Bradycardia
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 originates in 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 node or "A-V node", (located between the atria) and through the bundle of His (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 less than 50 beats per minute) may be "normal" in athletes and in other people who lead a physically active lifestyle. In these individuals, 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 may be a form of cardiac arrhythmia (a heart-generated abnormality of heart rate or rhythm). This cardiac arrhythmia may be due to a dysfunction of the sinus node, or it may be related to some disturbance in the passage of heartbeat signals through the A-V node and bundle of His. Also, bradycardia may sometimes occur as a side effect of medications, including propranolol, atenolol, metoprolol, sotalol, verapamil and diltiazem. In addition, bradycardia may be seen in patients suffering from certain underlying medical illnesses that are not specifically related to the heart, such as:
- hypothyroidism (an abnormally low level of thyroid hormones)
- severe liver disease
- hypothermia (an abnormally low body temperature)
- typhoid fever
- brucellosis (an infectious disease marked by fever, sweating and weakness transmitted to humans by direct contact with diseased animals or through ingestion of infested meat, milk or cheese).
Although bradycardia is a symptom in itself, it may occur with associated symptoms, such as dizziness, weakness, lack of energy or fainting spells.
If bradycardia is due to an underlying medical illness, there will be additional symptoms that are specific to that medical illness. For example, patients whose bradycardia is due to severe hypothyroidism may also 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 will also 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 pulses. 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 will also 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. However, because some forms of bradycardia come and go, a one-time office ECG may be normal. If this is the case, ambulatory electrocardiography may be required. During ambulatory electrocardiography, the patient wears a portable ECG machine (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 ECG 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 may also order blood tests for cholesterol and certain liver enzymes, which are often 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 will usually subside as soon as the drug that triggered the bradycardia is metabolized or excreted in the urine. Bradycardia due to hypothyroidism will resolve promptly after treatment with replacement doses of thyroid hormones. Certain forms of bradycardia due to cardiac arrhythmias may be cured immediately with a permanent pacemaker (see below).
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 slow heart beat. The following are conditions that produce bradycardia that do require treatment:
- Cardiac arrhythmias due to sinus node dysfunction - In patients 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 due to A-V node problems - In patients with A-V node problems, the passage of heartbeat signals may be blocked to different degrees (See: Cardiac Arrhythmias). Some patients with less severe degrees of A-V block ("heart block") require no treatment. Others may simply be monitored with frequent ECGs, especially if they are asymptomatic and have a heart rate that is adequate for their daily activities. Patients with the most severe form of A-V block (called "third degree" A-V block) may 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, liothyronine or liotrix) as well as thyroid extract.
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 persons with bradycardia, the prognosis depends on the underlying disorder. For example, the prognosis is usually good in patients with hypothyroidism, since treatment with thyroid hormones can promptly 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 strain on the heart.) At one time, the prognosis for individuals 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 patients with cardiac arrhythmias.
For more information about bradycardia, you can contact:
American Heart Association (AHA)
7272 Greenville Ave.
Dallas, TX 75231
Toll-free (800) 242-8721
Fax: (214) 706-2139
http://www.americanheart.org/
National Heart, Lung and Blood Institute (NHLBI)
6701 Rockledge Dr.
P.O. Box 30105
Bethesda, MD 20824-0105
Phone: (301) 592-8573
E-Mail: NHLBIinfo@rover.nhlbi.nih.gov
http://www.nhlbi.nih.gov/
American College of Cardiology
9111 Old Georgetown Rd.
Bethesda, MD 20814
Phone: (301) 879-5400
http://www.acc.org/
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