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Introduction Hypoxic challenge tests (HCT) are recommended by the British Thoracic Society to assess the requirement for in flight supplemental O 2. During an HCT, the FiO2 is reduced to 15% replicating the PiO2 experienced during air travel. Various HCT methods exist, potentially affecting clinical outcomes. In the UK, two main techniques are used, the ‘Venturi mask method’ (V-HCT), which employs 100% N2 via a 40% O2 venturi barrel, and the ‘Mouthpiece method’ (M-HCT), which uses pre-mixed 15% O2 via a mouthpiece. We compared these methods in elderly individuals with airflow obstruction, hypothesising that the M-HCT would induce hyperventilation (assessed via transcutaneous CO2 – tCO2) and potentially yield false-negative results.Methods Participants with self-reported COPD or asthma, aged over 60 and naïve to HCT, attended the University of Winchester for a screening visit followed by two experimental visits. Spirometry during the screening visit confirmed airflow obstruction. During the experimental visits, participants completed both V-HCT and M-HCT in a random order, with continuous measurements of tCO 2, SpO2, and HR. They also reported any claustrophobia and answered, ‘If asked to perform a HCT by your clinician, what test would you rather perform?’.Results Twelve participants completed both HCT methods (see table 1 for demographics). Five found the M-HCT claustrophobic, while none did for the V-HCT, and all preferred the V-HCT. In contrast to our hypothesis, tCO2 levels were similar between methods, with no significant difference from baseline to the final three minutes (M-HCT: 5.1 to 5.0 kPa; V-HCT: 5.1 to 5.1 kPa). Furthermore, ten participants had lower SpO2 nadirs during the M-HCT. Statistical analysis on the grouped data was not feasible as two participants’ SpO2 dropped <83% during the M-HCT, necessitating an FiO2 increase as per ethical guidelines.Abstract P19 Table 1Participant demographicsConclusion Patients preferred the V-HCT, reporting less claustrophobia than the M-HCT, but no evidence of hyperventilation during either method was shown. The reason for lower SpO 2 with the V-HCT in ten participants remains unclear, although room air entrainment through the mask’s expiratory ports, raising the FiO2 is a possible explanation. Future studies are required to accurately measure the true FiO2 inhaled during the Venturi technique.