A 72-year-old woman with a history of hypertension presents with acute onset severe breathlessness while lying flat. She is agitated, tachypnoeic at 32 breaths/min, with oxygen saturations of 86% on air, bilateral basal crackles, and pink frothy sputum. Blood pressure is 190/110 mmHg, pulse 120 bpm. What is the most appropriate immediate management step?