Atrial fibrillation (A-tre-al fi-bri-LA-shun), or
AF, is the most common
arrhythmia
(ah-RITH-me-ah). An arrhythmia is a problem with the speed or rhythm of the
heartbeat. A disorder in the hearts electrical system causes AF and other
types of arrhythmia.
AF occurs when rapid, disorganized electrical
signals in the hearts two upper chambers, called the atria (AY-tree-uh),
cause them to contract very fast and irregularly (this is called fibrillation).
As a result, blood pools in the atria and isnt pumped completely into the
hearts two lower chambers, called the ventricles (VEN-trih-kuls). When
this happens, the hearts upper and lower chambers dont work
together as they should.
Often, people who have AF may not even feel
symptoms. However, even when not noticed, AF can lead to an increased risk of
stroke. In many patients, particularly when the rhythm is extremely rapid, AF
can cause chest pain,
heart
attack, or
heart
failure. AF may occur rarely or every now and then, or it may become a
persistent or permanent heart rhythm lasting for years.
Understanding the Heart's Electrical System
The heart has an internal electrical system that
controls the speed and rhythm of the heartbeat. With each heartbeat, an
electrical signal spreads from the top of the heart to the bottom. As it
travels, the signal causes the heart to contract and pump blood. The process
repeats with each new heartbeat.
Each electrical signal begins in a group of cells
called the sinus node, or sinoatrial (SA) node. The SA node is located in the
right atrium, which is the upper right chamber of the heart. In a healthy adult
heart at rest, the SA node fires off an electrical signal to begin a new
heartbeat 60 to 100 times a minute. (This rate may be slower in very fit
athletes.)
From the SA node, the electrical signal travels
through special pathways to the right and left atria. This causes the atria to
contract and pump blood into the hearts two lower chambers, the
ventricles. The electrical signal then moves down to a group of cells called
the atrioventricular (AV) node, located between the atria and the ventricles.
Here, the signal slows down just a little, allowing the ventricles time to
finish filling with blood.
The electrical signal then leaves the AV node and
travels along a pathway called the bundle of His. This pathway divides into a
right bundle branch and a left bundle branch. The signal goes down these
branches to the ventricles, causing them to contract and pump blood out to the
lungs and the rest of the body. The ventricles then relax, and the heartbeat
process starts all over again in the SA node.
For more information, see the Diseases and
Conditions Index article on
How
the Heart Works, which contains an animation that shows how the heart's
electrical system causes the heart to pump blood.
Understanding the Electrical Problem in Atrial
Fibrillation
In AF, the hearts electrical signal begins in
a different part of the atria or the nearby pulmonary veins and is conducted
abnormally. The signal doesnt travel through normal pathways, but may
spread throughout the atria in a rapid, disorganized way. This can cause the
atria to beat more than 300 times a minute in a chaotic fashion. The
atrias rapid, irregular, and uncoordinated beating is called
fibrillation.
The abnormal signal from the SA node floods the AV
node with electrical impulses. As a result, the ventricles also begin to beat
very fast. However, the AV node cant conduct the signals to the
ventricles as fast as they arrive, so even though the ventricles may be beating
faster than normal, they arent beating as fast as the atria. The atria
and ventricles no longer beat in a coordinated fashion, creating a fast and
irregular heart rhythm. In AF, the ventricles may beat up to 100175 times
a minute, in contrast to the normal rate of 60100 beats a minute.
When this happens, blood isnt pumped into the
ventricles as well as it should be, and the amount of blood pumped out of the
ventricles is based on the randomness of the atrial beats. In AF, instead of
the body receiving a constant, regular amount of blood from the ventricles, it
receives rapid, small amounts and occasional random, larger amounts, depending
on how much blood has flowed from the atria to the ventricles with each
beat.
Most of the symptoms of AF are related to how fast
the heart is beating. If medicines or age slow the heart rate, the effect of
the irregular beats is minimized.
AF may be brief, with symptoms that come and go and
end on their own, or it may be persistent and require treatment. Or, AF can be
permanent, in which case medicines or other interventions cant restore a
normal rhythm.
The animation below shows atrial fibrillation. Click
the "start" button to play the animation. Written and spoken explanations are
provided with each frame. Use the buttons in the lower right corner to pause,
restart, or replay the animation, or use the scroll bar below the buttons to
move through the frames.
The animation shows how the
hearts electrical signal begins in a different place in the heart,
causing the atria to beat very fast and irregularly.
Outlook
People who have AF can live normal, active lives.
For some people, treatment can cure AF and return their heartbeat to a normal
rhythm. For people who have permanent AF, treatment can successfully control
symptoms and prevent complications. Treatment consists primarily of different
kinds of medicines or nonsurgical procedures.