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Bradycardia is the medical term for an abnormally slow heart rate, generally a resting heart rate lower than 60 bpm (for infants, 100 bpm), although only instances where the heart slows to under 50 bpm are considered a serious symptom. Moreover, a slow heart rate alone is generally not symptomatic of any serious disease, particularly in individuals who commonly engage in aerobic exercise such as long distance running or cycling. However, when combined with fatigue, weakness, dizziness, lightheadedness, fainting, chest discomfort, palpitations or shortness of breath, it is usually the sign of a serious underlying disorder.

Relative bradycardia describes any heart rate above 60 bpm that is unusually low for the situation. For example, a heart rate of 70 bpm is appropriate for someone at rest, but not someone engaging in exercise.

Bradycardia is considered to be an arythmia and can have several causes, which are generally split between those related to the heart itself and those related to all other causes. Use of sedative drugs is a common cause. However, it can also indicate a metabolic disorder, an electrolyte imbalance, and even autoimmune diseases.

When a patient presents with bradycardia and the cause is unknown, the physician's first steps are usually to take a comprehensive metabolic panel and test blood gases. Atropine is a common first treatment.