A 60-year-old woman with a history of ischaemic heart disease presents to A&E with progressive dyspnoea. She has had previous admissions with heart failure in the past year. She has a respiratory rate of 34 breaths/minute and a systolic blood pressure of 90 mmHg. Her oxygen saturation is 88% on room air. On examination, she looks pale and sweaty, and has widespread crepitations over both lung fields. Oxygen by face mask was commenced. What is the SINGLE most appropriate next step in management?