The heart is a complex muscle with two chambers (left and right atrium) above two larger chambers (left and right ventricle) that are the prime pumpers. The beating action is regulated by the heart's natural pacemaker -- the sinus node, a small piece of tissue at the top of the right atrium that sends out electrical pulses about once a second. Similar to a bucket brigade, the pulses stream into negatively charged muscle fibers lining the heart, creating a sudden electrical change beginning in the right atrium and extending into the ventricles. When the electrical pulses reach the ventricles, they contract, forcing the blood out. Thus, although the heartbeat seems like a single muscle contraction, it actually more resembles the wave phenomenon at a baseball game.
Research has shown that arrhythmia is triggered by an electrical impulse that arrives at the wrong time. The erratic beating starts when the mistimed signal travels in a continuous loop throughout the heart's muscle fibers -- setting off what is called reentrant arrhythmia. If the reentrant arrhythmia occurs in the ventricles, the result is sudden cardiac death, which claims 250,000 U.S. lives each year. When it occurs in atrial tissue -- known as atrial fibrillation, the condition can be treated with medication or electrical pulses.
Researchers have reproduced these results with dog hearts, but they still don't know why a small electric jolt throws a large muscle, like the heart, into a chaotic state. Instead, nothing should happen, Chay says, similar to when a person ignores a tap on the shoulder. And the key involves discovering why a mistimed pulse causes problems and why a properly timed pulse restores the normal heartbeat.
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