Bradycardia is an abnormally slow heart rate of less than 60 beats per minute. A normal heartbeat is between 60 and 100 beats per minute.
Here's what happens during a normal heartbeat: The electrical signal that starts a heartbeat comes from the heart's sinus node, the natural pacemaker located in the upper portion of the right atrium. From the sinus node, the heartbeat signal travels to the atrioventricular (A-V) node, located between the atria, and then through the bundle of His (pronounced "hiss") -- a series of modified heart-muscle fibers located between the ventricles -- to the muscles of the ventricles. This triggers a contraction of the ventricles and produces a heartbeat.
Bradycardia, even as low as 50 beats per minute, can be normal in athletes and other people who are physically active. In these people, regular exercise improves the heart's ability to pump blood efficiently, so fewer heart contractions are required to supply the body's needs.
In other cases, bradycardia can be a form of cardiac arrhythmia, a heart-rate abnormality. Cardiac arrhythmia can be caused by a problem in the sinus node, or it can be related to some disturbance in the passage of heartbeat signals through the A-V node and bundle of His. Bradycardia can occur with toxic levels of certain drugs, such as digoxin (Lanoxin) and narcotics. Also, bradycardia sometimes is a side effect of certain medications, including propranolol (Inderal), atenolol (Tenormin), metoprolol (Toprol-XL), sotalol (Betapace), verapamil (Calan, Isoptin, Verelan) and diltiazem (Cardizem, Dilacor-XR). Bradycardia also occurs in some people who have certain medical illnesses not related to the heart, such as:
An abnormally low level of thyroid hormones (hypothyroidism)
An abnormally low body temperature (hypothermia)
A very high blood potassium level