Several studies have compared the correlation between peak VO2 and ISWT and 6MWT distances, but this study is the first to perform a meta-analysis of the correlation. This meta-analysis provided further evidence that the 6MWT and ISWT distances had significant correlations with peak VO2 measured using CPET in patients with COPD. When comparing the correlation coefficients with peak VO2, the ISWT showed a stronger correlation than the 6MWT. These results suggest that ISWT reflects a stronger correlation with peak VO2 measured using CPET than 6MWT in patients with COPD.
To date, many studies have confirmed that 6MWT and ISWT are valid, reliable, and responsive to therapeutic interventions [8, 24]. However, the 6MWT and ISWT provide quite different protocols. The 6MWT is a self-paced submaximal test and can be continuous or intermittent. This test is sensitive to methodological variations, such as encouragement, oxygen supplement, and circumstances (e.g., wheeled walking aid, circular/straight track, indoors/outdoors) . By contrast, ISWT is an externally paced maximal exercise test [5, 8, 24]. This feature may be an advantage in circumstances where methodological variation is a concern if the test is performed by various sites or operators . In addition, where the larger space requirements of 6MWT preclude its use, ISWT can be a useful alternative . 6MWT requires a 30-meter walking course, but ISWT only requires a short course of 10-meter walking course. The protocol of ISWT is more standardised than that of 6MWT, and the proposal of incremental values is also clear in the shuttle walk test. In addition, ISWT shows a linear change of lung gas exchange including peak VO2, but 6MWT shows an exponential change . Furthermore, the walking distance in ISWT has been reported to be reliable and a good indicator for predicting re-hospitalisations in patients with moderate to severe COPD . Therefore, ISWT has better features than 6MWT.
Thus, we aimed to determine the correlation between peak VO2 and 6MWT and ISWT distances through a meta-analysis and to compare the correlation coefficient of both field tests. In previous studies, both field tests were confirmed to show a relatively strong correlation with peak VO2 through a meta-analysis. Notably, the correlation coefficient of ISWT was stronger than that of 6MWT. In a subgroup analysis, we found that both field tests had lower correlation coefficients in the high exercise capacity group. That is, when the exercise capacity was good, the correlation between the distance of field tests and peak VO2 decreased. This may be related to the “ceiling effect” that occurred in 6MWT. In a related study comparing bronchodilator-induced changes in exercise capacity with the 6MWT and the shuttle walk test, 6MWT showed less responsive for detecting changes in COPD patients with high exercise capacity . In addition, the heterogeneity of ISWT was very low in all analyses. Although there was a limitation to the small number of the included ISWT studies, the low heterogeneity could support the good reproducibility of ISWT. On the contrary, 6MWT showed high heterogeneity, and I2 value was higher, particularly in the high exercise capacity group. ISWT was proven superior to 6MWT for evaluating COPD patients with high exercise capacity.
Despite the advantages of ISWT, it is underutilised to evaluate exercise capacity and effectiveness for treatment and respiratory rehabilitation, and to predict prognosis in chronic respiratory diseases. Many studies have already demonstrated its superiority in pre-operation evaluation, respiratory rehabilitation, and cardiopulmonary function evaluation [24, 27–29]. ISWT has not been introduced in many countries including South Korea. 6MWT is also a good field test . However, considering the result of this study and other studies,[5, 6, 11, 12, 23] the use of ISWT is recommended because a stronger correlation with peak VO2 in CPET was found in this study using a meta-analysis.
This systematic review and meta-analysis has several limitations that should be mentioned. Firstly, we only included a small number of studies. Secondly, in the included studies, exercise modalities in CPET were heterogeneous. Peak VO2 was measured using a treadmill in three studies and a cycle ergometer in nine studies. Several studies have reported that the peak VO2 measured using a cycle ergometer is lower than that using a treadmill, but both peak VO2 measured with these devices show a significant correlation [30, 31]. In addition, the objective of this study was not to analyse the absolute value of the peak VO2, but to investigate the correlation between the peak VO2 and the field test results. Therefore, the difference in CPET exercise method is believed to not significantly affect the results of this study. Finally, for more convincing evidence with regard to the correlation between peak VO2 and field tests, more qualified prospective studies are required. In addition, future studies should verify whether the correlation coefficient of ISWT is superior to that of 6MWT as a primary outcome after calculating the sample size with the superiority study design.