Heart Rhythm Disorders
The primary function of the heart is to supply blood and nutrients to the body. The regular beating, or contraction, of the heart moves the blood throughout the body. Each heartbeat is controlled by electrical impulses traveling through the heart. In the normal heart these electrical impulses occur in regular intervals. When something goes wrong with the heart’s electrical system, the heart does not beat regularly. The irregular beating results in a rhythm disorder, or arrhythmia.
The electrical system regulating heartbeat consists of two main areas of control and a series of conducting pathways, similar to the electrical wiring in a house (See Multimedia File 1).
- The sinoatrial, or SA, node is located in the right atrium. It is the source and main control and is the source of each heartbeat. The SA node responds to the body's overall need for blood and increases the heart rate when necessary, such as during exercise, emotional excitement, or illness such as fever. The SA node is sometimes called the "natural pacemaker" of the heart.
- Electrical impulses leave the SA node and travel through special conducting pathways in the heart to the other area of control, the atrioventricular (AV) node. The AV node provides a pathway for impulses from the atria to the ventricles. It also creates a delay in conduction from the atria to the ventricle. This causes the atria to contract first and allow the ventricles to fill with blood before they contract themselves.
- The delay ensures proper timing so that the lower chambers have time to fill completely before they contract.
Normally, the heart beats 60-100 times a minute. This state is called "normal sinus rhythm" or "normal rhythm." Depending upon the needs of the body, it may beat faster (sinus tachycardia) due to stress or slower (sinus bradycardia) such as during sleep.
Arrhythmias are abnormalities of the heartbeat. There are many types of arrhythmias, and they are classified by where they begin, (the atria, AV node, or the ventricles). Generally speaking, those that do not originate from the ventricles are called supraventricular arrhythmias while those that come from the ventricles are called ventricular arrhythmias.
The following are some of the more commonly encountered arrhythmias, starting with the supraventricular arrhythmias.
- Premature atrial contractions (sometimes called PACs, APCs, or premature supraventricular contractions): PACs are an extra heart beat arising from the atria, outside of the SA node, that causes the heart to contract earlier than expected. This is a very common occurrence in all ages and usually is not serious.
Supraventricular tachycardia, or paroxysmal SVT: SVTs are regular, rapid heartbeats resulting from abnormal electrical impulses above the ventricle (usually the atria or the AV node).
- Sick sinus syndrome: This syndrome is characterized by irregular firing of the SA node resulting in both slower and faster-than-normal heart rates.
Atrial fibrillation: A common condition caused by electrical impulses discharged at a rapid rate from many different areas of the atria. It usually causes a fast and irregular heartbeat.
Atrial flutter: A condition caused by a rapid discharge of abnormal electrical impulses from the right atrium. Typically, the right atrium fires at a rate of 300 beats per minute, but only every other beat is conducted through the AV node, meaning that the ventricular rate is classically 150 beats per minute.