A 75-year-old man with known systolic heart failure (ejection fraction 25 percent) presents with acute onset orthopnoea, pink frothy sputum, and severe respiratory distress. Blood pressure is 180/100 mm Hg, heart rate 120 beats per minute, oxygen saturation 86 percent on room air. Chest auscultation reveals diffuse crackles. What is the most appropriate initial management step in the emergency department?