Why did I nearly faint during a workout?

Dr Mark Porter / 31 July 2017

Dr Mark Porter advises a reader who experienced a light-headed episode during a gym workout.



Q: After a rather sedentary life I have decided to heed my doctor’s advice and exercise regularly. But after a recent session on an exercise bike I felt very light headed and had to lie down.

Other gym users said I turned ghostly white. I recovered after about ten minutes but felt washed out for the rest of the day.

My GP has been very thorough and I ended up seeing a cardiologist who diagnosed post-exertional low blood pressure – essentially I nearly fainted. It was warm in the gym that day and I was pushing hard. How can I avoid it happening again?

A: Faints are caused by low blood pressure and / or an abnormal reflex (vas-vagal) that lowers the heart rate leading to decreased blood flow to the brain.

Coping with fainting: understanding the causes and symptoms

They are common with at least half of all women and a quarter of men experiencing one at some stage.

Recognised triggers include emotional stress (the person who faints at the sight of a needle), standing for long periods (the guardsman on parade) and over doing it during a work out.

Unlike more sinister causes of light-headedness during exertion – such as underlying heart problems – faints tend not to be a problem during strenuous exercise, but come on shortly afterwards. And an attack is more likely if you are tired, dehydrated, or working out in an overly hot environment.

Dehydration: strategies for prevention

The signs of an impending faint tend to develop over a few minutes and typically include excessive sweating, feeling a bit wobbly and looking pale. If the victim doesn’t sit or lie down, then the next stage is likely to involve keeling over.

The minute they hit they floor, the abnormally low pressure in the circulation no longer has to fight gravity to reach the brain, and normal function is restored. They are likely to feel groggy for a while afterwards, but normally come round very quickly once they are laying flat.

Prevention is the key. Don’t overdo it, if you are feeling tired. Drink plenty of fluids before and during your work out. And cool down slowly after strenuous activity – standing still encourages blood to pool in the legs making fainting more likely, so walk around.

What to do if someone feels faint

If someone does feel faint, lay them on the floor for at least ten minutes before letting them get back up. And, if they are in the gym or out on a run, send them home and suggest they book in to see their GP to double check it was just a vaso-vagal and nothing more serious. I am glad to hear you were given the all clear.

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The material is for general information only and does not constitute investment, tax, legal, medical or other form of advice. You should not rely on this information to make (or refrain from making) any decisions. Always obtain independent, professional advice for your own particular situation.