Blood pressure and fainting

A drop in blood pressure can reduce blood flow to the brain briefly, especially after standing, during illness, or when dehydrated. Fainting can also occur with low blood sugar, anaemia, or an abnormal heart rhythm such as atrial fibrillation. Light-headedness, nausea, sweating, blurred vision, or warmth may come before the blackout, although warning symptoms are not always present.

Lie flat and raise your legs if possible, then loosen tight clothing and stay still until fully alert. Do not stand quickly or drive until a clinician has assessed the event. Record what you were doing, your position, warning symptoms, injuries, medicines, food and fluid intake, and how long recovery took. A clinician may compare lying and standing blood pressure and review your heart rhythm.

Call emergency services if fainting occurs during exercise, causes a significant injury, follows chest pain or breathlessness, involves a sustained irregular heartbeat, or recovery is delayed. Urgent assessment is also needed for repeated episodes, fainting without warning, pregnancy, or known heart disease.

Ask whether your medicines, hydration, blood sugar, blood count, or heart rhythm could be involved and whether you need monitoring or specialist review.