The 4-metre gait speed in COPD: responsiveness and minimal clinically important difference

Response to pulmonary rehabilitation

Of the 430 patients referred for pulmonary rehabilitation, 120 were deemed non-completers as they failed to attend ≥50% of supervised sessions. Nine patients completed, but failed to attend assessment post-rehabilitation; hence, data from 301 (70%) participants (165 male and 136 female) were available for analysis. Baseline characteristics are presented as median (interquartile range): age 71 (64–76) years, forced expiratory volume in 1 s (FEV₁) 49 (32–63)% predicted, body mass index 26.7 (23.4–30.9) kg·m⁻², Charlson index 1 (1–2) and COTE index 0 (0–1). table 1 shows the changes in 4MGS, ISW, MRC, CAT, CRQ and SGRQ following pulmonary rehabilitation. There was no significant correlation between change in 4MGS and number of pulmonary rehabilitation sessions attended or baseline 4MGS. A significant increase in mean 4MGS with pulmonary rehabilitation (0.08 m·s⁻¹) was demonstrated, which correlated significantly with change in ISW (r=0.26; p<0.001), and change in SGRQ, CRQ and CAT (r= −0.18, r=0.26 and −0.16; p<0.01). The effect size with PR for both the 4MGS and ISW in the whole cohort was 0.4.

Changes in 4MGS and ISW with pulmonary rehabilitation stratified according to baseline 4MGS (arranged into quartiles) are shown in figure 1. For patients in the lowest quartile for baseline 4MGS (<0.77 m·s⁻¹), the mean change in 4MGS with pulmonary rehabiliation was 0.11 m·s⁻¹ and the mean change in ISW was 61 m. For this subgroup, effect size for 4MGS and ISW were 1.0 and 0.6, respectively. With pulmonary rehabiliation, patients in the highest quartile for baseline 4MGS (≥1.05 m·s⁻¹) improved mean 4MGS by 0.02 m·s⁻¹ and mean ISW by 53 m, corresponding to an effect size of 0.2 and 0.4, respectively.

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Figure 1–

Scatterplot showing individual values and mean (solid line) change in a) 4-m gait speed (4MGS) and b) incremental shuttle walk (ISW) with pulmonary rehabilitation stratified according to baseline 4MGS quartiles (Q1: <0.77 m·s⁻¹; Q2: 0.77–0.91 m·s⁻¹; Q3: 0.92–1.04 m·s⁻¹; Q4: >1.05 m·s⁻¹). Dashed lines represent an improvement and decline by the minimal clinically important difference (0.11 m·s⁻¹ for 4MGS; 47.5 m for ISW).

Longitudinal change in 4MGS

Of 200 patients who consented for the study, 38 did not return for 12-month follow-up (12 died, six moved out of the area, 14 declined and six were no longer contactable). The remaining 162 patients (89 male and 73 female) had a median age of 70 (64–76) years. table 2 shows changes in 4MGS, ISW, MRC, CAT, SGRQ and FEV₁ % pred from baseline to 12 months. There was a significant reduction in mean change of the 4MGS (−0.04 m·s⁻¹) and median change in ISW (−30 m) over 12 months. FEV₁ % pred did not change significantly within the same time period. There was a significant positive correlation between change in 4MGS and change in ISW (r=0.45; p<0.001) (fig. 2), and significant negative correlations with CAT and SGRQ over time (r= −0.26 and r= −0.34; p<0.001).

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Figure 2–

Relationship between change in 4-m gait speed (4MGS) and change in incremental shuttle walk (ISW) over 1 year.

Minimal clinically important difference

The MDC95 was 0.11 m·s⁻¹. Using an anchor-based approach, the mean (95% CI) change in 4MGS in those achieving the MCID in the ISW following pulmonary rehabilitation was 0.11 (0.09–0.13) m·s⁻¹. For those “feeling better” after pulmonary rehabilitation, the mean change in 4MGS was 0.08 (0.05–0.10) m·s⁻¹. Using the most conservative estimate for the MCID (0.11 m·s⁻¹), 40% of all patients undergoing pulmonary improved their 4MGS by the MCID, compared with 51% achieving the MCID in ISW. In the frail group, 55% increased their 4MGS by the MCID compared to 47% achieving the MCID in ISW. In the longitudinal study, patients with a >47.5-m decline in ISW over 12 months showed a mean change in 4MGS of −0.11 m·s⁻¹.

Previous studies

Recovery of function or walking ability is a common goal after rehabilitation. Several previous studies have utilised the usual gait speed as an outcome measure, particularly in patients who have received rehabilitation or exercise interventions after recent hospitalisation, hip fracture, spinal cord injury or stroke [4–6, 19, 20]. Palombaro et al. [5] pooled data from three studies on 92 patients who had gait speed measured before and after an exercise trial post-hip fracture, and demonstrated a MDC95 of 0.08 m·s⁻¹. Based on clinical expert opinion and anchoring against the Timed “Up & Go” Test [5], the MCID was estimated at 0.10 m·s⁻¹. Barthuly et al. [4] studied elderly patients undergoing short-term rehabilitation following an acute hospital admission. Using an anchor-based method (against four different comparators), the MCID for the usual gait speed was estimated between 0.10 and 0.18 m·s⁻¹ with an associated MDC95 of 0.13 m·s⁻¹.

The usual gait speed has also been demonstrated to be responsive to natural recovery after stroke [5, 21], and to the natural ageing process in community-dwelling older persons [22, 23]. In an observational study of older adults, Perera et al. [22] estimated the smallest real difference in gait speed decline to be 0.05 m·s⁻¹ and the MCID to be between 0.05 and 0.12 m·s⁻¹, while Onder et al. [23] demonstrated a 5.2% change in gait speed from baseline to 1 year in moderately to severely disabled females.

Our study is the first to examine the responsiveness of the 4MGS in a COPD-specific population. A strength of the study was that we used two “interventions” to examine responsiveness: firstly, we were able to prospectively show significant improvement in the 4MGS with pulmonary rehabilitation; and secondly, we were also able to demonstrate a statistically significant decline in the 4MGS with time (>12 months). Changes in 4MGS correlated with changes in ISW, an established field walking measure of exercise capacity, providing further evidence of the longitudinal validity of the 4MGS in COPD. The magnitude of change was similar to those seen in other elderly populations [5, 22]. Another strength of the study was that we used both multiple anchor- and distribution-based methods to calculate the MCID of the 4MGS, allowing us greater confidence in the validity of our estimate of the MCID. As the relationships between change in 4MGS and change in ISW and Likert scale in response to pulmonary rehabilitation were modest (r=0.26 and r=0.20, respectively), caution is required in interpreting the results, but reassuringly, the estimates were consistent (0.08–0.11 m·s⁻¹), similar to values obtained with longitudinal decline and distribution-based approach, and similar to MCID estimates of the 4MGS in other populations [5, 22]. This lends greater weight to the inferences drawn from our data.

Limitations of the study

There are a number of limitations to determining the MCID, which are not unique to our study. Distribution-based approaches compare the change in the outcome to some measure of variability, but do not address the clinical importance to the patient. However, distribution-based methods are a well-accepted approach to estimating clinically important differences in outcome measures [24]. In contrast, anchor-based methods do take into account patient experience, but require a suitable external criterion that correlates significantly with the outcome measure of interest. Investigators have suggested an arbitrary minimum r-value of between 0.3 and 0.5 [25], although lesser degrees of association are needed to generate convincing inferences when multiple anchors are used (as in our study) [26]. Furthermore, our demonstration of r=0.45 between ISW change and 4MGS change with time is in line with these arbitrary parameters [27], and in line with studies used to determine the MCID of the 6-min walk distance in patients with COPD [28]. For a multisystem outcome variable such as 4MGS, it is unlikely that a single clinical anchor could capture the same construct [24], hence our decision to use multiple anchors in different clinical situations with corroboration from a distribution-based approach.

Our study also highlighted a limitation associated with the 4MGS, namely a ceiling effect. This is likely to explain the slightly lower r-value (r=0.26) seen between change in 4MGS and change in ISW with pulmonary rehabilitation. Although this does not affect the potential utility of the 4MGS as a simple screening tool for exercise intolerance [11] or as an outcome measure in frail individuals, the 4MGS may be less useful when testing the effects of intervention in high functioning patients. Figure 1 highlights the fact that the 4MGS was less responsive to intervention in those with better 4MGS at baseline, and for patients in the highest quartile, the effect size was small (d=0.2). This has been well described in the gerontology literature [29]. Conversely, our data also suggest that the 4MGS was particularly responsive in individuals with the lowest baseline 4MGS. A response effect size of 1.0 was seen in those within the lowest quartile for baseline 4MGS (<0.77 m·s⁻¹); this corresponds closely to the cut-off of 0.80 m·s⁻¹ that is commonly used in gerontology to identify frail individuals [1]. Although it is possible these findings could be secondary to “regression to the mean”, it is noticeable that ISW response to pulmonary rehabiliation was similar across the 4MGS quartiles. No relationship was observed between baseline gait speed and change in gait speed with pulmonary rehabilitation, suggesting that 4MGS change is not linearly dependent on baseline 4MGS value.

Certainly, expert consensus groups have identified usual gait speed as the most suitable functional outcome measure for pharmacological trials in frail older adults [7]. In patients with COPD, the 4MGS may therefore have a particular role in the assessment of poorly functioning or frail individuals, for whom longer (both in time and distance) tests of exercise capacity are more difficult or impractical. Typical scenarios might be the critical care setting, hospitalised in-patients, those recovering from exacerbation or housebound patients. Future prospective studies in different populations are needed to confirm this hypothesis.

A further limitation to our study is that patients lost to follow-up or those who did not complete pulmonary rehabilitation were excluded from the final analysis. Although the completion rate of 70% is in line with, if not better than, rates described from other UK services, it is conceivable that the exclusion of the non-completers may have biased estimates of responsiveness of the 4MGS. The non-completers did show slower baseline 4MGS than the completers, despite similar lung function parameters (table S1 in the online supplementary material).

Gait speed as an outcome measure in COPD

The test–retest and interobserver reliability and concurrent validity of the 4MGS in patients with COPD have recently been reported [11]. The current study extends these observations by demonstrating responsiveness of the 4MGS and proposing a clinically meaningful change, further pre-requisites for an outcome tool. We propose that the 4MGS has potential as a simple assessment tool in COPD.

A significant advantage of the 4MGS is simplicity. Assessment time is typically 1–2 min and the test requires only a stopwatch. The 4MGS is also readily acceptable to patients as walking is a familiar activity, the test is easy to understand and the short distance may relieve any anxieties associated with having to complete a maximal or near-maximal test. Furthermore, it requires little space, which means the 4MGS could be adopted in most clinical environments. In comparison, well-established field walking tests in COPD, such as the 6-min walk or the shuttle walk, are associated with learning effect and need to be repeated after adequate rest (typically, a second test is performed on a separate day or at least after an adequate rest, e.g. 30 min). Space can also limit their widespread use in some clinical environments. For example, the 6-min walk requires a 30-m course. Some investigators would also argue that the 4MGS may be more indicative of activities of daily living. In a recent study, Shimada et al. [30] compared several physical performance tests to predict the need for help with personal care in 10351 adults aged ≥65 years. Usual gait speed was more strongly correlated with the demand for personal care than other performance measures, with an odds ratio of 5.9 and an area under curve of 0.92–0.94.

In summary, our study demonstrates that the 4MGS improves with pulmonary rehabilitation and declines with time in COPD, with an estimated MCID of 0.11 m·s⁻¹. The 4MGS is a simple assessment tool that shows potential as a quick, practical functional outcome measure in patients with COPD that is suitable for use in most healthcare settings. The 4MGS may be particularly useful in frail individuals with COPD.