According to the American Heart Association, up to 5 percent of people over age 65 have atrial fibrillation, which occurs when the two upper-chambers of the heart quiver rather than beat normally. Because blood isn't pumped fully out of the chambers, it can pool and form clots.

A rapid pulse, also known as tachycardia, can be caused by numerous factors, some benign, some serious. According to the American Heart Association, a resting heart rate of over 100 beats per minute is considered tachycardia, and tachycardia is a type of arrhythmia, or abnormal heart rate or rhythm.