The average heart rate is 60 to 100 beats per minute, or BPM. But no one has a heart rate that stays at the same rate all the time. Differences or variations in heart rate, called arrhythmias, are common, and most commonly occur as a consequence of heart disease, the Merck Manual reports. Arrhythmias can also be a consequence of stress, increased activity or taking certain medications. Arrhythmias can be benign or serious.
Bradycardias
A heart rate less than 60 BPM is bradycardia. Bradycardia is commonly seen in athletes; people taking drugs called beta blockers, used to treat high blood pressure; and in diseases like hypothyroidism. Problems within the heart's electrical conduction system can also cause bradycardia. The sinus node, located in the upper part of the right atrium, normally sets the pace of the heartbeat; it sends out electrical impulses that pass across the top chambers of the heart, the atria. The signal then passes through the atrioventricular, or AV node, located between the atria, and the lower chambers, the ventricles.
Problems within the sinus node can cause bradycardia. The sinus node may fire slowly, or the impulses can be blocked or slowed in the AV node. If the sinus node doesn't fire at all, the ventricles may set their own signal, which is usually slower than normal, and results in bradycardia. Bradycardia can also be caused by illnesses or situations which stimulate the vagus nerve; these include diarrhea, vomiting, pain, hunger and fatigue. Vagus nerve stimulation is called a vagal response, and can be serious enough that a person can pass out from it, or, in rare cases, die, the Merck Manual states.
Tachycardia
Tachycardia, or heart rate faster than 100 BPM, can be normal is some situations; any situation that elicits a "fight or flight" adrenaline response causes the heartbeat to increase. This is temporary, though, and slows as soon as the excitement or stress subsides. Medications, alcohol and diseases such as hyperthyroidism can all cause tachycardia.
Some forms of tachycardia related to cardiac abnormalities can be quite serious. Rapid atrial contraction such as atrial flutter and atrial fibrillation are usually less serious than ventricular tachycardia. The most serious tachycardia is ventricular fibrillation, where the ventricles quiver because the normal electrical impulses have been disrupted. Trauma such as being struck by lightning or heart attack can cause ventricular fibrillation, the Mayo Clinic states.
Irregular Heartbeat
Many people experience irregular heartbeats called premature ventricular contractions, or PVCs. PVCs occur when the ventricles fire prematurely; people who experience PVCs may describe them as a "thud" in their chest. Medications, drugs, alcohol, stress and anxiety can all increase the rate of PVCs; heart damage and infections can also increase the number of PVCs, the Mayo Clinic reports. PVCs alternating with a regular beat is called bigeminy; trigeminy is the term for a PVC that occurs every third beat. PVCs aren't harmful unless they occur so frequently that they disrupt the normal blood flow of the heart.
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