There's certainly a difference between not being able to breath and feeling like you're not getting oxygen, which is what I imagine Floyd was actually experiencing. I've experienced a panic attack before and it certainly feels like you cant breath, no matter how deeply and effectively you are breathing, it still feels like not enough. He was more than likely having a heart attack and I'm no doctor but a heart that's freaking out doesn't pump blood very well, it may even be the part of a heart attack that kills us, again, no doctor.