What is atrial fibrillation?
Atrial fibrillation (AF) is the most common, abnormal rhythm of the heart.
The heart contracts (beats) and pumps blood with a regular rhythm, for example, at a rate of 60 beats per minute there is a beat every second. The heart may beat faster or slower with a shorter or longer interval between beats, but at any one rate the interval between beats is constant. This regular rhythm occurs as a result of regular electrical discharges (currents) that travel through the heart and cause the muscle of the heart to contract. In AF, the electrical discharges are irregular and rapid and, as a result, the heart beats irregularly and, usually, rapidly.
AF is common; half a million new cases are diagnosed yearly in the U.S., and billions of dollars are spent annually on its diagnosis and treatment.
What causes atrial fibrillation?
Normal function of the heart
The heart has four chambers. The upper two chambers are the atria, and the lower two chambers are the ventricles. Blood returning to the heart from the body in the superior and inferior vena cava contains low levels of oxygen and high levels of carbon dioxide. This blood flows into the right atrium and then into the adjacent right ventricle. After the ventricle fills, contraction of the right atrium pumps additional blood into the right ventricle. The right ventricle then contracts and pumps the blood to the lungs where the blood takes up oxygen and gives off carbon dioxide. The blood then flows from the lungs to the left atrium and into the adjacent left ventricle. Contraction of the left atrium pumps additional blood into the left ventricle. The left ventricle then contracts and pumps the blood to the rest of the body. The heartbeat (pulse) that we feel is caused by the contraction of the ventricles.
The ventricles must deliver enough blood to the body for the body to function normally. The amount of blood that is pumped depends on several factors. The most important factor is the rate of contraction of the heart (the heart rate). As the heart rate increases, more blood is pumped. In addition, the heart pumps more blood with each beat when the atria contract and fill the ventricles with additional blood just before the ventricles contract.
With each beat of the heart, an electrical discharge (current) passes through the electrical system of the heart. The electrical discharge causes the muscle of the atria and ventricles to contract and pump blood. The electrical system of the heart consists of the SA node (sino-atrial node), the AV node (atrio-ventricular node) and special tissues in the atria and the ventricles that conduct the current.
The SA node is the heart's electrical pacemaker. It is a small patch of cells located in the wall of the right atrium; the frequency with which the SA node discharges determines the rate at which the heart beats. The electrical current passes from the SA node, through the special tissues of the atria and into the AV node. The AV node serves as an electrical relay station between the atria and the ventricles. Electrical signals from the atria must pass through the AV node to reach the ventricles.
The electrical discharges from the SA node cause the atria to contract and pump blood into the ventricles. The same discharges then pass through the AV node to reach the ventricles, traveling through the special tissues of the ventricles and causing the ventricles to contract. In a normal heart, the rate of atrial contraction is the same as the rate of ventricular contraction.
At rest, the frequency of the electrical discharges originating from the SA node is low, and the heart beats at the lower range of normal (60-80 beats/minute). During exercise or excitement, the frequency of discharges from the SA node increases, increasing the rate at which the heart beats.
Function of the heart during atrial fibrillation
During AF, electrical discharges are not generated solely by the SA node. Instead, electrical discharges come from other parts of the atria. These abnormal discharges are rapid and irregular and may exceed 350 discharges per minute. The rapid and irregular discharges cause ineffective contractions of the atria. In fact, the atria quiver rather than beat as a unit. This reduces the ability of the atria to pump blood into the ventricles.
The rapid and irregular electrical discharges from the atria then pass through the AV node and into the ventricles, causing the ventricles to contract irregularly and (usually) rapidly. The contractions of the ventricles may average 150/minute, much slower than the rate in the atria. (The ventricles are unable to contract at 350/minute.) Even at an average rate of 150/minute, the ventricles may not have enough time to fill maximally with blood before the next contraction, particularly without the normal contraction of the atria. Thus, AF decreases the amount of blood pumped by the ventricles because of their rapid rate of contraction and the absence of normal atrial contractions.
Heart rate during atrial fibrillation
In a heart that is beating normally, the rate of ventricular contraction is the same as the rate of atrial contraction. In AF, however, the rate of ventricular contraction is less than the rate of atrial contraction. The rate of ventricular contraction in AF is determined by the speed of transmission of the atrial electrical discharges through the AV node. In people with a normal AV node, the rate of ventricular contraction in untreated AF usually ranges from 80 to 180 beats/minute; the higher the transmission, the higher the heart rate.
Some older people have slow transmission through the AV node due to disease within the AV node. When these people develop AF, their heart rates remain normal or slower than normal. As disease in the AV node advances, these people can even develop an excessively slow heart rate and require a permanent pacemaker to increase the rate of ventricular contractions.
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