Background The effect on Paco₂ of high concentration oxygen therapy when administered to patients with severe exacerbations of asthma is uncertain.

Methods 106 patients with severe exacerbations of asthma presenting to the Emergency Department were randomised to high concentration oxygen (8 l/min via medium concentration mask) or titrated oxygen (to achieve oxygen saturations between 93% and 95%) for 60 min. Patients with chronic obstructive pulmonary disease or disorders associated with hypercapnic respiratory failure were excluded. The transcutaneous partial pressure of carbon dioxide (Ptco₂) was measured at 0, 20, 40 and 60 min. The primary outcome variable was the proportion of patients with a rise in Ptco₂ ≥4 mm Hg at 60 min.

Results The proportion of patients with a rise in Ptco₂ ≥4 mm Hg at 60 min was significantly higher in the high concentration oxygen group, 22/50 (44%) vs 10/53 (19%), RR 2.3 (95% CI 1.2 to 4.4, p<0.006). The high concentration group had a higher proportion of patients with a rise in Ptco₂ ≥8 mm Hg, 11/50 (22%) vs 3/53 (6%), RR 3.9 (95% CI 1.2 to 13.1, p=0.016). All 10 patients with a final Ptco₂ ≥45 mm Hg received high concentration oxygen therapy, and in five there was an increase in Ptco₂ ≥10 mm Hg.

Conclusion High concentration oxygen therapy causes a clinically significant increase in Ptco₂ in patients presenting with severe exacerbations of asthma. A titrated oxygen regime is recommended in the treatment of severe asthma, in which oxygen is administered only to patients with hypoxaemia, in a dose that relieves hypoxaemia without causing hyperoxaemia.

Clinical trial number ACTRN12607000131459.