AbstractElectrical alternans concomitant with pericardial effusion has been considered a pathognomonic sign suggestive of a large effusion with cardiac tamponade, particularly if there is P wave alternans as well as QRS alternans. However, the mechanism of this phenomenon remains controversial. A patient with pericardial effusion secondary to adenocarcinoma of the lung with metastases, pericardial effusion, electrical alternans, and cardiac tamponade was studied by echocardiography, right and left heart catheterization, and pericardiocentesis. Hemodynamic data were consistent with cardiac tamponade. The echocardiogram demonstrated a large anterior and posterior pericardial effusion. Noncongruous motion of the septum and posterior wall was recorded at a rate equal to the heart rate. In addition, congruous motion of the septum and posterior wall was recorded at a rate that was half the heart rate and corresponded to the electrical alternans. The congruous movement disappeared after pericardiocentesis, as did the electrical alternans. The electrical alternans is synchronous with and due to the pendulous movement of the heart within the pericardial sac, as demonstrated by echocardiogram and cineangiograms.