example,the FEV1 correlates weakly with the degree of dyspnea,20 andthe change in FEV1 does not reflect the rate of decline in patients'health.30 More important, prospective observational studiesof patients with COPD have found that the degree of dyspnea8and health-status scores31 are more accurate predictors of therisk of death than is the FEV1. Thus, although the FEV1 is importantto obtain and essential in the staging of disease in any patientwith COPD, other variables provide useful information that canimprove the comprehensibility of the evaluation of patientswith COPD. Each variable should correlate independently withthe prognosis of COPD, should be easily measurable, and shouldserve as a surrogate for other potentially important variables.
In the BODE index, we included two descriptors of systemic involvementin COPD: the body-mass index and the distance walked in sixminutes. Both are simply obtained and independently predictthe risk of death.7,9,10 It is likely that they share some commonunderlying physiological determinants, but the distance walkedin six minutes contains a degree of sensitivity not providedby the body-mass index. The six-minutewalk test is simpleto perform and has been standardized.12 Its use as a clinicaltool has gained acceptance, since it is a good predictor ofthe risk of death among patients with other chronic diseases,including congestive heart failure23 and pulmonary hypertension.24Indeed, the distance walked in six minutes has been acceptedas a good outcome measure after interventions such as pulmonaryrehabilitation.32 The body-mass index was also an independentpredictor of the risk of death and was therefore included inthe BODE index. We evaluated the independent prognostic powerof body-mass index in our cohort using different thresholdsand found that values below 21 were associated with an increasedrisk of death, an observation similar to that reported by Landboand coworkers in a large population study.10
The Global Initiative for Chronic Obstructive Lung Disease andthe American Thoracic Society recommend that a patient's perceptionof dyspnea be included in any new staging system for COPD.1,3Dyspnea represents the most disabling symptom of COPD; the degreeof dyspnea provides information regarding the patient's perceptionof illness and can be measured. The MMRC dyspnea scale is simpleto administer and correlates with other dyspnea scales20 andwith scores of health status.21 Furthermore, in a large cohortof prospectively followed patients with COPD, which used thethreshold values included in the BODE index, the score on theMMRC dyspnea scale was a better predictor of the risk of deaththan was the FEV1.8
The BODE index combines the four variables by means of a simplescale. We also explored whether weighting the variables includedin the index improved the predictive power of the BODE index.Interestingly, it failed to do so, most likely because eachvariable included has already proved to be a good predictorof the outcome of COPD.
Our study had some limitations. First, relatively few womenwere recruited, even though enrollment was independent of sex.It probably reflects the problem of the underdiagnosis of COPDin women. Second, there were differences among the three countries.For example, patients in the United States had a higher mortalityrate, more severe dyspnea, more functional limitations, andmore coexisting conditions than patients in Venezuela or Spain,even though the severity of airflow obstruction was relativelysimilar among the patients as a whole. The reasons for thesedifferences are unknown, because there have been no systematiccomparisons of the regional manifestations of COPD. In all threecountries, the BODE index was the best predictor of survival,an observation that renders our findings widely applicable.
Three studies have reported the effects of the grouping of variablesto express the various domains affected by COPD.33,34,35 Thesestudies did not include variables now known to be importantpredictors of outcome, such as the body-mass index. However,as we found in our study, they showed that the FEV1, the degreeof dyspnea, and exercise performance provide independent informationregarding the degree of compromise in patients with COPD.
Besides its excellent predictive power with regard to outcome,the BODE index is simple to calculate and requires no specialequipment. This makes it a practical tool of potentially widespreadapplicability. Although the BODE index is a predictor of therisk of death, we do not know whether it will be a useful indicatorof the outcome in clinical trials, the degree of utilizationof health care resources, or the clinical response to therapy.
We are indebted to Dr. Gordon L. Snider, whose guidance, comments,and criticisms were fundamental to the final manuscript.
Source Information
From the COPD Center at St. Elizabeth's Medical Center, Tufts University School of Medicine, Boston (B.R.C., V.P.P.); Bay Pines Veterans Affairs Medical Center, Bay Pines, Fla. (C.G.C.); Hospital Miguel Servet, Zaragoza, Spain (J.M.M.); Hospital Nuestra Senora de La Candelaria, Tenerife, Spain (C.C.); Hospital Universitario de Caracas and Hospital Jose I. Baldo, Caracas, Venezuela (M.M.O., R.A.M.); and Boston University School of Public Health, Boston (H.J.C.).
Address reprint requests to Dr. Celli at Pulmonary and Critical Care Medicine, St. Elizabeth's Medical Center, 736 Cambridge St., Boston, MA 02135, or at bcelli{at}copdnet.org.
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