Introduction and objectives Commercial airline travel poses a recognised risk to patients with respiratory disease, including in those with asthma. Hypoxic challenge testing (HCT) is typically employed to mitigate this risk by dictating in-flight oxygen requirement. The objective of this work was to evaluate the role of HCT in patients with severe asthma.
Methods A retrospective analysis was performed of all BTS/SIGN Asthma Step 5 adult individuals under the Royal Brompton Hospital severe asthma service, who completed HCT between 2007 and 2014. In line with British Thoracic Society recommendations, under hypoxic conditions a reduction in PaO2 to <6.6 kPa was reported as a positive result. A PaO2 level of 6.6–7.5 kPa was considered borderline and supplemental oxygen was advised if co-existent evidence of hyperventilation. Electrocardiograph monitoring was performed in all patients during the HCT.
Results Of the 37 patients studied, 21 (57%) had a positive HCT. Individuals with a positive HCT had a lower PaO2 under normoxic conditions (10.1 kPa v 11.4 kPa, p < 0.01), but similar PaCO2 level (4.80 kPa v 4.91 kPa, p > 0.05). Baseline oxygen saturation was poorly predictive of the need for supplementary oxygen and two-thirds of patients, for whom supplementary oxygen was recommended, had a baseline SpO2 level of greater than 95%; approximately half of these individuals de-saturated to less that 90% on HCT (Figure 1). Lung function was more obstructed in the positive HCT group (predicted FEV1 (52% v 78%, p < 0.01). Across the entire cohort, HCT was associated with a mean rise in heart rate (HR) of 5 bpm and there was no evidence of dysrhythmia or change in QTc. A combination of any two of: baseline PaO2 ≤ 10.5 kPa, FEV1 ≤ 60% predicted and PEF ≤ 350 L/min predicted the need for in-flight oxygen with a sensitivity of 90% and a specificity of 69%.
Conclusions In patients with severe asthma, baseline oxygen saturation level is poorly predictive of the need for in-flight oxygen. Our findings indicate that a HTC should be considered for all BTS/SIGN Step 5 asthmatics in whom air travel is being considered and should certainly be recommended in those with impaired lung function.