Is an individual prediction of maximal work rate by 6-minute walk distance and further measurements reliable in male patients with different lung diseases?

Background: In patients with chronic lung diseases, the work rate for endurance training is calculated by the maximal work rate (Wmax). Because the assessment bears side effects, a prediction by easier accessible tests would be of practical use.

Objective: We addressed the reliability of predicting Wmax on the basis of the 6-min walk distance (6MWD) test and a set of further parameters in patients with different lung diseases.

Methods: Baseline data of a longitudinal study including 6MWD, Wmax, peripheral muscle force, lung function, fat-free mass and dyspnea (Modified Medical Research Council score) of 255 men with occupational lung diseases (104 asthma, 69 asbestosis, 42 silicosis, 40 chronic obstructive pulmonary disease) were evaluated.

Results: 6MWD correlated with Wmax (r = 0.51, p < 0.05). The product of 6MWD and body weight, in particular fat-free mass, led to an improvement in the correlation of Wmax with 6MWD. Muscle force, lung function and Modified Medical Research Council score correlated moderately but significantly with Wmax (p < 0.05 each). The maximum correlation gained by including 6MWD and further parameters in the prediction equations was r = 0.76 in patients with obstructive lung function impairment and r = 0.61 in asbestosis patients. The residual standard deviations of Wmax predicted by the calculated equations ranged between 20 and 28 W, and the 95% prediction intervals of Wmax ranged between ±47 and ±65 W.

Conclusions: A reliable prediction of individual Wmax by 6MWD or related measures and therefore a replacement by other tests is not possible. Nevertheless, it may be useful for the comparison of average values in epidemiological and clinical studies.