syncope from abdominal crunches

[corninATL]

I get a severe head rush and near blackouts every time i attempt abdominal crunches.  My heart compensates by beating very hard but it takes a frightenly long time to recover.  Even just 1 or 2 crunches it happens.   I also get it to a lesser degree from weight bearing exercises which use abs indirectly,   but not from exercises that don't use the abs nor from cardio.     I am going to admit I'm 70 lbs over weight but I'm able to easily do 30 mins of cardio...I'm like an athletic person under a layer of fat if that makes sense.   I had an EKG, stress test and echo cardiogram done, they were normal

[Cleveland Clinic]

when you do a cruch or bare down you decrease the blood return to the heart which causes your heart to speed up a little and beat harder.  This in return causes a vagal response which causes you to nearly pass out.  

[ed34]

Are you warming up first to get your heart going? You do know if you jump straight into intense exercise you run the risk of creating heart rhythm problems? It sounds like your blood pressure is dropping sharply because you aren't doing enough of a gentle warm up to get the heart 'gradually' increasing in rate. As you get fitter, it will have the ability to respond much quicker.