Arrhythmias, also known as cardiac arrhythmias, are irregularities in the heartbeat, including when it is too fast or too slow.[2] A resting heart rate that is too fast ¨C above 100 beats per minute in adults ¨C is called tachycardia, and a resting heart rate that is too slow ¨C below 60 beats per minute ¨C is called bradycardia.[2] Some types of arrhythmias have no symptoms.[1] Symptoms, when present, may include palpitations or feeling a pause between heartbeats.[1] In more serious cases, there may be lightheadedness, passing out, shortness of breath, chest pain, or decreased level of consciousness.[1] While most cases of arrhythmia are not serious, some predispose a person to complications such as stroke or heart failure.[2][3] Others may result in sudden death.[3]

Reference
Wiki: Arrhythmia