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 un-synchronized,

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.