got a character you wanna make pass out (or very narrowly avoid it)? good! here's some thoughts:
head: light-headedness, dizziness, etc are all what come to mind, but there can be an overwhelming sensation of pressure. it's like a soft blanket used for violence, to smother their brain into submission.
eyes: never forget the classic fade to black or tunnel vision, but make sure to remember seeing stars! tiny bright floaters before and after passing out, and even if they don't experience a complete syncope. sometimes it can feel like their eyes are being squeezed in a vice grip, and they may end up hunched over, hands pressed to their eyes in a desperate attempt to relieve the pain as they focus more on that than the fact that passing out is imminent.
mouth: maybe it's dry, but the sensations of numbness and even tingling can dance across the tongue, lips, and teeth. maybe lead has replaced all of their muscles in their tongue, and speaking feels like a struggle as the mouth and brain shout across a phone line that's quickly losing reception.
ears: when the world gets put on a muffled semi-mute, that can be distracting. odd distortions happen, with sounds making it across the barrier but not quite lighting up the parts of the brain that would acknowledge the importance of what's being heard. yeah, it can be like being underwater, if underwater also came and went and let sounds cross at random.
chest: no matter how much air is frantically gulped down, it's never going to be enough. there's a hunger for oxygen, a hunger that simply isn't sated no matter how they try. it burns, it aches, it begs to be acknowledged and taken care of but nothing seems to actually fix it.
abdomen: imagine the feeling of a roller-coaster drop, but it's just the internal organs taking the plunge. it's an overwhelming rush of sensation that really shouldn't be there, and it can get overwhelming.
limbs: the overwhelming burn of a muscle that's at the end of its rope pops up before sensation is lost. going limp and numb would be a blessing compared to the shriek of the tissues insisting that something is very wrong. it seems to be burning the bridges that tell limbs what to do; the response time is incredibly slow. limbs can also give out before the brain does, leaving somebody on the floor before they even fully lose consciousness.
