Atrial fibrillation (AF) is the most common arrhythmia occurring in people. When the atria (the top chambers of the heart) are in fibrillation, they beat at 400 to 600 beats per minute. Because this is so fast, the atria just quiver rather than help the ventricles (bottom chambers) pump the blood. Fortunately, it is the bottom left chamber that pumps the blood out to the body and makes the pulse. The pulse is determined by the number of AF beats that make it to the bottom.
The AV node is the normal pathway heartbeats use to go from the top part of the heart to the bottom. When the heartbeat is normal, the AV node lets every beat make it to the bottom, resulting in the top and the bottom being coordinated. However, if the heart rate gets too fast, as in AF, the AV node filters out many of the beats and the pulse is therefore not too fast.
AF arises from the left atrium in about 90% of patients – the pulmonary veins (PV) being the most common source. These veins bring oxygenated blood from the lungs back to the left atrium. Until recently, doctors assumed that the atrial muscle stopped right where the pulmonary veins meet the left atrium. We now know that threads of atrial tissue called fascicles can go up into the pulmonary veins for some distance. These fascicles are where AF usually originates. When the fascicle starts firing rapidly, the fast firing follows the fascicle to the left atrium and causes AF.
Types of AF
There are 3 types of atrial fibrillation: paroxysmal, persistent, and permanent (or chronic).
- Paroxysmal atrial fibrillation: brief episodes of abnormal heart rhythm that stop on their own.
- Persistent atrial fibrillation: long periods of abnormal heart rhythm during which a normal rhythm can be restored with medication or an electrical shock.
- Permanent atrial fibrillation (also known as chronic atrial fibrillation): abnormal heart rhythm, where even medications and electrical shocking cannot restore normal rhythm.
Consequences of AF
When AF occurs, the heart does not pump blood as efficiently to the body. This is due to several factors. First, when the top part of the heart no longer works in harmony with the bottom, the pumping efficiency declines. Second, whenever the pulse is rapid and irregular, the efficiency declines further. Finally, when the atria fibrillate, the walls just quiver rather than pump vigorously. After 48 hours, blood can clot and stick to the wall of the left atrium, usually in a finger-like projection called the left atrial appendage. If the blood clot breaks off, it has about an 85% chance of being pumped up the arteries in the neck leading to the brain causing a stroke. In the other 15% of the time, the clot may go down another artery causing a heart attack or the loss of the pulse in the arm or leg. The increased risk of stroke with AF underscores the importance of recognizing this heart rhythm problem.