Who should have a hypoxic challenge test

Introduction: Some patients with respiratory disease require oxygen when flying at altitude but it is difficult to predict which. The 50 metre walk previously favoured is unverified and prediction equations are generally based on patients with COPD. There is little pretest guidance for who should undergo hypoxic challenge testing. We audited our hypoxic challenges to identify predictors of a positive test. Method: We looked at 197 patients who had challenge tests over an 18 month period. Details on diagnosis, lung function, PaO2, and shuttle walk distance were analysed. Results: The average age of patients was 52 years. Cystic fibrosis (34%) and interstitial lung disease (29%) were the commonest conditions. A total of 42% of tests were positive - most COPD patients had a positive test and approximately half of the ILD patients did. Very few patients with CF did (6%). Pretest PaO2 was the most accurate predictor of a positive (See table). FEV1 was not an accurate predictor of a positive result. Only 19/197 had a walk test within 3 years of the audit - all had a walking distance of >65 metres, with no correlation between distance and positive results. Discussion: Relative hypoxia and poor lung function before the test is perhaps unsurprisingly correlated with a positive test but by no means 100% discriminatory. A number of patients would be missed by using this approach alone or just the 50 metre walk. This audit would question whether patients with CF require ‘routine’ flight assessments.