A 35-year-old man with known asthma presents with an acute exacerbation. Baseline spirometry (FEV~1~ 65% predicted) shows FEV~1~ 45% predicted during the attack. After 2 hours of standard therapy (systemic corticosteroids, oxygen, nebulized salbutamol), FEV~1~ is 52% predicted with persistent symptoms. Which investigation is most appropriate to assess severity and guide further management?