What Is It?
Bradycardia is an abnormally low heart rate of less than 60 beats per minute.
In normal adults, the heart beats regularly at a rate of 60 to 100 times per minute, and the pulse (felt at the wrist, neck, or elsewhere) matches the contractions of the heart’s ventricles (the heart’s two powerful lower chambers). The signal for 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 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 lead physically active lifestyles. In these people, regular exercise maximizes the ability of the heart 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 dysfunction of 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. Also, bradycardia sometimes occurs as 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). In addition, bradycardia may be seen in people suffering from certain underlying medical illnesses that are not specifically related to the heart, such as:
- An abnormally low level of thyroid hormones (hypothyroidism)
- Severe liver disease
- An abnormally low body temperature (hypothermia)
- Typhoid fever
- Brucellosis (an infectious disease marked by fever, sweating and weakness that is transmitted to humans by direct contact with diseased animals or through ingestion of contaminated meat, milk or cheese)
Although bradycardia is a symptom in itself, it can occur with other symptoms, such as dizziness, weakness, lack of energy or fainting spells.
If bradycardia is the result of an underlying medical illness, there will be additional symptoms that are specific to that medical illness. For example, people whose bradycardia is due to severe hypothyroidism also can have constipation, muscle cramps, weight gain (often despite poor appetite), very dry skin, hair that is thin and dry, an abnormal sensitivity to cold temperatures and other symptoms related to low levels of thyroid hormones.
Your doctor will ask about your family history of heart disease, cardiac arrhythmias and fainting spells. He or she also will review your current symptoms and your personal medical history, including your use of medications that may cause bradycardia.
During the physical examination, your doctor will check your heart rate and rhythm, together with your pulse. This is because certain cardiac arrhythmias cause a mismatch of the pulse, which reflects the activity of the ventricles, and the heart sounds. Your doctor also will check for physical signs of thyroid abnormalities (enlarged thyroid gland, very dry skin, hair that is thin and dry).
To further evaluate your bradycardia, your doctor will order an electrocardiogram (EKG). However, because some forms of bradycardia come and go, a one-time office EKG may be normal. If this is the case, ambulatory electrocardiography may be required. During ambulatory electrocardiography, the patient wears a portable EKG machine called a Holter monitor, usually for 24 hours. If your symptoms are infrequent, you may wear a monitor for longer. You will be taught to press a button to record your EKG reading when your symptoms occur.
Depending on the results of your physical examination, other tests may be necessary to rule out medical illnesses that produce bradycardia. For example, if you have symptoms and physical signs of hypothyroidism, your doctor may order blood tests to measure levels of thyroid hormones and TSH, a pituitary gland hormone that stimulates the thyroid. Your doctor also may order blood tests for cholesterol and certain liver enzymes, which often are elevated in people with hypothyroidism.
The duration of bradycardia depends on its underlying cause. For example, normal bradycardia in a well-trained athlete will persist as long as the athlete maintains his or her accustomed level of exercise. When bradycardia occurs as a side effect of medication, it usually will subside as soon as the drug that triggered the bradycardia is used by the body or excreted in the urine. Bradycardia resulting from hypothyroidism will resolve promptly after treatment with replacement doses of thyroid hormones. Certain forms of bradycardia resulting from cardiac arrhythmias can be cured with a permanent pacemaker.
There are no general guidelines to prevent all forms of bradycardia. When bradycardia occurs as a side effect of medication, the problem can be prevented by either switching the drug or reducing its dose.
In most cases, bradycardia in healthy, well-trained athletes does not need to be treated. In fact, in most people, bradycardia does not require treatment unless patients have symptoms that are clearly due to acslow heartbeat. The following are conditions that produce bradycardia that requires treatment:
- Cardiac arrhythmias resulting from sinus-node dysfunction — In people with frequent, severe symptoms of sinus-node dysfunction, the main treatment is usually a permanent pacemaker, an implanted device that generates electrical impulses to regulate the heartbeat.
- Cardiac arrhythmias resulting from A-V node problems — In people with A-V node problems, the passage of heartbeat signals may be blocked to different degrees. Some patients with less severe degrees of A-V block require no treatment. Others may simply be monitored with frequent EKGs, especially if they have no symptoms and have a heart rate that is adequate for their daily activities. People with the most severe form of A-V block (called third-degree A-V block) can be treated with a special dual-chambered permanent pacemaker, a pacemaker with one electrode in the atrium and one in the ventricle.
- Hypothyroidism — Hypothyroidism is treated with replacement doses of thyroid hormones. Synthetic forms of these hormones are available — levothyroxine (Synthroid, Levothroid and others), liothyronine (Cytomel, Triostat) or liotrix (Thyrolar) — as well as thyroid extract.
When To Call A Professional
Call your doctor if your pulse is less than 60 beats per minute. This is especially important if you have been suffering from dizziness, weakness, lack of energy or fainting spells.
In well-trained athletes with normal bradycardia, the efficiency of the heart is often a reflection of overall good health, and the prognosis is excellent. In other people with bradycardia, the prognosis depends on the underlying disorder. For example, the prognosis is usually good in people with hypothyroidism, since treatment with thyroid hormones can relieve bradycardia and other symptoms related to low hormone levels. In some elderly patients, hormone dosages may need to be increased very slowly over several weeks to prevent straining the heart. At one time, the prognosis for people with third-degree A-V block was poor, with 50 percent of patients dying within one year of diagnosis. Now, however, the availability of permanent pacemakers has improved the prognosis for many people with cardiac arrhythmias.
Diseases and Conditions Center
All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.