A 52-year-old man with moderate persistent asthma presents to the clinic with complaints of tremor in his hands and palpitations that started 2 days after initiating a new bronchodilator. His baseline FEV₁ is 65% predicted. On examination, heart rate is 102 bpm, regular rhythm, and fine tremor is noted in outstretched hands. He denies chest pain or dyspnea at rest. His previous controller was inhaled corticosteroid monotherapy. What is the most likely cause of his symptoms?