In a healthy heart, the sinoatrial node⏤or SA node⏤initiates all electrical impulses in the atria.

In atrial fibrillation, electrical impulses are initiated randomly from many other sites called " ectopic sites " in and around the atria, commonly near the roots of pulmonary veins.

These unsynchronized chaotic electrical signals cause the atria to quiver⏤or fibrillate⏤rather than contract.

Although the atrial rate during atrial fibrillation can be extremely high, most of the electrical impulses do not pass through the atrioventricular, the AV node, to the ventricles.

This is due to refractory properties of the cells of the AV node.

Those that do come through are irregular.

Ventricular rate, or heart rate, is therefore irregular and can range from slow⏤less than 60⏤to rapid⏤more than 100⏤beats per minute.

On an ECG, atrial fibrillation is characterized by absence of P-waves and irregular narrow QRS complexes.

Reminder: P-wave represents electrical activity of the SA node that is now obscured by activities of multiple ectopic sites.

The baseline may appear undulating or totally flat depending on the number of ectopic sites in the atria.

In general, larger number of ectopic sites results in flatter baseline.

As the atria do not function properly, the heart puts out less blood, and heart failure may occur.

The most common complication of atrial fibrillation, however, is the formation of blood clots in the atria.

As the atria do not empty completely into the ventricles, the blood may stagnate inside the atria and blood clots may form.

These clots may then pass into the bloodstream, get stuck in small arteries, and block them.

When a blood clot blocks an artery in the brain, a stroke may result.