I've been diagnosed with orthostatic hypotension, aggrivated by low volume (read dehydration). I passed out at the hospital early one Monday morning, the day after a race. Ironically I had taken my dad there for a procedure and when I stood up to leave his bedside so he could change....conk. Woke up to "CODE PURPLE- GI... CODE PURPLE- GI" blaring on the PA system and about 4 people standing around me.

After several follow up visits and something called a tilt table which does exactly what is sounds like, the need to drink and drink and drink and drink and drink some more eliminated that problem. Evidently, I have low resting BP and HR to begin with, and coupled with even mild dehydration will lead to passing out.

Get checked out before it happens again- especially if you're driving.