Atrial Fibrillation

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Overview

Atrial fibrillation is an abnormality of heart rhythm in which chambers of the heart contract in a disorganized manner, producing an irregular heart rate. Atrial fibrillation can lead to heart failure (shortness of breath, edema, palpitations) and chest pains and, when left untreated, can lead to stroke.

Normal Blood Flow in the Heart
The heart has a right side and a left side and each side has a chamber that receives blood returning to the heart (atrium) and a muscular chamber that pumps blood out of the heart (ventricle). Atria are relatively thin-walled chambers, whereas the ventricles are much more muscular. Blood passes from the atria into the ventricles through two processes. During the resting phase, when the ventricles are not contracting, the tricuspid and mitral valves open. Some of the blood that has accumulated in the atria passively flows through the tricuspid and mitral valves into the right and left ventricles, respectively. The atria then contract, pumping blood out and into the ventricles. Once the ventricles fill with blood, they contract, pumping blood into the lungs and to the body.

Atrial Fibrillation
Contractions of the different chambers of the heart are normally organized in a specific manner. An electrical impulse travels through the heart's chambers and sets off contractions. The heart's "spark plug" is a small area of specialized heart tissue called the SA node, located in the right atrium.

Each time this tissue "fires," an impulse travels first through the right and left atria, signaling these chambers to contract and pump blood into the ventricles, and then travels down into a patch of another specialized heart tissue located between the atria and the ventricles, called the AV node. Electrical-wire-like specialized tissue conducts the impulse down into the ventricles, where it signals the right ventricle to contract and to pump blood out and into the lungs, and signals the left ventricle to contract and pump blood out to the rest of the body. Normal sequence of electrical activation of the chambers of the heart is called sinus rhythm.

In atrial fibrillation, sinus rhythm does not occur. Instead, multiple "wavelets" of electrical impulses travel randomly through the atria, leading to random activation of different parts of the atria at different times. Because the tissues of the right and left atria are not stimulated to contract in an organized manner, the walls of the atria quiver.

Lack of organized contraction by the atria causes several detrimental results. First, because less blood is pumped into the ventricles, there is less blood circulating throughout the body and blood accumulates in the lungs, causing shortness of breath (dyspnea) and other symptoms of heart failure.

Second, because the heart is no longer pumping blood into the ventricles, the blood in the atria (particularly in a small part of the left atrium—the left atrial appendage) becomes relatively stagnant. There is a small risk that, over time, the stagnant blood will form a blood clot. If a blood clot forms, it may eventually enter the left ventricle and be pumped out into the body. If this happens, the clot may travel to the brain, block the flow of blood in a cerebral artery, and cause a stroke.

Third, atrial fibrillation can create chest pain (angina). Multiple disorganized wavelets of electrical activity bombard the AV node with electrical impulses. When many electrical impulses are conducted through the AV node down into the ventricles, the ventricles contract very rapidly, producing a very fast heart rate. When the ventricles contract too rapidly, less blood is pumped into the body and blood may "back up" into the lungs.

Rapid contraction increases the ventricles' demand for oxygen. The demand may exceed the ability of the coronary arteries to supply the ventricles with oxygen-rich blood, causing angina.

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