Sinus Bradycardia

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Sinus bradycardia is the most common type of bradycardia (slow heart rate).

In most circumstances, sinus bradycardia is entirely normal. In other cases, however, sinus bradycardia can be related to an underlying medical problem that requires therapy, or may be due to diseased or damaged sinus node, in which case it may require treatment with a pacemaker.

What Is Sinus Bradycardia?

It is the heart's electrical impulse that generates and coordinates the heart best.

This electrical impulse is generated in the sinus node, a tiny nest of cells located at the top of the right atrium. When the sinus node is producing these electrical impulses at a relatively reduced rate, the heart rate becomes slow, and sinus bradycardia occurs.

Normal vs. Abnormal Sinus Bradycardia

At rest, the electrical impulses generated by 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 most 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 doctors “officially” classify as 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 (that is, if the rate becomes so slow that not enough blood is being pumped by the heart), symptoms can develop.

Most typically, these symptoms are lightheadedness, fatigue, poor exercise tolerance, and/or 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?

When sinus bradycardia is producing symptoms, it is always considered to be abnormal. Abnormal sinus bradycardia can be either transient, or persistent.

Transient sinus bradycardia is most often caused by increased tone in the vagus nerve. Stimulation of the vagus nerve will slow down the sinus node, causing a slowing of the heart rate. Vagal nerve stimulation is often produced by various gastrointestinal problems (especially nausea or vomiting), or in response to pain or sudden emotional stress. Sinus bradycardia caused by vagus nerve stimulation is considered "physiological" (as opposed to pathological), because it is a normal response, and it disappears as soon as the elevated vagal tone subsides.

An abnormal sinus bradycardia that is persistent is most often caused by intrinsic sinus node disease — disease within the sinus node itself. Usually, sinus node disease is due to a type of fibrosis (scarring) within the sinus node, which is a common manifestation of aging.

Intrinsic sinus node disease is usually seen in people who are 70 years old or older.

In people with intrinsic sinus node disease, the heart rate is often inappropriately low not only at rest, but also during exertion. As a result, they may have poor exercise tolerance, or episodes of lightheadedness (or even syncope) when they exert themselves. If their condition is severe, they may even have symptoms while at rest. People with symptomatic sinus node disease are often said to have "sick sinus syndrome."

Persistent abnormal sinus bradycardia can also be caused by various cardiac and non-cardiac medical conditions.

Cardiac conditions associated with persistent sinus bradycardia include  coronary artery diseasepericarditismyocarditis, and trauma due to injury or to cardiac surgery.

Non-cardiac medical problems may also cause persistent or recurrent sinus bradycardia.

These include amyloidosishypothyroidismdysautonomia, and various types of infections (including Lyme disease, Chagas disease, and Rocky Mountain spotted fever). Brain disorders, especially those associated with increased intracranial pressure or stroke, can produce sinus bradycardia.

Sinus bradycardia may also be caused by hypoxia (low blood oxygen levels), as often happens with obstructive sleep apnea.

Drugs are another common cause of sinus bradycardia, especially beta blockerscalcium channel blockers, antiarrhythmic drugs, opioids, lithium, and various chemotherapy drugs.

When Should Sinus Bradycardia Be Treated?

Sinus bradycardia should be treated whenever it is not due to a transient or treatable condition, and 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 usually takes care of the slow heart rate.

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 may include the placement of a pacemaker.

A Word From Verywell

Sinus bradycardia is most often a normal phenomenon that does not require an extensive medical evaluation, or specific treatment.

But if you have sinus bradycardia that is producing symptoms, you will need to work with your doctor to determine why you have it. There may be an important underlying cause that needs to be identified and treated. If it turns out you have intrinsic sinus node disease that is producing troublesome symptoms, you can be confident that a pacemaker will completely eliminate those symptoms.

Sources:

Epstein AE, DiMarco JP, Ellenbogen KA, et al. 2012 ACCF/AHA/HRS Focused Update Incorporated into the ACCF/AHA/HRS 2008 Guidelines for Device-based Therapy of Cardiac Rhythm Abnormalities: a Report of the American College of Cardiology Foundation/American Heart Association Task Force on Practice Guidelines and the Heart Rhythm Society. J Am Coll Cardiol 2013; 61:e6.

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.

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