Patients admitted over a 1 year period with acute exacerbations of cystic fibrosis (CF) and hypercapnia (Paco₂ ⩾45 mm Hg, n=14) were compared with a control group with hypercapnic exacerbations of COPD (n=49). The CF group were significantly younger (23 v 69 years) with a lower body mass index (18.3 kg/m²v 25.0 kg/m²) but no significant difference in FEV₁ (% predicted 27 v 34).

Using the Stinebaugh and Austin reference diagram for acid-base classification, a mixed respiratory acidosis and metabolic alkalosis was present in 71% of patients with CF and in only 22% of those with COPD (p<0.01). Both the plasma chloride and sodium levels were significantly lower in the CF group, together with a greater degree of hypoalbuminaemia.

Metabolic alkalosis is associated with sodium and chloride depletion and is likely to relate to the basic chloride channel defect in CF. A low albumin level may also contribute to a metabolic alkalosis due to its role as a weak non-volatile acid. Resulting respiratory compensation for this metabolic alkalosis results in hypoventilation and increased hypercapnia. This study is the first to show a significant role for a metabolic alkalosis in contributing to hypercapnic respiratory failure in patients with CF. Attention to fluid and electrolyte replacement in this group may aid treatment.