Electrical alternans is an electrocardiographic

Electrocardiogram

Electrocardiography is a transthoracic interpretation of the electrical activity of the heart over a period of time, as detected by electrodes attached to the outer surface of the skin and recorded by a device external to the body...

 phenomenon of alternation of QRS complex

QRS complex

The QRS complex is a name for the combination of three of the graphical deflections seen on a typical electrocardiogram . It is usually the central and most visually obvious part of the tracing. It corresponds to the depolarization of the right and left ventricles of the human heart...

 amplitude or axis between beats and a possible wandering base-line. It is seen in cardiac tamponade

Cardiac tamponade

Cardiac tamponade, also known as pericardial tamponade, is an emergency condition in which fluid accumulates in the pericardium ....

 and severe pericardial effusion

Pericardial effusion

Pericardial effusion is an abnormal accumulation of fluid in the pericardial cavity. Because of the limited amount of space in the pericardial cavity, fluid accumulation will lead to an increased intrapericardial pressure and this can negatively affect heart function...

 and is thought to be related to changes in the ventricular

Ventricle (heart)

In the heart, a ventricle is one of two large chambers that collect and expel blood received from an atrium towards the peripheral beds within the body and lungs. The Atria primes the Pump...

 electrical axis due to fluid in the pericardium

Pericardium

The pericardium is a double-walled sac that contains the heart and the roots of the great vessels.-Layers:...

, as the heart essentially wobbles in the fluid filled precardial sac.

Generally electrical alternans can be seen with tamponade, and narrow AV junctional reenterant tachycardia with an accessory pathway (such as WPW syndrome). A similar phenomenon, pseudo-alternans, can be seen in bigeminal PVC in the PR interval, alternans pre-excitation, and alternans bundle branch block

Bundle branch block

A bundle branch block refers to a defect of the heart's electrical conduction system.-Anatomy and physiology:The heart's electrical activity begins in the sinoatrial node , which is situated on the upper right atrium. The impulse travels next through the left and right atria and summates at the...

. For the most part however, the most serious condition to rule out is tamponade.