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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.