What Is Bradycardia?

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 (possibly a majority) of healthy people have a resting heart rate below 60. So having bradycardia is not necessarily a bad thing, or even an abnormal thing. Bradycardia at rest is often a sign of good health.

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 almost always caused either by sinus node dysfunction or heart block.

If the heart rate is abnormally slow, several of the body’s organs may not function normally, and various symptoms may result. In general, the symptoms of abnormal bradycardia tend to become worse with efforts at 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:

• Lightheadedness or dizziness (especially with exertion)
• Easy fatiguability
• Syncope (fainting) or near-syncope
• Dyspnea (shortness of breath)
• Chest pain or discomfort
• Confusion

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.

Bradycardia is often caused by a sudden increase in the activity of the vagus nerve. This is the nerve in the brain that helps regulate the control of the heart, lung, 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 is required. 

On the other hand, persistent abnormal bradycardia can be caused by various medical conditions. These include:

• Coronary artery disease
• Pericarditis
• Myocarditis
• Cardiac trauma due to injury or to cardiac surgery
• Amyloidosis
• Hypothyroidism
• Dysautonomia
• 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
• Hypoxia (low blood oxygen levels), as often happens with obstructive sleep apnea
• Various drugs, including beta-blockers, calcium channel blockers, antiarrhythmic drugs, opioids, lithium, and various chemotherapy drugs

Of the two general types of bradycardia, sinus bradycardia is by far the more common.

The heartbeat is 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.

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: Transient sinus bradycardia is most often caused by the 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 acute pain or sudden emotional stress.

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

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.

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. 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.

The second general type of bradycardia is heart block. In contrast to sinus bradycardia, which in most people is actually quite normal, 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 not all of the electrical impulses are reaching the ventricles, the heart rate becomes slower than it is supposed to be.

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

Transient Heart Block: Transient heart block can occur (as with transient sinus bradycardia) with episodes of increased vagal tone. This type of transient heart block is most often seen in younger, healthier people whose vagal tone becomes elevated because of nausea, sudden pain, or sudden stress. This heart block is considered to be benign, and almost never requires treatment beyond treating (or avoiding) the events that caused the elevated vagal tone.

Persistent Heart Block: Persistent heart block is a more serious matter because it tends to become worse (and may become life-threatening) as time goes by. With heart block, however, even when the underlying condition is persistent, the bradycardia itself may be intermittent.

This means that sometimes, maybe even most of the time, the resting heart rate is actually in the normal range; but the heart rate can suddenly drop to symptom-producing levels without any apparent reason or trigger (because the underlying condition is persistent). Moreover, the block may be partial or complete.

This fact often makes heart block somewhat more challenging to diagnose than sinus bradycardia. Whether the bradycardia is there all the time or is intermittent, however, persistent heart block virtually always requires treatment.

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 event due to an increase in vagal tone. This can almost always be accomplished by simply taking a careful medical history.

A 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 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.

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 quite helpful in determining whether sinus node disease is actually producing symptoms or not. 

Sinus bradycardia that is not reversible and is producing symptoms should be treated with a permanent pacemaker.

If left untreated, a heart block may eventually lead to heart failure, cardiac arrest, or sudden cardiac death.

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.