Why does postural hypotension cause fainting?

Blood can pool in the legs and abdomen when a person stands. Usually, the body tightens blood vessels and adjusts the heartbeat quickly enough to maintain circulation to the brain. If this response does not keep up, vision may dim, hearing may seem distant, nausea or sweating can appear, and a person may collapse. Consciousness commonly returns once they are lying flat, but a collapse still needs attention.

Fainting is a very common presentation of Postural Hypotension. A blackout after standing has several possible causes, so it should not be assumed to be due to blood pressure without assessment.

If you feel a faint coming on, lie down straight away and raise your legs, or sit with your head lowered if lying down is not possible. Tell someone nearby and do not try to walk through the feeling. After an episode, stand only with support and avoid driving until a clinician has advised that it is safe.

Someone who has fainted should be placed on their back with their legs raised if they are breathing normally. Loosen tight clothing and check for injury. Do not give food or drink until they are fully alert.

Call emergency services for a person who does not wake promptly, is not breathing normally, has persistent chest pain, has had a serious injury, or has seizure-like movements that do not stop. Urgent assessment is also appropriate after fainting during exercise, while lying down, or with palpitations, severe headache, weakness, or speech problems. A first episode or recurrent fainting warrants a medical appointment.

Describe what you were doing before you fainted, any warning symptoms, how long recovery took, medicines you use, and whether anyone saw the event. Ask whether heart rhythm checks, blood-pressure measurements in different positions, or other tests are appropriate.