What Is Bradycardia?

When a Slowed Heart Rate Means Serious Trouble

Bradycardia is the medical term for a heart rate that is slower than is considered normal. In medical textbooks, bradycardia is usually defined as a resting heart rate that is below 60 beats per minute.

However, many healthy people have a resting heart rate between 50-60 beats per minute, particularly athletes or sleeping people. So having a low heart rate is not necessarily a bad thing, or even an abnormal thing.

On the other hand, bradycardia can be a significant problem if the heart rate becomes “too slow,” that is, if it is so slow that the heart is not able to pump enough blood to supply the body’s needs. This type of abnormal bradycardia is medically concerning and requires careful evaluation and treatment.

Bradycardia slow enough to cause clinical problems is often caused by sinus node dysfunction or heart block.

Normal vs. Abnormal Values

At rest, the sinus node typically generates electrical impulses at a rate of between 60 and 100 times per minute. So a resting heart rate within this range is called “normal sinus rhythm.”

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 (i.e.: between 50-60 beats per minute).

So healthy young people and even older people 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.

While this constitutes sinus bradycardia, it is a “physiologic” form of sinus bradycardia — which means the heart rate is appropriate to the body’s needs, and thus, the sinus bradycardia is normal.

Sinus bradycardia is considered a problem if the heart rate is too slow to meet the body’s needs. If the heart rate becomes so slow that not enough blood is being pumped by the heart, symptoms can develop. If the sinus bradycardia is producing symptoms, it is abnormal and needs to be treated.

Symptoms of Bradycardia
Verywell / JR Bee

Symptoms

If the heart rate is abnormally slow, several of the body’s organs may not function normally, and various symptoms may result. The symptoms of abnormal bradycardia tend to become worse with exertion (because the body’s needs become greater when you exert yourself), but symptoms may also be present during rest if bradycardia is severe.

Symptoms that can result from bradycardia include:

If bradycardia is associated with any of these symptoms, the cause of the bradycardia must be determined, and treatment must be given to return the heart rate to normal.

While sinus bradycardia can produce significant symptoms, the risk of dying from it is relatively low when asymptomatic.

Causes

Of the two general causes of bradycardia (sinus node-mediated or heart block), sinus node bradycardia is by far the more common.

Sinus Node

The heartbeat is normally generated and coordinated by the heart's electrical impulse, and the 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 is said to be present.

Causes of sinus bradycardia can be either transient or persistent. Persistent causes are more likely to require treatment.

Transient Sinus Bradycardia

Transient sinus bradycardia is most often caused by the increased tone in the vagus nerve, such as during sleep. The vagal nerve helps regulate the control of the heart, lungs, and digestive tract. Once the vagal tone is restored to normal, the heart rate also returns to normal, so no permanent treatment of the bradycardia itself may be required.

Sinus bradycardia that is 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.

Persistent Sinus Bradycardia

An abnormal sinus bradycardia that is persistent is most often caused by intrinsic sinus node disease — disease within the sinus node itself. Usually, intrinsic sinus node disease is due to a type of fibrosis (scarring) within the sinus node, which is a common manifestation of aging. So 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 can be inappropriately low both at rest and during exertion. People with symptomatic disease are often said to have sick sinus syndrome or sinus node dysfunction, during which the heart rate can fluctuate between bradycardia and tachycardia (rapid heart rate).

In addition to intrinsic sinus node disease, several other medical conditions can cause sinus bradycardia, including

Heart Block

The second general type of bradycardia is heart block. In contrast to sinus bradycardia, heart block is always an abnormal condition. 

Heart block occurs when the heart’s electrical impulses are partially or completely blocked as they travel from the atria of the heart to the ventricles. Because the sinus node that normally determines the heart rate is in the atrium, a block between the atria and ventricles causes a change in how fast the heart beats.

When the ventricles of the heart can't get information from the sinus node about how fast to beat, they use information from another special part of the heart between the atria and ventricles called the AV node - this results in a potentially dangerous bradycardia.

As with abnormal sinus bradycardias, a heart block can be either transient or persistent.

Transient Heart Block

Transient heart block can occur with certain conditions such as Lyme disease, thyroid dysfunction or drug toxicity (particularly digitalis). In these cases, a temporary pacemaker may be helpful as well as treatment of the underlying condition.

Persistent Heart Block

Persistent heart block can result from many conditions, including genetics, congenital disorders, sarcoidosis and amyloidosis.

If there is suspicion for a structural disorder like cardiomyopathy, imaging with a trans-thoracic echocardiogram may be recommended.

Persistent heart block, especially if it is symptomatic or complete, is more likely to require permanent treatment.

A stress test may be needed for those who only have symptoms during exertion to find out if a pacemaker would be helpful.

A partial block occurs when the electrical signals to the heart are delayed or intermittently stopped. A complete block occurs when the signals stop completely and is more likely to require pacemaker therapy.

Diagnosis

The evaluation of bradycardia is usually pretty straightforward. First, the doctor needs to examine an electrocardiogram (ECG) while the bradycardia is present, to determine whether it is due to sinus bradycardia or heart block.

Then, the doctor must determine whether the bradycardia is likely to be persistent, or instead whether it is a transient cause such as infection. This can often be accomplished by simply taking a careful medical history.

stress test can be helpful in bringing out either sinus node disease or heart block that becomes apparent only during exertion. Prolonged ambulatory ECG monitoring can also be helpful in diagnosing bradycardias that occur only intermittently.

An electrophysiology study (a specialized type of cardiac catheterization) can be quite definitive in diagnosing both sinus node disease and heart block, but it is not commonly necessary to perform this invasive testing to make the diagnosis.

Treatment

The treatment of bradycardia depends on whether it is sinus bradycardia or heart block, and whether it is reversible or not.

Reversible bradycardias may be due to the transient elevations in the vagal tone, which we have already discussed. In such cases, treatment consists of avoiding the kinds of conditions that cause the vagal tone to become elevated. 

Persistent bradycardia can also be reversible if it is caused by drug therapy, an infectious disease, pericarditis, myocarditis or hypothyroidism. In these cases, aggressively treating the underlying problem often takes care of the slow heart rate.

If sinus bradycardia is reversible, or it is producing no symptoms, it can usually be managed simply by periodic follow-up evaluations.

However, sometimes in older people, sinus node disease only produces symptoms during exertion, when the heart rate fails to increase as it should with exercise. So a stress test may be helpful in determining whether sinus node disease is actually producing symptoms or not. 

Sinus bradycardia caused by either heart block or sinus node dysfunction that is not reversible and is producing symptoms may be treated with a permanent pacemaker.

In some cases, such as partial AV block caused by a myocardial infarction (heart attack), a trial of a temporary pacemaker may be done to determine if the AV block is permanent or reversible.

If left untreated, some bradycardias may rarely cause cardiac arrest.

A Word From Verywell

Bradycardia is often a normal phenomenon that does not require an extensive medical evaluation or specific treatment. But if you have either sinus bradycardia that is producing symptoms, or heart block whether there are symptoms or not, you will need to work with your doctor to determine why you have it and to decide if a pacemaker may be required. 

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