A 58-year-old man with advanced lung cancer presents to the emergency department with dyspnea, hypotension (BP 90/60 mmHg), elevated JVP, and muffled heart sounds. An ECG is obtained and shows the pattern marked **A** in the diagram. Which of the following best explains the mechanism underlying this ECG finding?
A. Acute pericarditis with fibrin deposition creating a diffuse inflammatory substrate
B. Acute myocardial infarction with diffuse subendocardial ischemia affecting all coronary territories
C. Severe hyperkalemia causing progressive prolongation of the PR interval and QRS duration
D. The heart swings within a large pericardial effusion, causing beat-to-beat changes in electrical axis orientation
Explanation
Why "The heart swings within a large pericardial effusion, causing beat-to-beat changes in electrical axis orientation" is right
The pattern marked A (low voltage QRS + electrical alternans) is the hallmark ECG finding of cardiac tamponade due to a large pericardial effusion. Electrical alternans—beat-to-beat alternation of QRS amplitude—occurs because the heart literally swings within the fluid-filled pericardial sac, changing the orientation of the electrical axis with each heartbeat. This is a relatively specific (though not sensitive) sign of large pericardial effusion ± tamponade. The low-voltage QRS complexes (<5 mm in limb leads) result from the insulating effect of the pericardial fluid surrounding the heart. In this patient's clinical context (advanced malignancy, Beck's triad of hypotension, muffled heart sounds, and elevated JVP), the diagnosis is acute cardiac tamponade requiring urgent pericardiocentesis.
Why each distractor is wrong
Acute myocardial infarction with diffuse subendocardial ischemia: Acute MI produces regional ST elevation or depression and T-wave changes in specific territories, not diffuse low-voltage QRS with beat-to-beat alternation. The clinical presentation (no chest pain, no troponin history mentioned) and the specific pattern of electrical alternans rule out MI.
Severe hyperkalemia: Hyperkalemia produces peaked T waves, prolonged PR interval, widened QRS, and loss of P waves—not electrical alternans or isolated low-voltage QRS. The ECG pattern in A is incompatible with hyperkalemia.
Acute pericarditis with fibrin deposition: Acute pericarditis causes diffuse ST elevation with PR depression (as shown in option B), not low-voltage QRS with electrical alternans. Electrical alternans is not a feature of uncomplicated pericarditis; it indicates a large effusion with hemodynamic compromise.
High-YieldNEET PG
Electrical alternans = heart swinging in pericardial fluid = large effusion ± tamponade = urgent pericardiocentesis.
ESC Guidelines on Pericardial Diseases 2015
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