Heart rate over 100 (as high as 240) beats per minute. When it is a normal response to exercise or stress, it is no danger to healthy people, but when it originates elsewhere, it is an arrhythmia. Symptoms include fatigue, faintness, shortness of breath, and feeling the heart thumping. It may subside within minutes or hours with no lasting ill effects, but in serious heart, lung, or circulatory disease it can precede atrial fibrillation or heart attack and demands immediate medical attention. Tachycardias can be treated by an electric shock to the heart, by antiarrhythmic drugs, and by pacemakers.

This entry comes from Encyclopædia Britannica Concise.
For the full entry on tachycardia, visit Britannica.com.

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