(07-04-2014 06:36 AM)Caddyshack Wrote: So should we set the lower pressure at 95%? If, for instance, the average pressure for 95% of the night was 10, would we set the lower pressure at 9.5?
If so, does the higher pressure follow a similar formula?
No. It definitely does not work this way. Or rather, it's not that simple.
The 95% pressure is the level the pressure was AT or BELOW for 95% of the night. 95% is a statistics term and whether or not it has any medical implications in your particular case for your PAP therapy depends on a lot of other variables, not all of which are numbers gathered by the PAP machine.
That said: The long term 95% (or 90%) pressure level pulled from a week of autotitration on a wide-open APAP data is often used as a good estimate of what pressure to use in fixed CPAP mode. If the 95% pressure is 10cm, then setting a fixed CPAP to 10cm provide enough pressure to prevent almost all the apneas and hypopneas from occurring even during extended periods of supine REM sleep when the OSA is at its worst.
(07-04-2014 06:51 AM)JimZZZ Wrote: This is a good question that I too have asked. To my knowledge, so far, the selection of a pressure range is so personalized with so many variables that a rule of thumb is difficult to establish. Please tell me I'm wrong.
When setting an APAP range, there are a lot of variables. One variable that a lot of people do use is the 95% pressure level. But how people use that level varies quite a bit.
One reasonable rule of thumb is that the minimum
pressure setting should not be too far below the 95% pressure level. If you need 14cm of pressure at certain times throughout the night, setting the min pressure at 5cm may cause the machine to have trouble gracefully responding if a cluster of events gets started when the pressure is at 5cm, but needs to be close to 14cm. It takes time for a PAP to increase the pressure, but it also takes time for the airway to respond to a pressure increase and stabilize. So as counter intuitive as it sounds, sometimes setting the minimum pressure setting too low can actually increase the 95% setting because the machine winds up responding aggressively enough to clusters of events by trying to quickly increase the pressure, but the increase in pressure happens so rapidly that the airway can't stabilize before more events and another pressure increase occur.
In practice it seems that most people tend to find that the minimum pressure setting usually needs to be within about 4 or 5 cm of the 95% level, and many people will say that the minimum pressure setting should be within 1-2 cm of the 95% pressure level.
There seems to be a lot more debate on whether the 95% pressure level is useful for deciding where to set the maximum pressure setting.
A lot of people get concerned if their 95% (or 90%) pressure level equals their max setting: There's this concern that if you're constantly bumping up against your maximum pressure setting, that you need to allow the machine to go higher (and higher and higher) if it wants to. In some situations, a modest increase in max pressure may indeed be warranted: If you're getting a lot of clusters of OAs or if you're snoring during the times when the pressure is at the max setting, increasing the max setting a bit may help the machine prevent the events from happening.
But if you don't have a whole lot of events when the machine is at its maximum pressure setting, increasing the max pressure might not do much in terms of making your therapy more optimal.
I'll offer myself as an example: In my case, my titrated IPAP/EPAP pressures 8/6 cm (on multiple bi-level titrations). My stomach cannot tolerate these pressures all night long, every single night and it is difficult for me to fall asleep when my pressure is that high because the pressure is high enough to trigger aerophagia problmes. By using the Auto BiPAP range that I do (min EPAP = 4, max IPAP = 8, min PS = 2, max PS = 4), I can get to sleep without getting too much aerophagia, and I don't usually wake up with a rock hard stomach in the middle of the night. However, my median IPAP = 8 on most nights (and so my 95% IPAP also equals 8 as well.) My AHI is less than 2 on the majority of nights, and unless there's a visit from the aliens or I've got the flu, my AHI is always less than 5. The IPAP pressure increases are caused by flow limitations and the PR "search algorithm."
As for my EPAP pressures: The median EPAP is often, but not always equal to 4cm, my min EPAP pressure setting. The 95% EPAP bounces around from night to night. Some nights, my 95% EPAP = 4, other nights it equals 6 (my maximum possible EPAP). My long term 95% EPAP is usually around 5.5, which is just a smidgeon under my max EPAP pressure setting of 6cm. And most of the EPAP increases are driven by snores and not clusters of events.
Conventional wisdom on PAP forums would say that I need to increase the max IPAP (and hence the max EPAP) setting so there's room for my machine to continue to increase the IPAP/EPAP pressures if it wants to. And I've experimented with increasing the max IPAP a number of times over the last 3 years. Each time the same thing happens:
- My 95% IPAP and EPAP pressure levels go UP, sometimes substantially above 8/6, with most of the additional pressure increases being caused by flow limitations, the PR search algorithm, and snoring ...
- My AHI remains about the same OR it goes UP either because of additional random centrals OR because my breathing becomes unstable during a large pressure increase.
- My Flow Limitation Index remains about the same or goes down slightly. (My long term FLI is about 2.7 Flow Limitations/hour; hence there's not a whole lot of room to improve the FL numbers by pressure increases.)
- My aerophagia becomes significantly worse.
- The number of nighttime wakes goes UP, primarily because of aerophagia-related arousals and wakes.
- My overall sleep gets WORSE, not better, and I typically feel worse during the daytime.
In other words, for me there is no real advantage to increasing max pressure settings to the point where my 95% pressure level is less than my maximum pressure setting. And so I've come to accept that for me
it's reasonable to have
my median IPAP pressure level
= my 95% IPAP pressure level
= my max IPAP pressure setting
= my titrated IPAP pressure