The 365 consecutive patients with symptomatic COPD had been recruited to the body-mass index, airflow obstruction, dyspnea, and exercise capacity (BODE) protocol at the Bay Pines Veterans Administration Health Care System (n = 278) and St. Elizabeth’s Medical Center (n = 87) between 1994 and 2005. The human research committees approved the study, and all of the patients gave informed consent. The study enrollment criteria were as follows: smoking history of > 10 pack-years; diagnosis of COPD; FEV1/FVC ratio of < 0.70; and response to bronchodilator of < 12% or 200 mL. The patients were part of a larger cohort, the results of which have been reported earlier. The cohort consisted primarily of patients attending pulmonary clinics, which had no contraindications for pulmonary rehabilitation, had been clinically stable for 6 weeks prior to entry into the study, and were able to perform all of the required tests. The CPET and the 6MWD were performed on separate days. Comorbidity was determined with the Charlson score.
Pulmonary Function and Exercise Tests
The FEV1 and FEV1/FVC were calculated following American Thoracic Society recommendations. The results were expressed as absolute and percent predicted values. Lung volumes were obtained using body plethysmography. Improve your lung volumes with My Canadian Pharmacy.
The 6MWD test was completed as recommended by the American Thoracic Society. Oxygen saturation was continuously monitored. Supplemental O2 was provided to 121 patients (33%) to maintain arterial oxygen saturation at > 90%. The inclusion or exclusion of these patients in the analysis did not modify any of the results.
Exercise ergometry was performed using a 1-min incremental protocol. Patients started with a 2-min period of unloaded pedaling at 60 cycles per minute, followed by a loading rate increment of 10 W/min. The patients were encouraged to cycle until discomfort or exhaustion for at least 2 min. Heart rate was monitored with a 12-lead ECG, and BP was measured with the cuff technique. Minute ventilation (Ve) and its components were measured using a pneumotachograph. The concentrations of expired oxygen and carbon dioxide were analyzed breath by breath. These measures and flow signals were integrated to yield 30-s averages of Ve, tidal volume, respiratory rate, Vo2, carbon dioxide output, and gas exchange ratio. The predicted maximal Vo2 was calculated using standard equations. Maximal voluntary ventilation was directly determined > 12 s. The heart rate reserve was calculated as the predicted maximal heart rate (maximal heart rate = 220 beats/min — age) minus the observed peak heart rate. Dyspnea was measure as per the modified Medical Research Council (MMRC) dyspnea scale.
For continuous variables, the data are presented as the mean ± SD and range. The differences in physical characteristics, pulmonary function test results, and exercise parameters were evaluated using t tests for independent variables. Receiver operating characteristic (ROC) curves were used to identify the best sensitivities and specificities for both 6MWD and peak Vo2 to determine survival in COPD patients. The Pearson correlation test was used to assess the relationship between the 6MWD and peak Vo2 and other clinical variables. Mortality was evaluated over time using Kaplan-Meier analysis. Differences in death rate at different 6MWD thresholds were evaluated by the log-rank test. Univariate and logistic regression analyses were used to investigate the relationship between the clinical variables and mortality. A probability value of < 0.05 was considered to be significant. Statistical analysis was completed with a commercial software package (GB Stat; St. Louis, MO).