Panic attacks CANNOT cause you to pass out. Fainting is caused by insufficient oxygen getting to the brain due to a drop in blood pressure. You fall to the ground because when you're horizontal, its easier for the heart to get blood to your brain because you're not working against gravity.
During a panic attack, your blood pressure goes UP not DOWN. You also hyperventilate, so you've got plenty of oxygen in your blood.
If you are truly losing consciousness, it is not a panic attack. See a doctor regardless.
This is actually a technique to teach folks suffering from panic attacks that their fears about passing out are unfounded. Its impossible. I've been told this in the past, but I most recently read it in this book:
When Panic Attacks: The New, Drug-Free Anxiety Therapy That Can Change Your Life [Paperback]
David D. Burns M.D. (Author)