An 80-year-old man with known severe COPD and stable ischaemic heart disease presents to the emergency department with a 2-day history of worsening shortness of breath, increased cough productive of yellow sputum, and ankle swelling. He is usually independent but now struggles to walk across a room. On examination: Alert but distressed. BP 130/85 mmHg, HR 110 bpm, RR 28 breaths/min, Temp 37.8°C, SpO2 88% on room air. Chest examination reveals diffuse wheezes and crackles bilaterally, reduced air entry at the bases. JVP is elevated to 5 cm above the sternal angle. Mild pitting edema to the knees. ECG shows sinus tachycardia, no acute ischaemic changes. Chest X-ray shows hyperinflation, flattened diaphragms, increased bronchial markings, and mild interstitial prominence. Given this presentation, what is the most appropriate initial management step?