A 69-year-old woman with long-standing hypertension presents to the emergency department with acute onset severe dyspnoea at rest. She is distressed, sitting upright, respiratory rate 32 breaths/min, oxygen saturation 90% on 10 L/min via mask, blood pressure 190/110 mmHg and bilateral basal crackles. ECG shows sinus tachycardia. What is the most appropriate immediate management priority?