The rhythm of the normal heart is controlled by the electrical impulses forms within the SA node and produces a heart rate between 50 and 100 beats/minute during rest. A deviation from or a disturbance of the normal sinus rhythm is called arrhythmia. An arrhythmia may occur when depolarization is initiated by other pacemaker cells of the heart than those of the SA node, thus altering the formation of the electrical impulses. Another mechanism which produces arrhythmia is when the conduction of the electrical impulses is altered.
The reentry phenomenon is another impulse conduction problem that sustains arrhythmia and is caused by changes in the refractory period and propagation speed of the heart. When neighboring areas of the myocardium have different refractory periods, the electrical impulse may depolarize an area which is receptive, while another area remains inactive since the cells are still in refractory from the previous cardiac cycle. Once the inactive area has recovered, the impulse may be accepted and the area can serve as a pathway back to the area which was initially depolarized. Reentry occurs if the later area has had time to recover and can be depolarized again and continues until the cells become unreceptive. The resulting movement of the electrical impulse is often circular and is said to occur in a reentry circuit.
The classification of cardiac arrhythmia involves the site of its origin and its rate. The significance of an arrhythmia differs widely and may be benign, symptomatic, life threatening or fatal. There are four main types of arrhythmia: premature (extra) beats, supraventricular arrhythmias, ventricular arrhythmias, and bradyarrhythmias.
Premature beats are the most common type of arrhythmia. They are harmless most of the time and often don’t cause any symptoms. When symptoms do occur, they usually feel like a fluttering in the chest or a sensation of a skipped beat. Most of the time, premature beats need no treatment, especially in healthy people.
Premature beats that occur in the atria are called premature atrial contractions, or PACs. Premature beats that occur in the ventricles are called premature ventricular contractions, or PVCs. In most cases, premature beats occur naturally, not due to any heart disease. But certain heart diseases can cause premature beats. They can also happen because of stress, too much exercise, or too much caffeine or nicotine.
Supraventricular arrhythmias are tachycardias (fast heart rates) that start in the atria or the atrioventricular (AV) node. Types of supraventricular arrhythmias include atrial fibrillation (AF), atrial flutter, supraventricular tachycardia (SVT), and WolffParkinson-White (WPW) syndrome.
AF is the most common type of serious arrhythmia. It’s a very fast and irregular contraction of the atria. AF occurs when the heart’s electrical signal begins in a different part of the atrium than the sinoatrial (SA) node or when the signal is conducted abnormally. When this happens, the electrical signal doesn’t travel through the normal pathways in the atria, but instead may spread throughout the atria in a fast and disorganized manner. This makes the walls of the atria to quiver very fast (fibrillate) instead of beating normally. As a result, the atria aren’t able to pump blood into the ventricles the way they should.
In AF, electrical signals can travel through the atria at a rate of more than 300/minute. Some of these abnormal electrical signals can travel to the ventricles, causing them to beat too fast and with an irregular rhythm. AF is not usually life threatening, although it can be dangerous when it causes the ventricles to beat very fast.
These are arrhythmias that start in the ventricles. They can be very dangerous and usually need immediate medical attention. Ventricular arrhythmias include ventricular tachycardia (VT) and ventricular fibrillation (VFib). Coronary artery disease, heart attack, weakened heart muscle, and other problems can cause ventricular arrhythmias.
Ventricular tachycardia is a fast, regular beating of the ventricles that may last for only a few seconds or for much longer. A few beats of ventricular tachycardia often don’t cause problems, but ventricular tachycardia episodes that last for more than just a few seconds can be dangerous. Ventricular tachycardia can turn into other, more dangerous arrhythmias, such as v-fib.
V-fib occurs when disorganized electrical signals make the ventricles quiver instead of pump normally. Without the ventricles pumping blood out to the body, a person will lose consciousness within seconds and will die within minutes if not treated. To prevent death, the condition must be treated immediately with defibrillation, an electric shock to the heart. V-fib may happen during or after a heart attack, or in a heart that is already weak because of another condition. Health experts think that most of the sudden cardiac deaths that occur every year, are due to v-fib.
Bradyarrhythmias are arrhythmias in which the heart rate is slower than normal. If the heart rate is too slow, not enough blood reaches the brain, and the person can lose consciousness. In adults, a heart rate slower than 60 beats per minute is considered a bradyarrhythmia. Some people normally have slow heart rates, especially people who are very physically fit. For them, a heartbeat slower than 60 beats per minute is not dangerous and doesn’t cause symptoms. But in other people, bradyarrhythmia can be due to a serious disease or other condition.
Bradyarrhythmias can be caused by heart attack, conditions that harm or change the heart’s electrical system (such as underactive thyroid gland or aging), an imbalance of chemicals or other substances (such as potassium) in the blood, or even some medicines (such as beta blockers).
Bradyarrhythmias can also happen as a result of severe bundle branch block. Bundle branch block is a condition in which the electrical signal traveling down either or both of the bundle branches is delayed or blocked. When this happens, the ventricles don’t contract at exactly the same time, as they should, and the heart has to work harder to pump blood to the body. The cause of bundle branch block is often an existing heart condition.