Heat Syncope (Fainting in the Heat)

What to do if someone faints in the heat

1. Lay the person flat on their back in the shade or a cool place.
2. Raise their legs about 12 inches (30 cm) to help blood return to the brain.
3. Loosen tight clothing and remove heavy layers.
4. Cool them with a fan, cool damp cloths or water on the skin.
5. When fully awake, give sips of cool water or a sports drink.
6. Have them get up slowly, sitting for a few minutes before standing.
7. Check their temperature if possible. Call 911 if they do not wake quickly, are confused, or have a temperature of 40 °C (104 °F) or higher.

How heat syncope happens

In the heat, the body cools itself by widening blood vessels in the skin and sweating. This moves blood away from the core. When you stand still, gravity pulls blood into the leg veins, and the leg muscles are not pumping it back. Sweating reduces blood volume further. Together, these lower blood pressure and blood flow to the brain, causing a faint.

The faint itself is protective: once you are lying flat, blood flow to the brain returns and you recover. Heat syncope is most common in the first few days of hot weather, before the body adjusts (acclimatizes).

Symptoms

Before fainting, people usually feel:

• light-headedness or dizziness
• blurred, gray or tunnel vision
• nausea
• sweating, pale or clammy skin
• weakness, yawning
• a fast heartbeat

The faint itself lasts seconds to a minute or two. Body temperature is usually normal or only mildly raised, which separates heat syncope from more serious heat illness.

Who is at risk

• Older adults, especially those taking blood pressure medicines or diuretics
• People who are not used to the heat, such as travelers or the first heat wave of summer
• People standing for long periods: soldiers on parade, workers, spectators, people in queues
• Athletes immediately after stopping exercise in the heat
• Dehydration, from vomiting, diarrhea, alcohol or not drinking enough
• Pregnancy
• People with heart disease or conditions affecting blood pressure control, such as orthostatic hypotension

Heat syncope vs other heat illnesses

Other causes of fainting to consider

Heat syncope is diagnosed when fainting clearly happens in a hot setting, with typical warning signs and quick recovery. Doctors consider other causes when the history does not fit:

• Vasovagal syncope, triggered by pain, the sight of blood or emotion
• Dehydration from illness
• Heart rhythm problems or structural heart disease, especially if fainting happens during exercise, without warning, or with chest pain
• Low blood sugar
• Seizures

Medical evaluation

After a first faint, especially in older adults or anyone with heart disease, a doctor may check:

• blood pressure lying and standing
• an ECG
• blood tests for salts, kidney function and blood sugar
• a medicine review

Further heart tests are done if the faint happened during exercise, without warning, or with palpitations or chest pain.

Prevention

• Drink regularly in hot weather, before you feel thirsty. Pale yellow urine is a good guide.
• Acclimatize gradually to heat over 1 to 2 weeks before strenuous activity.
• Avoid standing still for long periods in the heat. If you must, shift your weight and flex your calf muscles regularly.
• Get up slowly from sitting or lying.
• After exercise, keep moving slowly to cool down rather than stopping suddenly.
• Wear light, loose clothing and seek shade.
• Limit alcohol, which increases fluid loss.
• Review medicines with your doctor before hot weather if you take blood pressure pills or diuretics.

Frequently asked questions

What is heat syncope?

Heat syncope is fainting or near-fainting caused by heat. Blood vessels widen and blood pools in the legs, reducing blood flow to the brain, especially when standing still for long periods or getting up quickly in hot weather.

What should you do if someone faints from the heat?

Lay them flat in the shade, raise their legs, loosen clothing and cool them with fans or damp cloths. Once fully awake, give sips of water and help them up slowly. Call 911 if they stay confused, do not wake quickly or have a very high temperature.

What is the difference between heat syncope and heat exhaustion?

Heat syncope is a brief faint with quick recovery when lying down and little rise in body temperature. Heat exhaustion is a more general illness with heavy sweating, weakness, headache, nausea and a raised temperature, which may include fainting.

Is heat syncope dangerous?

Usually not. People typically recover within minutes of lying down. Dangers include injuries from falling and missing a more serious problem such as heat stroke or a heart condition. Get emergency help for confusion, high temperature or chest pain.

How can I prevent fainting in the heat?

Drink fluids regularly, avoid standing still for long periods, flex your leg muscles, get up slowly, keep moving after exercise, wear light clothing, seek shade and acclimatize gradually to hot weather.