CPAP pressure calculations

[leswaller]

Can anyone tell me the calculation or the algorithm used to calculate the correct pressure for a CPAP user

[James @ Intus]

Assuming you aren't talking about the very complex algorithms for APAP, the fixed pressure for CPAP isn't formulated at all. It's just by means of titration so the doctor closely monitors progress and keeps tweaking the pressure. That is repeated (in an ideal situation) until you arrive at the lowest pressure with the best reduction in AHI.

There isn't a magic formula as one mega obese person's AHI of 65 will drop to sub 5 on pressure 8 whilst the next slightly overweight person's AHI of 25 struggles to drop to 10 at pressure 13. Of course, there are rules of thumbs, averages etc. but you can't translate another patient's needs onto your own needs. An experienced doctor who has had a good look at you will know a decent pressure to start at, often 8, 9 or 10 and then they'll call you in after a few weeks and up it if not all snoring/apnoea events have disappeared. Repeat this until "cured" and that's your formula.

This is why trials are done on APAPs because these automatic machines arrive at the optimum pressure faster (and with less expensive clinical involvement) than manual titration. Then they give patients a fixed pressure CPAP set at the pressure found by the trial-APAP.

Problem is of course, if you then loose/gain weight or have a cold, your pressure needs may change but you can't walk into your clinic with your CPAP under the arm every week to adjust the pressure. That's why APAPs are so popular. They will change with you as and when you need it.

[leswaller]

Hi James, thanks for the swift reply. Bearing your comments in mind, do you have any suggestions on the following scenario. When I was diagnosed all I had was a simple overnight sleep study followed by an outpatient appointment at which I was given my CPAP machine (a bog standard Resmed S8 Escape) preset (presumably by guess work). I did not have a titration study. Since then I have succesfully used the machine, my weight has gone down, but I feel that the machine is not fully compensating for all the apnea events as I occasionally wake momentarily during the night. Although it must be said that the number of events have reduced significantly. I know how to get into the clinical menus and hence I know the pressure the machine is set at. I was considering increasing the pressure to see if this had any further effect on reducing the apnea events, but I wanted to have a feel for how much to increase it by - hence my question about the pressure calculations. Buying an APAP machine is not an option.
All suggestions greatfully received.

[James @ Intus]

"Easy does it!"

Not saying anyone should be "hacking" their own machine so try and get medical feedback on what you do where you can. But if you do, and there isn't much you can break or do to hurt yourself, take small steps. One of our patients recorded his snoring and choking sounds every night and if on 3 nights in a row it persisted, he owuld up the pressure by half a cm. After 2 weeks he got to a pressure where it was all gone and he was sleeping restfully at 10.5 instead of the guessed 8 he got from his clinic.

So I'd keep a log so you can reference what you did against the outcomes. A nice excel spreadsheet would do, you can even post nice screenshots of your graphs here!

Then when you have been a few months at what you believe to be the perfect pressure, you can consider renting a finger pulse test to double-check your blood oxygen stays where it belows, hence indicating you breathe properly.

So, to answer it in short, easy does it!

[leswaller]

Thanks James, I will proceed in 0.5cm increments. If I dont post again you will know that I have inflated like a balloon and taken off

[Tigers Fan]

Do not shoot down flying objects in your bedroom!

Fear not, Les! before you are inflated and take off, one of two things will happen :-

1. you'll have to tighten the headgear straps to the point of collapsing your skull

or

2. more likely, the mask lifts and the leakage rate is high enough to awaken the household and ........

I've kept my APAP set at Nurse Ali's first guesstimate of 10 cm - minimum pressure and acts like a CPAP. I sleep fine and the APAP does its complicated algorithm thing as needed until, for whatever reason like forgetting to switch on the humidifier, my mouth dries out and walls stick together or some such. The APAP then decides I need to fly to hospital for immediate resussitation and does its damdest to inflate me. I don't know whether it does it by increasing pressure in increments of 0.5 cm or just cranks up to max, but at 20 cm I'm halfway down the runway. Having also learned how to get to the Clinician's menu,I'm doing the opposite to you - reducing the max air pressure setting a little at a time, looking for the cross-over point between getting the therapy and needing to wake up for a mouthful of water!

Yes, a simple solution seems obvious - but on cooler nights the moisture condenses out of the humidified air. And other times, my mouth just dries out for the fun of watching the result!

Tigers Fan

[leswaller]

Tigers Fan
I have this mental picture of you being accelerated across the bedroom pursued by an angry APAP machine on full bore
My bog standard CPAP was set at 8.5 which was probably how it was taken out of the box. I am now up to 10 which I think is probably about right judging by last night's sleep. I will keep it at this for a few nights and see how I get on