Breathlessness during exercise in COPD: how do the drugs work?
- Correspondence to:
Professor P M A Calverley
Department of Medicine, Clinical Sciences Centre, University Hospital Aintree, Liverpool L9 7AL, UK; pmacalliverpool.ac.uk
- chronic obstructive pulmonary disease
- salmeterol
- pressure-time product
- dynamic hyperinflation
- respiratory muscles
Salmeterol reduces breathlessness during exercise without necessarily changing exercise duration
The inability to exercise because of distressing breathlessness is one of the most frequent problems experienced by patients with chronic obstructive pulmonary disease (COPD)1 and is a major determinant of impaired quality of life.2 Our understanding of why this occurs and how best to treat it has improved significantly in the last decade. At one level the problem appears relatively straightforward. Exercise invariably involves an increase in whole body oxygen consumption and carbon dioxide production, which requires an appropriate rise in alveolar ventilation if arterial carbon dioxide tension is to remain constant. In patients with COPD the ability to increase minute ventilation is restricted as is the capacity to empty their lungs quickly, hence exercise limitation occurs at a lower workload than in age matched healthy subjects. Although there is much truth in this simple scheme, it does not do justice to the many complex adaptive responses that patients use to cope with their chronic airflow obstruction, nor does it explain the variability seen in both the duration of exercise and the intensity of breathlessness in patients with apparently similar levels of airflow obstruction.
ADAPTATIONS TO REDUCED EXERCISE CAPACITY IN COPD
Unsurprisingly, the general relationship of ventilatory capacity, commonly established indirectly from the forced expiratory volume in 1 second (FEV1),3 is not simple. While the 12 minute walking distance is broadly related to the severity of airflow obstruction,4 spirometric measurements are not very precise indicators of exercise capacity in the individual patient. Differences in ventilation-perfusion matching during exercise mean that individuals with more “wasted ventilation” achieve their maximum sustainable ventilation sooner for the same degree of alveolar ventilation.5 Differences in pre-morbid fitness are also relevant. Patients who are less fit progress to anaerobic metabolism (and hence increased carbon dioxide production) …
