ZocdocAnswersWhen I got blood taken out of me why did i pass out?

Question

When I got blood taken out of me why did i pass out?

I am a 23 year old male. I am 6'3 and weigh in about 240 pounds. Some doctors have told me that i have high blood pressure. When i got the blood taken out of me the doctor checked my blood pressure. She told me that it was normal and checked it again. Both times she said it was normal. Im not taking any type of medication so im not sure why i would just pass out. Also my arm is really bruised from where she drew blood out too. Thanks for any help you could offer me.

Answer

Passing out in the setting of having one's blood drawn is a very common phenomenon. Patients that experience this reaction are said to have situational syncope, with syncope referring to the loss of consciousness and the ability to maintain one's posture. In this type of syncope, certain situations trigger a response in the body known as a vasovagal reflex. The vagus nerve is one of the body's autonomic nerves (not controlled consciously) and can be triggered by different stimuli (such as the sight of blood or needles, urinating, or having a bowel movement). When the nerve is stimulated, it can cause a decrease in a person's heart rate and blood pressure, causing a brief period when the brain is under-perfused with blood, causing a brief lack of oxygen and subsequently causing the person to pass out. This often happens in the setting of having blood drawn, but doesn't physically have to do with the amount of blood taken; it is usually just the sight of the blood and needle (especially in someone who hasn't had their blood taken before). Blood pressure and heart rate usually rapidly recover in these circumstances. A small bruise at the sight where the blood was taken is also a common occurrence; applying ice if it is sore can help.

This answer is for general informational purposes only and is not a substitute for professional medical advice.

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