Sinus Bradycardia

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Sinus bradycardia is the most common type of bradycardia, or slow heart rate.

In sinus bradycardia, the sinus node - the structure in the right atrium that generates heart’s electrical impulses - is firing at relatively reduced rate. Thus, the heart rate becomes “slow.”

In most circumstances, sinus bradycardia is entirely normal.

In other cases, however, sinus bradycardia can be caused by a diseased or damaged sinus node and may require treatment with a pacemaker.

Normal vs. Abnormal Sinus Bradycardia

At rest, the electrical impulses from the sinus node typically occur between 50-60 and 100 times per minute. This is called "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 50-60 times per minute is called "sinus bradycardia."

Sinus bradycardia is often completely normal. The healthy body is very good at regulating the heart rate to be whatever it needs to be to support the body’s functions. And often, this normal heart rate is within the range of what is “officially” sinus bradycardia. So 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) for many people to have heart rates in this range while sleeping.

However, if the heart rate is too slow to meet 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.

What Causes Abnormal Sinus Bradycardia?

Transient, abnormally slow sinus bradycardia is most often 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.

The most common cause of persistent sinus bradycardia 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."

    Other cardiac conditions can also produce persistent sinus bradycardia. These conditions include coronary artery disease, pericarditis, myocarditis, and trauma due to injury or to cardiac surgery. Sinus bradycardia is also associated with more generalized medical conditions, including amyloidosis, hypothyroidism, dysautonomia, and various infections. Drugs are another cause of sinus bradycardia. These drugs include beta blockers, calcium channel blockers, and antiarrhythmic drugs.

    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, either at rest or during exertion.

    How Is Sinus Bradycardia 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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