Hypercapnia at admission, regardless of acidosis, may worsen the outcome of hospitalised patients with chronic obstructive pulmonary disease exacerbations.

In patients with exacerbation of chronic obstructive pulmonary disease (ECOPD), the presence of respiratory acidosis is considered "a marker of severity", identifying the need for ventilation treatment. The evaluation of ECOPD patients with hypercapnia, even without acidosis, may provide information on patients not commonly defined as "at risk". We retrospectively assessed 407 hospitalised patients with ECOPD, divided into three groups at admission: patients with normocapnia (N = 176), hypercapnia (PaCO2 > 45 mmHg and pH ≥ 7.35, N = 126), and acidosis (PaCO2 > 45 mmHg and pH < 7.35, N = 105). Data on general, clinical, laboratory and microbiological characteristics, and on outcomes (mortality up to 3 years), were collected. Patients with hypercapnia and acidosis had similar functional and clinical characteristics to their normocapnic peers, but of greater severity. The mortality rate at 1 year was similar between hypercapnic and acidotic patients (24% and 25%), and higher than that recorded in the normocapnic group (14%). Similar trends were observed in mortality from 6 months to 3 years. The presence of hypercapnia and acidosis was significantly associated with an increased and independent risk of death at 1 year. A threshold value of PaCO2 ≥ 55 mmHg was associated with 1-year mortality. In a multivariate analysis considering the new threshold and compared to normocapnia, the presence of hypercapnia (PaCO2 ≥ 55), regardless of acidosis, was independently associated with a higher mortality risk at 1 year. In hospitalised ECOPD patients, hypercapnia itself, even when compensated, is associated with a worse prognosis up to an intermediate-term follow-up. The need to ventilate these patients should be evaluated, regardless of the presence or absence of acidosis.