Electrical alternans is a phenomenon seen on the electrocardiogram with alternation in the amplitude of QRS complexes. The term electrical alternans totalis is used when the amplitudes of all the waves (P, QRS and T) show alternating amplitude. Electrical alternans totalis is seen in cardiac tamponade and is thought to be due to the heart swinging movement of the heart within the pericardial cavity. Electrical alternans may sometimes be associated with its mechanical counter part: pulsus alternans. But most often the two are unrelated. Another situation in which electrical alternans is seen is with supraventricular ectopic bigeminy. The ectopic beat often has a lower QRS amplitude, possibly due to the lower ventricular volume at the onset of systole (Brody’s effect).
Isolated alternans of the ST segment and T wave also may occur. T wave alternans can be macroscopic or microvolt T wave alternans which can be detected only with special equipment. ST segment and T wave alternans have been reported in vasospastic angina and is thought to be the harbinger of life threatening arrhythmias. T wave alternans (both macroscopic and microvolt) have also been linked with ventricular arrhythmias and sudden cardiac death. T wave alternans has been noted in congenital long QT syndrome preceding torsades des pointes.