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(Chest. 2005;128:55-61.)
© 2005 American College of Chest Physicians

Encouraged 6-min Walking Test Indicates Maximum Sustainable Exercise in COPD Patients*

Alejandro Casas, MD; Jordi Vilaro, PT; Roberto Rabinovich, MD; Anamaria Mayer, PT; Joan Albert Barberà, MD; Robert Rodriguez-Roisin, MD, FCCP and Josep Roca, MD

* From Servei de Pneumologia (Drs. Casas, Rabinovich, Barberà, Rodriguez-Roisin, and Roca, and Ms. Mayer), Hospital Clínic, IDIBAPS, Universitat de Barcelona; and EUIF Blanquerna (Dr. Vilaro), Universitat Ramon Llull, Barcelona, Spain.

Correspondence to: Josep Roca, MD, Servei de Pneumologia, Hospital Clínic, Villarroel 170, 08036 Barcelona, Spain; e-mail: jroca{at}clinic.ub.es


   Abstract
TOP
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Study objectives: In patients with moderate-to-severe COPD, an encouraged 6-min walking test (6MWT) is a high-intensity submaximal exercise protocol that shows an oxygen uptake (O2) plateau after the third minute of the test. This last feature prompted the hypothesis that self-paced walking speed is set to achieve "maximal" sustainable O2, namely "critical power" or "critical speed."

Patients and methods: Eight patients with moderate-to-severe COPD (mean age, 68 ± 7 years [± SD]; FEV1, 50 ± 13% predicted; PaO2, 69 ± 8 mm Hg) underwent the following tests on different days in order: (1) encouraged 6MWT; (2) standard incremental shuttle test to identify peak walking speed; (3) four different high-intensity, constant walking speed tests to exhaustion to calculate critical walking speed; and (4) timed walking test at critical walking speed (CWS) to examine sustainability of the exercise.

Results: 6MWT and CWS showed similar results (mean of last 3 min): O2 (1,605 ± 304 mL/min vs 1,584 ± 319 mL/min), minute ventilation (47 ± 12 L/min vs 48 ± 11 L/min), respiratory exchange ratio (0.89 ± 0.1 vs 0.90 ± 0.1), heart rate (130 ± 18 beats/min vs 131 ± 16 beats/min), Borg dyspnea score (5.4 ± 1.3 vs 5.5 ± 2.4), and walking speed (1.49 ± 0.1 m/s vs 1.44 ± 0.1 m/s, respectively).

Conclusion: This study supports that 6MWT indicates maximum sustainable exercise that might be related with its predictive value in COPD patients.

Key Words: COPD • critical power • exercise testing • exercise tolerance • 6-min walking test • sustainable submaximal exercise


   Introduction
TOP
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Impairment of exercise tolerance in COPD patients has important implications on health-related quality of life,123 hospitalization rate,45 and survival.67 Consequently, exercise testing is progressively being considered an essential component in the routine clinical assessment of functional status.

Recommendations on exercise testing in lung function laboratory settings are well established,8910 and the role of incremental cardiopulmonary cycling exercise as the "gold standard" for evaluation of exercise tolerance is acknowledged. We are facing, however, a widespread clinical use of simple exercise protocols31112 because of their simplicity, applicability, and low cost. Moreover, timed walking tests have been shown to predict survival6 and utilization of health-care resources45 in COPD patients. Therefore, it is suggested that simple exercise tests may be useful for staging of the disease.7

In a study13 conducted in patients with moderate-to-severe COPD, we reported the 6-min walking test (6MWT) as a high-intensity. submaximal exercise protocol that shows an exponential oxygen uptake (O2) increase up to a plateau during the last 3 min of the test. From these results, we hypothesized that the self-paced walking speed during the test is set to achieve "maximal" sustainable exercise, which, by definition, corresponds to patient’s critical power,14 or critical walking speed (CWS), defined as the maximum sustainable walking speed.

In the current study, we estimated the CWS in eight patients with moderate-to-severe COPD. The physiologic responses during the 6MWT were compared with those obtained during walking at CWS in the same patients. Identity between 6MWT and CWS may help to explain the high predictive value of 6MWT in chronic disorders.1516 An ancillary purpose of the investigation was the analysis of O2 profiles in four different common clinical exercise protocols performed in the same patients: incremental cycling exercise, 6MWT, incremental shuttle test, and stair-climbing test.10


   Materials and Methods
TOP
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Eight men with clinically stable COPD were recruited for the study. All of the men had a lack of exacerbations in the preceding 6 weeks. Oxyhemoglobin desaturation during exercise was an exclusion criteria. All patients were receiving inhaled, long-acting ß2-agonists and ipratropium bromide. Five of patients were regularly treated with inhaled steroids, but none of the eight patients had received systemic steroids in the last 3 months prior to the study. The study design included two phases: (1) comparison between 6MWT and walking at CWS, and (2) analysis of physiologic responses during the four different clinical exercise protocols.

Comparison Between 6MWT and Walking at CWS
All patients performed, in sequence, the following timed walking tests carried out in a 10-m course to allow comparability among exercise protocols: (1) encouraged 6MWT (6MWT carried out in a 10-m course [6MWT-10]); (2) standard incremental shuttle test; (3) four high-intensity timed-walking tests at different constant speeds to calculate CWS (Fig 1 ), done in random order on separate days; and (4) an additional timed walking test at CWS (Fig 1), carried out to demonstrate the sustainability of the effort at least for 20 min and to compare with the physiologic responses during 6MWT-10.



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  		Figure 1. Calculation of CWS in a representative patient. Top, A: O2 profiles in four high-intensity tests at constant walking speed (L1, 1.56 m/s; L2, 1.58 m/s; L3, 1.67 m/s; and L4, 1.75 m/s, which correspond to approximately 90%, 95%, 100%, and 105% of the peak walking speed during a standard incremental shuttle test, respectively). In three of the four tests, maximum whole-body O2 was reached at exhaustion. The fifth constant walking speed test was done at CWS (LCWS, 1.50 m/s). Center, B: The asymptote of the hyperbolic relationship between walking speed (y-axis) and time to exhaustion (L1 to L4 respectively, 1,060 s, 589 s, 328 s, and 225 s) indicates the CWS in the patient. Bottom, C: Linear relationship between walking speed and 1/time to exhaustion. The Y-intercept corresponds to CWS.

 
Calculation of CWS
The four different high-intensity, constant-walking, speed exercises (Fig 1) were specifically chosen at approximately 90% of the peak walking speed (L1), 95% of the peak walking speed (L2), 100% of the peak walking speed (L3), and 105% of the peak walking speed (L4) obtained during the incremental shuttle test. The CWS for a given patient corresponds to the asymptote of the hyperbolic function defined by walking speed and time to exhaustion17181920 (Fig 1, center, B). This hyperbolic function is mathematically transformed into a linear relationship when the reciprocal of time to exhaustion (1/time) is used in the x-axis (Fig 1, bottom, C). CWS is defined by the y-intercept obtained using the equation proposed by Neder et al.14

Physiologic Responses During Different Clinical Exercise Protocols
Besides the incremental shuttle test indicated above, three additional clinical exercise protocols were done in random order on separate days, in the same patients: incremental cycling exercise, 6MWT-90, and stair-climbing test. In all instances, the time elapsed between tests ensured achievement of resting conditions before each protocol.

Measurements
Lung function at rest2122 was carried out in all patients. A telemetric portable system (K4b2; Cosmed; Pavona di Albano, Italy) was utilized for on-line exercise measurements in all exercise protocols.1323 A standard incremental shuttle test to volitional fatigue was performed according to the established protocol.24 As indicated above, two types of 6MWT were performed in the present study: 6MWT carried out in a 10-m course (6MWT-10) and 6MWT-90; every 30 s, the patients were encouraged by an experienced physiotherapist to continue walking as far as possible.325 During the stair-climbing test, patients were instructed to climb as far as possible at a brisk pace without the use of railings and to stop at their symptom-limited maximum.26 Incremental cycling test was carried out following recommendations previously reported.10 Before and at the end of all tests, participants were shown a modified Borg scale printed on a card and asked to indicate their current degree of shortness of breath and leg discomfort on a scale of 0 = nothing at all, to 10 = very, very severe.27

Statistical Analysis
Results are expressed as mean ± SD or as mean ± SEM. Comparisons between 6MWT-10 and CWS were done using Student paired t tests and Bland and Altman analyses.28 One-way analysis of variance (ANOVA) for repeated measurements was carried out to examine O2 profiles at the different exercise protocols. A p value < 0.05 was considered as statistically significant.


   Results
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
The study group (mean age, 68 ± 7 years; range, 57 to 77 years) showed moderate-to-severe ventilatory dysfunction (FEV1, 1.63 ± 0.33 L; 50 ± 13% predicted; range, 32 to 67% predicted). Four patients were classified as Global Initiative for Chronic Obstructive Lung Disease (GOLD) type 2, three patients were GOLD type 3, and one patient was GOLD type 4.8 Seven of the eight patients showed air trapping (residual volume, 137 ± 26% predicted; range, 80 to 169%). On average, they presented moderate hypoxemia (PaO2, 69 ± 8 mm Hg; range, 60 to 82 mm Hg) with a mean PaCO2 of 41 ± 4 mm Hg (range, 38 to 49 mm Hg). Only two patients showed moderate hypercapnia (48 mm Hg and 49 mm Hg, respectively), but none of them showed exercise-induced hypoxemia. Incremental cycling exercise exhibited moderately low peak O2 (O2peak): 20.7 ± 2.4 mL/min/kg (range, 17 to 26 mL/min/kg).

Assessment of CWS
Individual relationships between walking speed and time to exhaustion during the four high-intensity, constant-walking speed tests (Fig 2 , top, A) showed the expected hyperbolic function indicated in Figure 1, center, B (L1 to L4). All patients presented an excellent linear correlation between constant walking speed expressed as a percentage of peak incremental shuttle and the reciprocal of time to exhaustion (Fig 2, bottom, B). Accordingly, calculated CWS was 1.44 ± 0.15 m/s, and the corresponding O2 plateau was 1,584 ± 319 mL/min. Sustainability of walking at CWS for at least 20 min (Fig 1, top, A) was confirmed in all subjects.



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  		Figure 2. Individual relationships between walking speed and time to exhaustion for the four high-intensity tests at constant walking speed done in each COPD patient (Pat). Top, A: The hyperbolic relationship between the two variables. Bottom, B: The x-axis shows the reciprocal of time to exhaustion (1/time) that explains the linear transformation of the relationship between the two variables. In this graph, walking speed is expressed as percentage of peak walking speed measured during the incremental shuttle test. Individual coefficients of correlation are indicated.

 
Three out of four high-intensity, constant-walking speed tests presented similar O2 values at exhaustion (L2, 1,715 ± 316 mL/min; L3, 1,655 ± 284 mL/min; and L4, 1,660 ± 239 mL/min), indicating that maximum walking O2 (mean value, 1,677 mL/min), calculated as the average of the highest O2 for each subject regardless of the speed of the tests (L2, 1.52 ± 0.15 m/s; L3, 1.59 ± 0.15 m/s; and L4, 1.67 ± 0.17 m/s), was identified in all patients.

Comparisons Between CWS and 6MWT-10
During the last 3 min of the 6MWT-10, O2 showed a plateau in all patients, as reported by others.13 Similarly, steady-state conditions (O2, minute ventilation [E], and heart rate [HR]) during walking at CWS were also observed in each patient. Consequently, comparisons between walking at CWS and 6MWT-10 were based on mean values of the 3 last min of each test. Figure 3 , top, A, displays a strong correlation in O2 results between CWS (1,584 ± 319 mL/min) and 6MWT-10 (1,605 ± 304 mL/min), wherein all patients fell close to the identity line (r = 0.93, p < 0.001). Likewise, Figure 3, bottom left, C, indicates an excellent concordance of O2 between the two exercise tests (– 21 ± 282 mL/min). Figure 3, top right, B, and bottom right, D, do not show statistically significant differences in walking speed (– 0.046 ± 0.14 m/s) between 6MWT-10 (1.49 ± 0.16 m/s) and calculated CWS (1.44 ± 0.15 m/s) [r = 0.90, p < 0.01]. E (47 ± 12 L/min vs 48 ± 11 L/min, 6MWT and CWS, respectively); carbon dioxide output (CO2) [1,441 ± 326 mL/min vs 1,420 ± 292 mL/min]; respiratory exchange ratio (RER) [0.89 ± 0.1 vs 0.90 ± 0.1]; HR (130 ± 18 beats/min vs 131 ± 16 beats/min); and symptom score were also similar between 6MWT-10 and CWS. Borg dyspnea scores at the end of the two tests were 5 ± 1 (range, 4 to 8) and 6 ± 2 (range, 2 to 9), respectively; likewise, the Borg leg scores were 4 ± 2 (range, 2 to 6) and 4 ± 2 (range, 1 to 8).



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  		Figure 3. Top left, A: Individual relationships between O2 during CWS (y-axis) and O2 during 6MWT-10 (x-axis) [r = 0.93, p < 0.001]. Top right, B: Individual relationships between CWS (y-axis) and walking speed during 6MWT-10 (x-axis) [r = 0.90, p < 0.01]. Bland and Altman analyses for these two variables (O2 and walking speed) are shown (bottom left, C, and bottom right, D, respectively). A high concordance between 6MWT-10 and CWS was observed in all instances.

 
Clinical Exercise Protocols
Figure 4 displays O2 profiles during incremental cycling exercise, incremental shuttle test, 6MWT-90, and stair-climbing test measured in the same patients. While 6MWT-90 showed a plateau in O2 after the third minute of the test,13 O2 increased up to the end of the test in the other three exercise protocols. The figure shows an abrupt raise in O2 during stair climbing (O2 at min 1 was 80% of O2 peak) that contrasted with the steady increase of O2 during both incremental cycling and incremental shuttle tests (O2 at min 1 were 35% and 34% of O2peak, respectively). Detailed information on physiologic responses at the end of each of the four tests is provided in Table 1 .



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  		Figure 4. Mean O2 profiles of the eight COPD patients during four different clinical exercise protocols (mean ± SEM): incremental cycling (open circles), incremental shuttle (solid circles), 6MWT-90 (solid triangles), and stair climbing (open squares).

 

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  		Table 1. Comparison Among Clinical Exercise Protocols*

 
The two types of 6-min walking protocols—6MWT-10 and 6MWT-90—displayed a plateau in O2 after the third minute of the test. 6MWT-10, however, showed higher O2 (1,605 ± 304 mL/min vs 1,429 ± 227 mL/min; p = 0.04), lower walking speed (1.49 ± 0.16 m/s vs 1.63 ± 0.14 ms; p = 0.0005), and shorter walked distance (mean difference, – 54 ± 1 m; p = 0.0002) than 6MWT-90.


   Discussion
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
The current study showed similar physiologic responses between encouraged 6MWT-10 and walking at CWS, which indicates that patients with moderate-to-severe COPD set their walking speed during the test in order to achieve critical O2. The level of O2 achieved at CWS may indicate the integrated response of the systems involved in O2 transport/O2 utilization that ultimately determine the highest sustainable level of exercise. These results might constitute the underlying explanation for the high prognostic value of the 6MWT.

As described above, the three highest constant walking speeds (L2 to L4 in Fig 1) showed similar O2 at exhaustion (on average, 1,677 mL/min). Moreover, both stair climbing (1,693 ± 256 mL/min) and incremental shuttle tests (1,651 ± 243 mL/min) also presented similar peak O2 results (Table 1), thereby further confirming identification of maximum O2max in all these patients. Maximum O2 indicates the maximum capacity of the system for O2 transport/O2 utilization. It is of most interest that critical O2 (O2 at CWS, 1,584 mL/min) was reported close to 94% of maximum O2 in the present study, as shown by other authors14 in COPD patients during incremental cycling.

Does CWS Indicate Critical Power?
While incremental cycling protocols910 are designed to facilitate the assessment of the relationship between O2 and work rate throughout the test, timed walking tests present important limitations in this regard. It is well accepted that the three most important factors determining energy requirements during encouraged 6MWT are walking speed, ergonomics of the test, and body weight. Since body weight remains unchanged, the other two factors modulate intrasubject variability of energy requirements during the test. The current study provides information on the relationships between these two factors (walking speed and ergonomics) and O2.

We observed proportionality between O2 and walking speed during the assessment of CWS, which supports the contention that walking speed was the key determinant of work rate in our patients. This is valid provided that the analysis is done with walking tests carried out in the same corridor length (10-m course in the study), such that similar ergonomics can be reasonably assumed. Moreover, verification of steady-state conditions during exercise at CWS, alike the equivalence between 6MWT-10 and CWS, further indicates the pivotal role of walking speed modulating energy requirements in these patients. The statement requires that self-paced walking speed was constant throughout the time intervals of the encouraged 6MWT, as it was proven in the present study and in previous reports.1329

As described in the "Results" section, 6MWT-10 showed lower efficiency (higher O2 at lower walking speed) than 6MWT-90, which illustrates that the ergonomics of the test cannot be neglected as a determinant of energy requirements. Clearly, the 10-m course imposed a higher number of turns than 6MWT-90 during the test, which decreased the average speed and increased O2. The impact of number of turns during the test on the walking distance has been also reported by Sciurba et al.30 These results fully support the appropriateness of the current study design that used 6MWT-10 for comparisons with CWS, whereas 6MWT-90 was used to compare with other clinical exercise protocols. It is of note that O2 during 6MWT-90 (1,429 ± 227 mL/min) was approximately 85% of O2max. Further insights on standardization of 6MWT are clearly beyond the scope of the present investigation.

Exercise Protocols: Physiologic and Clinical Implications
The most striking feature of encouraged stairs climbing (Fig 4, Table 1) was that approximately 80% of O2peak was already achieved within the first minute of the test. This finding rises major concerns on the safety of the test. Moreover, it does not fit with a desirable smooth increase in work rate that may eventually facilitate the analysis of physiologic responses throughout the test. These considerations together with inherent problems for an appropriate standardization of this test limit the usefulness of stairs climbing as a routine clinical test. Incidentally, it has been reported26 that nonencouraged stairs climbing protocols may generate a O2 plateau, likely indicating self-adjustment of the patient to achieve sustainable exercise.

Incremental cycling exercise and incremental shuttle test showed similar O2 profiles (Fig 4, Table 1). At peak exercise, however, incremental cycling showed higher CO2 and higher RER (p < 0.01 each) than the incremental shuttle test. Likewise, symptoms score (dyspnea and leg) were also higher (p < 0.05 each) during cycling exercise compared to incremental shuttle test. The differences between these two tests could be explained by higher blood lactate levels, hence ventilatory requirements, during incremental cycling compared to timed walking tests.13 In the current study, O2peak (1,661 ± 180 mL/min) during incremental cycling was similar to O2 at peak incremental shuttle (Table 1), despite the fact that the amount of exercising muscle mass during cycling exercise is significantly smaller than in timed walking tests and the characteristics of the exercise are different. As described above, maximum O2 was observed at exhaustion in most of the exercise protocols carried out in the present study (Fig 1, Table 1), irrespective of the amount of exercising muscle mass. A potential explanation for this finding is that the ceiling of whole-body O2 in COPD patients is mainly defined by the degree of pulmonary impairment and not by the amount of exercising muscle mass. The level of exercise is clearly below the vascular constraints to preserve systemic BP observed in healthy man, in whom the amount of exercising muscle mass modulates O2max.3132

The characteristics of the physiologic responses during encouraged 6MWT have been described in the current study and by others.13 Standardization and clinical implications of the test are also reported.3910 The incremental shuttle test is properly standardized,24 and it is widely used for evaluation of patients with chronic respiratory and cardiac disorders.3334

In summary, the current study demonstrates that physiologic responses during 6MWT in patients with moderate-to-severe COPD are equivalent to those seen walking at critical power, which corresponds to approximately 90% of maximum O2 in these patients. The relationship between 6MWT and CWS may likely explain the high predictive value of 6MWT that, together with its simplicity, applicability, and acceptable reproducibility,3035 prompts its recommendation as the most suitable choice for conventional clinical assessment of COPD patients, as part of the multidimensional evaluation of the disease severity.7


   Acknowledgements
 
The authors thank Jose Luis Valera, Felip Burgos, Conxi Gistau, and all the technical staff of the lung function laboratory for their skillful support during the study; Rosa Antonucci, visiting scientist from Universita di Roma, La Sapienza (Italy) for her contribution to the study; Elena Gimeno, PT, from EUIF Blanquerna, Universitat Ramon Llull, and Carme Hernandez, coordinator nurse of the Integrated Care Unit, are acknowledged for their outstanding work supervising the logistics of the study. Finally, the authors thank Cosmed (Italy) and Laboratorio Dr Esteve SA for the material support to conduct the study.


   Footnotes
 
Abbreviations: ANOVA = analysis of variance; 6MWT = 6-min walking test; 6MWT-10 = 6-min walking test carried out in a 10-m course; 6MWT-90 = 6-min walking test carried out in a 90-m course; CWS = critical walking speed; GOLD = Global Initiative for Chronic Obstructive Lung Disease; HR = heart rate; L1 = 90% of the peak walking speed; L2 = 95% of the peak walking speed; L3 = 100% of the peak walking speed; L4 = 105% of the peak walking speed; RER = respiratory exchange ratio; CO2 = carbon dioxide output; E = minute ventilation; O2 = oxygen uptake; O2peak = peak oxygen uptake

Alejandro Casas was a Pre-doctoral Research Fellow from Fundación Neumológica Colombiana (Colombia) supported by CHRONIC (IST-1999/12158) from the European Union; and, Anamaria Mayer was a Research Fellow from Federal University of Sao Paulo (Brasil) supported by CAPES and E-Remedy (IST-2000/25146).

Supported by grants FIS 00/0281 from the Fondo de Investigaciones Sanitarias; and Comissionat per a Universitats i Recerca de la Generalitat de Catalunya (1999 SGR 00228); and Red Respira C03/11.

Received for publication November 14, 2003. Accepted for publication January 4, 2005.


   References
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 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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C. G. Cote, V. Pinto-Plata, K. Kasprzyk, L. J. Dordelly, and B. R. Celli
The 6-Min Walk Distance, Peak Oxygen Uptake, and Mortality in COPD
Chest, December 1, 2007; 132(6): 1778 - 1785.
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