I have vasovagal syncope, and I had a couple years when I also had anemia and I would pass out pretty frequently. I used to get that infamous “sense of impending doom” right before passing out. Several times I was 100% certain I was dying (and weirdly calm about it). So I’ve wondered since if it might be caused by a gradual, steady decline of blood pressure - not just BP being low, but BP *continuing to drop,* which is apparently what was happening to me at those times.

Subjectively it feels a bit like a balloon is deflating, or like you’re spiraling down. Like you’ve been flying your whole life and never realized it, but now you’re landing at last. Sometimes things literally start going dark (dark spots in front of your eyes). Emotionally it’s complete bizarre: there’s this overwhelming sense of *wrongness* - things just feel... not right, and very different than how they should feel. The first time it happened was worst because I knew that whatever was happening, it had never happened before in my entire life, so that right there seemed like really bad news. There is a sense inevitability and powerlessness. And an intense sense of vulnerability - sometimes I felt like a big animal was stalking me and I had to find a small dark cave and hide and be very quiet. I was always desperate to know that at least someone was with me, yet also certain they couldn’t help. 

Whatever it is, think it taps into some very ancient instincts for what to do when badly wounded: hide, curl up, hope that someone will guard you.