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?

Explanation

## Clinical Presentation Analysis The patient presents with classic **sympathomimetic side effects**: tremor, palpitations, and tachycardia occurring shortly after initiating a new bronchodilator. These findings are pathognomonic for excessive β₂-adrenergic receptor stimulation. ## Mechanism of β₂-Agonist Side Effects **Key Point:** β₂-agonists (short-acting and long-acting) stimulate β₂-adrenergic receptors on bronchial smooth muscle (therapeutic effect) but also on cardiac myocytes, skeletal muscle, and the CNS (unwanted effects). ### Peripheral Effects of β₂ Overstimulation | System | Effect | Clinical Manifestation | |--------|--------|------------------------| | Cardiac | Increased heart rate, contractility | Tachycardia, palpitations | | Skeletal muscle | Tremor via β₂ on muscle fibers | Fine tremor (especially hands) | | Metabolic | Increased lipolysis, glycogenolysis | Hyperglycemia, hypokalemia | | CNS | Stimulation | Anxiety, insomnia, headache | **High-Yield:** Fine tremor is a **dose-dependent side effect** and typically resolves with dose reduction or continued use (tolerance develops). It is NOT a sign of overdose toxicity but rather expected pharmacology. ## Why This Is NOT an Acute Coronary Event **Clinical Pearl:** Although β₂-agonists can increase myocardial oxygen demand, the patient: - Denies chest pain or dyspnea at rest - Has regular rhythm (no arrhythmia) - Presents with benign tremor and palpitations (not anginal symptoms) - Has no risk factors mentioned for ACS These findings are consistent with **sympathomimetic excess**, not cardiac ischemia. ## Tachyphylaxis vs. Acute Overstimulation **Warning:** Tachyphylaxis (loss of response) develops over *days to weeks* with chronic use and presents as *loss of bronchodilation*, not acute onset of side effects. This patient has acute symptoms within 2 days of initiation — this is acute overstimulation, not tachyphylaxis. ## Management Approach 1. Reassure patient that tremor is expected and usually self-limiting 2. Consider dose reduction if symptoms are intolerable 3. Advise taking dose in morning to minimize sleep disturbance 4. Monitor for hypokalemia (especially if concurrent diuretic use) 5. Tolerance to tremor typically develops within 1–2 weeks **Mnemonic:** **BETA** side effects = **B**radycardia (rare, reflex), **E**xcitability (tremor, anxiety), **T**achycardia, **A**rrhythmias (potential, especially with high doses or cardiac disease).