A 55-year-old woman with known lung cancer presents with palpitations and dyspnea. ECG shows a regular narrow-complex tachycardia at 155 bpm with alternating QRS amplitude from beat to beat. What is the most likely underlying condition?
- A Acute myocardial infarction
- B Pericardial effusion with cardiac tamponade ✓
- C Hypokalemia
- D Digitalis toxicity
Correct answer: B. Pericardial effusion with cardiac tamponade
Explanation
Electrical alternans (beat-to-beat alternation of QRS amplitude) in the setting of tachycardia is pathognomonic for pericardial effusion with cardiac tamponade. The mechanism involves the heart swinging within a large pericardial fluid collection. While electrical alternans can rarely be seen in other conditions, the combination with tachycardia strongly suggests tamponade.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.