The heart has four chambers: two atria at the top that collect blood when the heart relaxes, and two ventricles on the bottom, which pump it out to the body. Arrhythmias that start in the atria aren't usually lethal, although they can still cause trouble. One-third of all hospitalizations for arrhythmia are due to atrial fibrillation, a condition that makes the atria quiver instead of pump. This can cause blood pooling and clotting, and slightly increase the risk of a stroke.
Arrhythmias that start in the ventricles are more dangerous. In ventricular fibrillation, if the heart isn't getting enough oxygenperhaps because a heart attack or heart failure has cut off the supplythe lower chambers can start quivering instead of beating regularly. Very little blood will get where it needs to go, and the heart can go into cardiac arrest. Ventricular fibrillation kills 300,000 Americans each year.