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How did the 02 come through the machine? You can put an O2 port at the machine end of the big tube or the face mask end of the tube. The O2 goes and gets delivered.

If the O2 generator doesn't have enough pressure, then I could see where the CPAP could drive it back but that shouldn't happen on any modern O2 delivery system.

One other issue is the O2 via CPAP needs to be at a higher flow rate than via a cannula since the cannula O2 tens to be breathed in at a lower "wastage" rate than with CPAP which is actively forcing the O2 out of the vent ports. Any data recording CPAP device will tell you the flow rates and that can be combined using simple ratios of the O2 flow rate to show what you're getting. If I remember right when I was last doing these calculations, the flow rate can be 20% or a 1:5 ratio when I was looking at my mothers data. The solution that worked for her was about 15L/min for a few days and then drop it off.

When my husband was in the the hospital with some mild congestive heart failure and low bl pressure,they started him on 02 at 2L through the nasal canula but at home on the concentrator he needed 2.5L. It's been reduced back to 2L the last month since his cpap pressure and leak issue has stabilized. His adapter is attached to the hose on the humidifier end where there is no movement or chance of kinking. At the mask end there's too much possibility of getting tangled up. I also replace the 25ft tubing every month per RT's instructions and make sure it lays flat.