Plotting an inverse bell curve manually is not practical ... it takes so much data to see the trend ... basically for a decent time period, something like weeks or months, you'd have to look at all the time spent at a given pressure and determine the overall AHI for all the time at that pressure, then repeat for all pressures. Wish I could find my old screenshots ...
So here is a SH screenshot from earlier this year when I was still on my APAP ...
Sorry, the image attachment doesn't seem to work, or I'm too dumb with my 25 years of IT experience to figure it out lol ....
I have the machine range set to 17-20 cmH2O. I'm only giving it a range of 3 cmH2O to operate in. Pretty tight confines, and actually it could be tightened up more as I'll explain later.
What I want to point out here is the 'Pressure' line on the pressure graph. You'll notice the vast majority of the time I'm at 17 cmH2O. In fact, if you look at the 95% number in the summary, it is 17 cmH2O, which is also my Min pressure. Cruising along at 17 cmH2O is preventing the vast majority of my issues. What you want to work towards is having a graph like this, where your minimum pressure is preventing the vast majority of your issues. This is basically what a straight CPAP will do.
But, in this case, we have the advantage of range that an APAP provides. So again looking at the pressure line, you see it periodically depart the 17 cmH2O pressure and goes up. This is the APAP detecting issues and adjusting the pressure accordingly to try to resolve them. When it feels the issue has been resolved, it will drop back down towards the minimum pressure of 17 cmH2O. This is what all the spikes on the pressure line represent.
You'll notice even though I have the machine set to operate in the 17-20 cmH2O range, this night it only used pressures in the 17cmH2O to 18.5 cmH2O range and resulted in an AHI of 0.61. The peaks never came close to the 20 cmH2O I was allowing it. If I look at other nights earlier this year, this pressure pattern is very typical for me. As I mentioned earlier, the higher a pressure departs (either lower or higher) from your 90%/95% ideal pressure the more potential you have for issues. In this case I want to give the APAP enough space to deal with the odd issue, which it does between 17 cmH2O and 18.5 cmH2O every night for months on end. So, I could reduce the high end of the range down to 19 as it rarely ever gets to 19. It gets to 18.5 on a regular basis to deal with issues not handled by the minimum pressure, so it should just be set to 0.5 cmH2O above it's normal peak to keep it from running away on me to higher pressures, which we know increases AHI and also causes additional mask leaks, both which will reduce your quality of sleep.
Hope this makes some sense.
So, what you want to do to get your machine to the point it is handling your issues as well as my example. To get there you need to:
1. Keep moving your min pressure up until you find the spot where your min pressure is the same as your 95% pressure and
2. Keep adjusting your max pressure (upwards or downwards) to the point where it is barely above (0.5-1.5 cmH2O) above your typical spikes on your pressure graph.