We read with interest the recent paper by Bonnevie et al. (1),“Six-minute stepper test to set pulmonary rehabilitation intensity in patients with COPD− A retrospective study.” The central idea of the study was to use the heart rate (HR) obtained from the six-minute stepper test (6MST) to adjust the training intensity for patients with COPD. For that, the authors compared the mean HR during the first (HR1–3) and last (HR4–6) three minutes of the 6MST with the HR at the ventilatory threshold (HRvt) from the cardiopulmonary exercise test (CPET). The study has an interesting premise since the exercise intensity is a crucial aspect to obtaining the benefits of aerobic training, and simple strategies to prescribe it are still needed. However, some aspects are subject to criticism.

First, HR is not suitable for determining target exercise training intensity for patients with COPD, because, even at maximum cardiopulmonary exercise testing, the peak HR is commonly low, i.e., they present with a high HR reserve (2). In addition, although the 6MST is time-limited, it is possible that, for some patients with COPD, it is a maximum test, and, for others, it is a submaximal test, as observed with the six-minute walk test in this population (3). Therefore, the HR obtained from the 6MST should be used cautiously to prescribe aerobic training in patients with COPD. To minimize possible imprecisions of this method, we suggest, in future studies, to associate dyspnea ratings to regulate the intensity of exercise training (4).

Second, the external validity of adjusting the training intensity based on HR is reduced when the ventilatory threshold is not always detected in patients with COPD (5). In the present study, the authors had the elegance of describing that HR at the ventilatory threshold (HRvt) from CPET was not identified some patients (more than one-third). How, then, should the intensity be adjusted for these patients? Additionally, a previous study (5) showed a wide variability on HRvt when expressed as a percent of attained peak HR, predicted peak HR, and HR reserve in patients with COPD.

Third, patients in the study by Bonnevie et al. (1) seemed to have presented mild impairment of functional capacity, since they performed a higher number of steps in the 6MST than did patients with COPD in the original study, but presented with similar performances to those observed in healthy subjects (6). Since cardiovascular responses to exercise in patients with COPD differ significantly depending on the severity of COPD (7), it would have been interesting to see data from the CPET to explain the possible causes of the exercise limitation in the patients studied. In relation to CPET, how was the CPET conducted in seven patients that used long-term oxygen?

Fourth, the analysis to answer the main research question was doneby correlating HR from 6MST withHRvt from CPET. However, this seems to be not appropriate when the objective is the comparability of the methods (8). Instead, the Bland-Altman analysis would be more informative(9).

Although we recognize the relevance of the Bonnevie's et al. study, it seems not be clinically justifiable using the HR estimated from a constant workload test (6MST) to target the intensity of exercise training in patients with COPD undergoing pulmonary rehabilitation. Since maximal exercise capacity is rarely limited by the cardiocirculatory system in patients with COPD, the use of this parameter may result in improper training stimulus. In this context, to solve the lack of evaluation centers for pulmonary rehabilitation, we suggest using either the incremental shuttle walk test (10) or the modified incremental step test (11) to determine the appropriate exercise intensity, based on a percentage of the speed or number of steps, respectively.