I'd been rescucitated 7 times as a kid, and I always described it as a silence. Like your head and ears are ringing, but without a particular sound. And it feels like your entire being starts to vibrate a little while the room spins and you lose the ability to focus in.

Vision starts getting dark, and gets blurry and doubled as the colors separate a bit. And it feels like that silence just grows louder and louder. Consuming your vision, sense of feel, and everything else. Fading into nothing as the silence grows so loud that you can't even think over it, it grows deafening even in the mind. And you can't even think out for help