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Why do events occur with APAP machines
#1
I am having a hard time understanding this. APAP machine is supposed to automatically adjust pressure to prevent apneas.
Why do apneas happen at all when using an APAP machine ? 

I have just started on my therapy and my AHI is all over the place (all below 5) - but I want to understand what's causing these apneas and why the APAP machine isn't preventing them already ?
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#2
The machine relies on sensing common precursors of obstruction such as snores, flow limitations and volume changes in order to raise pressure ahead of events. Sometimes those precursors are not present, pressure is too low for the machine to respond fast enough, and sometimes, well, you know, s*** happens. Also no APAP is programmed to stop CA events. Without seeing any information on your pressure range, or data on what happened ahead of the event, it's impossible to say why you are having events. We can optimize APAP therapy, but zero events are actually rare.
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#3
In order to fully answer this you'll need to show data from sleepyhead, but...


Since you are using a dreamstation it appears to me that your min pressure is too low to prevent obstructive events.  APAP machines do not adjust pressure until the apnea event is over, however they may change it due to snoring, flow limitations etc.  

PR machines are notoriously less aggressive than resmed machines, and as such, you need to be closer to your therapeutic pressure for them to be as effective. Don't get me wrong, when you have the pressure settings correct, treatment by a PR machine is comfortable and excellent.
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#4
Funny you say that. My prescription says pressure should be 10. The specialist did mention I may require close to 11 - 'but we'll monitor' (over a period of 4 months).
Initially I set my min pressure to 8.5 and max at 20. I had the lowest AHI reading with this.
Figured an APAP shouldn't need a min and max pressure - so I started reducing my min pressure by a cm a day and I had the highest AHI reading when my min was set to around 6.
Funnily enough, the 90% pressure has been dropping steadily over the last 5 days I had my therapy on. Began with 9.5 and lowest was 7.5. 
All this is just a data set from the last 5 nights. I don't want to be in a situation where my apneas would be prevented with a lower pressure - but I keep pumping a higher pressure.
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#5
Fyi. 90% pressure means that your pressure was at that value or BELOW 90% of the time.
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#6
The Respironics machines are so slow to respond, that as you reduce minimum pressure, the average and 90% will also drop as the machine constantly stays below your therapeutic optimum. With your titration results, I would not put the minimum pressure less than 8.5-9.0 or about 2-cm below your 90%. If you were using a Resmed Autoset, you could probably get away with pressure around 6.5-7.0. Yes, the machine are auto-adjusting, but you have to give them a reasonable head-start or you will have events.
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#7
(04-10-2017, 09:44 PM)bonjour Wrote: Fyi. 90% pressure means that your pressure was at that value or BELOW 90% of the time.

Thank you for this info - how would you know the pressure that or below ? Now that I google for it, I can read about this in various forums - but how are people coming to know that the 90% pressure means it could have also been below the saod reading ? 
I did set my min to 9 last night and had the lowest AHI in the past week. Thanks again for posting - I would have never known about this otherwise. You have touched my understanding of therapy for a life time. 

(04-11-2017, 08:43 AM)Sleeprider Wrote: The Respironics machines are so slow to respond, that as you reduce minimum pressure, the average and 90% will also drop as the machine constantly stays below your therapeutic optimum.   With your titration results, I would not put the minimum pressure less than 8.5-9.0 or about 2-cm below your 90%.  If you were using a Resmed Autoset, you could probably get away with pressure around 6.5-7.0.  Yes, the machine are auto-adjusting, but you have to give them a reasonable head-start or you will have events.

Reasonable head start is just a hit/miss depending on various pressures one sets ? 

What is the correct way to set min and max pressure on an APAP machine ?
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#8
As a rule of thumb, when someone is starting on APAP therapy, my recommendation will be to set the minimum pressure at the average pressure, or 90% pressure minus 2-cm, and that recommendation will vary by brand. Whichever is higher (average or 90%-2.0) seems to work best for the Dreamstation. It really depends. When we have data, we can see some people with very consistent pressures and a need to control obstructive events, while another person may be all over the place in pressure and have what appears to be mixed or central events. I can't give you a hard rule, since people are involved, and they all seem to have an individual need.
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#9
(04-10-2017, 08:23 PM)BHappy Wrote: I am having a hard time understanding this. APAP machine is supposed to automatically adjust pressure to prevent apneas.
Why do apneas happen at all when using an APAP machine ? 

I have just started on my therapy and my AHI is all over the place (all below 5) - but I want to understand what's causing these apneas and why the APAP machine isn't preventing them already ?

because of ashby's law of requisite variety
we are unable to control enough variables to fully control everything about breathing

this is basic servo control theory plus systems theory saying we cant do everything 

there are several feedback loops going on to control our breathing
and some are internal and not practical to diddle 
others are external and we can do some things

even if we wired you up with the whole nine yards of a PSG there could be things that we could not control
without invasive procedures that are still not guaranteed to work
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#10
An APAP machine is nice in that it responds to events...but it is not predictive. As a result, they chase events, rather than actively prevent them.

You can have an APAP set for a pressure range of 6-20 cm H20 (common for a DME to do this out of the box and used to get a bit of a rough titration on pressure in the first few days)...but if you need a pressure of 12 cm H20, you'll have events until it responds enough to have you at the pressure you need. When you stop having events, the machine then wants to return to the minimum pressure so it starts dropping the pressure and you get events again. The cycle continues unless the settings are changed to better reflect the actual therapy you need.

APAPs are wonderful when they are set up correctly and can help for the times when you need that adjustment because of how you're sleeping, the head cold you have, it's a full moon (or whatever might influence your sleep apnea at the time). But just like any other computer driven hardware, they need to be set up right (correct base pressure and range) otherwise garbage in = garbage out.
INFORMATION ON APNEA BOARD FORUMS OR ON APNEABOARD.COM SHOULD NOT BE CONSIDERED AS MEDICAL ADVICE. ALWAYS SEEK THE ADVICE OF A PHYSICIAN BEFORE SEEKING TREATMENT FOR MEDICAL CONDITIONS, INCLUDING SLEEP APNEA. INFORMATION POSTED ON THE APNEA BOARD WEB SITE AND FORUMS ARE PERSONAL OPINION ONLY AND NOT NECESSARILY A STATEMENT OF FACT.
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