Assessment of disability
Disability is defined by the World Health Organization as 'any restriction or lack of ability to perform any activity within the range of normal for a human being' . In COPD, structural and physiological impairment of the respiratory system is associated with varying degrees of disability. However, the clinical assessment of the COPD patient in the past has relied heavily on the quantification of physiological impairment with little attention given to the assessment of the consequent disability. The increasing realization that common spirometric measures of pulmonary impairment correlate only weakly with exercise intolerance, symptom intensity, and quality of life, has prompted a search for better evaluative methods. This review focuses on the interface between physiological impairment and disability in COPD and forms the basis for a more comprehensive clinical assessment of the symptomatic patient.
Why does decrement in FEV1 not correlate precisely with disability?
The forced expiratory volume in 1s (FEV1) is the most common test of physiological impairment in COPD, and has stood the test of time. It is a simple reproducible test, it is of unquestionable diagnostic utility, it is useful in following the course of the disease, and is a valuable prognostic indicator. The term COPD, however, encompasses heterogeneous pathophysiological derangements of the small and large airways, lung parenchyma and capillary bed in highly variable combinations and these diverse structural abnormalities are unlikely to be reflected in one simple spirometric test. Therefore, it is not surprising that the FEV1, which is a crude measurement of overall physiological impairment, has been shown repeatedly in research studies to correlate poorly with measures of disability such as symptom intensity and exercise capacity in COPD [2-4]. This poor statistical correlation is borne out by common clinical observation. Thus, patients with the same measured FEV1 (expressed as percentage predicted) may vary greatly in their level of disability; patients may deteriorate clinically, either acutely (e.g. during infective exacerbations) or chronically, while preserving spirometric FEVj. Moreover, patients may achieve considerable improvements in symptoms and exercise endurance as a result of interventions such as bronchodilators, oxygen therapy or exercise training, with little or no change in the FEV1 [5-7]. These observations collectively attest to the fact that disability is multifactorial and often independent of the FEVj. Factors that determine the level of disability (or its change over time) in a given individual include: the level of expiratory flow limitation, gas exchange abnormalities, ventilatory demand, extent of thoracic overinflation, extent of mechanical loading of the inspiratory muscles, degree of ventilatory muscle and peripheral muscle weakness/deconditioning, and cardiac factors. Moreover, the level of disability is also profoundly influenced by interactions among multiple physiological, psychological, social, and environmental factors.
Spirometric FEV1 is prone to measurement artifact because a forced manoeuvre, initiated from total lung capacity, introduces gas compression effects, airway compression effects, and results in an altered pattern of lung emptying compared with that which occurs during normal tidal breathing over a range of operating lung volumes. Spirometric FEV1 gives no information about the extent of prevailing expiratory flow limitation, the extent of dynamic lung hyperinflation (DH) required to maximize expiratory flow rates and therefore does not provide an assessment of the 'dynamic' expiratory flows available under conditions of increased ventilation such as exercise (Fig. 5.1). All of these factors can vary greatly for a given FEV1 and contribute importantly, either singly or in combination, to symptom generation, ventila-tory limitation and exercise capacity .
Despite the multifactorial nature of disability in COPD, it is reasonable to assume that the degree to which an individual is disabled ultimately reflects the extent of ventilatory mechanical abnormalities present. Given the limitations of the FEV1 as a measure of mechanical impairment, additional physiological measurements such as dynamic lung volumes, together with direct measurements of symptom intensity and exercise impairment, are used increasingly to clinically evaluate patients and to determine the success of therapeutic interventions. A variety of parameters can be employed to comprehensively assess impairment, disability and handicap in the symptomatic COPD patient (Table 5.1).
What causes dyspnoea in COPD?
In a recent American Thoracic Society Consensus Statement , dyspnoea was defined as 'a term used to characterize a subjective experience of breath-
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