electrical alternans

Electrical alternans occurs when there is a large pericardial effusion causing the heart to swing like a pendulum within the pericardial fluid (the 'swinging heart' phenomenon). With each beat, the heart's position relative to the chest wall electrodes changes, causing the QRS amplitude/axis to alternate. This is a highly specific but not very sensitive sign of cardiac tamponade, especially when combined with sinus tachycardia and low voltage QRS complexes.

Pathophysiology: A large pericardial effusion allows the heart to rotate freely within the pericardial sac with each cardiac cycle, changing its electrical vector relative to fixed surface electrodes. This produces alternating QRS amplitudes on the ECG, typically occurring at half the heart rate (2:1 alternans pattern).

Clinical significance: Electrical alternans is one of Beck's triad-adjacent findings (though not part of the classic Beck's triad itself, which includes hypotension, jugular venous distension, and muffled heart sounds) associated with cardiac tamponade. It is highly suggestive of significant pericardial effusion when present, and should prompt urgent echocardiography to assess for tamponade physiology and consideration for pericardiocentesis if hemodynamically significant.

Other causes: Electrical alternans can rarely be seen in severe tachyarrhythmias, myocardial ischemia, or severe heart failure due to alternating action potential duration or conduction patterns (mechanoelectrical alternans), but the classic board association is with pericardial effusion/tamponade.

Differentiation: Total electrical alternans (P, QRS, and T wave amplitude changes) is more specific for tamponade than QRS-only alternans, which can be seen in other conditions like SVT with aberrant conduction.

Management: Identification of electrical alternans should prompt immediate clinical correlation with vital signs (hypotension, tachycardia), physical exam findings (JVD, pulsus paradoxus, muffled heart sounds), and echocardiography to confirm tamponade physiology (RV diastolic collapse, IVC plethora, respiratory variation in mitral/tricuspid inflow velocities).