ASSESSMENT OF FITNESS IN PATIENTS WITH CYSTIC-FIBROSIS AND MILD LUNG-DISEASE

Background - Maximal exercise testing is used in patients with cystic fibrosis to assess functional status and prognosis. The lactate thresh old is an index of aerobic fitness with significant advantages over ma ximal exercise tests. This study was undertaken to determine if the la ctate threshold might be identified, non-invasively, in adult patients with cystic fibrosis and mild lung disease by measurement of ventilat ory and gas exchange parameters. Methods - Ten subjects with mild cyst ic fibrosis (forced vital capacity (FVC) >70% predicted) and 10 health y controls undertook an incremental exercise test on a bicycle ergomet er. Ventilation and gas exchange parameters were measured continually and arterialised venous blood pH, carbon dioxide tension (PCO2), and l actate concentrations were measured at intervals throughout the tests. Results - In subjects with cystic fibrosis there was no significant d ifference between the mean gas exchange and lactate thresholds (mean d ifference 1.0 (95% confidence interval (CI) of the mean -1.5 to 3.44) ml/kg/min). In contrast, there was a significant difference between th e mean ventilatory and lactate thresholds (3.8 (95% CI 0.9 to 6.7) ml/ kg/min). Arterialised venous PCO2 increased significantly during the e xercise tests. In healthy subjects the mean differences between these thresholds were not significantly different from zero and PCO2 fell si gnificantly during the tests. Conclusions - The ventilatory threshold significantly overestimates the lactate threshold in subjects with cys tic fibrosis induced lung disease because of impaired carbon dioxide e xcretion during exercise. However, the gas exchange threshold may be u sed to determine the lactate threshold in this patient group.