Abstract

Background: Carbon dioxide (CO₂) retention during exercise is uncommon in mild to moderate lung disease in cystic fibrosis (CF). The ability to deal with increased CO₂ is dependent on the degree of airflow limitation and inherent CO₂ sensitivity. CO₂ retention (CO₂R) can be defined as a rise in P_ETCO₂ tension of ⩾5 mm Hg with exercise together with a failure to reduce P_ETCO₂ tension after peak work by at least 3 mm Hg by the termination of exercise.

Aim: To ascertain if carbon dioxide retention during exercise is associated with more rapid decline in lung function.

Methods: Annual spirometric and exercise data from 58 children aged 11–15 years, with moderate CF lung disease between 1996 and 2002 were analysed.

Results: The mean FEV₁ at baseline for the two groups was similar; the CO₂R group (n = 15) was 62% and the non-CO₂ retention group (CO₂NR) was 64% (n = 43). The decline in FEV₁ after 12 months was −3.2% (SD 1.1) in the CO₂R group and −2.3% (SD 0.9) in the CO₂NR group. The decline after 24 months was −6.3% (SD 1.3) and −1.8% (SD 1.1) respectively. After 36 months, the decline in FEV₁ was −5.3% (SD 1.2) and −2.6% (SD 1.1) respectively. The overall decline in lung function was 14.8% (SD 2.1) in the CO₂R group and 6.7% (SD 1.8) in the CO₂NR group. Using the primary outcome measure as a decline in FEV₁ of >9%, final multivariate analysis showed that the relative risks for this model were (95% CIs in parentheses): ΔP_ETCO₂ 11.61 (3.41 to 24.12), peak V˙O₂ 1.23 (1.10 to 1.43), and initial FEV₁ 1.14 (1.02 to 1.28).

Conclusion: Results show that the inability to defend carbon dioxide during exercise is associated with a more rapid decline in lung function.