I have had panic attacks for the past 6 yrs & in that time I have fainted 3 times. For most people it is true that when you are having a panic attack, your blood pressure rises & makes you feel light-headed, like you might faint, but typically you don't. However, I am hypoglycemic, so, sometimes when adrenaline is released in my system, it can cause my blood sugar to drop, thus resulting in fainting. I was extremely confused by this as it sounds you are as well. So, I talked to my doctor about it & this was his explanation. I keep a small snack with me at all times, usually a pack of crackers & if I get too anxious, I eat the snack so that I can at least ease my mind about not fainting during a panic attack. I hope this helps & that you are doing o.k.
