[Functional capacity (dyspnea) and quality of life in patients with chronic obstructive lung disease (COPD): instruments of assessment and methodological aspects]

Abstract

Functional capacity (dyspnea) and quality of life are important criteria for the assessment of disease impact and treatment outcome in patients with chronic obstructive pulmonary disease (COPD). We will review measures for dyspnea and quality of life with particular emphasis on their methodological properties. Because asthma and COPD exhibit important differences and are therefore not comparable, we discuss only those measures which have been verified methodologically in patients with COPD. A critical review of current measures for dyspnea reveals at best an only fair association between perceived dyspnea and physiological lung function, sharing not more than 30% of common variance. Moreover, the affective state of the patients, their individual adaptation level and further psychological variables serve as mediators between lung function and perceived dyspnea. However, several valid and reliable measures for dyspnea are available for research and clinical practice. While the term "quality of life" is rather broad and unspecified, many researches in the field prefer the more restrictive term "health-related quality of life (HRQOL), that is the quality of life as affected by health status. The concept of HRQOL encompasses the impact of the individual's health on his or her ability to perform activities of daily living such as social role functioning, home management, social and family relationships, self-care, mobility, recreation and hobbies. In the past 15 years there has been an increasing body of literature on the measurement of HRQOL in patients with COPD. Several disease-specific and generic instruments for the use in COPD patients have been published. There is a growing consensus about the methodological criteria a given instrument has to fulfill. These are validity, sensitivity (for change) and reliability. They have to be tested in patients suffering from the illness for which the HRQOL instrument is planned to be applied in clinical studies or routine. The disease-specific instruments are supposed to be more sensitive to small therapeutically induced changes. However, the empirical results in patients with COPD are mixed and do not clearly favour disease-specific instruments. Lung function, dyspnea measures and exercise tolerance as well do not correlate strongly with HRQOL. Most associations cover only between 10% and 16% of shared variance. Exercise tolerance is not well associated with lung function but correlates better with HRQOL. Nowadays we can rely on several measures for dyspnea and HRQOL which have been thoroughly verified methodologically in COPD patients. However, some disease-specific and generic instruments exhibit substantial flaws, so that they need to be revised and it seems better not to use them at present. Many methodological and conceptual questions remain unanswered in the field indicating a strong need for more research.