A 72-year-old man with known left ventricular systolic dysfunction presents with acute onset severe breathlessness. He is sitting upright, distressed, with bilateral crackles to the mid zones, oxygen saturations 88 percent on air, pulse 110 bpm and blood pressure 140/80 mmHg. After starting high-flow oxygen and positioning him upright, what is the most appropriate intravenous drug to give next?