Atrial Fibrillation
Complete electrical anarchy. Not one loop, not one focus, but dozens of tiny wavelets crashing, splitting, and colliding in a chaotic sea.
To understand atrial fibrillation (AFib), you must forget everything you know about organized circuits and singular pacemakers. AFib is an electrical riot.
It requires two components to exist: a spark to start the fire, and dry kindling to keep it burning. The sparks are the triggers, and the kindling is the substrate.
The Sparks: Pulmonary Veins
The most common triggers for AFib reside in the pulmonary veins. These four vessels carry oxygenated blood from the lungs into the left atrium.
During embryonic development, sleeves of atrial muscle extend outward into these veins. These myocardial sleeves are electrically abnormal. Their resting membrane potentials are less negative than healthy atrial tissue, hovering closer to the threshold of depolarization.
Because of this instability, cells inside the pulmonary veins frequently fire off rapid, spontaneous impulses (triggered beats). When these premature sparks exit the vein and crash into the left atrium, they can ignite the riot.
The Kindling: A Fragmented Atrium
But a spark alone does not guarantee a fire. If a premature beat from a pulmonary vein hits a perfectly healthy, uniform left atrium, it will simply sweep across the tissue and die out.
To sustain fibrillation, the sparks need an abnormal architecture—a substrate. Over time, factors like hypertension, aging, structural heart disease, and sleep apnea stretch the atrial walls. This mechanical stretch leads to microscopic scarring (fibrosis).
Recall from Volume I how gap junctions neatly couple healthy cells side-by-side. Fibrosis destroys this elegant coupling. Collagen weaves between the muscle fibers, turning a smooth electrical highway into a maze of roadblocks.
When a spark hits this fibrotic maze, the wavefront breaks apart. It splits, turns, and shatters into dozens of independent mini-wavelets.
The Multiple Wavelet Theory
Once shattered, how do these wavelets survive without extinguishing each other? They survive because the electrical properties of the fibrillating atrium change.
The refractory period becomes very short, and conduction velocity slows down. Together, these two factors create a tiny electrical "wavelength" (Wavelength = Conduction Velocity × Refractory Period).
A smaller wavelength means a single wavelet occupies very little space. Instead of one large wavefront sweeping across the chamber, the atrium can now fit dozens of miniature wavelets simultaneously. They drift chaotically—colliding, dividing, and spinning around tiny islands of scar tissue. The atrium no longer contracts; it quivers.
The Sieve: Protecting the Ventricles
The left atrium is shivering at an astonishing 400 to 600 impulses per minute. If every one of these wavelets traveled down into the ventricles, the heart would immediately degenerate into ventricular fibrillation, a fatal rhythm.
But the AV node acts as a brilliant biological filter. When hundreds of chaotic wavelets bombard the top of the AV node, it simply cannot process them all. Its long refractory period blocks the vast majority of these impulses.
Some wavelets enter the node but die halfway down. This is called concealed conduction (a concept we first met in Volume II). Even though these aborted wavelets don't reach the ventricles, they succeed in resetting the AV node's refractory timer, further delaying the next successful beat.
Ultimately, only a completely random, unpredictable few impulses manage to weave their way completely through the AV node to the His bundle. This creates the classic ECG signature of AFib: an irregularly irregular ventricular response with absolutely no discernible P waves.
How do we cure a rhythm that has no single organized circuit? We don't chase the chaotic wavelets across the atrium. We focus on the source of the sparks.
In the EP lab, we perform Pulmonary Vein Isolation (PVI). We construct a continuous, circular line of ablation lesions completely surrounding the ostia (openings) of the pulmonary veins.
By creating this ring of scar tissue, we electrically quarantine the veins. The triggered sparks inside the sleeves may continue to fire indefinitely, but they hit the scar boundary and cannot exit to ignite the rest of the atrium. The fire goes out.
Key Takeaways
- The Sparks: AFib is most often triggered by rapid, spontaneous impulses firing from myocardial sleeves inside the pulmonary veins.
- The Kindling: A diseased, fibrotic atrium provides the necessary substrate by breaking a single wavefront into dozens of tiny wavelets.
- Wavelength: Short refractory periods and slow conduction shrink the electrical wavelength, allowing multiple wavelets to coexist simultaneously.
- The AV Node's Role: The AV node is bombarded 400-600 times a minute but blocks most impulses, including many via concealed conduction, leading to an irregularly irregular pulse.
- The Cure (PVI): Ablation works by electrically disconnecting the pulmonary veins from the rest of the atrium, trapping the triggers behind a wall of scar.