Bradycardia is a heart rate that's slower than normal. There are two main types of bradycardia—sinus bradycardia and heart block.

Sinus bradycardia is defined as a resting heart rate of under 60 beats per minute that arises from the sinus node, which sets the heart rhythm. Sinus bradycardia can be normal for some people, but it may need treatment if it causes symptoms. Heart block, an abnormal type of bradycardia, may lead to serious symptoms and outcomes.

This article explains bradycardia symptoms and their causes. It discusses how bradycardia is diagnosed and how both types of heart rate problems are treated.

What Is Sinus Bradycardia?

Sinus bradycardia is a slow heart rate that involves the sinoatrial (or sinus) node, often referred to as "the heart's natural pacemaker." The sinus node is the part of the heart that produces electrical signals that trigger each heartbeat.

At rest, the sinus node typically generates electrical impulses for a heart rate between 60 and 100 times per minute, which is 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. People also have lower heart rates when sleeping.

Slow heart rates without symptoms usually are no cause for concern. 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.

What Are the Symptoms of Sinus Bradycardia?

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:

• Fatigue or feeling weak
• Dizziness or lightheadedness
• Confusion
• Fainting (syncope) or near-fainting
• Shortness of breath (dyspnea)
• Tires easily during exercise
• Chest pain

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

If bradycardia happens for an extended period of time without treatment, it can lead to complications such as:

• High blood pressure
• Angina
• Cardiac arrest

If you have any symptoms of bradycardia, talk to your healthcare provider to determine the cause. Proper treatment can return the heart rate to normal.

Is Sinus Bradycardia Serious?

It's important to know the dangers because sinus bradycardia can be serious.

A slow heart rate that causes symptoms, such as chest pain or shortness of breath, requires medical attention. This can lead to life-threatening cardiac arrest. Over time, sinus bradycardia also may cause high blood pressure.

Early diagnosis and treatment are important.

What Causes Sinus Bradycardia?

Several medical conditions can cause sinus bradycardia, including:

• Sinus node dysfunction (sick sinus syndrome), which is due to the electrical system of the heart aging and doesn't reliably trigger every heartbeat, causing slow and irregular beats.
  
• 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

These causes of sinus bradycardia fall into two categories:

• Sinus node bradycardia: a problem with the generation of the signal for the heart to beat
• Heart block: a problem with the transmission of that signal to the ventricles of the heart

Of the two, sinus node bradycardia is more common.

Sinus Node Bradycardia

When the sinus node produces electrical impulses at a relatively reduced rate, the heart rate becomes slow. Sinus bradycardia causes can be either transient (meaning that they don't last long) or persistent, which is 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. The vagal tone tells you how well the vagus nerve is functioning. 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.
  

Heart Block

The second 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 (upper chambers) to the ventricles—the lower chambers that pump blood out of the heart. A block between the atria and ventricles causes a change in how fast the heart beats.

Partial blocks occur when the electrical signals to the heart are delayed or intermittently stopped. A complete block happens when the signals stop entirely.

When the heart's ventricles don'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 bradycardia, a heart block can be transient or persistent.

Transient heart block can occur with certain conditions such as:

• Lyme disease
• Thyroid dysfunction
• Drug toxicity (particularly digitalis, a medication used to treat certain heart conditions)

Persistent heart block can result from many conditions, including:

• Genetics
• Congenital disorders
• Sarcoidosis
• Amyloidosis
  

Risk Factors

Since bradycardia can be caused by damage to the heart, certain health conditions and factors associated with heart disease may increase the risk of bradycardia, such as:

• High cholesterol
• Obesity
• Smoking
• Older age
• Coronary artery disease
• Diabetes
• High blood pressure
• Heart valve disease
• Cardiomyopathy

How Is Sinus Brachycardia Diagnosed?

An electrocardiogram (ECG) will show that a slow heart rate is present. ECG tests will help to determine whether a slow heart rate is due to sinus bradycardia or heart block.

The healthcare provider will then 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 taking a careful medical history.

If other 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 to diagnose these cases by identifying whether 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 wearing a Holter monitor. It can also help diagnose bradycardias that occur intermittently.
• Electrophysiology study: In this procedure, a long, thin catheter is threaded up through a vein to the heart, called cardiac catheterization. However, it is usually not necessary to perform this invasive testing to make a diagnosis.

How Is Sinus Bradycardia Treated?

The treatment of slow heart rate depends on whether the cause is sinus bradycardia or heart block and whether it's reversible or not. Treatment may include lifestyle changes, medication changes, or an implanted device called a pacemaker. 

Reversible

A physician might treat transient sinus bradycardia by recommending you avoid 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 partial and complete heart blocks are persistent, your healthcare provider may recommend a permanent pacemaker.

In some cases, healthcare providers 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 healthcare providers determine if the block is permanent or reversible.

When to See a Healthcare Provider

If you have sinus bradycardia, you should see your healthcare provider annually. Call your provider if your symptoms change noticeably or if your symptoms start to affect your daily life and routine.

You should seek immediate medical care if you experience:

• Chest pain
• Shortness of breath
• Dizziness, feeling lightheaded, or fainting

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 (more common) and heart block. Treatment may involve correcting any underlying causes or implanting a pacemaker.