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.