chevron-left chevron-right download

Syncope (Fainting)

Syncope (pronounced sin-cop-ee) refers to fainting or a sudden loss of consciousness. Approximately 20 to 30% of people will suffer an episode of syncope in their lifetime.

Mechanism of Syncope:

Syncope occurs when the brain does not receive sufficient blood flow to maintain its normal function. Syncope can occur if the blood flow to the brain is interrupted for as little as 5 to 6 seconds.

Causes of Syncope:

Many causes of syncope are not dangerous but any episode of syncope can cause injury or death if it occurs under certain circumstances for example, whilst driving, climbing or descending stairs or a ladder or while standing on a hard surface such as concrete. Some of the more common causes of syncope are listed below:

  1. Vasovagal or vasodepressor syncope:

This is one of the most common causes of syncope. It is caused by a “mix-up” in the nerve signals which control the heart rate and the blood pressure. Insufficient blood flow to the brain can occur either because the heart is beating to slowly or because the blood vessels dilate allowing the blood pressure to fall. Some situations that trigger vasovagal syncope include: psychological stress (such as it might occur with an injury or the sight of blood or receiving injection), bleeding, dehydration, urinating, coughing or having a bowel movement. In some cases no trigger can be identified. Vasovagal syncope most often occurs when a person is standing, although it can occur when the person is seated. Often the patient experiences a warning before fainting including sweating, feeling hot, dizziness, nausea and tunnel vision. Typically the period of unconsciousness lasts less than a couple of minutes, but the patient may have another episode if he/she tries to stand too quickly following the episode. Many patients report that they feel tired for several hours after one of these episodes.

  1. Abnormal Heart Rhythms:

Abnormal heart rhythms can cause syncope. When the heart beats too slowly, insufficient blood maybe pumped to the brain. Surprisingly very rapid beating of the heart can also cause syncope. This occurs because the heart does not have time to fill between heart beats, which results in it being unable to pump blood to the brain. Some abnormal heart rhythms which can cause syncope include ventricular tachycardia, atrial fibrillation, atrial flutter and supraventricular tachycardia.

  1. Obstruction of Blood Flow from the Heart:

Syncope can occur when the heart cannot pump blood to the brain because of an obstruction. This can occur with narrowing of the aortic valve (aortic stenosis), hypertrophic cardiomyopathy and pulmonary embolism (a blood clot in the lungs).

  1. Postural hypotension:

Postural hypotension occurs when the blood pressure is insufficient when standing. The most common cause of this is medication, especially medications that are used to lower blood pressure such as fluid tablets. Dehydration may also cause hypotension. Some causes of dehydration include: insufficient intake of fluids, prolonged exercise in a hot environment and vomiting and diarrhoea.

Diagnosis of syncope:

The most important method of diagnosing the cause of syncope is for the doctor to ask detailed questions about the circumstances of the syncope. For example, if the patient has had repeated episodes of syncope whilst giving blood the most likely explanation is a simple faint or vasovagal syncope.


The doctor will measure your blood pressure in the lying and standing positions to determine whether it is low or falls upon standing. He may also measure your heart rate and listen to your heart in order to detect problems with the heart valves.

The electrocardiogram or ECG

Abnormalities of the electrocardiogram may help the doctor determine if an abnormal heart rhythm is present or if there is heart disease.

The echocardiogram

The echocardiogram is a harmless painless test which uses ultrasound to examine the heart structure. This might detect abnormalities of the heart muscle or heart valves.

Heart rhythm monitoring:
  • The Holter monitor. The Holter monitor is a device which is used to record the ECG electrocardiograms for between 24 and 72 hours. The device is about the size of a mobile phone and several electrodes are stuck to the skin and connected to the device with wires. The device is usually attached to a belt or strap, so that the patient can wear it for most of the day. Most Holter monitors also have a button which the patient can press to record when he/she is feeling faint or dizzy. After the monitoring period is finished the device is connected to a computer and an ECG for the recording period is uploaded. If the patient has an episode of syncope during the recording period the heart rhythm at the time of the episode can then be examined. The major limitation of the Holter monitor is that syncope occurs fairly infrequently and we have to be lucky to capture an episode during the monitoring period.
  • Event recorders. Event recorders can be kept by the patient for several weeks. This is more likely to capture an abnormal heart rhythm. Several types are available. Some are attached to the patient via electrodes like in a Holter monitor where others are held against the chest if the patient notices symptoms.
  • Implantable loop recorders. The implantable loop recorder is a small device about the size of a computer USB stick which is implanted under the skin. This device records the ECG continuously. The ECG will be saved to the memory of the device if certain limits are exceeded or if the patient activates the device with an activator (a small device like a computer mouse). The loop recorder battery can last for over three years, so it is likely if the patient has a syncopal episode that the heart rhythm will be recorded. If a loop recorder is implanted, it is important that the patient carries the activator with them at all times.
Tilt Table Test

This is a test which is used to help detect vasovagal syncope. The patient is asked to lie on a flat table which is then tilted upright. The patient’s legs are restrained with straps. The heart rate and blood pressure are monitored and in some patients prone to vasovagal syncope a sudden drop in blood pressure and/or heart rate is recorded. This test has limited usefulness because the diagnosis of vasovagal syncope is usually made upon questioning rather than with testing.

Cardiac Electrophysiological Study (EPS)

This test is performed if your doctor thinks that the syncopal episodes are caused by an abnormal heart rhythm. This test is always performed in a hospital. The test might be performed under sedation or general anaesthetic. The doctor places wires into a large vein in the groin known as the femoral vein. The wires are then passed up into the heart. The wires have electrodes which allow the doctor to record electrical signals from the heart to determine whether abnormal heart rhythms are likely to occur.

Exercise stress test

This is performed if the patient has syncope during exertion such as, when walking, running or climbing stairs. During this test the patient is placed on a treadmill and the heart rhythm and blood pressure are recorded during exercise

Treatment of syncope:

Vasovagal syncope. The most important aspect of treatment of vasovagal syncope is to avoid situations that trigger syncope. The patient should avoid dehydration by drinking plenty of water and by adding a small amount of salt to the diet. The addition of salt helps the body retain fluid. The patient should avoid standing still for long periods in hot rooms as might occur during a cocktail party. If the patient is aware that they faint after injections or having blood taken they should insure that they sit down during these procedures and that these procedures are undertaken in an area which allows the patient to lie down immediately if they feel unwell. A patient who is prone to vasovagal syncope should lie down immediately if they feel an episode coming on. Muscular contraction (such as repeated clenching of the fists or calves) may help prevent an episode.

Postural hypotension

When the postural hypotension is due to drugs, these drugs should be withdrawn and replaced with other medications which are less likely to cause the problem. When the postural hypotension is due to problems with the nerve supply to the lower limbs, medications which increase the constriction of blood vessels or increase the blood volume might be used.

Abnormal heart rhythms

When syncope is caused by slow heart rhythms the usual treatment is a pacemaker. This is a small device placed under the skin of the upper chest near the collarbone. A pacemaker detects if the heart is beating too slowly, then stimulates the heart with small, painless electric impulses to make it beat faster.

Fast heart rhythms

Many fast heart rhythms can be cured with the technique of catheter ablation. In this technique a thin a wire is advanced from the veins in the groin up into the heart and a small burn is placed in the heart to destroy the short-circuit which is causing the fast heart rhythm.

Ventricular tachycardia

When syncope is due to ventricular tachycardia the treatment may be catheter ablation as described above or an implantable defibrillator. The implantable defibrillator is like a larger version of the pacemaker which can terminate fast abnormal heart rhythms with small pulses of electricity (overdrive pacing) or with an electric shock, which returns the heart rhythm to normal.

© Dr Mark McGuire 5/1/2016 (version 1.3)

Version: 1.3 (5/1/16) Copyright Dr Mark McGuire: all rights reserved, may not be copied in part or whole without my express permission.

More info: Please click on the links below Syncope
Heart Rhythm Society