Shuttle Walking Test and 6-Minute Walking Test Induce a Similar Cardiorespiratory Performance in Patients Recovering from an Acute Exacerbation of Chronic Obstructive Pulmonary DiseaseVagaggini B. · Taccola M. · Severino S. · Marcello M. · Antonelli S. · Brogi S. · De Simone C. · Giardina A. · Paggiaro P.L.
Cardio-Thoracic Department, Pneumology Section, University of Pisa, Pisa, Italy Corresponding Author
Background: The incremental shuttle walking test (SWT) has recently been proposed as a more valid and reproducible alternative to the conventional 6-min walking test (6MWT) in the evaluation of exercise tolerance in patients with chronic obstructive pulmonary disease (COPD). Objective: To compare the cardiorespiratory performance obtained during two sessions of SWT with that obtained during two sessions of 6MWT. Methods: We examined 18 patients (forced expiratory volume in 1 s: 48 ± 14%) recovering from an acute exacerbation of COPD that had required hospitalization. In the same afternoon, each patient performed two SWT and two 6MWT, with an interval of at least 30 min between each test; the sequence of the tests was randomized. Results: Mean walking distance was greater in the second SWT test than in the first SWT. The changes from baseline in systolic blood pressure, heart rate, respiratory rate, oxygen saturation and dyspnea Borg index at the end of the test were similar between the two 6MWT and the two SWT. There was a highly significant correlation between walking distances measured during SWT and during 6MWT (ρ: 0.85, p < 0.0005). Neither SWT nor 6MWT correlated with functional data of COPD. Conclusions: SWT, though being considered to be closer to a submaximal exercise test than 6MWT, does not induce a greater cardiorespiratory performance than 6MWT in patients recovering from acute exacerbation of COPD.
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Exercise limitation is a prominent complaint in patients with chronic obstructive pulmonary disease (COPD). However, the correlation between exercise capacity and disease severity, in terms of lung function and gas exchange impairment, is generally poor . The reduction in exercise tolerance is due to different factors: abnormal lung mechanics (e.g. airflow limitation or dynamic hyperinflation), inefficient pulmonary gas exchange, peripheral muscle weakness , abnormal oxygen transport and energy production in the exercising muscle . It is widely known that pulmonary rehabilitation can improve exercise tolerance and consequently the quality of life and daily living [4, 5].
The 6-min walking test (6MWT) is frequently used to evaluate the functional status as well as the effects of pharmacologic therapy and rehabilitation programs in COPD patients [4, 6]. 6MWT is a self-paced test, and the degree of the therapist encouragement can be crucial in setting walking speed. The test is reliable, safe and inexpensive , but it is not considered a submaximal exercise test. More recently, an incremental shuttle walking test (SWT) has been developed . It is an externally paced test, less influenced by the therapist encouragement, and it has been proposed as a more reproducible test to evaluate exercise tolerance. Studies comparing the performance obtained during SWT with that obtained during 6MWT had shown a moderate correlation between 6MWT and SWT, with a higher heart rate observed at the end of SWT . Another study compared SWT performance with that obtained by the conventional treadmill test, showing a highly significant correlation between the VO2max value obtained during the treadmill test and that obtained during SWT . Thus, SWT is considered to be closer to a submaximal exercise test than 6MWT.
The aim of our study was to compare the cardiorespiratory performance during 6MWT and SWT in a group of COPD patients examined before hospital discharge after acute exacerbation of COPD. Our hypothesis was that SWT, being an incremental submaximal test, might induce a greater stress on the cardiorespiratory system than 6MWT. In order to verify this hypothesis, 18 COPD patients underwent two 6MWT and two SWT, and some cardiorespiratory parameters were measured before and after each test.
Patients and Methods
We examined 18 inpatients (15 males and 3 females, mean age 67 ± 8.2 years), with moderate-to-severe COPD (forced expiratory volume in 1 s, FEV1: 48 ± 14%). Table 1 shows the demographic and functional findings of the patients. All of them were in the recovery phase of an exacerbation of COPD that had required hospitalization. At the time of the study, they were treated with bronchodilators and/or inhaled corticosteroids, and were examined at least 14 days after the beginning of the exacerbation. Pulmonary function tests and blood gas analysis were performed the day before the study protocol.
Table 1. Demographic and functional characteristics of 18 COPD patients
Patients had never before performed an 6MWT or SWT. After a training test, patients performed two 6MWT and two SWT during a 1-day session, in a randomized order (6MWT1-SWT1-6MWT2-SWT2: 9 patients; SWT1-6MWT1-SWT2-6MWT2: 9 patients). The tests were performed at intervals of 30 min to enable complete recovery of dyspnea (Borg index: value of the previous test ±1). Oxygen saturation and heart rate were monitored throughout the test by a pulse oxymeter applied to a fingertip. Before and within 60 s after the end of both 6MWT and SWT, heart rate, blood pressure, respiratory rate, oxygen saturation and dyspnea Borg index were measured by the same therapist.
Six-Min Walking Test. The 6MWT was performed according to the recommended method . The therapist sat half way in a 38-meter hospital corridor in a visible position, issuing standardized instructions and encouragement every 30 s during the test. Patients were instructed to walk at maximum speed for 6 min; the start and the end points of the course were delineated to demonstrate the patients where to turn around and where to walk back; the patients were allowed to stop anytime and anywhere until they had rested enough to start walking again. Patients stopped the test either when they experienced severe dyspnea or at the end of the 6-min interval.
Shuttle Walking Test. The SWT was performed according to a previously reported method . In summary, the patient had to walk up and down a 10-m course delineated by two cones set 0.5 m from either end. The speed at which the patient walked was dictated by an audiosignal played on a tape cassette. Every minute the speed of walking was increased by a small increment. The only verbal contact with the therapist was the advice given each minute to increase the walking speed slightly. The test was terminated either: (a) by the patient, when he or she was too breathless to maintain the required speed; (b) the therapist, if the patient failed to complete a shuttle in the time allowed (i.e. when he was farther than 0.5 m from the cone at the time when the beep sounded), or (c) attainment of 85% of the predicted maximal heart rate derived from the formula [210 – (0.65 × age)].
Statistical Analysis. All indices are expressed as means ± SD. Data comparison among different tests was performed by means of ANOVA, while data comparison between baseline and the end of exercise was performed by means of the paired t test. Changes from baseline were computed for each test and compared by means of ANOVA. The relationship between meters of 6MWT2 and SWT2 was calculated by the Spearman rank correlation coefficient. A p value < 0.05 was considered significant.
After the training test, all patients found it easy to pace themselves and no difficulties were encountered in administering both tests.
Significantly more meters were covered with the 6MWT than with the SWT in both the first test (432.6 ± 86.4 vs. 272.8 ± 101.9 m, respectively, p = 0.0002) and in the second test (447.2 ± 80.8 vs. 312.6 ± 119.2 m, respectively, p = 0.0003; fig. 1). Distances covered during 6MWT2 and SWT2 were longer than distances covered during 6MWT1 and SWT1, respectively, but the difference was statistically significant only for SWT2 (fig. 1).
Fig. 1. Mean walking distance (m) during the two 6MWT (▧ = 6MWT1; ■ = 6MWT2) and during the two SWT (▧ = SWT1; ■ = SWT2).
Table 2 shows the mean values of systolic and diastolic blood pressure, heart rate and respiratory rate at the beginning and at the end of each exercise test. There was no difference in baseline values among different tests, suggesting that resting time between tests was adequate. At the end of each test, systolic blood pressure, heart rate and respiratory rate significantly increased; the increase was not significantly different among the various tests. In particular, no difference was observed between values at baseline and after 6MWT2 and SWT2.
Table 2. Cardiorespiratory parameters before and after 6MWT and SWT
SatO2 was similar at the beginning of each test, and significantly decreased at the end of each test, without any significant difference among the various tests (table 3). The dyspnea Borg index showed a similar increase at the end of the exercise test in both 6MWT and SWT. The value of the dyspnea Borg index obtained at the end of the second test was significantly higher than that measured at the end of the first test in 6MWT (p = 0.02), but not in SWT (p = 0.09). There was a significant relationship between the distances walked in 6MWT2 and in SWT2 (ρ = 0.85, p < 0.0005; fig. 2).
Fig. 2. Correlation between the distances walked in the 6MWT and SWT.
Table 3. Oxygen saturation and dyspnea Borg index before and after 6MWT and SWT
No correlation was observed between 6MWT and SWT performance and clinical and functional findings of COPD patients, such as the duration of the disease, FEV1, forced vital capacity, total lung capacity and resting arterial blood gases.
We demonstrated that the cardiorespiratory response after SWT was similar to that obtained after the standard 6MWT regarding increases in heart rate, respiratory rate, systolic blood pressure, dyspnea Borg index and decrease in oxygen saturation. This fact suggests that the subjects developed a similar cardiorespiratory performance in both tests. Therefore, our data are not in agreement with the assumption that SWT induces a greater increase in cardiopulmonary parameters, because it is an incremental test and not a self-paced exercise test as 6MWT. The limited number of subjects studied may however attenuate the implication of these results.
A previous study reported that maximum heart rate and the dyspnea Borg index were higher at the end of SWT than at the end of 6MWT in a group of 15 patients with severe COPD . Furthermore, maximum heart rate achieved at the end of both 6MWT and SWT seems slightly lower in our patients in comparison with the data reported by Singh et al. [8, 9]. There is no relevant difference in the level of severity of COPD between our study and other studies, at least regarding FEV1. However, FEV1 is a poor predictor of symptoms and disability in COPD patients , and similar FEV1 values may not reflect similar levels of dyspnea or quality of life. Another possible explanation for the discrepancy between our results and the previous ones might be the different phase of the disease: our patients were observed in the recovery phase of an acute exacerbation of COPD, and not in the stable phase as the patients examined by Singh et al. [8, 9]. After an acute exacerbation of COPD, dyspnea and lower limb muscular weakness may add to the cardiorespiratory limitation, thus affecting the results of performance.
In two other studies 6MWT and SWT were compared in different patient cohorts, e.g. patients with cystic fibrosis  and heart failure ; in the latter, both tests were also compared with a maximal treadmill peak oxygen consumption. Both papers showed a correlation between the results obtained in the two tests, although SWT correlated better with peak oxygen consumption than 6MWT . Peak heart rate obtained after both tests was higher than peak heart rate obtained in our study . This fact might be due to the different patient groups, with patients with cystic fibrosis being younger and with less airflow limitation.
In several studies, maximum oxygen uptake measured after 6MWT or after SWT correlated with maximal oxygen uptake during a cardiopulmonary exercise test using cycloergometer or treadmill [9, 13, 14, 15], suggesting that both tests express the ability to perform an exercise in patients with chronic flow limitation. In COPD patients, it is believed that the extent of an impairment can be detected more clearly by SWT than by traditional 6MWT, emphasizing the possible beneficial effects of any treatment. This hypothesis is based on theoretical considerations and on the results of few studies comparing the two tests in patients with different diseases [8, 12, 13]. By contrast, our data show that the two tests are equivalent when used in a group of patients with COPD of moderate-to-severe degree recovering from an acute exacerbation. Furthermore, these differences might be explained by the different stages of the disease in patients at the time of observation (exacerbation vs. stable phase of COPD).
The correlation between 6MWT and SWT is considerably higher than that reported by other authors , confirming the similarities between these two tests in our experience.
According to previous studies [8, 9, 13, 16, 17], our study confirms the feasibility and the good repeatability of SWT even for patients with no previous experience with this test, which can then be used as a routine method to assess exercise tolerance in COPD patients.
In the second test, maximum distance was significantly longer than in the first SWT but not in the 6MWT. It may be argued that the SWT is more difficult, and the ‘learning effect’ is thus more pronounced than in the 6MWT. However, this ‘learning effect’ for both tests has been reported previously [10, 13, 16], but it is considered to be marginal for SWT because of the lack of encouragement by the therapist, which could influence patient response. When 6MWT is performed in a standardized manner, as in the present study, with similar encouragement by the therapist, the reproducibility is quite high and probably similar to that of SWT. Indeed, in a recent study in a high percentage of rehabilitation centers, the standardization of 6MWT is quite poor .
At the end of the second test, the dyspnea Borg index was higher than at the end of the first test, after both 6MWT and SWT, possibly due to fatigue which developed during the first test. However, there was no significant difference among the dyspnea Borg index obtained before each test, since the time interval between the tests was sufficient to enable recovery.
No correlation was observed between the performance obtained in 6MWT or SWT and main functional parameters of the patients, in particular FEV1 or arterial blood gas levels, confirming previously reported data [1, 19]. Thus, exercise tests must be performed in these patients in order to assess their disability, because this information cannot be derived from traditional functional findings.
In conclusion, in patients recovering from an acute exacerbation of COPD, SWT and 6MWT induce a similar cardiorespiratory performance. Both tests are reproducible and feasible even in hospitalized patients. Further studies are required to compare these two tests in patients at different phases of COPD and in the evaluation of treatment effects on COPD patients.
Dr.sa Barbara Vagaggini
Dipartimento Cardiotoracico, Ospedale Cisanello
Via Paradisa, 2
IT–56100 Pisa (Italy)
Tel. +39 050 995366, Fax +39 050 580124, E-Mail firstname.lastname@example.org
Received: July 19, 2002
Accepted after revision: May 12, 2003
Number of Print Pages : 6
Number of Figures : 2, Number of Tables : 3, Number of References : 19
Respiration (International Review of Thoracic Diseases)
Founded 1944 as ‘Schweizerische Zeitschrift für Tuberkulose und Pneumonologie’ by E. Bachmann, M. Gilbert, F. Häberlin, W. Löffler, P. Steiner and E. Uehlinger, continued 1962–1967 as ‘Medicina Thoracalis’ as of 1968 as ‘Respiration’, H. Herzog (1962–1997)
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