Multiple attempts have been made to determine functional capacity in patients who have chronic obstructive pulmonary disease (COPD). Laboratory tests that assess functional capacity are time consuming and costly, and are not well accepted by patients. In an attempt to simplify functional assessment in patients with COPD, the 12-minute walk test was developed. This test was later shortened to a six-minute walk distance that was shown to be reliable and valid as a test. However, this test did not take into consideration factors that can influence the results, such as stride length and body weight. A formula that included these variables was developed, but because of its complexity, it is not useful. Carter and associates reduced this complex formula to one that multiplies body weight by distance walked to equal work as a method of determining functional capacity. They used this formula in a trial to determine its validity as an instrument assessing functional capacity in patients with COPD. They also evaluated gender in this equation.

Patients with moderate to severe COPD were eligible for the trial. Other inclusion criteria were the ability to undergo exercise testing, hypoxemia corrected by oxygen administration, and being in a nonacute phase of COPD with a stable medication regimen. Patients were tested using pulmonary function studies, a peak effort ramp cardiopulmonary exercise test with gas exchange, and a six-minute walk distance. The six-minute walk was done in a measured and marked 100-foot (30.5-m) corridor. Patients were instructed to walk from end to end at their own pace, attempting to cover as much distance as possible. During the walk, the authors recorded time and distance walked, dyspnea score, information on leg fatigue, and heart rate for each minute walked. Patients were allowed to rest but were encouraged to proceed with the walk when they recovered.

There were 124 participants who completed the study. At baseline, the men had a significantly higher pack-year history compared with the women. The men walked for a significantly longer average distance than the women. The average six-minute workload in men was 35,370 kg per m compared with 25,643 kg per m in women. The six-minute walk test correlated better with various pulmonary function studies when compared with the walk distance alone. The six-minute walk test was more sensitive in differentiating patients with low or high work capacity.

The authors conclude that the six-minute walk test can be used to assess functional capacity in patients with COPD. This test is better than the walk distance at assessing function. The authors add that the test also is better correlated to metabolic expenditure than the walk distance.