What Is Bradycardia?

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Bradycardia is the medical term for a heart rate that is slower than expected. Bradycardia is a resting heart rate that is below 60 beats per minute.

Having a low heart rate is not necessarily a bad thing or even abnormal. Many healthy people have a resting heart rate of 50 to 60 beats per minute. For instance, athletes, in particular, are known for having a slow heart rate.

However, bradycardia can be a significant problem if the heart rate becomes so slow that the heart stops pumping enough blood for the body. Therefore, this type of abnormal bradycardia is medically concerning and requires careful evaluation and treatment.

Sinus node dysfunction and heart block often cause bradycardia that is slow enough to cause clinical problems.

This article explains the symptoms, causes, diagnosis, and treatment of bradycardia.

"Normal" Values

The sinus node is in the right atrium and is the natural pacemaker of the heart. At rest, the sinus node typically generates electrical impulses at a rate of between 60 and 100 times per minute. A resting heart rate within this range is called a normal sinus rhythm.

Sinus bradycardia is a heart rate between 50 to 60 beats per minute. While technically out of the normal range, these values can be entirely normal for some people.

A healthy body is very good at regulating the heart rate to support the body’s functions, and often, a lower heart rate is within this range.

Physiological bradycardia is considered a harmless form of sinus bradycardia. In other words, the heart rate is appropriate to the body’s requirements and is not a medical concern.

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.

When the heart rate becomes too slow to pump enough blood, it needs treatment. Sinus bradycardia that produces symptoms is a sign you should seek medical care.

Symptoms of Bradycardia
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Bradycardia Symptoms

If the heart rate is abnormally slow, several of the body’s organs may not function normally. A heart rate that is too slow results in various symptoms, including:

These symptoms worsen with exercise because the body’s needs increase when you stress it. However, symptoms may also be present during rest if bradycardia is severe.

If bradycardia is associated with any of these symptoms, you should talk to your doctor to determine the cause. Appropriate treatment can return the heart rate to normal.

The risk of dying from bradycardia is relatively low when there are no symptoms. However, rarely, some symptomatic bradycardias may cause cardiac arrest if left untreated.

Causes

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

Sinus Node Bradycardia

The heartbeat is usually generated and coordinated by the heart's electrical impulse. This impulse originates in the sinus node—a tiny nest of cells located at the top of the right atrium.

When the sinus node produces these electrical impulses at a relatively reduced rate, the heart rate becomes slow, resulting in sinus bradycardia.

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

  • Transient sinus bradycardia: An increased tone in the vagus nerve, such as during sleep, often leads to this type of low heart rate. This nerve helps regulate the heart, lungs, and digestive tract. Once the vagal tone returns to normal, the heart rate also returns to normal. Therefore, you may require no permanent treatment of the bradycardia itself.
  • Persistent sinus bradycardia: Intrinsic sinus node disease (disease within the sinus node itself) most often causes a persistent type of sinus bradycardia. Usually, intrinsic sinus node disease is due to fibrosis (scarring) within the sinus node, a common result of aging. Therefore, intrinsic sinus node disease usually occurs in people who are 70 years old or older.

Intrinsic Sinus Node Disease

In people with intrinsic sinus node disease, the heart rate can be inappropriately low. This slow heart rate can happen both at rest and during exertion.

People with symptomatic disease are often said to have sick sinus syndrome or sinus node dysfunction. As a result, their 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 wholly or partially blocked as they travel from the heart's atria to the ventricles. Since the sinus node that usually 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 heart's ventricles can't get information from the sinus node about how fast to beat, they use information from another particular part of the heart between the atria and ventricles called the AV node. This broken communication results in potentially dangerous bradycardia.

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

  • Transient heart block can occur with certain conditions such as Lyme disease, thyroid dysfunction, or drug toxicity (particularly digitalis, a medication used to treat certain heart conditions). In these cases, treatment of the underlying condition is necessary. Sometimes, treatment may include a temporary pacemaker.
  • Persistent heart block can result from many conditions, including genetics, congenital disorders, sarcoidosis, and amyloidosis.If your doctor suspects a structural disease like cardiomyopathy, they may recommend imaging with a trans-thoracic echocardiogram (an ultrasound to see the heart in motion). Persistent heart block, primarily symptomatic or complete, is more likely to require permanent treatment. Your doctor may recommend a stress test if you only have symptoms during exertion to find out if a pacemaker would be helpful.
  • Partial blocks occur when the electrical signals to the heart are delayed or intermittently stopped. A complete block happens when the signals stop entirely and is more likely to require pacemaker therapy.

Diagnosis

Evaluating bradycardia is usually pretty straightforward. But, first, a healthcare provider needs to examine an electrocardiogram (ECG) while the slow heart rate is present. An ECG will help determine whether the slow heart rate is due to sinus bradycardia or heart block.

Then, the healthcare provider must determine whether the bradycardia is likely to be persistent or due to a transient (temporary) cause, such as an infection. This determination can often be accomplished by simply taking a careful medical history.

Tests may include:

  • Stress test: In some people (mainly older people), sinus node disease or heart block may only produce symptoms during exertion. A stress test can help diagnose these cases. That's because it can identify whether or not the heart rate increases as it should in response to an exercise challenge. (Without this, such cases may seem asymptomatic.)
  • Prolonged ambulatory ECG: With this test, you will do normal daily activities while monitored. It can also help diagnose bradycardias that occur only intermittently.
  • Electrophysiology study: A specialized cardiac catheterization can be pretty accurate in diagnosing sinus node disease and heart block. However, it is usually not necessary to perform this invasive testing to make the diagnosis.

Treatment

The treatment of slow heart rate depends on whether the cause is sinus bradycardia or heart block and whether it is reversible or not.

Reversible

You might treat transient sinus bradycardia by avoiding the kinds of conditions that cause the vagal tone to become elevated. For example, treating sleep apnea or adjusting medication are some things that might resolve transient bradycardia.

Persistent bradycardia can also be reversible if it is caused by certain things, including:

  • Drug therapy
  • Infectious disease
  • Pericarditis
  • Myocarditis
  • Hypothyroidism

In these cases, aggressively treating the underlying problem often takes care of the slow heart rate.

If sinus bradycardia is reversible or produces no symptoms, you can usually manage it by observing it at periodic follow-up evaluations.

Permanent

When heart block or sinus node dysfunction causes bradycardia and does not produce symptoms, your doctor may recommend a permanent pacemaker.

In some cases, doctors will do a trial of a temporary pacemaker. For example, when a partial AV block is caused by a myocardial infarction (heart attack), using a temporary pacemaker can help doctors determine if the block is permanent or reversible.

Summary

Bradycardia (low heart rate) is often normal. A medical condition doesn't always cause it, but it can. Sinus bradycardia that produces symptoms and heart block, whether there are symptoms or not, should be evaluated by a doctor. Those conditions may require treatment, such as a pacemaker.

A Word From Verywell

If you have a low heart rate, you may feel worried. Rest assured that for a lot of people, bradycardia is a normal heart rate. Other times, though, it can be a sign of a medical condition that needs treatment. So, if you experience symptoms like light-headedness, fatigue, fainting, shortness of breath, or chest pain, be sure to have it evaluated.

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11 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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