A 72-year-old man presents to the emergency department with acute severe dyspnoea while lying flat. He has a history of ischaemic cardiomyopathy. On examination he is distressed, tachypnoeic at 32 breaths/min, blood pressure 170/100 mmHg, oxygen saturation 86% on room air and widespread crackles to the upper lung zones. What is the most appropriate immediate management step in addition to high-flow oxygen and intravenous access?