MRCP-0952

A 73 year old woman attends COPD clinic for review. Her blood gases were checked on her last visit two months back. The test was repeated again today. The paO2 on both occasions was 6.8 kPa. There is no CO2 retention on 28% O2. She stopped smoking around 6 months ago and is maintained on combination inhaled steroids and long acting b2-agonist therapy. What is the next best step in management?