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Introduction Hypercapnic respiratory failure (HRF) is associated with prolonged inpatient hospital stay and has impact on morbidity and mortality. Most patients with HRF can be controlled by non-invasive ventilation (NIV) however, some patients continue to have refractory HRF despite optimization of NIV. It is thought that acetazolamide can reduce metabolic alkalosis which then may reduce hypercapnia,1 however there are limited literature with mixed outcomes about benefit of acetazolamide in these cases.2 A retrospective review of patients prescribed acetazolamide for this purpose was performed. Method Patients with HRF who were in Respiratory Support Unit (RSU) from June 2019 to February 2025 were screened and patients who received acetazolamide for 3 days were identified. Results 16 patients were identified, 11 patients were excluded due to reasons of unable to find exact prescription, already on acetazolamide for other reasons and non-HRF. 5 patients (4 Females) were identified for analysis. Reasons for HRF and RSU admission were Obesity hypoventilation syndrome/Obstructive Sleep Apnoea and Exacerbation of COPD. Mean NIV days till acetazolamide trial was 14 (8–21). There was a mean reduction of serum bicarbonate by 13.3 mmol/L and CO2 by 2 kPa post-trial. Mean RSU-need bed days was 19. The duration of hospital admission was confounded by other factors like social care needs. All 5 patients were discharged back to their homes. 3 patients are still alive and 2 has passed away at different times during further admissions with different reasons.Abstract P232 Table 1 Conclusion There was improvement of serum bicarbonate and CO2 levels for refractory HRF patients with good NIV compliance who received acetazolamide. No evidence of harm demonstrated. If instituted sooner, it might reduce RSU bed stay. Further randomized studies are needed to see its impact on RSU stay and mortality.References Bemand TJ et al. Acetazolamide for metabolic alkalosis complicating respiratory failure with chronic obstructive pulmonary disease or obesity hypoventilation syndrome: a systematic review. Thorax 2023;78:1004–1010. Rialp Cervera G, et al. Effects of early administration of acetazolamide on the duration of mechanical ventilation in patients with chronic obstructive pulmonary disease or obesity-hypoventilation syndrome with metabolic alkalosis. A randomized trial. Pulm Pharmacol Ther. 2017;44:30–37.