A 72-year-old man with known ischaemic heart disease presents with acute pulmonary oedema: severe dyspnoea, frothy sputum, respiratory rate 32 breaths/min, blood pressure 178/100 mmHg, oxygen saturation 88% on room air and bilateral basal crackles. ECG shows sinus tachycardia. What is the most appropriate immediate management priority in the emergency department in addition to oxygen titrated to saturations 94-98%?