The human heart has two upper chambers or "atria"; The right atria receives venous non-oxygenated blood from the body and via the right ventricle pumps the blood to the lungs. The left atria receives the freshly oxygenated blood from the lungs and via the left ventricle pumps the blood out to the body. The left atria receives oxygenated blood from the lungs via blood vessels called the pulmonary veins. Atrial fibrillation almost always requires participation of the left atria in order for the arrhythmia to occur.
In order for atrial fibrillation to occur, two mandatory things are required. Something in the heart has to start the arrhythmia up or "initiate" or "induce" the episode. Then, once the heart is in atrial fibrillation, something else in the heart has to keep the heart in fibrillation. The structures in the heart which keep the heart fibrillating once the arrhythmia is started by an initiator are called "propagators" for atrial fibrillation. The left atria, and in particular the regions in and around the pulmonary veins.
A CT scan of the human left atria with four pulmonary veins attached
Atrial fibrillation can be considered analogous to a variety of commonly observed phenomena. For example, in order for a lawn mower to run, you absolutely need two things; a fueled up and turned on engine capable of running for more then just a few turns (i.e. a propagator) and someone to pull the started cord to get the process going (i.e. an initiator). Without both of the above, you won't be cutting your lawn. An avalanche of snow is another example in nature in which both an initiator (i.e. an unfortunate skier) and a propagator (i.e. a mountain slope filled with snow) are both needed to cause a problem.
A simplified mode for sustained atrial fibrillation
In essentially every normal person, atrial fibrillation can be started by rapidly pacing the heart. In people who don't have clinical atrial fibrillation however, after a short period of time (seconds to minutes) atrial fibrillation generally terminates on its own. Thus normal people lack propagators for atrial fibrillation and even when started, the arrhythmia cannot continue for long and eventually stops on its own.
In patients with atrial fibrillation however, once fibrillation is started by an initiating factor, the arrhythmia tends to sustain; in some patients for many minutes, in others for hours or days or even longer. Patients with atrial fibrillation therefore have both the initiators and the propagators for their arrhythmia. For a more detailed discussion on initiators and propagators for atrial fibrillation, click on the links above.
The left atria and in particular the regions in and around the pulmonary veins are important sources for both the initiators and the propagators of atrial fibrillation. The pulmonary veins are considered a vital part of the arrhythmia substrate needed to allow atrial fibrillation to start and, once started, to continue. Essentially all of the surgical and catheter based curative approaches for atrial fibrillation involve delaying or eliminating electrical conduction from the pulmonary veins and regions immediately adjacent to these structures in the left atria.
What causes atrial fibrillation to start and propagate in the regions around the pulmonary veins? Although the answer to this question is not completely known, what is known for certain is that factors that can cause the left atria (and thus the attached pulmonary veins) to stretch will cause pulmonary vein initiators and propagators to form. Any condition elevating left atrial pressure can thus be considered a potential cause for atrial fibrillation. A history of high blood pressure is very common in patients with atrial fibrillation. Regurgitation or stenosis of the heart valve is also commonly seen in patients with atrial fibrillation. All of these conditions cause elevated left atrial pressures and left atrial stretch. Finally, getting older is also commonly associated with atrial fibrillation. We all know that parts of the body start to feel the effects of gravity with aging and things we can see will start to sag and stretch (did you know that the tip of you nose slowing sags and droops down over decades or life presumably due to the effects of gravity). In the heart, gravity is not the issue but since birth the structures of the heart are under continuous pressure from the approximate 100,000 daily heart beats. Over many years this pressure can cause the heart to stretch, thus potentially explaining the exponentially increased incidence of atrial fibrillation in older patients.