Tachycardia:

Tachycardia comes from the Greek words tachys (rapid or accelerated) and kardia (of the heart). Tachycardia typically refers to a heart rate that exceeds the normal range for a resting heartrate (heartrate in an inactive or sleeping individual).

In humans, this rate is usually based upon age, sometimes it can be very dangerous depending on how hard the heart is working and the activity:
1-2 days: >159 beats per minute (bpm)

3-6 days: >166 bpm

1-3 weeks: >182 bpm

1-2 months: >179 bpm

3-5 months: >186 bpm

6-11 months: >169 bpm

1-2 years: >151 bpm

3-4 years: >137 bpm

5-7 years: >133 bpm

8-11 years: >130 bpm

12-15 years: >119 bpm >15 years - adult: >100 bpm

When the heart beats rapidly, the heart pumps less efficiently and provides less blood flow to the rest of the body, including the heart itself. The increased heart rate also leads to increased work and oxygen demand for the heart (myocardium), which can cause a heart attack (myocardial infarction) if it persists. This occurs because the decreased flow of necessary oxygen to the heart causes myocardial cells to begin to die off. Acutely, this leads to angina; and chronically to ischemic heart disease.