People with arrhythmia can have resting heart rates of 200 or higher.
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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.

Bradycardia——an unusually slow heartbeat of less than 60 bpm (beats per minute)——also starts in the atria. In most cases the heart's "natural pacemaker"—a cluster of cells in the upper right chamber that regulates the heart's electrical activity—simply isn't firing correctly. Fit people have low heart rates, too, because their hearts are especially strong and efficient. In other people, bradycardia can cause dizziness and fainting.

Arrhythmias that start in the ventricles are more dangerous. In ventricular fibrillation, if the heart isn't getting enough oxygen—perhaps because a heart attack or heart failure has cut off the supply—the 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.

Ventricular tachycardia occurs when the ventricles start pounding away at up to 200 bpm—a disruption that can cause dizziness and breathlessness. The normal resting heart rate for adults is between 60 bpm and 100 bpm. In many cases, a person with ventricular tachycardia will briefly lose consciousness and faint.