Fainting at the sight of blood

The sight, smell or thought of blood may activate the body’s vasovagal reflex. Heart rate and blood pressure can drop, causing warmth, nausea, sweating, tunnel vision, dizziness or ringing in the ears before a brief loss of consciousness. Lying down usually helps restore blood flow to the head. This trigger does not by itself identify the cause of every faint; dehydration, low blood sugar, anaemia or a heart rhythm problem can also contribute.

Tell someone, move away from hazards and lie flat with your legs raised if possible. If lying down is not practical, sit and lower your head between your knees. Tense the muscles of your legs, arms and abdomen for short intervals, then relax and repeat. Do not stand quickly, drive or handle sharp equipment until you feel fully recovered. Afterward, take fluids and a light snack if you are able to swallow normally.

Seek urgent help if the person does not wake promptly, has trouble breathing, chest pain, a severe headache, an irregular pounding heartbeat, a seizure, serious injury or fainting during exercise. Arrange a medical review after a first unexplained episode, repeated episodes, or an episode without the blood trigger. Assessment may include the circumstances, medicines, blood pressure and heart rhythm.

Ask whether your pattern fits a vasovagal faint, whether medicines or fluid loss could be lowering your blood pressure, and whether you need an examination or heart-rhythm check.