Our specific hypotheses were as follows: (1) the continuous method provides high test-retest reliability that is at least comparable to the discrete method, (2) the slopes of the linear regressions for power production, oxygen consumption (Vo2), and minute ventilation (Ve) as independent variables and breathlessness as the dependent variable are higher in patients with COPD than in healthy subjects, and (3) the x-intercepts and absolute thresholds are lower for patients with COPD compared with healthy subjects.
Inclusion criteria for all participants were as follows: age > 45 years, ability to exercise on the cycle ergometer, and no history of clinically important comorbid disease. Patients with COPD were recruited from the outpatient clinics of our institution. The diagnosis of COPD was based on American Thoracic Society criteria, and each patient complained of exertion breathlessness. All healthy subjects were recruited from family members or friends of the patients, they denied any cardiorespiratory complaints, and they had normal spirometry results. Institutional review board approval was obtained, and all participants gave written informed consent.
Each subject participated in three visits over a 5-day time period. At visit 1, spirometry and 12-lead ECG (to exclude any major rhythm abnormality) were performed to ensure that the subject met the inclusion criteria. Next, the participant was familiarized with the equipment and protocol by pedaling on the cycle ergometer for 5 min, and practiced using the computer system to provide ratings of breathlessness.
At visit 2 (2 days after visit 1) and visit 3 (2 days after visit 2), spirometry was performed using a standard system (Collins model CPL; Warren E. Collins; Braintree, MA) to ensure that lung function was stable. The highest values for FVC and FEVX were selected from a minimum of three FVC maneuvers. Predicted values for spirometry were taken from Crapo et al. Next, subjects performed two incremental exercise tests separated by a 60-min rest. The method for measuring breathlessness (discrete or continuous) was assigned by an alternating schedule. At visit 2, 12 patients and 12 healthy subjects used the discrete method first and the continuous method second; the other subjects rated breathlessness in the opposite order. At visit 3, the method for measuring breathlessness was reversed for each participant.