Course length of 30 metres versus 10 metres has a significant influence on six-minute walk distance in patients with COPD: an experimental crossover study

Questions: Do patients with chronic obstructive pulmonary disease (COPD) achieve a different distance on the six-minute walk test (6MWT) conducted on a 10 m course versus on a 30 m course? When assessing the distance on a 6MWT conducted on a 10 m course, is it valid to use existing reference equations that were generated on longer courses? Design: A randomised double-crossover experimental study. Participants: Forty-five patients with COPD in primary physiotherapy care. Intervention: All patients performed a 6MWT twice over a 10 m course and twice over a 30 m course. The 6MWTs were performed in accordance with the American Thoracic Society guidelines. Outcome measures: 6MWD was assessed and predicted distance was calculated based on a range of reference equations. Results: The 6MWD on the 10 m course was 49.5 m shorter than on the 30 m course, which was statistically significant (95% CI 39.4 to 59.6). By using existing reference equations for a 6MWT conducted on the 10 m course, the predicted distance is highly overestimated (with a range of 30% to 33%) and the average distance as a percentage of the predicted value is 8%(pred) lower compared to a 6MWT conducted on the 30 m course, resulting in a worse representation of a COPD patient's functional exercise capacity. Conclusion: This study shows that the impact of course length on the 6MWD and on the use of reference equations in patients with COPD is substantial and clinically relevant (based on the most conservative published minimum clinically important difference). Therefore, existing reference equations established for a 6MWT conducted over a 30 m (or longer) course cannot be applied to predict the distance achieved on the 6MWT on a 10 m course, which is frequently used in primary care physiotherapy practices for patients with COPD.