What causes fainting?

Written by: Dr Boon Lim
Edited by: Cal Murphy

Syncope (pronounce sin-cope-pee) is the medical term for fainting, blacking out, or loss of consciousness. Many people have experienced this phenomenon, passing out in a crowd, or on public transport. But why does this happen to certain people? Dr Boon Lim, a leading London cardiologist and expert on syncope explains what causes fainting:

What are the causes of fainting?

Syncope is caused by a combination of:

  • Low blood pressure
  • The heart not pumping enough blood to the brain
  • Reduced peripheral vascular resistance (i.e. the blood vessels become too “relaxed” and are able to accommodate much more volume than usual). This effectively causes the blood circulating around our body to start to pool in very unhelpful places – typically this build-up of blood is most pronounced within the lower limbs and the stomach and intestinal bed.


How do these factors make you faint?

Let’s suppose, for the average, normal-sized, male patient, the circulating volume of blood is approximately five litres. When standing up, gravity pulls blood down into the lower limbs, which can act as huge reservoirs, removing approximately 800 ml of blood from the main circulation, therefore leaving less blood circulating around the rest of the body, particularly to the heart and brain.

With the heart receiving less blood (15-20% less) than usual, less blood is pumped with each heartbeat (i.e. a lower stroke volume). This diminished stroke volume is sensed by stretch receptors in the neck and heart, which send a signal to the brain to cause it to activate the fight or flight response (also called a reflex sympathetic surge) in the form of a burst of adrenaline.

Because of this adrenaline surge, we develop multiple symptoms before progressing to syncope, which include:

  • Palpitations
  • Anxiety
  • Sweatiness
  • A need to cool down, or get breath of fresh air
  • Clamminess
  • Feeling hot
  • Nausea
  • Feeling light-headed

As blood continues to pool, there comes a point in time when even with maximum adrenaline levels, blood pressure cannot be maintained, and as it falls, blood flow to the brain decreases, and you lose consciousness.


Situations to avoid

If you are prone to fainting, certain situations may be more likely to trigger an episode, such as:

  1. Being dehydrated
  2. Standing for long periods, e.g. on crowded public transport
  3. Feeling unwell, with an intercurrent illness, such as a chest infection
  4. Getting up quickly from a sitting or lying position


For more advice on how to prevent fainting, see Dr Lim’s top tips to help with syncope.


Dr Boon Lim is an expert in syncope and leads the renowned Imperial Syncope Diagnostic Unit based at Hammersmith Hospital in London. He has multiple research interests in syncope to improve quality of life for patients who suffer with this condition. If you’d like to see Dr Lim, visit his profile and request an appointment! Before your appointment, there are seven things Dr Lim recommends that you try to make a note of to help diagnose the cause of syncope.

By Dr Boon Lim

Dr Boon Lim is one of London's leading cardiologists and electrophysiologists. He specialises in heart rhythm disturbances, pacing and syncope at Imperial College and at his Harley Street clinic. He leads the established Imperial Syncope Diagnostic Service at Hammersmith Hospital and is frequently invited to national and international meetings to share his experience. He has a special interest in the mapping and ablation of atrial fibrillation (AF) and is highly-skilled at using modern 3-D mapping technologies.

Dr Lim has been awarded the prestigious Top Doctors Award in each of it's editions in 2017, 2018, and 2019-20. The Top Doctors Awards are peer-nominated by other medical specialists of the doctors that they would recommend to friends and family in times of medical need.

Dr Lim was awarded several distinguished prizes during his medical training at Cambridge University where he obtained a double First Class Honours Degree. He has continued his passion for education and research serving as an Honorary Senior Lecturer at Imperial College where he supervises several PhD students. He is very actively engaged in all aspects of research with a particular interest in developing the best techniques for treating atrial fibrillation. He is invited to speak both nationally and internationally to discuss his findings and to teach other physicians about the best techniques for complex mapping and ablation of atrial fibrillation and other complex arrhythmias.

He leads a very active syncope research team based at the Imperial Syncope Diagnostic Unit and is looking to improve healthcare delivery for patients through use of effective online education to help improve the quality of life for patients across the UK. 

Dr Lim is the author of the Penguin Life Experts book 'Keeping Your Heart Healthy' published in September 2021.

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