Arrhythmias Channel

Overview

Heart rhythm problems (arrhythmias) occur when there is a malfunction in the electrical impulses within the heart that coordinate how it beats. As a result, the heart beats too fast, too slowly, or irregularly.

Most people have experienced occasional, brief, usually harmless arrhythmias, such as a skipped, fluttering, or racing heartbeat. Although these heart palpitations may feel scary, most people who experience them have nothing wrong with their heart and need no treatment. But more than 4 million Americans, most over age 60, experience heart arrhythmias that may cause bothersome-sometimes even dangerous-signs or symptoms. These may include shortness of breath, fainting-and at the extreme, even an unexpected loss of heart function, breathing, and consciousness that leads to death within minutes unless the person receives emergency medical treatment to restart the heart. So it's important to figure out when an arrhythmia is serious and merits medical attention.

The good news is that physicians have many new tools for treating troublesome arrhythmias. And patients can make lifestyle changes to protect themselves.

Need-to-know anatomy

Your heart, which is a muscle, is divided into four chambers-two atria at the top and two ventricles at the bottom. Normally, each heartbeat starts in the right atrium, where a specialized group of cells called the sinus node -the body's natural pacemaker-sends an electrical signal. The signal spreads throughout the right and left atria to the area between the two atria called the atrioventricular (AV) node . The AV node connects to specific pathways that conduct the signal to the ventricles below. As the signal travels, the heart muscle contracts. First the atria contract, which pumps blood into the ventricles. A fraction of a second later, the ventricles contract, pumping blood throughout the body. The combined action of the atria and ventricles results in a heartbeat. In an arrhythmia, this natural rhythm is disrupted.

Causes

Arrhythmias are classified by the part of the heart in which they begin and whether they produce an irregularly fast or slow heartbeat.

Tachycardias

Tachycardias are arrhythmias in which the heartbeat speeds up. They're further classified by where in the heart they begin.

Atrial tachycardias

Several different kinds of arrhythmia originate in the atrium.

• Atrial fibrillation : About 2.2 million Americans experience atrial fibrillation, making it the most common medically significant arrhythmia. It occurs more often in people age 60 and older, and while it's not generally fatal, it may lead to further problems.

In a healthy heart, the electrical impulses in the atria are coordinated, but in atrial fibrillation, they occur randomly and very, very quickly; in fact, the atria can beat up to 400 beats per minute, many times higher than the normal rate of about 70 beats per minute. This hyperfast beating can last for a few minutes, an hour, or persist for days.

The electrical activity that's gone haywire in the atria usually stays there and doesn't entirely spread to the ventricles-the larger, more muscular chambers that pump blood to the lungs (right ventricle) and the rest of the body (left ventricle). The atrioventricular node (AV node) is a group of cells that conduct electrical activity from the atria to the ventricles. In atrial fibrillation, the AV node prevents most electrical impulses from progressing through the ventricles, though enough may get through that the ventricles will contract at a faster-than-normal rate.

• Atrial flutter : Like atrial fibrillation, atrial flutter involves abnormal electrical activity in the atria (usually in the right atrium). Unlike atrial fibrillation, the activity in an atrial flutter is more coordinated and controlled. It's caused by a short circuit in the heart's electrical activity.
• Supraventricular tachycardia (SVT ): SVTs are caused by electrical activity from a premature heartbeat that gets "stuck" in a repeating pattern, and come in an alphabet soup of acronyms. "Supraventricular" means "above the ventricles"-where the handful of arrhythmias known as SVT begin. The two most common kinds of SVTs are atrioventricular reentry tachycardia (AVRT) and atrioventricular nodal reentry tachycardia (AVNRT). A less-common type of SVT is atrial tachycardia (AT). Both children and adults can experience these kinds of abnormal rhythms. In AVRT, an abnormal "bridge," or connection of tissue between the atria and ventricles, called an accessory pathway, makes it possible for abnormal heart rhythms to occur. AVRT occurs in people with Wolff-Parkinson-White (WPW) syndrome (see below). AVNRT occurs when there is a short circuit of the electrical impulse within the AV node. People experiencing SVT feel their heart racing rapidly (as much as 200 beats per minute); it starts out of the blue and ends just as abruptly after a few seconds or, in some cases, hours.
• Wolff-Parkinson-White Syndrome (WPW) : In this syndrome, there is an extra "bridge" that connects the atria and ventricles. A heartbeat may travel down the AV node to the ventricles, then return to the atria through the extra pathway. If the impulse continues to travel in a circular manner, it causes the heart to contract with each electrical cycle, which may result in a dangerously rapid heartbeat. Some people with WPW may develop atrial fibrillation (see above) with the electrical impulse conducting to the ventricles over both the normal AV node and the extra pathway, resulting in an extremely rapid heartbeat.

Ventricular tachycardias

Some forms of tachycardia originate in the ventricles-the lower, more muscular chambers of the heart that pump blood to the lungs (right ventricle) and the rest of the body (left ventricle):

• Ventricular tachycardia (VT) : VT is a rapid, regular heartbeat originating from a site in one of the ventricles instead of from the sinus node in the right atrium. Frequently, the site is an area of scar tissue in the heart's muscular wall caused by damage from a heart attack. During VT the heart rate is much faster than usual, ranging from 100 to 280 beats a minute. Some types of VT impair the heart's ability to pump blood. VT may cause such symptoms as palpitations, chest tightness, dizziness, shortness of breath, or loss of consciousness. In those cases, it may be life threatening and requires aggressive emergency medical care. Most VT is associated with other heart diseases, such as coronary artery blockage, cardiomyopathy or valvular heart disease. Some types of VT may occur in younger people who have no other heart disease. In these cases, exertion or exercise often triggers tachycardia. Generally, this kind of arrhythmia is less serious than it is in a person with existing heart disease. Still, some otherwise healthy people have significant symptoms when VT occurs. It requires medical attention.
• Ventricular fibrillation : Since the ventricles pump blood away from the heart, any malfunction can be extremely serious, even fatal. When electrical activity in the ventricles goes haywire and causes the heart muscle to contract quickly in a random fashion, the muscle appears to quiver instead of contract in its usual fashion. It cannot deliver blood to the lungs, brain, and other vital organs. Not surprisingly, ventricular fibrillation is very serious and is believed to be responsible for nearly 350,000 yearly U.S. fatalities from sudden cardiac death, some caused by a heart attack. The heart may be shocked back into a normal rhythm with a defibrillator, either an external one in the case of an unexpected attack, or an internal one (called an implantable cardioverter defibrillator) in the case of high-risk patients.
• *Long QT syndrome (LQTS) : In LQTS the heart's mechanical or pumping function is normal. The recharging system of the heart, however, is either slow, taking longer than normal to recharge, or inefficient (also called disorganized). An electrocardiogram can see the electrical activity of the heart and record it as waves. The different parts of the waves are labeled with different letters-P, Q, R, S, and T. The QT interval is a measure of the time it takes for the electrical system in the ventricles to recharge after each heartbeat. When the QT interval is longer than normal or the T waves look abnormal, your heart might beat abnormally. With a long QT interval, your ventricles are more vulnerable to spin electrically out of control into very fast, abnormal rhythms like ventricular tachycardia or fibrillation (see above). Symptoms appear if the abnormal rhythm lasts long enough, and include heart palpitations, fainting episodes, and even sudden death.

Bradycardias

When the heart beats too slowly (less than 60 beats per minute), the condition is known as a bradycardia. This isn't always a medical problem; people who are aerobically fit tend to have lower heart rates, sometimes below 60 beats per minute. Yet their hearts efficiently pump enough blood throughout the body. When the heart is not pumping sufficient blood because of a slow heartbeat, it may fall into one of the categories of bradycardia.

• Sick sinus : Normally the electrical messages in the heart begin in a cluster of cells in the right atrium, called the sinus node (SA node). The sinus node is your heart's natural pacemaker. If it is malfunctioning, it can cause the heart to beat too slowly or to alternate between fast and slow beating. This problem may be caused by a blocking of the electrical impulse near the SA node.
• Conduction block : The electrical pathway may also be blocked near the AV node, in between the atria and ventricles. A conduction block doesn't completely stop the heartbeat because another set of cells in the AV node or ventricles act as a secondary pacemaker. When this "backup" system goes into effect, the heartbeat is slower than when it is regulated by the SA node, which serves as the atrial pacemaker. Occasionally, the "backup" system fails to activate, and the heart temporarily stops beating, resulting in loss of consciousness. The block can be caused by heart disease, an injury to the area during heart surgery, a congenital defect, or the side effects of certain drugs.

Premature heartbeats

Unlike tachycardias and bradycardias, premature heartbeats signify no change in the speed of the heartbeat. They are extra heartbeats that occur in between regular heartbeats; they can begin in either the atria or the ventricles. They aren't usually serious, though they may cause another, more dangerous, form of arrhythmia in people with heart disease. Caffeine, cold medicines, and asthma medications can all prompt premature heartbeats.

Risk factors

While most people have heart palpitations at one time or another, certain factors make it more likely you'll experience them:

• Coronary artery disease (CAD) : a previous heart attack or other damage to the heart
• Cardiomyopathy : heart damage affecting the heart's rhythms
• Valvular heart diseases : problems with the valves between the heart's chambers and vessels
• High blood pressure : High blood pressure may change the structure of the heart and alter the electrical pathways.
• Advancing age : Increased age raises the chance for heart damage.
• Genetics : Some people have heart abnormalities from birth.
• Thyroid problems : Depending on whether the thyroid is over- or underactive, it may cause the heart to beat too quickly or too slowly
• Diabetes : increases the risk of high blood pressure and heart disease; blood sugar levels also influence heart rates
• Obstructive sleep apnea : This can cause certain types of arrhythmia.
• Electrolyte imbalance : An imbalance of minerals like sodium, potassium, magnesium, and calcium can alter the way the heart conducts electricity.