Exercise intolerance in severe COPD: A review of assessment and treatment

Exercise intolerance is common in persons with chronic obstructive pulmonary disease and can result from multiple physiologic factors, including dynamic hyperinflation, gas exchange abnormalities, and pulmonary hypertension. In the initial assessment, keep in mind that many patients underestimate the degree of their impairment. The 6-minute walk test is very useful in assessing the degree of exercise intolerance; when more extensive assessment is indicated, cardiopulmonary exercise testing (CPET) is the gold standard. CPET is particularly useful for defining the underlying physiology of exercise limitation and may reveal other causes of dyspnea, such as myocardial ischemia or pulmonary hypertension. Strategies for improving exercise tolerance range from the use of bronchodilators and supplemental oxygen to participation in a pulmonary rehabilitation program. (J Respir Dis. 2006;27(5):208-218) Chronic obstructive pulmonary disease (COPD) is a progressive disease characterized by airflow limitation that is not fully reversible.1 Airflow obstruction results from chronic airway inflammation, mucous plugging, and destruction of lung parenchyma that causes the loss of elastic recoil and early airway closure during exhalation.2 COPD is a major cause of morbidity and mortality. Most patients do not seek medical attention until they notice exercise intolerance, which is one of the first and most common symptoms of COPD. Patients are often trapped in a downward spiral, because their dyspnea and exercise intolerance lead to a sedentary lifestyle, resulting in deconditioning and worsening exertional dyspnea (Figure 1).3 In this article, we will review the physiology of exercise intolerance in persons with COPD. We will also describe the clinical evaluation and then offer a plan for effective management.PHYSIOLOGY OF EXERCISE INTOLERANCE COPD is a heterogeneous disease, and multiple factors contribute to exercise intolerance.4 The most important of these are dynamic hyperinflation and peripheral skeletal muscle weakness. Other factors include gas exchange abnormalities and pulmonary hypertension. Patients with COPD often have other smoking-related diseases, such as hypertension and heart disease, which also contribute to exercise intolerance. Dynamic hyperinflation The forced expiratory volume in 1 second (FEV1), which is used to measure the severity of airway obstruction, was initially considered the most important test of the potential for ventilatory limitation during exercise. However, this theory is being disproved. Evidence indicates that FEV1 is highly predictive of exercise intolerance if FEV1 and oxygen uptake (V.O2) are normalized to age, height, sex, and weight.5,6 O'Donnell and associates7 showed that improvement in exercise performance in patients who have taken nebulized ipratropium is not associated with FEV1, but correlates best with the change in inspiratory capacity. If the degree of airway obstruction, as measured by FEV1, is not a good predictor of exercise intolerance in persons with COPD, what causes the ventilatory limitation that occurs during exertion? Investigators have been exploring the concept of dynamic hyperinflation. Hyperinflation tends to develop secondary to expiratory flow limitations in persons with lung disease. During physical exertion, minute ventilation (Ve) increases to meet the metabolic demands of exercise by increasing the tidal volume, then the respiration rate. In obstructive lung disease, the increased tidal volume and respiration rate lead to a decreased expiratory time, which intensifies hyperinflation. Because total lung capacity (TLC) does not change during exercise, dynamic hyperinflation produces a larger functional residual capacity at the expense of the inspiratory reserve volume and capacity.8,9 The degree of hyperinflation that develops during physical activity can be reliably quantitated by measuring the decline in inspiratory capacity during cardiopulmonary exercise testing (CPET). O'Donnell and Webb10 showed that the perceived level of dyspnea (assessed by the Borg scale)