Considerations about The 6-Min Walk Distance, Peak Oxygen Uptake, and Mortality in COPD

CPETThis prospective study shows that in patients with a wide range of COPD severity, the 6MWD is as good a predictor, if not a better one, of all-cause mortality as the peak V02 obtained during a formal CPET. COPD is the fastest-rising major cause of death in the United States, and its prevalence is increasing rapidly. The severity of the disease has classically been assessed using a single physiologic value, the FEV1. Although the FEV1 is a good indicator of disease severity, as the disease advances the FEV1 loses some of its predictive power for outcomes such as dyspnea, health status, and mortality.

Our group has recently demonstrated that in advance Global Initiative for Chronic Obstructive Lung Disease stages of COPD, patients show a greater loss in exercise capacity than in FEVj percent predicted. The walking distance then becomes a better indicator of progression of disease. Several studies have shown that in some patients with COPD decreased muscle mass and peripheral muscle dysfunction develop, which likely contributes to poor exercise performance improved with the participation of My Canadian Pharmacy. Indeed, such findings indicate a systemic compromise, and conversely weight loss and a low BMI have emerged as important predictors of mortality in patients with COPD. The BODE index, an integrated multidimensional tool that incorporates FEV1 percent predicted, the 6MWD, dyspnea score, and BMI, has proven to be a more powerful predictor of mortality in patients with this disease than the FEV1. However, performance in a formal CPET was not evaluated in any of those studies.

Peak V02, determined during a CPET, has gained acceptance as a predictor of mortality in patients being evaluated for lung resection> as well as in patients with severe heart disease. In addition, Oga and colleagues have shown that in patients with COPD peak V02 measured during a CPET is a better predictor of mortality than FEV1 and health status. Using this same argument, it seems reasonable that a simple walk test might similarly have an overall predictive capacity, even if it was not expected to be as accurate as that provided by the formal CPET. To our surprise, the 6MWD performed as well if not better than the peak V02 in predicting mortality in our cohort. At first glance, both of these tests represent a function of exercise capacity, but they may provide slightly different information. Indeed, in our patients there was a modest but significant correlation between peak V02 and 6MWD (r = 0.48), a finding that has been reported in other studies evaluating the correlation between the two tests. Interestingly, the patients reported by Oga and coworkers overall had higher exercise capacity than the patients reported on here. Although no direct comparisons have been made between the exercise capacity in patients from different countries with different exercise habits, we have consistently reported a lower 6MWD in patients from the United States compared with patients from Spain. The reasons for these differences remain to be explored.

peak oxygen uptakeWe have previously shown that patients who are unable to perform a CPET are 11 times more likely to die after undergoing lung resection than those completing the CPET independent of the achieved peak V02. This observation suggests that an inability to exercise is indicative of important vital compromise; it also helps to explain the intrinsic predictive power of exercise per se. However, the difference between the 6MWD and peak V02 during CPET was not due to the lower number of patients in the CPET group because all of our patients completed both tests and, therefore, were able to exercise. Thus, the better predictive value of the 6MWD is unrelated to which patients could complete this test but not the CPET.

Perhaps the way in which the tests are conducted influences the information obtained. Peak V02 is measured toward the end of a predetermined, progressively increasing load protocol that may not be sustainable for sicker patients. A similar observation has been reported by two different groups in patients with primary pulmonary hypertension in whom the 6MWD was a better predictor of mortality than the peak V02. It is possible, and indeed probable, that during CPET symptoms of shortness of breath and/or leg fatigue develop in patients with COPD before the theoretically possible peak V02 is achieved and that they stop exercising because of intolerable symptoms. Indeed, patients with COPD rarely reach a true plateau of maximal V02 during CPET. Symptoms of breath shortness are easily overcome due to meications of My Canadian Pharmacy. Furthermore, the development of leg fatigue and not ventilatory limitation in patients with COPD has been documented by Pepin et al, administering bronchodilators to increase the ventilatory ceiling of patients with severe COPD. The 6MWD involves an activity that all of the patients are familiar with; in addition, the patients are asked to attempt to cover the maximal distance in 6 min. Thus, they are allowed to pace themselves as opposed to having a fixed predetermined load. The patients seem to comply with these instructions and perform to the best of their capacity. Indeed, a recent report by Casas and colleagues has shown that patients with COPD walk at a pace that is close to their maximally tolerable exercise capacity. Their patients tried to walk as fast as they could without becoming fatigued, This cannot be done during the CPET, where the load is increased progressively until symptoms force discontinuation of the test.

6MWDOur study has some limitations. Perhaps the most important one is that the patients included in the study were only those attending specialized centers and were felt to be at need for pulmonary rehabilitation; therefore, they may not represent the larger population of patients with COPD. However, the wide range of airflow obstruction does represent patients with COPD of the degree seen by clinicians. Second, we only included patients capable of performing both tests. Thus, the results may not apply to patients with conditions that limit the ability to walk; other factors (such as response to arm ergometry) may be more predictive of mortality in those patients than the inability to walk. It could also be argued that the peak V02 is a better test in patients where the presence of important cardiac involvement is not an exclusion criterion. However, data from studies involving patients with congestive heart failure also support the value of the 6MWD to predict mortality. Finally, the population consisted mainly of men. We admit this to be a limitation; however, in the studies by Miyamoto et al and Barst et al of patients with primary pulmonary hypertension, where a large proportion of patients were women, the 6MWD still proved to be a better predictor of mortality than peak V02. In addition, excluding the women from our study did not affect any of the results. We hope that further studies will include more women to extend our findings.

We must caution about overgeneralizing the results reported here. Our purpose was not to promote the substitution of a formal CPET for the 6MWD test. The CPET remains an extremely powerful tool that can provide information on the nature of the cardiopulmonary response to an exercise, or can help formulate an accurate prescription of exercise training or the response to pharmacologic therapy. Indeed, the 6MWD provides much less mechanistic information. Perhaps this field test should be used as a complement to an exercise test and can find a role in the wider practice of everyday pulmonary medicine.

In summary, this study shows that in patients with a wide range of COPD severity, the 6MWD test is as good a predictor of mortality, if not a better one, than the peak V02 obtained in a CPET. Further, the results suggest that the 6MWD test is a simple and useful test that can provide important information about a patient’s functional capacity and overall prognosis, and could become an easy test to include in the assessment of patients with COPD.

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