Prior to availability of blood gases, use a 28% Venturi mask at 4 l/min and aim for an oxygen saturation of 88–92% for patients with risk factors for hypercapnia but no prior history of respiratory acidosis. [Grade D]

Adjust target range to 94–98% if the Paco₂ is normal (unless there is a history of previous NIV or IPPV) and recheck blood gases after 30–60 min [Grade D]

Aim at a prespecified saturation range (from alert card) in patients with a history of previous respiratory acidosis. These patients may have their own Venturi mask. In the absence of an oxygen alert card but with a history of previous respiratory failure (use of NIV or IPPV), treatment should be commenced using a 28% oxygen mask at 4 l/min in prehospital care or a 24% Venturi mask at 2–4 l/min in hospital settings with an initial target saturation of 88–92% pending urgent blood gas results. [Grade D]

If the saturation remains below 88% in prehospital care despite a 28% Venturi mask, change to nasal cannulae at 2–6 l/min or a simple mask at 5 l/min with target saturation of 88–92%. All at-risk patients with alert cards, previous NIV or IPPV or with saturation <88% in the ambulance should be treated as a high priority. Alert the A&E department that the patient requires immediate senior assessment on arrival at the hospital. [Grade D]

If the diagnosis is unknown, patients aged >50 years who are long-term smokers with a history of chronic breathlessness on minor exertion such as walking on level ground and no other known cause of breathlessness should be treated as if having COPD for the purposes of this guideline. Patients with COPD may also use terms such as chronic bronchitis and emphysema to describe their condition but may sometimes mistakenly use “asthma”. FEV₁ should be measured on arrival in hospital if possible and should be measured at least once before discharge from hospital in all cases of suspected COPD. [Grade D]

Patients with a significant likelihood of severe COPD or other illness that may cause hypercapnic respiratory failure should be triaged as very urgent and blood gases should be measured on arrival in hospital. [Grade D]

Blood gases should be rechecked after 30–60 min (or if there is clinical deterioration) even if the initial Paco₂ measurement was normal. [Grade D]

If the Paco₂ is raised but pH is ⩾7.35 ([H⁺] ⩽45 nmol/l), the patient has probably got long-standing hypercapnia; maintain target range of 88–92% for these patients. Blood gases should be repeated at 30–60 min to check for rising Paco₂ or falling pH. [Grade D]

If the patient is hypercapnic (Paco₂ >6 kPa or 45 mm Hg) and acidotic (pH <7.35 or [H⁺] >45 nmol/l) consider non-invasive ventilation, especially if acidosis has persisted for more than 30 min despite appropriate therapy. [Grade A]