A 68-year-old man presents to the emergency department with acute severe dyspnoea, orthopnoea and pink frothy sputum. He has a history of ischaemic cardiomyopathy (LVEF 30%). On examination he is tachypnoeic, BP 160/95 mmHg, pulse 110 bpm, oxygen saturation 88% on room air, with widespread crackles. What is the most appropriate initial pharmacological management in addition to oxygen and positioning?