Before You Can Understand What Atrial Fibrillation Is

To understand this type of arrhythmia (abnormal heart rhythm) known as atrial fibrillation, you must first know about your heart and its electrical activity. The heart is a pump which sends oxygen-rich blood throughout your body. There are four chambers to that pump: each of the two upper chambers is called an atrium (atria is plural), and each lower chamber is called a ventricle.

Each chamber of the heart has its own function. The right atrium receives the "used up" blood supply from all parts of the body, and pumps it into the right ventricle. The right ventricle then pumps this blood supply into the lungs, where the blood receives oxygen. The now oxygen-rich supply is returned into the left atrium and pumped into the left ventricle. The left ventricle then pumps this new blood supply to all the parts of the body. And all this is just one heart beat!

Your heart needs an electrical system to trigger that pump and to keep the chambers contracting in a coordinated fashion. To do that, the heart has its own built-in pacemaker called the SA node (sinus node or sinoatrial node) located on the wall of the right atrium. It sends out impulses at about 60 to 100 beats per minute, to both the right and left upper chambers (atria) of the heart, telling them it's time to beat. The SA node sets the pace to meet your body's demands: During rest, the speed of the impulses originating from the SA node decreases, resulting in a slowing of the heart beat, and during exercise or excitement, the speed of impulses from the SA node increases, accelerating heart beating. The impulse then travels through the AV node (atrio-ventricular node), the "gatekeeper" of impulses, to the lower chambers of the heart (ventricles) via the His bundle, telling them it's their turn to beat. The sequence of this electrical event causes the atria to beat first and a split second later, the ventricles to beat. This is normal sinus rhythm.

During atrial fibrillation, the normal sinus rhythm is disrupted by a very rapid, uncontrolled and disorganized rhythm. Think of it as numerous little tornadoes wandering about, upsetting the normal flow of electrical activity. Instead of initiation of conduction from the SA node to the AV node in the normal sinus rhythm, the atrium's activity proceeds as multiple wavelets. As a result, the atria beat so rapidly (as fast as 350 beats per minute) they quiver (fibrillate). There is no effective contraction of the atrial muscles in this situation. The rapid atrial rate effectively overrides any activity from the SA node. In other words, these rapid wavelets take control of the atrium. The rapid, erratic electrical impulses reach the AV node, and some of them pass to the ventricles. This causes the ventricles to pump in a rapid and/or irregular pattern in response. If you try to feel your pulse during atrial fibrillation, you can tell that your heart is beating irregularly.

Recently, we have begun to recognize that there is more than one type of atrial fibrillation. For example, some cases of atrial fibrillation have been demonstrated to have a single focus, similar to a pacemaker. This abnormal focus is most commonly located somewhere near the wall of the left atrium often in the pulmonary veins. This focus may fire so rapidly that the atria cannot follow such a rapid rate in an orderly fashion. Conduction becomes fragmented and irregular, creating a pattern of atrial fibrillation. This has been called "focal atrial fibrillation" because there is a single focus that drives or triggers the arrhythmia. The other common type has been called "multi-wavelet" atrial fibrillation because there are multiple wandering eddies circulating in the atria. Both types may exist in a given person.

For example, a focal trigger from the pulmonary vein could initiate multiple wavelets. Think of this like dropping pebbles in a still pond. If the pebbles are dropped fast enough, the still surface of the pond becomes quite turbulent. The pebbles are like the focal triggers that start AF, and the turbulence is like the multiple wavelets that perpetuate AF.

Your doctors may use various terms to specify the type of atrial fibrillation you have, depending on how long you have had it, what other types of heart disease you have, and the pattern of your atrial fibrillation. For example, your atrial fibrillation is "paroxysmal" if you have recurrent episodes that stop on their own in minutes, hours or even days. This pattern may be erratic. You may have episodes only a few times a year, or you may have a number of episodes a day. The starting and stopping of this arrhythmia is usually very unpredictable and may cause patients significant symptoms and anxiety. Patients in this group may progress to the chronic form of atrial fibrillation, which is why we at the UCSF Atrial Arrhythmia Center favor an aggressive approach to treatment before your atrial fibrillation follows this course.

Atrial fibrillation is called "persistent" if you are in it for longer periods (days or weeks) but drugs or electrical cardioversion can restore your normal rhythm.

Your atrial fibrillation is "chronic" or "established" if the arrhythmia is sustained over a long period of time. Some cases of chronic atrial fibrillation can still be amendable to therapeutic intervention (treatment responsive) or resistant to all attempts at conversion to sinus rhythm (permanent).

Some cases of atrial fibrillation occur in the absence of any cardiac risk factors. This is termed "lone atrial fibrillation". Most patients with this form of atrial fibrillation tend to be somewhat younger. The arrhythmia in these individuals is usually paroxysmal, but can become chronic in some of these cases if not treated.

Signs and symptoms vary from person to person and depend on several factors, including the heart rate of the ventricles, your overall heart function, and any other on-going medical problems. Some people have no symptoms if the overall heart rate remains relatively slow. For others, the discomfort can be quite obvious. The most common symptom is palpitations (the fluttering sensation of your heart). With atrial fibrillation, the amount of blood your heart delivers per each heartbeat could be decreased by as much as 20 to 30% compared to one in a normal sinus rhythm. The decrease may be even greater in those with underlying heart disease. The result is less oxygen in your circulation and more blood being "backed up" in your heart. This will often create the feeling of anxiety, weakness, fatigue, dizziness, chest pain/pressure, shortness of breath, and decreased tolerance for exercise or even normal activity of daily living, significantly reducing the quality of your life.