Electrical alternans is a broad term that describes alternate-beat variation in the direction, amplitude, and duration of any component of the ECG waveform (ie, P, PR, QRS, R-R, ST, T, U). It was first recognized by Hearing in 1909 and further characterized by Sir Thomas Lewis in 1910 as occurring "either when the heart muscle is normal but the heart rate is very fast or when there is serious heart disease and the rate is normal." Kalter and Schwartz first identified electrical alternans on surface ECG in 1948.  Electrical alternans must be distinguished from mechanical alternans (eg, pulsus alternans), although both may coexist.
The pathophysiologic mechanisms that cause electrical alternans can be divided into 3 categories: (1) repolarization alternans (ST, T, U alternans), (2) conduction and refractoriness alternans (P, PR, QRS alternans), and (3) alternans due to cardiac motion. True electrical alternans is a repolarization or conduction abnormality of the Purkinje fibers or myocardium. The cellular mechanism behind electrical alternans is thought to be due to abnormal calcium cycling, either impaired release or impaired reuptake of sarcoplasmic reticulum calcium.  Electrical alternans due to cardiac motion is effectively artifact, as the heart swings in relation to the chest wall and electrodes, with a period twice that of the heart rate.
Repolarization alternans can be further subclassified as T-wave alternans and ST-segment alternans. T-wave alternans is associated with rapid changes in heart rate or prolongation of the QT interval. A long QT interval is associated with polymorphic ventricular tachycardia (ie, torsade de pointes); therefore, T-wave alternans is a possible precursor to torsade de pointes.
T-wave alternans has been reported with congenital long QT syndrome  , electrolyte imbalances (eg, hypocalcemia, hypokalemia, hypomagnesemia), treatment with quinidine or amiodarone  , hypertrophic cardiomyopathy, alcoholic cardiomyopathy, congestive heart failure, and acute pulmonary embolism. T-wave alternans has also been reported following cardiac resuscitation. Most importantly, the presence of T-wave alternans can be used as a predictor of ventricular tachyarrhythmic events, such as sudden cardiac death [5, 6, 7] , [8, 9, 10] sustained ventricular tachycardia, ventricular fibrillation, implantable cardioverter defibrillator (ICD) therapy for ventricular tachyarrhythmia, and cardiac arrest.
ST-segment alternans describes alternating levels of ST elevation, usually in the presence of myocardial ischemia. It has been reported with vasospastic angina pectoris, acute myocardial infarction, nonvasospastic angina pectoris, during exercise tests, during percutaneous transluminal coronary angioplasty (PTCA), and after subarachnoid hemorrhage.  ST alternans during acute ischemia has been associated with appearance of ventricular arrhythmia, including ventricular tachycardia and ventricular fibrillation.
Conduction alternans is an alternation of impulse propagation along any of the anatomic structures involved in conveyance of electrical impulse and is usually precipitated by changes in heart rate or input from nervous, humoral, or pharmacologic components. Conduction alternans may be seen in the setting of myocardial ischemia, atrial fibrillation, Wolff-Parkinson-White syndrome, rheumatic heart disease, acute pulmonary embolism, myocardial contusion, and left ventricular dysfunction. It may manifest on the surface ECG as alternation of the P wave, QRS complex, PR interval, R-R interval, or any combination of these. QRS alternans during narrow complex tachycardia has been suggested as a marker for orthodromic atrioventricular (AV) reentrant tachycardia using an accessory pathway as a retrograde limb.
Electrical alternans associated with cardiac motion is due to alternation in the position of the heart with relation to recording electrodes. The most common underlying disorder is an enlarged pericardial sac; however, not all pericardial effusions cause electrical alternans. The presence of pericardial disease and total electrical alternans (P, QRS, and T wave) frequently suggests cardiac tamponade, but total electrical alternans is seen in only 5-10% of patients with cardiac tamponade. Heart movement in patients with hypertrophic cardiomyopathy also may result in electrical alternans of this type.
Incidence of electrical alternans is estimated to be about 1-6 of 10,000 ECGs. Overall, QRS alternans is the most common type. ST alternans incidence has been reported as 5–7.7% of patients during PTCA using intracoronary ECG recordings. T-wave alternans was observed in 45% of patients with congenital long QT syndrome after examining 24-hour Holter monitors.
The finding of electrical alternans does not alter the morbidity and mortality rates of the underlying cause or association.
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