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Sinus Bradycardia

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Updated May 16, 2014

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

Sinus bradycardia is one of the two major causes of bradycardia, or slow heart rate. In sinus bradycardia, the slowing occurs because the heart's sinus node has reduced the rate at which it generates the electrical impulses that control the heart rhythm.

In many circumstances, sinus bradycardia is entirely normal. In other circumstances, sinus bradycardia causes the heart rate to become inappropriately low, which can lead to symptoms and may require a pacemaker.

At rest, the rate at which the sinus node produces electrical impulses (referred to as the "sinus rate") is between 60 and 100 times per minute. A sinus rate within this range is referred to as "normal sinus rhythm." When the sinus rate is faster than 100 times per minute, it is referred to as "sinus tachycardia." And a sinus rate slower than 60 times per minute is called "sinus bradycardia."

When the sinus node produces a heart rate that is too slow for the body's needs, symptoms can develop. Most typically, these symptoms are lightheadedness, fatigue, poor exercise tolerance, or syncope (transient loss of consciousness). Usually, sinus bradycardia is not considered to be a serious problem unless it is producing one or more of these symptoms.

When Is Sinus Bradycardia Considered Normal?

Quite often, sinus bradycardia is completely normal. Healthy young people, and even older people when they are in good physical condition, will frequently have resting heart rates in the 40s or 50s. It is also common (and normal) to have heart rates in this range while sleeping.

Transient sinus bradycardia is commonly caused by increased tone in the vagus nerves, which often occurs with nausea or vomiting, or in response to pain or sudden stress. This form of sinus bradycardia is a normal response to elevated vagal tone, and disappears as soon as the cause of the increased vagal tone has subsided.

What Causes Abnormal Sinus Bradycardia?

Several medical conditions can cause abnormal sinus bradycardia -- that is, sinus rates that are inappropriately reduced. The most common is intrinsic sinus node disease -- disease within the sinus node itself. Usually, sinus node disease is caused by an age-related fibrosis of the sinus node; it is a disorder of aging. Intrinsic sinus node disease is most commonly seen in people who are 70 years old or older.

People who have intrinsic sinus node disease, especially when it is associated with symptoms, are often said to have "sick sinus syndrome."

Additional cardiac conditions that can cause sinus bradycardia include coronary artery disease, pericarditis, and myocarditis. More generalized medical conditions can also produce sinus bradycardia, including amyloidosis, hypothyroidism, dysautonomia, and various infections. Sinus bradycardia can also result from several kinds of drugs, including beta blockers, calcium channel blockers, and antiarrhythmic drugs. Various forms of cardiac trauma, including trauma related to heart surgery, can produce sinus bradycardia as well.

When Should Sinus Bradycardia Be Treated?

Sinus bradycardia should be treated whenever it is producing a heart rate that is slow enough to cause fatigue, lightheadedness, or syncope.

All too often, doctors fail to recognize that an older patient who may have only mild sinus bradycardia while they are at rest might still need to be treated. This is because in some people, the chief problem caused by sinus node disease is not resting bradycardia, but rather, an inability to increase the heart rate appropriately during activity -- which can cause extreme fatigue during exertion. This condition is called "chronotropic incompetence."

Doctors can make the diagnosis of chronotropic incompetence simply by observing the patient's heart rate during exercise -- for instance, during a stress test. Because chronotropic incompetence is a fairly common condition in the elderly and is readily treatable (with a rate-responsive pacemaker), it is important for older people who are experiencing fatigue with even mild exertion to make sure their doctors do the proper evaluation.

How Is Sinus Bradycardia Treated?

If sinus bradycardia is producing symptoms, it should be treated.

The first step in treating sinus bradycardia is to determine whether it has a reversible underlying cause. If the sinus bradycardia is due to increased vagal tone, drug therapy, an infectious disease, pericarditis, myocarditis or hypothyroidism, then addressing the underlying problem is usually the only treatment that is necessary.

But if the sinus bradycardia is caused by intrinsic sinus node disease, or by some other medical problem that cannot be easily reversed, then treatment will require the placement of a pacemaker.

So if you have sinus bradycardia, you will need to work with your doctor to determine why you have it, and whether it is causing symptoms. While most people with sinus bradycardia do not require a pacemaker, if you do have symptoms from sinus bradycardia, you can be confident that a pacemaker will completely eliminate those symptoms.

Sources:

Tresch DD, Fleg JL. Unexplained sinus bradycardia: clinical significance and long-term prognosis in apparently healthy persons older than 40 years. Am J Cardiol 1986; 58:1009.

Epstein AE, DiMarco JP, Ellenbogen KA, et al. ACC/AHA/HRS 2008 Guidelines for Device-Based Therapy of Cardiac Rhythm Abnormalities: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Writing Committee to Revise the ACC/AHA/NASPE 2002 Guideline Update for Implantation of Cardiac Pacemakers and Antiarrhythmia Devices): developed in collaboration with the American Association for Thoracic Surgery and Society of Thoracic Surgeons. Circulation 2008; 117:e350.

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