Nowadays, the scientific foundations for pulmonary rehabilitation have been clearly established, and rehabilitation programmes have therefore become an essential part of the management of patients with chronic obstructive pulmonary disease (COPD). This has led to a new statement by the American Thoracic Society, in which they adopted the following definition: 'Pulmonary rehabilitation is a multidisciplinary programme of care for patients with chronic respiratory impairment that is individually tailored and designed to optimize physical and social performance and autonomy' . In common practice, this means that pulmonary rehabilitation aims to reduce symptoms, increase functional capacity and increase quality of life, with an awareness and acceptance of the fact that the level of impairment may not be changed. The term 'impairment' is derived from the World Health Organization (WHO), which in 1980 structured the various aspects of chronic disease by introducing the international classification of impairments, disabilities, and handicaps . This chapter explains how we can measure impairment, disability, and handicap in patients with COPD, and afterwards discusses several issues of pulmonary rehabilitation based on what is known in the literature.
According to the WHO, impairment is any loss or abnormality of psychological, physiological or anatomic structure or function . In respiratory patients, we measure impairment mostly by lung function tests, including forced expiratory volume in 1s (FEV1) the ratio FEV1/vital capacity (VC) and diffusion capacity. Inspiratory and peripheral muscle function are related to symptoms and may therefore also be important parameters for assessing the level of impairment. These measurements have become even more important, because in contrast to irreversible airflow obstruction, they can be improved by adequate training.
Disability is any restriction or lack (resulting from an impairment) of ability to perform an activity in the manner or within the range considered to be normal for a human being . As mentioned above, impaired lung function may lead to disability; however, the relation between the two is not very strong [3,4]. Therefore, the level of disability has to be assessed directly by exercise tests. These tests can be divided into two types: maximal exercise tests, usually performed on a bicycle ergometer, and submaximal tests such as walking tests for a given time or shuttle walking tests. These exercise tests are objective measurements, while disability is also a term used to describe the patient's feeling of the impact of COPD during his or her daily activities. A number of measures of subjective feelings of disability are used in COPD, which mostly include dyspnoea, such as the Borg ratio of perceived exertion , Mahler's dyspnoea index , the Medical Research Council (MRC) dyspnoea scale , and the Oxygen Cost Diagram (OCD) . Findings of low correlations between spirometry, on the one hand, and objective and subjective disability measurements on the other, may indicate a need to investigate disability directly both by exercise tests and subjective measurements.
Handicap is defined by the WHO as a disadvantage for a given individual resulting from an impairment or disability that limits or prevents the fulfilment of a role that is normal (depending on age, gender and social and cultural factors) for that individual . It has been shown that impaired lung function may result in an impaired quality of life [9-11], which can represent the subjective experience of a handicap.
Interestingly, health-care use by COPD patients appears to be more related to an impaired quality of life than to the severity of the lung disease itself . Therefore, it is important to assess quality of life as an important aspect of a chronic disease. Two general health measurements, the Quality of Well-Being Scale  and the Sickness Impact Profile  have been used in COPD patients. As these instruments are not sensitive enough to detect small changes  after therapy, Guyatt and co-workers developed the Chronic Respiratory Questionnaire (CRQ) , a disease-specific questionnaire, which was found to be sensitive in this respect. A disadvantage of the CRQ is that the questionnaire is not a standardized one, as the dimension 'dyspnoea' is strictly indivi-dualized—i.e. every patient has to quantify dyspnoea during activities that are important in their day-to-day life. This means that it is difficult to compare different studies by using the CRQ. For this reason, Jones and co-workers  developed a standardized disease specific questionnaire, the St George's Hospital Respiratory Questionnaire (SGRQ), which was found to be valid, repeatable, and sensitive. Neither the CRQ nor the SGRQ showed a strong relationship with lung function measurements or measurements of disability. The shared variance between the change in the CRQ with changes in walking distance was 27%, while the shared variance between CRQ and the MRC dyspnoea scale was only 10% . The shared variance between the SGRQ, on the one hand, and forced vital capacity (FVC) and 6-min walking distance on the other, was 18% and 37%, respectively . Therefore, it is not possible to predict the level of disability or quality of life for individual patients from their lung function [15,17,18]. This may not be entirely surprising, since patients may adapt to their disabilities and handicaps. Sometimes quality of life is surprisingly well maintained despite severe impairments and disabilities. As a consequence, one has to assess disability and quality of life directly.
By assessing all three aspects of a chronic disease—i.e. impairment, disability, and handicap —it is possible to focus on the specific problems of a patient and to determine which interventions in rehabilitation may be beneficial.
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