Dehydration and Passing Out: What to Know

When the body loses more fluid than it replaces, blood volume can fall. Standing up may then make it harder to deliver enough blood to the brain for a short time. Light-headedness, weakness, blurred vision, nausea, headache, thirst, a dry mouth, and reduced or dark urine can occur before a person faints. Dehydration may be linked to an illness such as Gastroenteritis or Heat Exhaustion, rather than being the only problem.

Passing out is a brief loss of consciousness and muscle control. A fall can cause injury, and fainting can also have causes unrelated to fluid loss, including heart or nervous-system problems. Do not assume dehydration explains a first episode, an unexplained episode, or fainting during exercise.

Lay the person flat and raise their legs if there is no suspected injury and breathing is normal. Loosen tight clothing, keep them cool, and check that they respond and breathe. Do not give food or drink until they are fully awake and able to swallow. Once alert, small frequent sips of water or an oral rehydration drink may help, particularly after fluid loss; follow the product directions.

If the person is not breathing normally, has a serious injury, remains unresponsive, or has a seizure, call emergency services and follow the dispatcher’s instructions. Stay with them and do not let them drive afterward. A person who feels faint should lie down promptly rather than trying to walk through it.

Seek urgent medical help after fainting with chest pain, a racing or irregular heartbeat, severe headache, trouble speaking, one-sided weakness, significant breathlessness, ongoing confusion, or heavy bleeding. Emergency assessment is also needed if the person fainted during exertion, was pregnant, has diabetes, has known heart disease, or sustained a significant head injury.

Arrange prompt medical assessment if fainting recurs, the cause is unclear, vomiting or diarrhoea prevents drinking, urine remains very low or dark, or dizziness does not settle with rest and fluids. A clinician may check blood pressure lying and standing, hydration, medicines, blood tests, and heart rhythm. Do not delay care in a child, older adult, or person who cannot reliably drink.

Ask what may have triggered the episode, whether any medicine can worsen low blood pressure, and how much fluid replacement is suitable for your health conditions. Mention the circumstances, warning symptoms, duration of unconsciousness, injuries, recent illness, urine changes, and whether anyone observed shaking or an irregular heartbeat. Ask whether you need an ECG, blood tests, or follow-up.