Patients with chronic obstructive pulmonary disease (COPD) develop dynamic hyperinflation (DH) during exercise,1,2 which has been invoked as one of the main mechanisms in the development of dyspnea.3–5

Cardiopulmonary exercise tests (CPET) provide information on the exercise capacity of these patients. Among them the 6 minute walk test (6MWT) is the most standardized field test.6 Some authors believe that 6MWT can be considered a maximum exercise test. In a study conducted in patients with moderate-to-severe COPD, Troosters et al.7 reported the 6MWT as a high-intensity submaximal exercise protocol which showed an exponential oxygen uptake (V¿O2) increase up to a plateau during the last 3min and these authors hypothesized that the self-paced walking speed during the test is set to achieve “maximal” sustainable exercise. These data are quite similar to those found by Casas.8 On the other hand, in the pulmonary function laboratory setting, incremental CPET on a treadmill is regarded as a more accurate way to reflect daily life exercise.9–12 Despite being based on the same type of exercise and not differences made for exercise performance evaluation, few studies have compared the 6MWT and the incremental CPET on a treadmill in COPD patients.13,14

The purpose of this study is to compare the perceptive, physiological responses and degree of dynamic hyperinflation in 6MWT and incremental CPET on a treadmill; and to evaluate how dyspnea is related to dynamic hyperinflation (DH) and other functional parameters in both tests.

From the results obtained in our study we can conclude that both types of walking exercise, continuous 6 minute walking and incremental workload CPET on a treadmill, entail development of dynamic hyperinflation (DH) which can be determined by measuring IC. Nevertheless, the different degree of DH on both tests, which is higher after the incremental CPET on a treadmill, can be attributed to different exercise intensities expressed by differences in heart rate, oxygen desaturation and symptoms. These data indicate that different exercise protocols elicit different responses, so that equivalences between test results are hard to make and that, in clinical practice, some clinical changes might be easier to demonstrate with one test modality and not with another.

In COPD patients, intolerance to exercise has a multifactorial etiology.23 The factors that contribute to this reduced capacity for exercise are peripheral vascular dysfunction, abnormalities in oxygen transport, ventilatory inefficiency, development of dynamic hyperinflation and resulting limitation because of symptoms (shortness of breath or leg discomfort).4 Hyperinflation as a cause of limitation to exercise is an accepted phenomenon demonstrated by different authors.5 In COPD patients, when respiratory frequency increases during physical exercise and expiratory time shortens, there is a gradual air entrapment that leads to dynamic hyperinflation, with a progressive decrease in IC. Determination of IC at rest and during exercise therefore provides valuable information on the COPD patient's ventilatory changes during exercise.24 In our study we have verified that there is a much higher degree of DH for the incremental treadmill CPET compared to the 6MWT, with a mean loss of 970mL and 190mL in IC after performing either test, respectively. We found a significant, inverse relation between post-exercise breathlessness and IC on both tests. A priori, these results could explain appearance of dyspnea upon both tests and the more intense sensation after the treadmill CPET compared to the 6MWT (6.6/10 vs 4.3/10 respectively). Nevertheless, there is no tight link between both parameters, as can be seen in Fig. 2C, where patients with quite similar levels of IC could have a quite different dyspnea sensation, resulting in a large scatter of the points. Classically, several mechanisms have been involved in the development of dyspnea during exercise in COPD.25,26 Based on the data of our study, we can confirm that although DH seems to be an important factor in the genesis of dyspnea in our patients, it is probably not the determinant factor for this complex sensation, particularly in steady state exercise during a 6MWT. There also exists a great deal of variability regarding dyspnea perception in patients after an intense incremental treadmill exercise, not closely related to DH, as can be seen in Fig. 2C.

Pepin and co-workers demonstrated that perceptual response was consistent with physiological findings.12,25 However, there are increasingly recognized discrepancies between pulmonary function data and symptoms in COPD patients, emphasizing the value of performing exercise testing. In our study, the only parameter predictive for dyspnea at the end of the different exercise tests was baseline mMRC dyspnea rating: (r=0.4, p 0.03 for 6MW-test and r=0.5, p 0.006 for the treadmill). We did not find a relationship (just in RV/TLC on the treadmill) between static functional parameters and different perception of dyspnea upon completing both tests. This fact underlines the known disproportions between different areas of disease involvement, namely between functional impairment and perceptual component in our case.

Besides the type of exercise, there is little information on studies that have compared different exercise testing protocols. Palange and colleagues have compared walking and cycling and demonstrated a greater level of breathlessness and lower leg fatigue after an incremental shuttle walking test (ISW) compared with an incremental cycling protocol.11 Other authors such as Mathur and co-workers found that dyspnea was the major symptom limiting both types of exercise when comparing incremental walking on a treadmill with incremental cycling.14 The finding that dyspnea is the primary symptom limiting walking exercise raises the question of whether a field 6MWT may be more appropriate or more sensitive to change when assessing the effects of therapeutic interventions on exercise-induced dyspnea.27,28 Marin et al. studied the development of DH during the 6MWT.29 In their study, they measured IC during the 6MW-test. This work established an inverse relation between IC and the sensation of shortness of breath reported at the end of the test (r=−0.49, p<0.00001), similar to the results of our study.

The responses found in our study in both tests enable us to state that they are not equivalent. We observed a maximal HR achieved during the treadmill CPET (HR treadmill=130±13bpm) that was higher than in the 6MWT (HR 6MWT=113±16bpm). SpO2 at completion of the treadmill CPET (SpO2=89±3%) was lower than when completing the 6MWT (SpO2=92±2%). Based on these data, we conclude that test intensities were not comparable, so that the 6MWT can be considered a submaximal test, contrary to what authors such as Casas have argued in the literature.8 These authors have evaluated patients with moderate to severe COPD and found that during the 6MWT patients showed an increase of oxygen uptake (V˙O2 max) up to a plateau maintained during the last 3min of the test. A similar physiological response was shown at critical walking speed (CWS), which indicates that patients with moderate to severe COPD set their walking speed during the test in order to achieve the maximal sustainable V˙O2, so that this test is considered a maximal stress. In the aforementioned study 8 COPD patients (FEV1=50±13%) were included; they underwent different exercise tests with different protocols. During the 6MWT they obtained a HR=130±18bpm and dyspnea measured with the Borg scale was 5.4±1.3. In our patients no such data of maximal cardio-respiratory stress could be observed during the 6MWT, in contrast to incremental treadmill CPET.

This study has several limitations. For instance, the COPD patients we studied may not always be homogenous between studies as we can suspect from discrepancies between aforementioned references or from striking differences between individuals. We have only included male patients, more prevalent in Spanish COPD populations, therefore our results cannot be extended for all COPD patients. The small number of patients included could be another limitation but the statistical analysis we have used, the bootstrap procedure, compensate for the limitation of this small number of patients included in the study.

IC was only evaluated prior to starting the exercise and at the end of the test. It was not possible to evaluate the response throughout the tests. This can be justified by the complexity of achieving reliable manoeuvers with acceptable reproducibility. Interbedded manoeuvers raise the possibility of changing DH levels and alter the perception of symptoms during the test, which could be impaired with respect to their usual performance.30,31 The degree of DH during the 6MWT may also have been underestimated because of the time taken by the patient upon completing the test to approach the spirometer to perform the IC maneuver, which in our case was avoided by keeping the same walking pace of the test for approaching the spirometer. However, we can state that the maneuver is simple to perform and suitable to demonstrate DH in daily clinical practice.32,33

Our study has been able to detect development of DH in COPD male patients even during submaximal exercise such as the 6MWT and more pronouncedly during maximal exercise such as the incremental treadmill CPET. However, these tests showed quite different physiological and perceptual responses, so that analogies for both test are difficult to set. Furthermore, the development of DH cannot be invoked as the only cause of the shortness of breath, especially on a six minute walk test.

The authors have no conflicts of interest to declare