BACKGROUND: Patients with respiratory disease are at risk of excessive hypoxemia in the hypobaric commercial aircraft cabin environment, and the consensus is that this is easily corrected with supplementary oxygen. However, despite the risks of hypercapnia with increasing inspired oxygen in some patients being well established, this issue is not currently addressed in medical guidelines for air travel.CASE REPORT: A 76-yr-old woman with chronic type 2 respiratory failure underwent hypoxic challenge testing (HCT) to assess in-flight oxygen requirements. She is stable on home ventilation, and baseline arterial blood gases showed mild hypoxemia (P_ao₂ 9.12 kPa), normal P_aco₂ (5.64 kPa) and pH (7.36) with 98% S_pO₂. HCT was performed delivering 15% F_Io₂ via a mask, and the patient desaturated to < 85%. HCT blood gases revealed significant hypoxemia (P_ao₂ < 6.6 kPa), indicating in-flight oxygen. Continuous oxygen at 2 L · min⁻¹ via nasal cannula corrected the hypoxia, although P_aco₂ increased to 6.9 kPa with reduction in pH to the threshold of severe respiratory acidosis (pH 7.25). The patient was advised against flying due to hypoxemia during HCT and the precipitous drop in pH on oxygen.DISCUSSION: It is possible to hyperoxygenate patients with type 2 respiratory failure in flight with the minimum level of supplementary oxygen available on many aircraft. In these cases P_aco₂ and pH should be scrutinized during HCT before recommending in-flight oxygen. No current guidelines discuss the risk of hypercapnia from in-flight oxygen; it is therefore recommended that this be addressed in future revisions of medical air travel guidelines, should further research indicate it.Spurling KJ, Moonsie IK, Perks JL. Hypercapnic respiratory acidosis during an in-flight oxygen assessment. Aerosp Med Hum Perform. 2016; 87(2):144–147.