The healthy heart is a marvel of efficiency, pumping approximately five quarts of blood through the body every minute by means of regular, forceful contractions of its four chambers. Each perceived thump of the heart actually consists of two beats, one by the upper chambers, or atria, and one by the lower chambers, or ventricles. The contractions are triggered by electrical impulses originating in the sinoatrial node, a specialized group of cells located in the heart's right atrium.

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Any disturbance in the normal beating pattern of the heart is called an arrhythmia, or irregular heartbeat. Practically everyone experiences some version of an arrhythmia on occasion, usually in the form of a mild palpitation or a "skipped" heartbeat. (In fact, what feels like a skipped beat is really an early beat, weak enough not to be felt, then a one- or two-second pause, followed by a relatively forceful beat; the delay between beats feels like a skip.) Mild, isolated disturbances of this sort are normally harmless. A recurrent arrhythmia, on the other hand, should be checked by a physician.
 
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There are two main categories of arrhythmia: tachycardia, meaning too fast a heartbeat, and bradycardia, meaning too slow a heartbeat. (Both conditions refer only to exceptional elevations or depressions of heart rate, not to the normal variance that occurs throughout the day depending on whether you are resting or active.) At rest, the heart beats somewhere between 60 and 100 times per minute. Tachycardia is defined as more than 100 beats per minute, while bradycardia is defined as fewer than 60 beats per minute. Both tachycardia and bradycardia can occur in surges or as persistent conditions.

Most arrhythmias fall into the tachycardia category. Some originate in the atria, some in the ventricles. Ventricular arrhythmias are typically more serious conditions. In fact, most cases of sudden cardiac death are due to ventricular arrhythmias, not heart attack, as was once thought. Particularly dangerous is ventricular fibrillation, in which activity within the ventricles is so uncoordinated that the ventricles are reduced to twitching, and virtually no pumping takes place. Ventricular fibrillation will cause death within a few minutes unless treated with CPR and defibrillation as soon as possible.

Bradycardias can be due to problems with nerves that control heartbeat, with the sinus node, where the heart's electrical impulses originate, or with actual transmission of those impulses through the heart. In the latter condition, known as heart block, electrical signals fail to reach the ventricles from the atria because of some conduction problem. Signals may be blocked continuously or intermittently. In a worst-case scenario, electrical activity within the heart shuts down completely and beating stops.

Symptoms
Keep in mind that occasional, isolated disturbances of the heartbeat are common and usually harmless. Signs of more serious arrhythmias include the following:

For tachycardia, or abnormally rapid heartbeat:

• Recurrent palpitations, defined as an uncomfortable awareness of your heartbeat. The palpitations may take the form of a strong pulse in the neck, a "flip-flopping" heart, or a fluttering, thumping, pounding or racing beat in the chest.
• Chest discomfort, weakness, fainting, sweating, shortness of breath, confusion, or dizziness.

For bradycardia, or abnormally slow heartbeat:

• Fatigue, shortness of breath, lightheadedness, or loss of consciousness.

September 2000, Reviewers: the Cleveland Clinic
 
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