Six-minute walk distance and work relationship with incremental treadmill cardiopulmonary exercise test in COPD

Cardiopulmonary exercise testing (CPET) is increasingly used to evaluate the overall impact of the illness on patients with chronic obstructive pulmonary disease (COPD). While laboratory tests of exercise performance are costly, the 6-min walk test (6-MWT) can be more easily performed. Although the main outcome commonly used in this field test is the distance walked in 6 min (6-MWD), this measure does not account for differences in body weight. Previous studies showed a good correlation between the work performed during the 6-MWT with incremental cycling CPET, an exercise modality more associated with quadriceps fatigability and with lower peak oxygen consumption than incremental walking tests.

Evaluate the correlation between the 6-MWD and its derivative body weight-walking distance product, an estimation of the work performed during the 6-MWT, with peak from a treadmill CPET.

Thirty COPD patients [forced expiratory volume in 1 s (FEV1) = 39 ± 13%; peak predicted] performed CPET to the limit of tolerance on a treadmill and 6-MWT, 48 h apart.6-MWD and work were correlated to resting and exercise functional variables.

The work of walking during the 6-MWT provided greater associations with peak than observed with 6-MWD. This was the case for FEV1, forced vital capacity, inspiratory capacity, lung diffusion capacity for carbon monoxide, peak , carbon dioxide output, minute ventilation and double product (r = 0.57, r = 0.57, r = 0.73, r = 0.7, r = 0.75, r = 0.65, r = 0.51 and r = 0.4, respectively; all P < 0.05).

A better association was found between the work estimated from the 6-MWT and peak achieved during CPET, in this case with a treadmill, than the 6-MWD alone.