Exercise testing in patients with cystic fibrosis: why and which?

While the determination of exercise capacity is viewed as clinical best practice in cystic fibrosis (CF), its measurement using a cardiopulmonary exercise test (CPET) remains as somewhat of a mystery for many CF clinicians. Two studies [1,2] in the current issue (of the Journal of Cystic Fibrosis) highlight the use of exercise testing in CF clinical practice. The role of retained exercise capacity (VO2max) and levels of physical activity in CF in maintaining health status is compelling. Using an accelerometer to determine levels of physical activity, Hebestreit et al. [3] found that more physically active CF patients had best preserved VO2max, FEV1 and nutritional status. These results are supported by a more recent review by Wilkes and colleagues [4] who reported that children with CF with increased activity demonstrated a reduced decline in pulmonary function. To date we know that there is a clear relationship between pulmonary dysfunction and exercise capacity in patients with CF i.e. those with more severe pulmonary disease have lower exercise capacity [5]. Interestingly the relationship between VO2max and the extent of radiological damage (modified Bhalla score on thin section chest HRCT) was stronger than with spirometry or body mass index [6] which suggests that the simple physiological measures used in the clinic setting may not be sufficiently sensitive to detect subtle changes in the lungs and in particular the airway. It has been suggested that in patients with mild pulmonary disease, parameters of exercise at peak or maximal performance may detect disease not identified by routine lung function testing [7]. Additional parameters obtained during formal exercise testing can therefore be useful in patients with CF. An example in children with CF; was the finding that the retention of CO2 during exercise was associated with more rapid decline in pulmonary function [8]. The study by GRUET et al. [1] highlights the usefulness of using CPET in CF and in particular the robustness of the oxygen uptake efficiency slope (OUES). The OUES, first described by Baba et al. [9] in the Journal of American College of Cardiology in 1996, has proven to be a useful measure in a number of clinical populations. In heart failure patients for example, the OUES has proven to be a valid substitution for maximal exercise capacity (VO2max) [10] and changes in OUES with exercise training have are well correlated with changes in other exercise related outcome measures [11].