The best things carried to excess are wrong (Charles Churchill (satirist) 1731–1764)

The use of oxygen for the management of patients with acute breathlessness, irrespective of cause, is well established in medical practice. The perception of benefit, even in the absence of measurement of oxygenation, and concerns over adverse outcomes from severe hypoxaemia have driven the use of high-concentration oxygen therapy over many years with little regard to possible harmful effects. While there have been many advocates for the cautious use of oxygen in chronic obstructive pulmonary disease (COPD) as a result of its propensity to promote hypercarbia, liberal use in asthma appears universal. This approach pervades student teaching through medical texts, even when there is significant respiratory input into the publication1 and extends to recent evidence-based guidelines on both asthma management and oxygen usage. The recently published British Thoracic Society/Scottish Intercollegiate Guidelines Network (BTS/SIGN) guidelines2 recommend administration of oxygen for acute exacerbations of asthma, stating ‘Many patients with acute severe asthma are hypoxaemic. Supplementary oxygen should be given urgently to hypoxaemic patients, using a face mask, Venturi mask or nasal cannulae with flow rates adjusted as necessary to maintain SpO₂ of 94–98%’, advice that is allocated a moderate to low level of evidence. Furthermore, they emphasise the use of oxygen therapy even in the absence of information on oxygenation, recommending that the ‘Lack of pulse oximetry should not prevent the use of oxygen’. Use of oxygen according to these recommendations is likely to result in a high fractional inspired oxygen, as ‘In hospital, ambulance and primary care, nebulised …