Fainting, or syncope, is a sudden, temporary loss of consciousness.

Healthcare professionals often use the term ‘syncope’ when referring to fainting because it distinguishes fainting from other causes of temporary unconsciousness, such as seizures (fits) or concussion. In most cases of fainting, the person who has fainted regains consciousness within a minute or two.

However, less common types of fainting can be medical emergencies. If a person who has fainted does not regain consciousness within two minutes, then medical assistance should be sought immediately and the patient placed in the Recovery Position.

Vasovagal Syncope

Vasavagal syncope is a form of fainting related to the temporary malfunction of the autonomic nervous system, which regulates heart rate and blood pressure, in response to a trigger, such as the sight of blood or extreme emotional distress. When heart rate slows, blood pressure drops, and the resulting lack of blood to the brain causes fainting and confusion.

Fainting may be preceded by feeling unwell, nauseous, and confusion. Vasovagal syncope is usually harmless and requires not treatment, but a person may injure themselve during an episode.

The Autonomic Nervous System

The autonomic nervous system comprises the brain, nerves and spinal cord, and regulates a number of automatic bodily functions, including heart rate and maintenance of blood pressure.

Fainting can occur when an external experience or circumstance triggers a temporary malfunction in the autonomic nervous system. The malfunction in the autonomic nervous system causes a drop in an individual’s blood pressure and a reduction in heart rate. This leads to a temporary interruption to blood supply to the brain.


Vasovagal syncope may be caused by:

  • sudden exposure to an unpleasant sight or experience, such as the sight of blood
  • standing for long periods of time
  • spending a long time in hot or stuffy environments
  • a sudden intense episode of stress, emotional upset, fear or anxiety
  • a sudden feeling of pain

Fainting and bodily functions

Fainting can occur when a bodily function or activity places a sudden strain on the autonomic nervous system. This kind of fainting is called situational syncope.

Possible causes of situational syncope include:

  • coughing
  • sneezing
  • swallowing
  • laughing
  • passing stools
  • urinating
  • some physical activities, such as weightlifting, although this could also be a sign of an underlying heart problem (see cardiac syncope, below)

Situational syncope may also occur after eating a meal or exercising.

Fainting and the carotid sinus

The carotid sinus is a collection of sensors in the carotid artery. It helps to regulate the flow of blood through the main artery in the neck, called the carotid artery, and into the brain.

Some people can develop a hypersensitive carotid sinus. This means that any physical stimulation of the carotid sinus can cause a sudden drop in blood pressure, resulting in fainting. This condition is known as carotid sinus syndrome.

Examples of physical stimulation that may affect the carotid sinus include: • turning the head to one side • wearing a tight collar • shaving over the part of the neck that contains the carotid sinus

Carotid sinus syndrome is very rare in people who are under 40 years of age. It is most common among older men.

Fainting and low blood pressure

Fainting can occur when the blood pressure falls suddenly as a person stands up. This drop in blood pressure is called orthostatic hypotension. It is more common in older people, and is the reason for fainting in 1 person in 10 aged 60 years or over who faint. It is a common cause of falls in elderly people.

When a person stands up after sitting or lying down, gravity naturally draws the blood down into the legs, reducing blood pressure. Usually, the nervous system counteracts this by making the heart beat faster and narrowing your blood vessels, both of which will stabilise your blood pressure.

However, in cases of orthostatic hypotension, something interrupts the process of stabilisation. This means that if a person stands up suddenly, the blood supply to the brain is interrupted, causing them to faint.

Some possible causes of orthostatic hypotension are explained below:

  • Dehydration (low blood volume). If a person becomes severely dehydrated (where the normal water content of the body is reduced), the amount of fluid in the blood will reduce along with the blood pressure. This will make it much harder for the nervous system to stabilise the blood pressure, increasing your chances of fainting. This can be caused by heavy bleeding, vomiting or diarrhoea.
  • Diabetes. Untreated diabetes (a condition where there is too much glucose in the blood) causes frequent urination, which can lead to dehydration. Excess glucose in the blood can also damage the nerves that help to regulate your blood pressure.
  • Medication. Some medicines can cause orthostatic hypotension in some people. These include diuretics, which increase the production and flow of urine from the body, beta-blockers and some types of antidepressants.
  • Neurological conditions. Some health conditions that affect the nervous system, such as Parkinson’s disease, can trigger orthostatic hypotension in some people.

Fainting and the heart

Fainting can occur when the blood supply to the brain is interrupted due to an underlying problem with the heart. This type of fainting is called cardiac syncope.

If you suspect that fainting as a result of a heart problem, the individual should be advised to see a doctor.

Possible causes of cardiac syncope include:

  • abnormal heart rhythms (arrhythmias)
  • narrowing of the heart valves (stenosis)
  • a heart attack, which is a medical emergency when the blood supply to the heart is suddenly blocked

Cardiac syncope becomes more common as people get older. For example, it is estimated that a third of people aged 60 years or over who have fainted may have fainted as a result of a heart problem.

Related Articles


SKYbrary Partners:

Safety knowledge contributed by: