 Dyspnea is the most troublesome symptom in COPD and it is felt that the main mechanism through which bronchodilators improve exercise tolerance is by reducing dyspnea. As such, dyspnea measurement appears a valid surrogate of exercise tolerance. One advantage of dyspnea measurement over the measurement of exercise endurance is that it does not require a maximal effort. In this regard, it may be a less noisy outcome than exercise duration. Dyspnea can be quantified during the 6-min walking test but pre and post-intervention comparisons are made difficult since the walking speed and thus the exercise stimulus is not controlled during the test. Another strategy to evaluate the effects of interventions on exertional dyspnea is to compare dyspnea at isotime while controlling the walking or cycling speed during the endurance shuttle walking test or the constant rate cycling test. One limitation of this approach is that pre and post intervention dyspnea measurement is not always obtained at the same time point since the duration of the test is variable. To overcome this problem, linear interpolation can be used to estimate of dyspnea. However, this approach is not as robust as when a real dyspnea score is directly obtained from the patients. 