Arterial Blood Gas

Hypoxaemia is the commonest finding and is often out of proportion to the level of hypercapnoea. (i.e. Type 1 respiratory failure); this contrasts with COPD patients in extremis (who have type 2 respiratory failure).

In pre-existing chronic lung disease or impaired conscious level hypercapnoea and hypoxia may develop. (i.e. Type 2 respiratory failure).

Acid base balance and lactate will aid assessment of tissue perfusion.