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Introduction This case describes a patient with type 2 respiratory failure. He had been receiving LTOT for several years. Recently, his condition declined, with capillary blood gas (CBG) tests consistently showing elevated carbon dioxide levels (PaCO 2 >8kPa). In accordance with NICE guidelines NG202, Non-Invasive Ventilation (NIV) was initiated.Method At follow-up, NIV was reportedly used19 hours/day, expressing significant discomfort when not used. A CBG was taken while on LTOT alone (5L/min). Results were: PaO 2 9.5kPa, PaCO2 6.7kPa, HCO3 25mmol/L, pH 7.36, and SpO2 94%. Although the PaCO2 was raised, it was comparable to previous results. However, he experienced notable breathlessness after one hour without NIV and insisted on recommencing.A follow-up CBG taken 30 minutes after restarting NIV (EPAP6, IPAP22, oxygen at 5L/min) showed: PaO2 19.1kPa, PaCO2 6.4kPa, HCO3 28mmol/L, pH 7.40, and SpO2 99%. The high oxygen level raised concerns, as excessive oxygen in patients with chronic hypercapnia can lead to CO2 retention and acidosis. The elevated PaO2 was likely due to the increased FiO2 delivered through NIV.Outcome Repeat CBG results on 2L/min and NIV showed optimal values: PaO 2 8.9kPa, PaCO2 7.0kPa, HCO3 27mmol/L, pH 7.40, SpO2 92%, aligning with British Thoracic society target SpO2 of 88-92% targets. The patient’s LTOT prescription was adjusted to 5L/min on LTOT and reduced to 2L/min while using NIV.NIV usage decreased to 10 hours per day, and the patient reported improved comfort and reduced breathlessness.Discussion This case emphasises the importance of careful oxygen titration in patients with chronic hypercapnia receiving NIV. Following this experience, a new departmental protocol is being discussed to ensure safe use of combined NIV and LTOT both on and off NIV to ensure safe and effective treatment. This case study was conducted as part of the PGCert in Sleep Medicine at UWE Bristol.