What Is Bradycardia?

Bradycardia is the medical term for a slower-than-normal heartbeat.

Technically, it means that a resting heart rate is below 60 beats per minute. The hearts of adults at rest normally beat between 60 and 100 times 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.

However, bradycardia can become a significant problem if the heart rate becomes so slow that the heart stops pumping enough blood to the body. This abnormal bradycardia requires careful evaluation and treatment.

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

"Normal" Values

The sinoatrial (or sinus) node is located in the right atrium of the heart. It produces the electrical signals that trigger each heartbeat, so it is often referred to as "the heart's natural pacemaker."

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 outside 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.

Physiological bradycardia is a form of sinus bradycardia. Among people who have it—including healthy young people and older people in good physical condition—their resting heart rate may hover in the 40s or 50s.

Still, their physiological bradycardia poses no medical concern. Their heart rate is appropriate to their body’s requirements. And this is all that really matters.

It is also common, and normal, for people to register low heart rates while they're sleeping.

However, 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.

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:

• Chest pain or discomfort
• Confusion
• Easy fatigue
• Fainting (syncope) or near-fainting
• Lightheadedness or dizziness (especially with exertion)
• Shortness of breath (dyspnea)

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

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

Causes

The general causes of bradycardia fall into two categories: sinus node-mediated and heart block. Of the two, 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 (meaning that they don't last long) 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 nerve's 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 (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. Intrinsic sinus node disease usually occurs in people who are 70 years of age or older.

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

• Amyloidosis (a rare disease in which protein deposits form in the heart muscle)
• Brain disorders, especially those associated with increased intracranial pressure or stroke
• Cardiac trauma due to injury or cardiac surgery
• Coronary artery disease
• Drugs like beta-blockers, calcium channel blockers, antiarrhythmic drugs, opioids, lithium, and some chemotherapy treatments
• Dysautonomia (a nervous system dysfunction)
• Hypothyroidism (low thyroid function)
• Hypoxia (low blood oxygen levels), as often happens with obstructive sleep apnea
• Myocarditis (inflammation of the heart muscle)
• Pericarditis (swelling and inflammation of the sac covering the heart)
  
• Various types of infections, including Lyme disease, Chagas disease, and Rocky Mountain spotted fever
  

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. The ventricles are the chambers that pump blood out of the heart.

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 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 exhibit symptoms only during exertion to find out if a pacemaker would help.
• Partial blocks occur when the electrical signals to the heart are delayed or intermittently stopped. A complete block happens when the signals stop entirely. It 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 can often be determined by simply taking a careful medical history.

If tests are needed, they may include:

• Stress test: In some people (mainly older people), sinus node disease or heart block may produce symptoms only 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 do normal daily activities while being monitored. It can also help diagnose bradycardias that occur only intermittently.
• Electrophysiology study: A specialized cardiac catheterization can be fairly accurate in diagnosing sinus node disease and heart block. In this procedure, a long, thin catheter is threaded up through a vein to the heart. However, it is usually not necessary to perform this invasive testing to make a diagnosis.

Treatment

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

Reversible

A physician might treat transient sinus bradycardia by recommending avoiding the kinds of conditions that trigger it. For example, treating sleep apnea or adjusting medications are some things that might resolve transient bradycardia.

Persistent bradycardia can also be reversed if it is caused by:

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

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

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 is defined as a heart rate of fewer than 60 beats per minute. The hearts of adults at rest typically beat between 60 and 100 times per minute.

A lower-than-normal heart rate poses no problem for many people unless certain symptoms appear. These symptoms include chest pain, dizziness, easy fatigue, and shortness of breath. Then it's time to consult a physician to determine the underlying cause.

The general causes of bradycardia fall into two categories: sinus node-mediated and heart block. The former is more common of the two. Treatment may involve correcting any underlying causes or implanting a pacemaker.

A Word From Verywell

You may feel worried if you learn you have a low heart rate. But rest assured that living with bradycardia is normal for many people. The difference between worry and contentment lies in knowing whether you have a medical condition that requires treatment. So if you experience symptoms like lightheadedness and shortness of breath, your body may be trying to tell you something. Answer the call by calling your physician.

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