Arrhythmias are generally divided into two categories: ventricular and supraventricular. Ventricular arrhythmias occur in the lower chambers of the heart, called the ventricles. Supraventricular arrhythmias occur in the area above the ventricles, usually in the upper chambers of the heart, called the atria. The irregular beats can either be too slow (bradycardia) or too fast (tachycardia).
Bradycardia
Bradycardia is a very slow heart rate of less than 60 beats per minute. It happens when the electrical impulse that signals the heart to contract is not formed in your heart's natural pacemaker, the sinoatrial node (SA node), or is not sent to the heart's lower chambers (the ventricles) through the proper channels.
Bradycardia most often affects elderly people, but it may affect even the very young. It may be caused by one of two sources: The central nervous system does not signal that the heart needs to pump more, or the SA node may be damaged. This damage might be related to heart disease, aging, inherited or congenital defects, or it might be caused by certain medicines—including those used to control arrhythmias and high blood pressure.
Tachycardia
Tachycardia is a very fast heart rate of more than 100 beats per minute. The many forms of tachycardia depend on where the fast heart rate begins. If it begins in the ventricles, it is called ventricular tachycardia. If it begins above the ventricles, it is called supraventricular tachycardia.
Ventricular Arrhythmias
Ventricular Tachycardia
Ventricular tachycardia is a condition in which the SA node no longer controls the beating of the ventricles. Instead, other areas along the lower electrical pathway take over the pacemaking role. Since the new signal does not move through your heart muscle along the regular route, the heart muscle does not beat normally. Your heartbeat quickens, and you feel as if your heart is "skipping beats." This rhythm may cause severe shortness of breath, dizziness, or fainting (syncope).
Ventricular Fibrillation
The most serious arrhythmia is ventricular fibrillation, which is an uncontrolled, irregular beat. Instead of one misplaced beat from the ventricles, you may have several impulses that begin at the same time from different locations—all telling the heart to beat. The result is a much faster, chaotic heartbeat that sometimes reaches 300 beats a minute. This chaotic heartbeat means very little blood is pumped from the heart to the brain and body and can result in fainting. Medical attention is needed right away. If cardiopulmonary resuscitation (CPR) can be started, or if electrical energy is used to "shock" the heart back to a normal rhythm, then the heart may not be too damaged. About 220,000 deaths from heart attacks each year are thought to be caused by ventricular fibrillation. People who have heart disease or a history of heart attack have the highest risk of ventricular fibrillation.
Premature Ventricular Contractions
A less serious type of ventricular arrhythmia is a premature ventricular contraction (PVC). As the name suggests, the condition happens when the ventricles contract too soon, out of sequence with the normal heartbeat. PVCs (sometimes called PVB for premature ventricular beat) generally are not a cause for alarm and often do not need treatment. But if you have heart disease or a history of ventricular tachycardia, PVCs can cause a more serious arrhythmia. Although most PVCs happen quickly and without warning, they can also happen in response to caffeine, which is found in coffee, tea, sodas, and chocolate. Some kinds of over-the-counter cough and cold medicines may also cause PVCs.
Supraventricular Arrhythmias
Supraventricular arrhythmias happen in the upper chambers of the heart. Generally, supraventricular or "atrial arrhythmias" are not as serious as ventricular arrhythmias. Sometimes, they do not even require treatment. Like PVCs, atrial arrhythmias can happen in response to a number of things, including tobacco, alcohol, caffeine, and cough and cold medicines. The disorder also may result from rheumatic heart disease or an overactive thyroid gland (hyperthyroidism).
Supraventricular Tachycardia
Supraventricular tachycardia is a rapid, regular heart rate where the heart beats more than 150 times per minute in the atria. Unlike other types of arrhythmias, supraventricular tachycardia does not start in the SA node.
Atrial Fibrillation
Atrial fibrillation is a fast, irregular rhythm where single muscle fibers in your heart twitch or contract. According to the National Institutes of Health (NIH), about 2.2 million Americans have atrial fibrillation. It is a main cause of stroke, especially among elderly people. Atrial fibrillation may cause blood to pool in the heart's upper chambers. The pooled blood can lead to the formation of clumps of blood called blood clots. A stroke can occur if a blood clot travels from the heart and blocks a smaller artery in the brain (a cerebral artery). About 15% of strokes happen in people with atrial fibrillation.
Wolff-Parkinson-White Syndrome
Wolff-Parkinson-White (WPW) syndrome is a group of abnormalities caused by extra muscle pathways between the atria and the ventricles. The pathways cause the electrical signals to arrive at the ventricles too soon, and the signals are sent back to the atria. The result is a very fast heart rate. People with this syndrome may feel dizzy, have chest palpitations, or have episodes of fainting.
Atrial Flutter
Atrial flutter happens when the atria beat very fast, causing the ventricles to beat inefficiently as well.
Premature Supraventricular Contractions
Also called "premature atrial contractions" (PACs), they happen when the atria contract too soon, causing the heart to beat out of sequence.
Heart Block
Heart block takes place when the SA node sends its electrical signal properly, but the signal is not sent through the atrioventricular (AV) node or lower electrical pathways as quickly as it should be. The condition is most often caused by aging or by the swelling or scarring of the heart that sometimes results from coronary artery disease. There are several types of heart block, and they are named by their degree of severity.
- First-degree heart block means that impulses are moving through the AV node too slowly.
- Second-degree heart block means that impulses are traveling through the heart's atria but are delayed in the AV node. Because of this delay, the ventricles do not beat at the right moment.
- Third-degree heart block means that no impulses are reaching the ventricles. To make up for this, the ventricles use their own "backup" pacemaker with its slower rate. Because a gap in time is likely to occur between the impulse from the atria and the impulse from the "backup" pacemaker in the ventricles, a person may faint. This is known as a Stokes-Adams attack. Third-degree heart block is very serious and can lead to heart failure or death.
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Updated July 2006
