A 75-year-old woman with known heart failure with reduced ejection fraction suddenly develops severe breathlessness at rest. She is sitting upright and distressed with a respiratory rate of 32 breaths per minute, oxygen saturation 88% on air, blood pressure 150/90 mmHg, pulse 110 beats per minute, elevated jugular venous pressure and bilateral basal crackles up to the mid-zones. A chest X-ray confirms pulmonary oedema. What is the most appropriate immediate intravenous medication?