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We thank Drs Arena and Lavie for their thoughtful comments regarding our study. We share their conclusion that development of more reliable means of assessing the functional status of patients with PAH would be beneficial. Doing so will likely require refinements in our tools to assess both functional capacity and functional performance.
Functional capacity measures a patient's potential for physical activity and has most often been assessed in patients with PAH by a 6-minute walk test. Although this test is simple and inexpensive, its reliability can vary substantially, “ceiling” effects have been noted, and a clinically relevant change in patients with PAH has not been defined.1 Cardiopulmonary exercise testing also has been used to evaluate functional capacity and, as noted by Arena and Lavie, may provide a more sensitive assessment than does the 6-minute walk test. Indeed, others have also noted its potential as a prognostic indicator in patients with PAH.2 We agree that further study is required to determine whether the additional expense and inconvenience associated with cardiopulmonary exercise testing (compared with the 6-minute walk test) will be justified by improved utility in research and patient care. Functional performance encompasses both physical and emotional aspects of a patient's day-to-day well-being and includes domains often measured in health-related quality-of-life tools. The difference between capacity and performance is a patient's “reserve.”3
Functional classification may be viewed as the interplay among functional capacity, performance, and reserve. Therefore, measurement of functional classification requires attention to all these factors. We applaud efforts such as those described by Arena and Lavie to improve accurate measurement of individual contributing components of functional class (eg, functional capacity). We hope these and efforts focused on other aspects of functional class will together help better standardize patient evaluations, research reporting, and ultimately patient care.