A 68-year-old man with severe chronic obstructive pulmonary disease presents with increasing breathlessness and purulent sputum. He has received controlled oxygen, nebulised bronchodilators, prednisolone and antibiotics. He is alert and cooperative, but remains tachypnoeic at 30 per minute. Arterial blood gas analysis shows pH 7.28, PaCO2 68 mmHg (35–45) and PaO2 58 mmHg (80–100) while receiving 28% oxygen. Which one of the following should be undertaken next?