Situational syncope why we faint under stress

If you faint at the sight of blood or needles, you are someone that experiences situational syncope. This type of temporary loss of consciousness is a response to a certain trigger or situation that causes you stress. It is very common and seen in up to 50% of patients in their lifetimes.

In this article, Dr Boon Lim, an award-winning cardiologist and one of the UK’s leading syncope specialists, explains what is happening to our bodies when we faint, how it can be avoided and offers some tips to help you cope when you feel a syncope episode beginning.

What happens when we faint?

Fainting is essentially caused by two things: low blood pressure and a low heart rate. This reduces the perfusion of blood and oxygen to the brain and leads to a transient loss of consciousness.

The type of fainting we are referring to is known as situational syncope or vasovagal syncope and occurs because your body overreacts to a certain trigger; whether that be at the sight of blood or any kind of emotional distress. It causes your heart rate to drop and blood to rush to your legs which lowers your blood pressure. As a result, less blood flows to your brain and you experience a transient loss of consciousness.


It is important to recognise another cause of fainting, known as cardiac syncope. This affects around 10 to 15% of patients that go to A&E. Cardiac syncope can be dangerous, and in some cases, fatal. It is very serious and should always be investigated.

Why do we faint?

There is an evolutionary aspect to why humans faint. The polyvagal theory suggests that the vagus nerve (part of your nervous system) can become very protective in certain situations, causing you to lose consciousness as a way to protect you from an observed threat.

If we take, for example, certain mammals like a possum, they often play dead when they are faced with something threatening. Right before this period, their blood pressure drops and they lay lifeless. They are in fact experiencing an episode of vasovagal syncope (situational syncope). In humans, when we face similar threats, such as the sight of blood or extreme emotional distress, or in the cases of many medical students after seeing graphic sights in the operating theatre, our body can overreact and we behave similarly to the possum. 


Can fainting and seizures be confused?

Quite often, to an untrained eye, an episode of situational syncope can appear like a seizure. This is because fainting can involve prolonged periods of bradycardia (slower heart rate) lasting 8-15 seconds. During this length of time, the brain isn’t supplied with sufficient blood and oxygen, so it can cause the body to shake and jerk in a similar way to someone having a seizure.

What do you feel before fainting?

Before fainting, you may experience presyncope. Usually, there is a strong adrenaline surge that brings about a hot flush, cramps, shortness of breath, palpitations, feeling weak at the knees, dizzy, etc. It happens when your brain isn’t getting enough oxygen due to the change in blood flow.

How can you avoid fainting?

Fainting can come on fairly abruptly for some, but for others, they may very well recognise that it is about to happen, so they can adopt strategies to abort the faint, for example, by sitting or laying down.

Being conscious of triggers (these are the events that cause you to faint) can help you too. If you are prone to fainting when having your blood taken or from prolonged standing and you can recognise the feeling, you should act accordingly; either by sitting or lying down to avoid it getting worse. It can be difficult to avoid syncope as you may not see it coming. 

Fainting can be dangerous, especially, if you are walking alone in the street or standing at the top of a flight of stairs. If you develop presyncope warning signs and you can take the relevant evasive action to avoid fainting, you may well be able to effectively manage it and keep yourself from falling over and hurting yourself.


There is nothing to be embarrassed about with fainting. You shouldn’t let the fear and anxiety around fainting get in the way of fulfilling what you want to do. The more times you can combat and conquer it by taking preventative strategies, the more you will be able to face stressful situations and continue with your day-to-day life.

Are there exercises to prevent fainting?

A very helpful exercise, and one that can work immediately, is called an isometric counterpressure manoeuvre. When you experience presyncope symptoms, you should hold your hands together and pull them slightly apart; clench your teeth; tense your buttocks, quadriceps and calves and cross your legs. It is an effective strategy in stabilising blood pressure by squeezing blood from your legs back to your heart.

If you feel like you are about to faint, you should sit down to prevent hitting your head when you fall. However, if you feel a slow onset of pre-syncope symptoms, the isometric counterpressure manoeuvre can be good enough to help you carry on with what you are doing without fainting or it can at least bide you enough time to ask for help and remove yourself from the situation.

There is an excellent video which describes isometric counterpressure manouevres and demonstrates clearly the blood pressure and heart rate changes by performing these simple exercises. Check this out on the Stop Fainting website. 

Top tip

Many patients come to me after trying many drugs and therapies from reputable clinics to help them with their problems fainting. I noticed that one prevention method they often failed to stick to is drinking enough water. Many people underestimate how important it is to drink sufficient amounts of water throughout the day and how well it can help prevent episodes of syncope.

Therefore, you should be making sure that you are drinking at least three litres of water per day and two of those ideally should be drunk before lunch, especially for frequent fainters.


Dr Boon Lim was recently invited to talk in a podcast on syncope in the BMJ’s Sharp Scratch. In this podcast, Dr Lim addresses medical students specifically talking about their best strategies to combat situational syncope, including syncope during blood taking, prolonged standing and how to handle the emotional stress of syncope.  

Listen to Dr Lim speaking to the BMJ student group here. 

You can also download the full podcast and on Spotify or Apple pods.

Dr Lim is the founder and creator of the Stop Fainting website which is a useful resource to help patients understand why they faint and how to prevent further episodes of fainting.

To make an appointment to see Dr Lim, go his Top Doctors profile. To read more about him and the conditions he specialises in, you can also visit his website.