Medicine: cardiac arhythmia

When the heartbeats become irregular

Heartbeats accelaration

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). It can be very dangerous depending on how hard the heart is working and the activity.  

ECG showing sinus tachycardia with a rate of about 100 beats per minute.

Slow heart rate

Bradycardia (Greek βραδυκαρδία, bradykardía, "heart slowness"), as applied to adult medicine, is defined as a resting heart rate of under 60 beats per minute, though it is seldom symptomatic until the rate drops below 50 beat/min. It may cause cardiac arrest in some patients, because those with bradycardia may not be pumping enough oxygen to their heart. It sometimes results in fainting, shortness of breath, and if severe enough, death.

Sinus bradycardia seen in lead II with a heart rate of about 50.