Arrhythmia describes an irregular heart beat.
The muscles of the heart contract in a coordinated fashion to insure blood is efficiently pumped into the arteries. The small right ventricle is the first to contract, followed soon after by all the other muscles of the other three ventricles.
However, if this rhythm is disrupted, the heart ceases to pump blood efficiently. As a result, the patient can easily become fatigued (especially when exerting themselves), can have chest pain, and may have difficulty standing suddenly.
Usually, an arrhythmia can be detected through the use of a stethoscope, as a healthy heart has a distinctive rhythm. However, in many cases, the arrhythmia is not present when the body is at rest. In such cases, a stress test with an EKG is usually required to determine if the heart is beating regularly.
Treating arrhythmia depends on the underlying cause. Primary arrhythmia (that which doesn't appear to have an underlying cause) can be treated with a pacemaker which provides an electric shock to the small right ventricle at regular intervals to keep it from contracting irregularly. When surgery is not possible and the arrhythmia is severe, a pacemaker wire is usually inserted through the chest and into the heart until a pacemaker can be installed.
Fibrillation, where the heart muscles fire randomly, is a severe form of arrhythmia. It usually must be treated with an electric defibrillator, which in theory will once again coordinate the rhythm of the heart.