well hold on, though. the card in question (presumably underworld dreams) is a triggered ability, which means it doesn't happen literally as soon as you draw a card. there is a delay between when you draw and when you take damage, and in that time you can cast instants in response to the ability, including whatever you drew. the other posters are correct that you lose as soon as you hit 0 (or, at least, close enough for these purposes) but in the specific case described, you have a window before you actually die, not because you're surviving with 0 health but because you're not at 0 health yet.
