A 76-year-old woman with known ischaemic heart disease and hypertension presents with acute severe breathlessness. She is sitting upright, using accessory muscles, with bilateral basal crackles to the mid-zones, a respiratory rate of 30/min, blood pressure 170/100 mmHg and oxygen saturation 90% on air. What is the most appropriate immediate pharmacological treatment in addition to high-flow oxygen and sitting her upright?