Blackouts and fainting

What is actually happening when you faint

The brain depends on a continuous supply of oxygen rich blood to function properly. If that supply drops, even for a short time, the body responds by shutting down briefly. That is what happens during a faint.

Most episodes last only a few seconds to a minute. People often recover quickly, although they may feel tired or unsettled afterwards. Many patients describe warning symptoms in the moments beforehand, but in some cases fainting happens with no warning at all, and that difference matters.

It is worth knowing that a blackout is a symptom rather than a diagnosis. The point of an assessment is to work out what caused this particular episode, not to label the episode itself.

• Light-headedness, or a feeling that you are about to go
• Nausea
• Blurred vision
• A brief loss of consciousness, usually followed by a quick recovery
• Feeling tired or unsettled for a while afterwards

The common causes that are not the heart

There are several common causes of fainting that have nothing to do with the heart. The most frequent is vasovagal syncope, where the body overreacts to a trigger such as pain, emotional stress, or standing for long periods. That reaction leads to a temporary drop in heart rate and blood pressure, and you faint.

These forms of fainting are generally benign, although they can still be distressing, and a first episode is frightening whatever the cause turns out to be.

Near fainting and dizziness have their own set of everyday explanations too, including anxiety, stress and the effects of some medicines. A spinning sensation in particular, known as vertigo, is usually related to the inner ear rather than the heart. None of that means every episode can be waved away, and the patterns that point the other way are set out further down this page.

• Vasovagal syncope, triggered by pain, emotional stress or standing still for a long time
• Dehydration
• Sudden changes in posture, such as standing up quickly
• Low blood sugar
• Heat exposure
• Anxiety or stress

When a blackout may be coming from the heart

Heart related fainting occurs when the heart cannot maintain adequate blood flow to the brain. One of the most important causes is an abnormal heart rhythm. If the heart beats too slowly or too quickly it may not pump effectively, and that can lead to a sudden drop in blood flow and a blackout. Heart block, atrial fibrillation and other arrhythmias can all present this way.

Conditions affecting the heart muscle or the valves can also lead to fainting. Severe valve disease or cardiomyopathy may limit the heart's ability to respond to increased demands, particularly during physical activity. Low blood pressure is another route to the same symptom, because the circulation cannot always be held steady.

Several of the conditions covered elsewhere on this site list fainting or blackouts among their symptoms, which gives a sense of the range involved.

• Ventricular tachycardia, where the lower chambers of the heart beat too fast to pump efficiently
• Second degree and third degree heart block, where the electrical signals from the upper chambers fail to reach the lower chambers
• Sick sinus syndrome and tachy brady syndrome, where the heart's own pacemaker becomes unreliable or the rate swings between slow and fast
• Cardiomyopathy and ventricular hypertrophy, which affect the heart muscle itself
• Low blood pressure, where the pressure inside the blood vessels is lower than the body needs

The patterns that make a cardiac cause more likely

Certain features make fainting more likely to be related to the heart, and these are the ones a cardiologist pays closer attention to. A sudden blackout with no warning symptoms at all is a particularly important pattern, and it can be a key indicator that the cause is cardiac rather than benign.

If any of the features below applies to you, do not wait for it to happen again. Fainting during exertion needs assessing straight away, and a blackout that came with no warning, or with palpitations or chest discomfort, needs same day medical assessment rather than a routine appointment, even if you feel completely well now.

A common mistake is to dismiss a single episode because you felt fine afterwards. The opposite also happens, and people become very anxious about something that turns out to be straightforward. Early evaluation often provides reassurance, and where a heart condition is identified it allows appropriate treatment to begin.

• Fainting that occurs during physical activity
• Fainting that happens without any warning
• Fainting associated with palpitations
• Fainting accompanied by chest discomfort
• Fainting in someone with known heart disease
• More than one episode, or an episode that caused an injury

What Dr Jogiya would do about it

When assessing fainting, the most important step is understanding the circumstances surrounding the episode. Dr Jogiya will usually ask detailed questions about what happened before, during and after the event, including what you were doing, whether you had any warning, and how long it took you to feel normal again. If somebody saw it happen, bring their account with you, because it often fills in the part you cannot remember.

One of the challenges with fainting is that it frequently occurs as a single event, and a short recording made in a quiet clinic room can easily miss a rhythm that comes and goes. That is why longer monitoring is so useful here. Capturing the heart rhythm at the time of symptoms is often the key to making an accurate diagnosis.

Which of these is worth arranging is decided at consultation, from your own history rather than from a page like this one, and in some cases additional tests are needed depending on the suspected cause.

• A 12 lead ECG, which looks at the electrical conduction of the heart from twelve different vectors
• Blood pressure measurements, including lying and standing readings
• An ECG monitor, also called a Holter monitor, worn from 24 hours up to 14 days to catch an intermittent rhythm
• An echocardiogram, an ultrasound scan of the structure and function of the heart including the valves and chambers
• A tilt test, which assesses the causes of dizziness, loss of consciousness or syncope by reproducing your symptoms while monitoring your heart
• A cardiac MRI, where the heart muscle needs looking at in greater depth

What you can do in the meantime

If you feel light-headed or sense that you may faint, sit or lie down immediately. Getting yourself down before you fall is the most useful thing you can do in the moment, because a fall from standing is how a faint causes an injury.

Keeping a short note of each episode is genuinely useful at a consultation. Write down the date, what you were doing, whether you had any warning, and how you felt afterwards.

If you drive, tell your GP or your specialist that you have blacked out. The DVLA has rules about driving after a loss of consciousness and they depend on the cause, so it is a question to ask rather than one to guess at.

These measures can reduce the likelihood of fainting in certain situations, but they do not replace medical assessment if episodes are recurrent or unexplained.

• Sit or lie down as soon as you feel it coming on
• Stay well hydrated
• Avoid standing for prolonged periods
• Rise slowly from sitting or lying positions
• Pay attention to warning symptoms rather than pushing through them
• Keep a note of when episodes happen and what you were doing