Oxygen therapy for asthma can elevate CO2

Patients with acute exacerbations of asthma randomised to receive high concentration oxygen therapy showed a greater rise in CO2 than those who received titrated oxygen to keep SpO2 > 93%.
This study has a few weaknesses but raises an interesting challenge to the dogma of high flow oxygen (and oxygen driven nebulisers) for all acute asthma exacerbations.
The suggested main mechanism for the elevation in CO2 is worsening ventilation/perfusion mismatching as a result of the release of hypoxic pulmonary vasoconstriction and a consequent increase in physiological dead space. The authors remind us that this has been demonstrated in other studies on asthma and acute COPD exacerbations. The authors infer that high concentration oxygen therapy may therefore potentially increase the PaCO2 across a range of respiratory conditions with abnormal gas exchange due to ventilation/perfusion mismatching
Some of the weaknesses include lack of blinding, recruiting fewer patients than planned, and changing their primary outcome variable after commencing the study (which the authors are honest about) from absolute CO2 to increase in CO2 (since it was apparent on preliminary analysis of the first few patients that presenting CO2 was the primary determinant of subsequent CO2). Furthermore, the CO2 was measured from a transcutaneous device as opposed to the true ‘gold standard’ of arterial blood gas analysis, although good reasons are given for this.
Despite some of these drawbacks this study provides us with a further reminder that oxygen is a drug with some unwanted effects and therefore its dose needs to be individualised for the patient.

Background The effect on Paco(2) 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 (Ptco(2)) was measured at 0, 20, 40 and 60 min. The primary outcome variable was the proportion of patients with a rise in Ptco(2) ≥4 mm Hg at 60 min.

Results The proportion of patients with a rise in Ptco(2) ≥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 Ptco(2) ≥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 Ptco(2) ≥45 mm Hg received high concentration oxygen therapy, and in five there was an increase in Ptco(2) ≥10 mm Hg.
Conclusion High concentration oxygen therapy causes a clinically significant increase in Ptco(2) 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.

Randomised controlled trial of high concentration versus titrated oxygen therapy in severe exacerbations of asthma
Thorax. 2011 Nov;66(11):937-41

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