A 60-year-old man with a history of ischaemic heart disease and hypertension presents with acute onset breathlessness, cough with pink frothy sputum, and bilateral crackles on chest auscultation up to the mid-zones. His heart rate is 110/min, BP 180/100 mmHg, and SpO2 88% on air. An ECG shows sinus tachycardia with evidence of an old inferior MI. What is the most appropriate initial pharmacological management in addition to oxygen?