The problem would be that you would need a very heavy gas in that mixture. Which would soon unmix, with the heavy gas at the bottom and breathable gas at the top.
Also be careful with breathing even minute amounts of such heavy gasses, as they will accumulate at the lowest parts of the lungs.
I remember a TV show where they breathed such a heavy gas to show what it does to your voice (it transfomed it way down, just like helium transforms it up). They had to stay upside down after that for some time to get the stuff out again.