The heart is divided into four chambers: two atria on top and two ventricles underneath. The role of the atria is to fill the ventricles with blood, which is then pumped to the lungs or to the rest of the body.
The normal beating of the heart is controlled by electrical signals sent from a particular segment of heart tissue called the sinus node. The sinus node acts as the heart's natural pacemaker. It is found near the top of the right atrium. Normally, the sinus node sends an electrical pulse that travels down the muscle tissue to the atrioventricular (AV) node (acting like a "toll booth"), which determines how many impulses are allowed to then pass to the ventricles.
An arrhythmia occurs when the heart's regular rhythm becomes irregular – it may speed up or slow down. Many arrhythmias exist, but those originating from the ventricles are generally more serious than from the atria.
Atrial fibrillation (AF) is one type of arrhythmia. Instead of the sinus node controlling the heart rate, different parts of the atria fire at the same time. This causes the atria to fibrillate, which is an uncoordinated quivering of the muscle. The atria pump blood less effectively but well enough to allow the ventricles to function. The irregular heartbeat is due to rapid beating of the atria (usually more than 350 beats per minute), and the irregular movement of the electrical signals through the AV node. Instead of every electrical signal being allowed through to the ventricles only certain electrical signals are allowed to pass.
AF is potentially dangerous because blood can pool in the atrium, which increases the risk that a blood clot will form. If this clot travels to the brain, it will cause a stroke. AF is the most common form of all harmful arrhythmias, affecting 6% of those over the age of 65, and the risk increases to 10% to 15% in people over 80. Men are more likely than women to be hospitalized for atrial fibrillation.
AF may be present in a number of different forms:
- paroxysmal AF: recurrent temporary condition that starts suddenly and then after a period of time (usually less than 7 days), the heart returns to a normal heartbeat without medical assistance
- persistent AF: the heart beats irregularly and requires medical assistance to return to a normal rhythm. This type of AF tends to last more than 7 days.
- permanent AF: the heart has an irregular rhythm that cannot return to normal with medication or other treatments
All three situations carry equal risk of blood clot formation.
The cause of AF is not always known, but possible causes can include any of the following:
- abnormal structure of the heart
- diseases that damage the valves of the heart
- high blood pressure
- thyroid disease
- heart failure
- previous stroke or transient ischemic attack (TIA)
- binge drinking
- increasing age
Knowing what factors can trigger an episode of atrial fibrillation, such as intense exercise, extreme emotion, or excessive alcohol intake, may help people to be proactive in their self-management of this condition.
Symptoms and Complications
Some people do not experience any symptoms. In this case, their doctor may recognize the condition during a regular medical examination.
When symptoms of AF are present, they typically include:
- chest discomfort
- rapid and irregular heartbeat
- lightheadedness or, in rare cases, fainting
- shortness of breath – this may occur at rest in more severe cases of AF
- reduced ability to exercise
People with AF who have right-sided heart failure may also have swelling of the arms and legs, weight gain, or ascites (abdominal swelling due to accumulation of fluid). This is a severe type of AF that requires immediate medical attention.
Everyone knows what it feels like to experience an occasional flutter of the heart, and usually it does not mean anything. See your doctor if you experience chest pains, feel faint, or notice your pulse to be faster than usual or irregular over a prolonged period.
AF is a progressive condition that can weaken the heart and its ability to pump blood, especially if it is not treated properly. Another possible complication of AF is the formation of blood clots that can cause a stroke. The risk of stroke depends on your age and other risk factors, but the presence of AF increases your risk of having a stroke by at least 5 times.
Making the Diagnosis
When the heart beats faster than normal, it's called tachycardia. AF is one type of tachycardia. If too many impulses are allowed through to the ventricles, your heart rate may be greater than 100 beats per minute. Conversely, if not many impulses are allowed through, your heart rate may, in fact, be slow (this is called bradycardia).
The stethoscope is a valuable instrument when it comes to detecting arrhythmias like AF, but there are modern tests that can pinpoint the problem. The electrocardiogram (ECG) prints a graph of the heart's electrical activity using small electrodes taped to the chest. The pattern on these graphs reveals the type of arrhythmia and can help the doctor diagnose AF. Since the arrhythmia might not occur at the hospital, there are portable ECGs that you can bring home. Some are constantly turned on over a specified period of time (these are called Holter monitors), while others are turned on by you when you feel palpitations (these are called event monitors or loop records). Some of these devices can download the heart signal data over a phone line or mobile phone.
Electrophysiologic study (EPS) is a more elaborate test. Thin tubes are inserted into a blood vessel in the leg and guided up to the heart. They hold electrodes that can find the heart tissue that may be overriding the signals from the sinus node.
Your doctor may also order blood tests to determine if another condition (e.g., thyroid hormone imbalances) is associated with your AF.
Treatment and Prevention
There are three main strategies to manage AF:
- rate control (preventing the heart rate from going too fast)
- rhythm control (trying to change the rhythm from AF back to normal sinus rhythm)
- anticoagulation (prevent clot formation and strokes with the use of blood thinners)
For rate control, several medications exist to help slow down a rapid heartbeat. With this strategy, the goal is to reduce the amount of impulses passing through the AV node and to slow the heart rate down. It should be noted that AF is still present when using this strategy.
Beta-blockers (e.g., atenolol*, metoprolol) and specific calcium channel blockers (e.g., diltiazem and verapamil) are medications used for rate control for people with a variety of heart conditions. These medications can be used for more than one purpose (e.g., they are also used to treat high blood pressure). Another medication, digoxin, has been used to treat arrhythmias for over 200 years. It is derived from a substance called digitalis.
In certain people, the rate is too difficult to control with medications. In this case, a specialized cardiologist (an electrophysiologist) will put in a cardiac pacemaker and arrange to do a procedure called AV node ablation (burn) a few weeks later. With special tubes inserted into a vein in your leg, heat is applied around the AV node of the heart to block all passage of electrical signals. The pacemaker then controls your heart rate, making it beat regularly.
Rhythm control can be achieved in different ways. The process of bringing the rhythm back to normal is called cardioversion; if medications are used in this process, it is called chemical cardioversion. Medications called antiarrhythmics (e.g., propafenone, flecainide, dronedarone) can convert an abnormal rhythm back to normal and prevent the abnormal rhythm from recurring. Alternatively, electrical cardioversion uses a machine called a defibrillator that gives a controlled electrical shock to the heart, converting the AF to a normal rhythm. Patients are given an anesthetic so they do not feel anything during the procedure.
A technique developed by electrophysiologists called pulmonary vein ablation (or pulmonary vein isolation) is also used. Using special catheters placed in the heart, doctors either ablate (burn) areas that cause AF or isolate the area so that it does not affect the other parts of the atrium. This option may offer a "cure" to AF that the other options cannot offer.
Anticoagulation helps prevent the formation of blood clots and is critical for people with AF because of the increased risk of stroke. In order to prevent blood clots, doctors often recommend medications that thin the blood. These may include anticoagulants (such as warfarin) or antiplatelet medications (such as ASA). Depending on your risk of developing a clot, your doctor may recommend either an antiplatelet or anticoagulant medication. Although the risk of bleeding increases with the use of these types of medication, the protection they offer to prevent stroke and other blood clot-related conditions most often outweighs the risk.
*All medications have both common (generic) and brand names. The brand name is what a specific manufacturer calls the product (e.g., Tylenol®). The common name is the medical name for the medication (e.g., acetaminophen). A medication may have many brand names, but only one common name. This article lists medications by their common names. For information on a given medication, check our Drug Information database. For more information on brand names, speak with your doctor or pharmacist.