Atrial fibrillation

Atrial fibrillation (AF) is the most common arrhythmia encountered in patients. When the atria fibrillate, they beat at 400 to 600 beats per minute. This is so fast that the atria just quiver rather than help the ventricles pump the blood. Fortunately, it is the bottom left chamber that pumps the blood out to the body and makes the pulse. The number of AF beats that make it to the bottom determines the pulse.

The role of the AV node in AF

The AV node is the normal pathway that lets heartbeats, from the top part of the heart, make it to the bottom. When the rhythm is normal, the AV node lets every beat make it to the bottom, resulting in the top and 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 clean oxygenated blood from the lungs back to the left atrium.

Types of AF

There are three main type of atrial fibrillation: paroxysmal; persistent; and permanent or chronic.

The first type is called paroxysmal because people with this type will go in and out of AF on their own. The episodes can last seconds, minutes, hours or days. They rarely require a cardioversion – which is the restoration of a normal heartbeat through electrical shock or use of medication – since they will go back into normal rhythm on their own.

The second type of AF is called persistent. People with this type of AF may be out of rhythm for long periods, even weeks or months. They require medications or cardioversion to get them back in rhythm, since they will not go back on their own. Medications or cardioversion are effective in getting them back in rhythm.

The third type of AF is called permanent or chronic since no matter what medications or how many cardioversions are tried, the person cannot be kept in normal rhythm.

Where AF usually originates

Until recently, doctors assumed that 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 pulmonary vein to the left atrium and causes AF.