Tachycardia is a higher-than-normal pulse rate. Serious arrhythmias, like ventricular fibrillation, are life-threatening and can require a shock from a defibrillator or a pacemaker to maintain a regular rhythm. Some causes of tachycardia are less serious but still require attention, including dehydration, fever, stress, fear, exercise, overactive thyroid, and anemia.

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