Defining “stable chronic hypercapnia” in patients with COPD: the physiological perspective

We read with interest the “European Respiratory Society guidelines on long-term home non-invasive ventilation for management of COPD” [1]. In recent years, several studies have attempted to improve outcomes by using home non-invasive ventilation in patients with end-stage COPD who suffer from chronic hypercapnic respiratory failure (CHRF). As noted in the guidelines, the definition of “stable chronic hypercapnia” following an acute exacerbation has historically been ambiguous, which has probably contributed to the inconsistent findings across clinical trials of home non-invasive ventilation [2]. A time period of 2–4 weeks is required before a state of “stable chronic hypercapnia” is likely to be present following an acute COPD exacerbation