Atrial fibrillation is the most common type of cardiac arrhythmia. 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 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.