Background Commercial aircraft are pressurised to ∼2438 m (8000 ft) above sea level that equates breathing 15% oxygen at sea level. A preflight hypoxic challenge test (HCT) is therefore recommended for children with cystic fibrosis or other chronic lung diseases and inflight oxygen is advised if pulse oximetric saturation (SpO₂) decreases <90%.

Objective Study responses to a modified HCT, encompassing various body positions and light physical activity, reflecting relevant activities of children during flight, with a view to challenge the evidence of the current cut-off.

Methods Oxygenation, heart rate and ventilation were observed in 34 healthy schoolchildren (17 boys) undergoing a modified HCT, alternating between breathing room air and 15% oxygen in nitrogen while seated, supine, standing and walking at 3 km/h and 5 km/h.

Results Nadir SpO₂ <90%, median (range), occurred in 9 subjects sitting, 89% (78–89%); 6 supine, 88.5% (87–89%); 9 standing, 89% (85–89%); 23 walking 3 km/h, 87% (74–89%); and 21 walking 5 km/h, 86% (74–89%). Total time <90% for these subjects in seconds was 20 (10–80) sitting, 30 (10–190) supine, 50 (10–150) standing, 80 (10–260) walking 3 km/h and 125 (10–300) walking 5 km/h. Light exercise in general led to lower SpO₂: 91% (77–96%), p<0.0001.

Conclusions A modified HCT led to moments of desaturation below 90% in various body positions at rest and during light physical activity in healthy schoolchildren. It is questionable whether the international recommended cut-off of 90% for children with chronic lung disease reflects clinical oxygen dependence during flights.