Reasons for Exercise Intolerance in Patients with Chronic Obstructive Pulmonary Disease

Abstract

INTRODUCTIONExercise intolerance in COPD patients is a major factor in reducing their overall levels of activity and negatively affecting their quality of life. A combination of various factors such as abnormalities of ventilatory mechanics, respiratory muscle function, alveolar gas exchange, and cardiac function are generally attributed to this reduction in exercise tolerance. This review examines the various different factors which may affect exercise tolerance in COPD patients and the evidence for each. PULMONARY MECHANICS AND GAS EXCHANGE Dynamic hyperinflation and maximal minute ventilationO'Donnell and Webb 1 reported that mechanical factors play an important, if not a predominant, role in limiting exercise tolerance in COPD patients. The mechanical limitation of maximal exercise ventilation in COPD patients is the consequence of both airway obstruction and of hyperinflation. 2 Expiratory airflow obstruction is the main pathophysiological result of the alveolar wall destruction and bronchiolar narrowing which characterises COPD. In moderate-to-severe COPD, resting expiratory airflows approach, or are equal to, maximal airflow. 3 In normal subjects, expiratory flow limitation may only occur during expiration at the highest work rates, whereas COPD patients with severe disease often show flow limitation at rest, 4 and less severe patients show flow limitation over most or all of expiration at low exercise levels.Prolongation of expiration together with a higher than normal exercise breathing frequency leads to further dynamic hyperinflation. O'Donnell and Webb 1 have shown that the dynamic hyperinflation developed by COPD patients during exercise is the most important contributor to exercise-induced dyspnoea. In a study of 23 patients with severe COPD, and ABSTRACT Exercise hyperpnoea is the consequence of the interaction of chemical, neural, muscular, haemodynamic and mechanical processes. The exact nature of the symptoms limiting exercise in chronic obstructive pulmonary disease (COPD) patients is not fully understood. Exercise limitation in COPD patients is generally attributed to a combination of abnormalities of ventilatory mechanics, respiratory muscle function, alveolar gas exchange, and cardiac function. COPD patients demonstrate widely variable exercise capacities, even among patients with similar levels of disease severity and it is likely that different factors are responsible for exercise limitation in different patients. The poor relationship between measurements of pulmonary function and exercise tolerance in COPD patients suggest the importance of non-cardiopulmonary factors in limiting exercise tolerance. The exact cause of exercise limitation in COPD patients is not fully understood and the attribution of limitation should be made with caution. Physical Therapy Reviews 2003; 8: 17-26 10 healthy age-matched control subjects, O'Donnell and Webb 1 investigated breathing pattern parameters, dynamic operational lung volumes, and Borg dyspnoea ratings during cycle ergometry to symptom-limited maxim...