Hypercapnia is regarded as a poor prognostic indicator in chronic obstructive pulmonary disease (COPD), but many patients hospitalized with hypercapnia associated with an acute exacerbation of COPD revert to normocapnia during recovery. We wished to determine if this reversible hypercapnia represents a distinct pattern of respiratory failure in COPD, or simply a stage in the progression to chronic hypercapnia. We therefore compared the long-term clinical progression and survival of COPD patients with reversible hypercapnic respiratory failure (defined as type 2.1) to those with normocapnic (PaCO₂ <50 mm Hg; type 1) and also to those patients with chronic hypercapnic (PaCO₂ >50 mm Hg) respiratory failure (defined as type 2.2).

We prospectively followed for 5 years a cohort of 85 patients who had been admitted as emergencies during a 1-year period to the respiratory unit of a University teaching hospital with an exacerbation of COPD complicated by respiratory failure (PaO₂ < 60 mm Hg). The main long-term outcome measures were survival and blood gas changes. Results: Sixty-eight (80%) patients survived the initial admission, and 17 (27%) survived 5 years. PaCO₂ rose substantially more during exacerbations in type 2.1 patients (mean 15.8 mm Hg), compared with type 2.2 (mean 6.8 mm Hg) and type 1 patients (mean 1.5 mm Hg). We analyzed 149 subsequent admissions among the survivors over the following 5 years. Type 2.1 patients had a better 5-year survival (28%) than type 2.2 (11% survival; P <0.05), and similar to type 1 patients (33% 5-year survival). Only 24% of reversible hypercapnic patients developed chronic hypercapnia during long-term followup.

The data support reversible hypercapnia being a distinct manifestation of respiratory failure in COPD, with a similar prognosis to that of normocapnic respiratory failure.