Correlation between carbon dioxide retention during exercise and the rate of decline of FEV₁ in cystic fibrosis

Progressive decline in lung function is an important cause of morbidity and mortality in patients with cystic fibrosis (CF). Markers to identify the patients at risk of rapid decline in their clinical condition are lacking. No study to date has used carbon dioxide (CO₂) retention during exercise to predict the rate of decline in respiratory function in CF.

In this study, the change in forced expiratory volume in 1 second (FEV₁) of 15 CF patients who retained CO₂ (measured by end tidal CO₂) during exercise testing (CO₂R group) and 43 who did not (CO₂NR group) was compared retrospectively over a 3 year period. The two groups were matched for age and body mass index. The results showed that the CO₂R group had significant declines in FEV₁ at 24 months (6.3% CO₂R group v 1.8% CO₂NR group, p<0.05) and 36 months (5.3% CO₂R group v 2.5% CO₂NR group, p<0.05). There was an almost 12-fold higher risk of a fall in FEV₁ of >9% in the CO₂R group over the following 3 years compared with the CO₂NR group.

The authors conclude that children with CF who exhibit CO₂ retention during exercise are at risk of a more rapid decline in lung function. However, this study is limited by the fact that Paco₂ values cannot be predicted accurately from end tidal Pco₂ in an individual person, particularly in patients with lung disease affecting ventilation/perfusion relationships. It should be noted that the CO₂R group had a slightly lower FEV₁ (62.3%, range 41–68%) than the CO₂NR group (64.7%, range 44–69%) at the commencement of the study.