A 52-year-old man with hypertension is started on metoprolol 50 mg daily. After 2 weeks, he reports fatigue, dizziness, and shortness of breath on exertion. His blood pressure is 110/70 mmHg. On examination, heart rate is 48 bpm and regular. Which investigation is most appropriate to assess the cause of his symptoms?

Explanation

## Clinical Context The patient presents with symptomatic bradycardia (HR 48 bpm) and exertional dyspnea following initiation of metoprolol, a non-selective beta-1 selective blocker. The symptoms suggest a conduction abnormality or excessive negative chronotropic effect. ## Why 12-Lead ECG Is the Investigation of Choice **Key Point:** ECG is the gold-standard, first-line investigation to evaluate bradycardia and assess for conduction abnormalities (AV block, sinoatrial block) induced by beta-blockers. **High-Yield:** Beta-blockers cause: - Decreased heart rate (negative chronotropic effect) - Slowed AV nodal conduction (negative dromotropic effect) - Risk of AV block (especially 2nd or 3rd degree) in susceptible patients **Clinical Pearl:** Symptomatic bradycardia with exertional dyspnea in the setting of beta-blocker initiation mandates immediate ECG to rule out high-degree AV block or severe bradycardia requiring dose reduction or drug discontinuation. ## Why Other Investigations Are Not First-Line | Investigation | Rationale for Not Being First-Line | |---|---| | Serum electrolytes & renal function | Useful for assessing drug clearance and secondary causes of bradycardia (e.g., hypothyroidism), but does not directly visualize conduction abnormalities | | Chest X-ray | Non-specific; would not identify the mechanism of bradycardia or conduction block | | Echocardiography | Indicated if structural heart disease (reduced ejection fraction, cardiomyopathy) is suspected, but not the first-line test for drug-induced bradycardia | **Mnemonic:** **ECHO-BRADY** — ECG first for bradycardia; Echocardiography only if structural disease is suspected.