Electrical alternans on ECG most strongly indicates which condition, prompting urgent evaluation?

• A. Ventricular tachyarrhythmia
• B. Pulmonary embolism
• C. Atrial fibrillation with rapid ventricular rate
• D. Large pericardial effusion with tamponade physiology

Answer: (D)

Electrical alternans indicates the heart is swinging within a large amount of pericardial fluid, causing beat-to-beat variation in the QRS amplitude (and sometimes axis). This pattern is classic for a large pericardial effusion with tamponade physiology, where the pericardial pressure impairs ventricular filling and can progress to hemodynamic compromise. The alternating QRS forces reflect the heart’s changing orientation relative to the ECG leads as it moves within the fluid.

This finding signals an urgent situation because tamponade is life-threatening and requires rapid assessment and intervention, such as pericardiocentesis if tamponade is confirmed. In contrast, ventricular tachyarrhythmia typically presents with a rapid, regular rhythm; atrial fibrillation with rapid ventricular rate shows an irregular rhythm without beat-to-beat QRS amplitude variation; and pulmonary embolism usually presents with tachycardia and dyspnea rather than electrical alternans.