A 20-year-old man presents to the Emergency Department with acute shortness of breath, wheeze, and chest tightness that started 1 hour ago after exposure to a cat, to which he is known to be allergic. He has used his salbutamol inhaler 10 puffs via spacer with minimal relief. On examination, he is tachypnoeic (RR 30/min), tachycardic (HR 120 bpm), and his peak expiratory flow rate (PEFR) is 45% of his best. He is able to speak in short sentences. According to BTS/SIGN guidelines, what is the most important immediate treatment?