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Patient education: Syncope (fainting) (Beyond the Basics)

David Benditt, MD
Section Editor
Peter Kowey, MD, FACC, FAHA, FHRS
Deputy Editor
Brian C Downey, MD, FACC
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Syncope, commonly known as fainting, refers to a sudden loss of consciousness, followed by a rapid and complete recovery. If you have symptoms of dizziness or lightheadedness, without loss of consciousness, this is often called presyncope, but these symptoms may be due to conditions that are unrelated to syncope.

Syncope should not be confused with sudden cardiac arrest. A person with sudden cardiac arrest also loses consciousness suddenly but will die without immediate medical attention. A person with syncope recovers quickly, almost always without treatment. However, injuries can occur during a syncopal episode, and recurrent episodes can be frightening. Moreover, in patients with heart problems, syncope may be a warning sign of more severe future heart arrhythmias and warrant thorough evaluation. It is important to determine the cause of syncope so that it can be prevented or treated in the future.

There are other reasons that a person may pass out, such as low blood sugar, hyperventilation, or seizures. These are not syncope but are important to assess and prevent.

Frequency — Syncope is surprisingly common in the population, but individual patients, if they have syncope, rarely experience more than one episode in a lifetime Approximately one-third of people have a syncopal episode at some point in their life. In most cases, syncope is not a sign of a life-threatening problem, although some people with syncope have a serious underlying medical condition. In non-elderly people, over 75 percent of cases of syncope are not associated with an underlying medical problem.

Risks — A person who suddenly and unexpectedly loses consciousness can be injured. Up to 35 percent of people who have syncope injure themselves; elderly people are more likely to be injured during a syncopal attack. (See 'Safety issues' below.)

Because of the risk of injury and the potential for serious underlying disease, any person who has a syncopal episode should seek medical attention.


To remain conscious, a supply of oxygen-rich blood must be pumped to the brain without interruption. If the brain is deprived of this blood supply, even for a brief period, loss of consciousness (passing out) will occur.

A number of medical conditions can cause syncope. Some of the most common are listed here.

Vasovagal syncope — One of the most common types of syncope is called vasovagal syncope or neurocardiogenic syncope. A variety of conditions can trigger vasovagal syncope, including physical or psychological stress, dehydration, bleeding, or pain. The heart rate slows dramatically and the blood vessels in the body expand, causing blood to pool in the legs, resulting in low blood pressure (hypotension). This causes a decrease in blood flow to the brain.

In some cases, vasovagal syncope is triggered by an emotional response to a stimulus, such as fear of injury, heat exposure, the sight of blood, or extreme pain. In other cases, it is caused by abnormal nervous system responses to activities such as urinating, having a bowel movement, coughing, or swallowing. In still other cases, no trigger can be identified.

In most cases of vasovagal syncope, you have some warning that you are near fainting. These signs include dizziness, feeling hot or cold, nausea, pale skin, "tunnel-like" vision, and profuse sweating. After the episode, symptoms may continue because of continued low blood pressure. Some people feel extremely tired.

Heart rhythm problems — A number of disturbances in the rate and/or rhythm of the heart can cause syncope. These disturbances are called arrhythmias.

The heart includes an area of specialized cells in the upper right chamber of the heart (right atrium) called the sinus node (figure 1). These cells send a series of regular electrical impulses to the atria that regulate the heart's rhythm and pace. These impulses travel in an organized way along conduction tissues within the heart muscle and then spread along smaller fibers that go to each muscle cell of the heart. The impulses cause the heart muscle cells to contract in an organized and regular way, generating an effective pumping of blood to all areas of the body.

Syncope can occur because of problems at several places in this system. The problems may be due to primary heart rhythm problems, underlying heart disease, use of a medication, or a transient abnormal communication between the heart and the nervous system The following are common rhythm problems that cause syncope.

Sinus bradycardia — Bradycardia means a slow heart rate. In sinus bradycardia, the heart rate is slower than normal. A dramatically slowed heart rate can decrease the blood supply to the brain by reducing the amount of blood that the heart can pump per minute.

Sometimes, sinus bradycardia occurs because of an abnormality in the sinus node itself. This is called sick sinus syndrome. In other cases, the slowed firing of the sinus node is due to medications. In still others, problems with the nerves that lead to the heart muscle and regulate the sinus node rate are to blame. For syncope to occur due to this problem, the heart usually stops for six or more seconds. This is known as asystole.

Heart block — Sometimes, part of the conduction system between the sinus node and the rest of the heart becomes disrupted due to heart disease. Most often, this occurs in the middle of the heart where a special set of fibers conduct the electrical impulse to the pumping chambers (the ventricles), preventing the normal flow of electrical impulses. If the electrical signal from the sinus node fails to get through the entire conduction pathway, the heartbeat can be interrupted. If the interruption is significant and the heart rate is too slow, it can impair blood flow to the brain.

Ventricular tachycardia — Tachycardia is a fast heart rate. The ventricles are the heart's main pumping chambers (figure 2). Ventricular tachycardia (VT) occurs when muscle in the ventricles send out their own rapid electrical impulses, taking over the rhythm normally controlled by the sinus node. The heartbeat that results from these abnormal impulses is also abnormal, and often fails to pump blood in an adequate way. The heart is racing but does not pump effectively, so blood flow to the brain may be decreased.

Most people with syncope due to VT have underlying heart disease, most commonly coronary heart disease. Treatment of people with syncope caused by VT usually includes an implantable cardioverter-defibrillator. (See 'Implantable cardioverter-defibrillator' below.)

Supraventricular tachycardia — Rapid heartbeats can originate above the ventricles (supraventricular tachyarrhythmias). This is not commonly associated with syncope, unless the heart rate is very rapid.

Blockage of blood flow from the heart — Any problem with the structure of the heart that interferes with the flow of blood can cause syncope. The two most common causes of outflow obstruction are hypertrophic cardiomyopathy and aortic stenosis.

Hypertrophic cardiomyopathy – Hypertrophic cardiomyopathy is an inherited condition in which the areas of the left ventricular muscle walls are thickened. In some cases, the condition can interfere with blood flow out from the left ventricle and can cause syncope. More information about hypertrophic cardiomyopathy, including treatment recommendations, is available separately. (See "Patient education: Hypertrophic cardiomyopathy (Beyond the Basics)".)

Mitral stenosis or aortic stenosis – In the normal circulation, blood flows through the mitral and aortic valves and into the body's largest artery, the aorta, to supply blood to the body (figure 2). Disease causing narrowing or obstruction of these valves will reduce the amount of blood being pumped and may predispose to syncope.

When severe, mitral or aortic stenosis can reduce blood flow to the brain and the rest of the body. Treatment of syncope caused by mitral or aortic stenosis often includes valve replacement surgery. (See "Indications for valve replacement in aortic stenosis in adults".)

Orthostatic hypotension — Hypotension means low blood pressure. Orthostatic hypotension refers to low blood pressure that occurs when a person stands or sits up. Due to the effects of gravity, this causes an inadequate amount of blood to the brain, leading to syncope.

Causes of orthostatic hypotension include the following.

Blood or fluid loss – Sometimes there is not enough blood in the body to ensure adequate flow to the brain when sitting or standing up. Low blood volume can be caused by blood loss or severe dehydration.

Medications – Certain medications can interfere with the normal mechanisms that maintain blood pressure. Examples include some antidepressants, certain blood pressure or heart medicines, or medicines containing opiates, such as morphine.

Illnesses that affect the nervous system – A number of illnesses can affect the specialized branch of the nervous system that helps maintain blood pressure (the autonomic nervous system). Examples are Parkinson disease, diabetes mellitus, the Shy-Drager syndrome, and amyloidosis.

Alcohol – Drinking alcohol can cause blood vessels to expand, causing blood pressure to fall and syncope to occur.

Carotid sinus hypersensitivity – Carotid sinus hypersensitivity is a condition in which reflexes lead to a slow heart rate and/or enlargement of blood vessels. This may be triggered by pressure on the carotid arteries (the main artery in the neck), and can lead to low blood pressure and syncope.

Neurologic disease (eg, Parkinsonism) that prevents blood vessels from constricting in compensation for upright posture.

Other causes — Less common causes of syncope include a heart attack, cardiac tumor, or blood clot in the arteries supplying the lungs.


There are three main ways to identify the causes of syncope: the medical history, the physical examination, and cardiac testing. A medical history and physical examination are recommended for anyone who has had syncope. Some people will also require cardiac testing.

Medical history — Gathering as much information as possible about events that occurred before, during, and after a syncopal episode can be helpful in determining the possible cause of syncope.

As an example, vasovagal syncope is suspected in a person who has warning signs of nausea or sweating. In contrast, a sudden loss of consciousness with no warning is more likely to be due to a heart rhythm problem. A person who has syncope during exertion is more likely to have an obstruction to blood flow (aortic stenosis or hypertrophic cardiomyopathy) or ventricular tachycardia as a cause.

Information about current medications and pre-existing medical conditions such as diabetes, heart disease, or psychiatric illness can help pinpoint the cause of syncope. If the person has abnormal body movements while unconscious and requires a long time to recover consciousness, the person may have had a seizure and not a true syncopal episode.

Physical examination — The clinician will measure your heart rate and blood pressure to help determine if a rhythm disturbance or low blood pressure caused the syncope. You may be asked to sit or stand while the blood pressure is measured to test for orthostatic hypotension. The clinician will listen to your heart for abnormal sounds that can be present in conditions such as aortic stenosis. You may have a test for blood in the stool to evaluate for blood loss, which could result in syncopal episodes.

If the cause of the syncope is not readily apparent, the clinician may perform special maneuvers to test your response. As an example, you may be asked to bear down as if having a bowel movement; abnormal heart sounds that occur in response to this maneuver can point to hypertrophic cardiomyopathy. The clinician may firmly massage your carotid artery (located in the neck) while your heart rate is closely monitored with an electrocardiogram (ECG or EKG). The heart's response to this maneuver can give clues to a possible diagnosis.

Testing — A number of medical tests are available to help determine the cause of the syncope.

Electrocardiogram — Anyone who have had an episode of syncope should have an ECG. An ECG can be performed in a clinician's office and takes only a few minutes. Sticky pads are placed on your chest, abdomen, arm, and leg, and are connected to a recording device with long, thin cables. This is not painful and there is no risk of electric shock with an ECG.

The ECG provides a picture of the electrical activity passing through the heart muscle. A normal ECG does not necessarily mean that syncope is not caused by a heart rhythm problem. Heart rhythm problems are often brief, come and go, and may not be present at the moment when the ECG is performed.

Rhythm monitoring — Heart rhythm monitoring may be recommended to diagnose rhythm problems that come and go and have not been detected with a routine ECG. This monitoring may be done at home or in the hospital.

Holter monitor – You may be asked to wear a monitoring device, called a Holter monitor, for 24 or 48 hours while performing normal daily activities at home. The device is connected to several long thin cables that are attached to your chest with sticky pads (similar to an ECG). The cables connect to a small, portable machine that can be attached to a belt or strap that is carried over the shoulder (figure 3).

However, this type of monitoring has limited use and provides a diagnosis in only about 2 to 3 percent of people with syncope. If you do not experience a syncopal episode while wearing the Holter monitor, the test may need to be repeated, or an alternate form of long-term monitoring may be recommended.

Event recorder – An event recorder may be recommended to capture rhythm problems associated with a syncopal episode. The advantages of an event recorder compared with a Holter monitor are its small size and the ability to monitor for abnormal rhythms for longer periods of time (usually one to two months).

Some devices require you to activate the recorder when you feel symptoms of a syncopal episode. However, if you lose consciousness and another person is not available to assist with the recording, the opportunity to "capture" the event on the monitor may be lost (figure 4).

Intermittent loop recorders – Intermittent loop recorders were developed to capture rhythm problems that occur before the device is activated. When you activate the monitoring device after regaining consciousness, the ECG recordings from the previous few minutes are retrieved and stored for analysis at a later time.

An implantable loop recorder (ILR) provides a way to monitor rhythms over an extended period of time (eg, 18 to 24 months). The ILR is implanted under the skin on the upper left chest area. It stores events automatically according to programmed criteria, or can be activated by the patient. The ILR may be most useful if your symptoms are infrequent and an arrhythmia is suspected, but other forms of testing are negative or inconclusive.

Echocardiogram — An echocardiogram is useful for identifying underlying structural heart disease such as hypertrophic cardiomyopathy or significant aortic stenosis. These findings alone do not conclusively establish the specific cause for syncope.

An echocardiogram uses ultrasound (sound waves) to obtain detailed pictures of your heart as it beats. A technician presses a transducer (wand) against your chest and abdomen. The transducer is attached to a recording device and monitor. You are awake during the procedure. An echocardiogram does not use radiation.

Upright tilt table test — This test is often done in healthy patients who have syncope. You lie on a flat table and are tilted at various angles while your heart rate and blood pressure are monitored closely (figure 5). Your response to the change in position can sometimes give clues about the cause of syncope.

Electrophysiology study — An electrophysiology study (EPS) may be performed if you have heart disease or if a rhythm problem is suspected.

Most people undergo EPS in a hospital setting. You will be given a sedative before the procedure but may be awake during testing. The physician uses a local anesthetic to numb a small area over a blood vessel, usually in the groin, and then threads small wires through the blood vessels into the heart using x-ray (fluoroscopic) guidance. Once in the heart, precise measurements of the heart's electrical function can be obtained.

Exercise testing — In some people, especially those with a history of syncope during exertion, an exercise test is useful. Your blood pressure, heart rate, and rhythm are monitored while exercising on a treadmill or bicycle.

Electroencephalogram — An electroencephalogram (EEG) is used to diagnose seizures, but may be part of the evaluation of unexplained "collapse" events. It involves the measurement of electrical activity in the brain. It can be performed in a provider's office or in a hospital, and generally takes approximately one hour. Multiple electrodes (small, flat metal discs) will be attached to your head and face with a sticky paste. The electrodes are connected to a recording device with long, thin wires. You must lie still and avoid speaking during the test.

An EEG is frequently obtained in people with syncope, but is rarely useful. It can be helpful if you have syncope and seizure-like activity.


Treatment of syncope is based upon the underlying cause. The goal of treatment is to prevent recurrences or more serious problems.

Vasovagal syncope treatment — Vasovagal syncope can usually be treated by learning to take precautions to avoid potential triggers and minimize the potential risk of harm. For example, if you faint while blood is being drawn, you may be instructed to lie down during the procedure. If you have a feeling that you will pass out during any activity, you should immediately lie down and elevate your legs.

Counter-pressure maneuvers — Counter-pressure maneuvers such as tensing your arms with clenched fists, leg pumping, and leg-crossing may stop a vasovagal syncopal episode, or at least delay it long enough that you can lie down with the feet elevated. Such maneuvers include:

Leg crossing while tensing the leg, abdominal, and buttock muscles.

Hand gripping, which involves gripping a rubber ball or similar object as hard as possible.

Arm tensing, which involves gripping one hand with the other while simultaneously moving both arms away from the body.

Medications — People with a heart rhythm problem may be started on medication to control the rhythm.

People with orthostatic hypotension may benefit from increasing the amount of blood fluid volume. Fludrocortisone (Florinef) is one medicine that is used to increase blood volume. Midodrine is a medication that constricts blood vessels that may be used in combination with fludrocortisone.

Pacemakers — A pacemaker is a small device that is implanted under your skin. Wires from the device are threaded to the heart where they emit impulses that help regulate the heartbeat. Pacemakers are often recommended if you have syncope caused by sinus bradycardia, carotid sinus hypersensitivity, or heart block. Some new pacemakers are directly implanted within the heart, but these are still only infrequently used. (See "Patient education: Pacemakers (Beyond the Basics)".)

Implantable cardioverter-defibrillator — In some people with serious, life-threatening ventricular arrhythmias (such as ventricular tachycardia) that cause syncope, a device called an implantable cardioverter-defibrillator (ICD) is used. The device is surgically implanted under the skin in your chest, similar to a pacemaker. It can sense when a life-threatening ventricular arrhythmia is occurring and administer an electric shock to correct the problem and potentially prevent the person from dying. (See "Patient education: Implantable cardioverter-defibrillators (Beyond the Basics)".)

Orthostatic training — In people with orthostatic hypotension and certain types of vasovagal syncope, orthostatic training may be useful to prevent syncope. Techniques are designed to decrease pooling of blood in the extremities, which can allow the blood pressure to drop when you stand. Methods to decrease this problem include the following:

Use of elastic compression stockings, which are worn on the feet and lower legs

Contraction of the leg muscles before and while standing

Rising to stand slowly and in stages


Passing out while driving or other activities can potentially harm both the patient and those around him/her. As a result, driving restrictions are sometimes recommended for certain people with syncope. This generally includes people who have a history of syncope that occurs without warning or known cause. Driving restrictions are governed by state or local laws.


Your health care provider is the best source of information for questions and concerns related to your medical problem.

This article will be updated as needed on our website. Related topics for patients, as well as selected articles written for health care professionals, are also available. Some of the most relevant are listed below.

Patient level information — UpToDate offers two types of patient education materials.

The Basics — The Basics patient education pieces answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials.

Patient education: Syncope (fainting) (The Basics)
Patient education: Pacemakers (The Basics)
Patient education: Orthostatic hypotension (The Basics)
Patient education: Bradycardia (The Basics)
Patient education: Sick sinus syndrome (The Basics)
Patient education: Atrial flutter (The Basics)
Patient education: Aortic dissection (The Basics)
Patient education: Time to stop driving? (The Basics)
Patient education: Vagal maneuvers and their responses (The Basics)
Patient education: Long QT syndrome (The Basics)

Beyond the Basics — Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are best for patients who want in-depth information and are comfortable with some medical jargon.

Patient education: Hypertrophic cardiomyopathy (Beyond the Basics)
Patient education: Pacemakers (Beyond the Basics)
Patient education: Implantable cardioverter-defibrillators (Beyond the Basics)

Professional level information — Professional level articles are designed to keep doctors and other health professionals up-to-date on the latest medical findings. These articles are thorough, long, and complex, and they contain multiple references to the research on which they are based. Professional level articles are best for people who are comfortable with a lot of medical terminology and who want to read the same materials their doctors are reading.

Approach to the adult patient with syncope in the emergency department
Carotid sinus hypersensitivity and carotid sinus syndrome
Syncope in adults: Clinical manifestations and diagnostic evaluation
Syncope in adults: Management
Reflex syncope in adults: Clinical presentation and diagnostic evaluation
Nonepileptic paroxysmal disorders in adolescents and adults
Syncope in adults: Epidemiology, pathogenesis, and etiologies
Upright tilt table testing in the evaluation of syncope
Indications for valve replacement in aortic stenosis in adults

The following organizations also provide reliable health information.

National Library of Medicine


National Heart, Lung, and Blood Institute


American Heart Association




The editorial staff at UpToDate would like to acknowledge Brian Olshansky, MD, who contributed to an earlier version of this topic review.

Literature review current through: Nov 2017. | This topic last updated: Wed Aug 09 00:00:00 GMT 2017.
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