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.