Electrical Alternans 

Updated: Sep 12, 2019
Author: Bharat K Kantharia, MD, FRCP, FAHA, FACC, FESC, FHRS; Chief Editor: Mikhael F El-Chami, MD 



Electrical alternans is a broad term that describes alternate-beat variation in the direction, amplitude, and duration of any component of the electrocardiograph (ECG) waveform (ie, P, PR, QRS, R-R, ST, T, U). It was first recognized over a century ago by Theodor Herring in 1909,[1] further characterized by Sir Thomas Lewis in 1910,[2] and subsequently identified on surface ECG in 1948 decades later by Kalter and Schwartz.[3]

Electrical alternans is not a distinct entity in itself, but rather an ECG sign of an underlying cardiac pathology. The component of the ECG displaying alternans may provide a clue in this regard. For example, electrical alternans related to the T wave (ie, T wave alternans) has shown its potential value in cardiac risk stratification and prediction of sudden cardiac death.

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 three main categories:

Alternans due to cardiac motion

Alternans due to cardiac motion is the most well-known mechanism of electrical alternans, encountered in large pericardial effusions and cardiac tamponade. The pendulous swinging motion of the heart in a fluid-suspended cavity in such situations effectively causes electrical alternans.[4] This motion with respect to the fixed overlying surface electrode[5, 6, 7, 8] results in total electrical alternans involving all portions of the electrocardiogram. Rarely, this phenomenon may be witnessed in hypertrophic cardiomyopathy as well.

Conduction alternans (P, PR, QRS)

Conduction via distinct alternate pathways with differing conduction and recovery characteristics can lead to the development of altering cycle lengths and can thus give rise to alternans. This is most classically seen in atrioventricular reentrant tachycardia (AVRT). It was once thought that the presence of electrical alternans recorded on ECG during supraventricular tachycardia (SVT) suggested AVRT rather than other forms of SVTs. However, it became evident that electrical alternans during SVTs was more likely a function of the heart rate rather than the mechanism of arrhythmia.[9] The rate of tachycardia remains the most important determinant of QRS alternans[10, 11] ; it may be encountered in a number of cardiac conditions associated with high rates of conduction, including atrial fibrillation, Wolff-Parkinson-White Syndrome, ventricular tachycardia, SVT, and accelerated idioventricular rhythm.[4]

Conditions such as cardiomyopathies or heart failure are associated with structural alterations of the cardiac tissue as well as generation of pathways with altered conduction characteristics.

Repolarization alternans (ST, T, U)

A number of cardiac conditions (eg, ischemia, heart failure) can exaggerate the inherent heterogeneity of the myocardial repolarization characteristics,[12, 13] serving as a substrate for ventricular arrhythmias and sudden cardiac death. This heterogeneity manifests as T-wave alternans, which is increasingly being utilized for risk stratification of sudden cardiac death (see the discussion in the Workup section under Other tests).

Alterations in calcium-handling dynamics in certain regions of the myocardium lead to changes in the repolarization characteristics of these areas, manifesting as electrical alternans. This occurs through direct effects on calcium conduction during the plateau phase of the action potential, and also through effects on calcium-sensitive sodium channels.[14, 15, 16, 17, 18, 19, 20, 21]  Cardiomyopathies, heart failure, and certain arrhythmias are associated with changing cycle lengths, which result in changing stroke volumes on a beat-to-beat basis. Consequently, the constantly changing mechanical stretch of the chamber walls affects the conduction of stretch-sensitive ion channels—a phenomenon known as mechanoelectric feedback (MEF).[22, 23] Cardiac memory of the preceding cycle lengths, a property known as hysteresis, plays a role in perpetuating alternans even after normalization of the heart rate.[19]


The reported causes or associations of electrical alternans have been mentioned already and are relisted here for convenience.

Alternans due to cardiac motion

Causes include the following:

  • Large pericardial effusion /cardiac tamponade

  • Hypertrophic cardiomyopathy

  • Emphysema / pneumothorax

  • Large bilateral pleural effusions

Repolarization alternans (ST, U, T)

ST alternans

  • Myocardial ischemia

  • Vasospastic angina pectoris

  • Acute myocardial infarction

  • Subarachnoid hemorrhage[24]

T-wave alternans

  • Congenital long QT syndromes

  • Cardiomyopathies

  • Treatment with quinidine or amiodarone[15]

  • Dyselectrolytemia (hypokalemia, hypomagnesemia, hypocalcemia)

  • Following cardiac resuscitation

Conduction alternans (P, PR, QRS)

Note the following causes:

  • Atrial fibrillation

  • Atrioventricular reentrant tachyarrhythmias

  • Wolff-Parkinson-White syndrome

  • Left ventricular dysfunction

  • Myocardial contusion

  • Acute pulmonary embolism

  • Ventricular tachycardia


United States data

Electrical alternans is estimated to be encountered in about 1 to 6 of 10,000 electrocardiograms (ECGs). Overall, QRS alternans is the most commonly reported, although it is only seen in 5%-10% of patients with tamponade. The incidence of ST alternans has been reported to be 5%-7.7% of patients during coronary intervention. T-wave alternans is observed in about 45% of patients with congenital long QT syndrome during Holter monitoring.


The finding of electrical alternans during supraventricular tachycardia does not in itself change the prognosis; however, some forms of electrical alternans have prognostic value. For example, obvious “macro-T wave” alternans indicates impending ventricular tachycardia and fibrillation leading to cardiac arrest (see the following image).

Electrical alternans. This electrocardiogram revea Electrical alternans. This electrocardiogram reveals macro-T wave alternans with switched polarity seen in all leads.

Similarly, electrical alternans seen during ventricular tachycardia is associated with significant hemodynamic instability (see the image below).

Electrical alternans. This electrocardiogram shows Electrical alternans. This electrocardiogram shows ventricular tachycardia from the right ventricular outflow tract (RVOT) region. Note the R wave alternans seen in the wide QRS complexes.

The presence of micro-T wave alternans may indicate a higher risk of sudden cardiac death and spontaneous ventricular arrhythmias. However, its greatest utility lies in its high negative predictive value (NPV) for future arrhythmias (>90% NPV for 1 year).[25]


Morbidity or mortality in the setting of electrical alternans is wholly related to the underlying cardiac pathology. T-wave alternans has been associated with higher risk of developing ventricular arrhythmias as well as sudden cardiac death.


Electrical alternans is frequently associated with serious underlying cardiac conditions. All complications are related to the underlying disease process.




History and physical examinations findings often provide a clue regarding the underlying cardiac pathology leading to electrical alternans. For example, electrical alternans in the setting of hypotension or worsening dyspnea in patients with pericarditis would indicate the development of cardiac tamponade. Similarly, ST-T alternans in the setting of chest pain, diaphoresis, and dyspnea would indicate severe myocardial ischemia. T-wave alternans observed in the setting of palpitations and syncope would point toward an underlying ventricular arrhythmia.

Physical Examination

Physical examination should be directed toward diagnosis of the underlying cardiac etiology for electrical alternans. The classical Beck triad of jugular venous distention, hypotension, and muffled heart sounds, or any of its individual features could arise from cardiac tamponade. Additional pulsus paradoxus may be noted. The presence of added heart sounds and signs of fluid overload suggest an underlying cardiomyopathy or heart failure.



Differential Diagnoses

  • Alternans Bundle Branch Block

  • Bigeminy

  • Digoxin Toxicity - Bidirectional Ventricular Tachycardia

  • Wide Varying Tidal Volumes in Severe Tachypnea



Laboratory Studies

Laboratory investigations in patients with electrical alternans are directed toward the underlying etiology as suggested by the patient's electrocardiogram and clinical presentation. Although elevated levels of cardiac enzymes such as the troponins, and biomarkers such as B-type natriuretic peptide (BNP) and N-terminal-pro-BNP (NT-pro-BNP), suggest myocardial ischemia and heart failure from cardiomyopathy, clinicians should evaluate the appropriate laboratory markers for malignancy, autoimmune disease, or renal failure in patients with large pericardial effusions, with or without tamponade physiology.[4]

Imaging Studies


Transthoracic echocardiography (TTE) is usually the first line of investigation in patients with electrical alternans, especially in the setting of hemodynamic instability in which a life-threatening pathology is suspected.[4]

The presence of a large pericardial effusion or features of tamponade-like diastolic collapse of the right-sided cardiac chambers can be identified.

Evidence of pulmonary embolism may be found, as indicated by right ventricular (RV) dysfunction or a relative hyperkinesis of the RV apex relative to the RV free wall (McConnells sign).

Echocardiography is also necessary for the evaluation of patients with hypertrophic cardiomyopathy, dilated cardiomyopathy, or congestive heart failure.

Chest radiography

A chest radiograph may reveal an enlarged cardiac silhouette, possibly indicating cardiomyopathy or a large pericardial effusion.

Evidence of the Westermark sign or Hampton hump may suggest pulmonary embolism as the cause of electrical alternans.

Other Tests


Electrocardiography (ECG) is the primary modality used for identification of electrical alternans. Any or all components of the electrical waveforms may exhibit alternans (see the following images).

Electrical alternans. This electrocardiogram shows Electrical alternans. This electrocardiogram shows a typical alternate-beat QRS electrical alternans. Note that the QRS voltage is low.
Electrical alternans. This electrocardiogram revea Electrical alternans. This electrocardiogram reveals supraventricular tachycardia with alternans. Note the phasic nature to the QRS morphology, particularly in the rhythm strip in V1.
Electrical alternans. This electrocardiogram shows Electrical alternans. This electrocardiogram shows ventricular tachycardia from the right ventricular outflow tract (RVOT) region. Note the R wave alternans seen in the wide QRS complexes.
Electrical alternans. This electrocardiogram revea Electrical alternans. This electrocardiogram reveals macro-T wave alternans with switched polarity seen in all leads.

T-wave alternans (TWA) can be detected based on stored ECGs of implantable cardioverter-defibrillators (ICDs).[26]  This entity often precedes the onset of torsades de pointes in congenital long QT syndrome (LQTS). Additionally, T-wave alternans often indicates a higher risk of progression to ventricular dysrhythmias in patients with any form of long QT syndrome or cardiomyopathy.[27, 28]

The presence of ST-T alternans is highly suggestive of ischemia in a large area of myocardium or the development of severe spasm of a proximal vessel.[29, 30]

Microvolt T-Wave alternans testing

Advances in ECG analysis software have led to the detection and analysis of T-wave alternans at a microvolt level (MTWA), which have significantly improved the sensitivity of testing, as visually evident T-wave alternans is quite uncommon. (For example, LQTS-MTWA [1.2%] vs MTWA [44%]).

Beta blockers are generally held for 24-48 hours prior to testing. However, this is often problematic, considering the population under study—those with cardiomyopathy/high arrhythmia burden.

T-wave alternans analysis tools (eg, spectral decomposition analysis, modified-moving average method) are used to quantify the extent of MTWA, which allows the categorization of patients into high, low, and indeterminate risk groups.[31, 32, 33, 34]

The main utility of MTWA testing is in its high negative predictive value (97%) for arrhythmias in the following year. It has been shown to be equivalent to invasive electrophysiologic testing in risk stratifying patients for placement of an implantable cardioverter-defibrillator (ICD).[25, 35, 36, 37]  Clinically, MTWA testing is used for risk stratification in two main populations, those with LQTS and patients undergoing evaluation for placement of ICDs for primary prevention.[27, 38, 39, 40, 41, 42, 43]

It remains unclear whether the disappearance of MTWA indicates an adequate degree of medical management of these patients.

Implantable loop recorder

Placement of a loop recorder may be of benefit in patients with T-wave alternans in whom there is a high suspicion of underlying dysrhythmias.

Cardiac magnetic resonance imaging

Advanced cardiac imaging could be of benefit for further evaluation of cardiomyopathies and heart failure of uncertain etiology.



Emergent pericardiocentesis must be performed in the setting of a large pericardial effusion or cardiac tamponade.

Cardiac catherization

Cardiac catheterization may be indicated to further evaluate or treat patients with myocardial ischemia or ischemic cardiomyopathy.

Electrophysiologic studies/ablative procedures

Electrophysiologic studies and possible ablative procedures may be required in the setting of atrioventricular reentrant arrhythmias/accessory pathways, etc.

ICD implantation

Patients with advanced heart failure or cardiomyopathies may require placement of an implantable cardioverter-defibrilator (ICD). This would benefit select patients with long QT syndrome, and it may be considered in those found to be at high risk by T-wave alternans testing.



Approach Considerations

It is important to remember that electrical alternans is just a sign of underlying pathology and not an entity in itself. Evaluation of the electrocardiographic component displaying alternans often provides a clue toward the direction of further investigation. The presence of T-wave alternans in the appropriate clinical setting can play a role in cardiac risk stratification.

Cardiology consultation is usually indicated.

Medical Care

Direct treatment toward correction of the underlying cause of electrical alternans, such as optimizing heart failure or anti-ischemic regimens in patients with cardiomyopathy or coronary disease. In the setting of long QT syndrome (LQTS), remove the offending drugs and correct concomitant dyselectrolytemias. Malignant pericardial effusions require aggressive treatment of the underlying malignancy with aggressive chemotherapy/immunotherapy regimens.

Diet and activity

No specific dietary or activity restrictions are required aside from those required for managing the underlying cause (eg, salt restriction for congestive heart failure; avoidance of stress and strenuous exercise for patients with congenital LQTS).


Surgical Care

Most diseases that cause true electrical alternans do not require surgical treatment. Pulmonary embolectomy may be required for unresolved large pulmonary emboli. Left-sided cervicothoracic sympathetic ganglionectomy may be required for patients with congenital long QT syndrome or malignant arrhythmias whose conditions remain refractory to drug therapy. Recurrent pericardial effusions may benefit from pericardiectomy.