INTRODUCTION: A flight trial was conducted to determine whether breathing 60% oxygen during high performance flight maneuvers using contemporary pilot flight equipment induces atelectasis and to explore whether cabin altitude had any influence on the extent of atelectasis identified.METHODS: On 2 separate days, 14 male aircrew flew as passengers at High [14,500-18,000 ft (4420-5486 m)] and Low [4000-6000 ft (1219-1829 m)] cabin pressure altitude in a Hawk T Mk1 aircraft breathing 60% oxygen. Sorties comprised 16 maneuvers at +5 Gz, each sustained for 30 s. Lung volumes (spirometry), basal lung volume (electrical impedance tomography, EIT), and peripheral oxygen saturation during transition from hyperoxia to hypoxia (pulmonary shunt fraction) were measured in the cockpit immediately before (Pre) and after (Post) flight.RESULTS: Forced inspiratory vital capacity (FIVC) was significantly lower Postflight after High (-0.24 L) and Low (-0.38 L) sorties, but recovered to Preflight values by the fourth repeat (FIVC4). EIT-derived measures of FIVC decreased after High (-3.3%) and Low (-4.4%) sorties but did not recover to baseline by FIVC4. FIVC reductions were attributable to decreased inspiratory capacity. Spo₂ was lower Postflight than Preflight in High and Low sorties.DISCUSSION: Breathing 60% oxygen during flight results in a 3.8-4.9% reduction in lung volume associated with a small decrease in blood oxygenation and an estimated pulmonary shunt of up to 5.7%. EIT measures suggest persisting airway closure despite repeated FIVC maneuvers. There was no meaningful influence of cabin pressure altitude. The operational consequence of the observed changes is likely to be small.Tank H, Kennedy G, Pollock R, Hodkinson P, Sheppard-Hickey R-A, Woolford J, Green NDC, Stevenson A.