Minimum pressure and APAP

[gary1001]

Something I was wonder recently was can the minimum pressure be set too low if you have a auto adjusting machine like the Resmed S9? In another words, wont the machine just increase the pressure anytime there is a problem oppose to having to set a lower limit?

[Nozzelnut]

For me, the minimum pressure I am comfortable starting with is 8. Less than that I feel like I'm not getting enough air through the mask.

As far as treatment, you can start with too low of pressure. The machine takes a little time to respond to events. If the pressure needed to correct that event, is much higher than current pressure, the machine will keep adding pressure; sometimes adding more than necessary; resulting in runaway pressure. Hope that makes sense. Doesn't usually cause a problem, but might wake you up due to mask leaks or put some air in your belly.

[Julie]

If it's too low, it can take too long to intercept an event that's high up. The low pressure should be set just 1-2 notches below the usual lowest pressure your machine reports (you do use software?), the 90-95% pressure.

[Wulfman...]

Something I was wonder recently was can the minimum pressure be set too low if you have a auto adjusting machine like the Resmed S9? In another words, wont the machine just increase the pressure anytime there is a problem oppose to having to set a lower limit?

If running in a range of pressures, the minimum pressure should be set to where it it can prevent almost all of the events. If set too low, it takes too long for the pressure to get to where it needs to be......and the events will occur......and then it drops back down to the minimum again. NO APAP will attempt to stop an apnea that's in progress.
The other problem is that with all the pressure changes, they disturb and disrupt the sleep patterns and stages. The user is left feeling like they haven't gotten any meaningful sleep.......and they haven't. And, their AHI and other indexes will be high.

If your minimum is set to "4", and you need a pressure of "10" or "15", you're going to get lousy therapy.
It doesn't "automatically" go from 4 to 10 or 15 because it doesn't know what pressure is needed. And, the time it takes to get there is too long.


Den

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[gary1001]

Thanks, that's really clear.

My 95% pressure is approx 7.0 however the lower pressure limit is set at 4.0. The exact reason I ask this question was I have been experiencing run away pressure lately so it sounds like the lower limit is too low. With a 95% of 7.0, what would you suggest as the lower pressure?

[LSAT]

Thanks, that's really clear.

My 95% pressure is approx 7.0 however the lower pressure limit is set at 4.0. The exact reason I ask this question was I have been experiencing run away pressure lately so it sounds like the lower limit is too low. With a 95% of 7.0, what would you suggest as the lower pressure?

Personally I would set the pressure at 5 min...10 max. (With a 95% at 7, 4 really is not that bad for a min)

[Wulfman...]

Thanks, that's really clear.

My 95% pressure is approx 7.0 however the lower pressure limit is set at 4.0. The exact reason I ask this question was I have been experiencing run away pressure lately so it sounds like the lower limit is too low. With a 95% of 7.0, what would you suggest as the lower pressure?

One thing about ranges of pressure is that it can be a "moving target". In other words, if you bump your minimum to 7 (and I think you should), your 95% may actually move higher than that. But, it depends on the types of events you're having and other possible (or unknown) factors.
If you're getting massive leaks and "runaway pressures", you my be wise to limit your maximum pressure.
Actually, you may find yourself increasing your minimum and lowering your maximum to a point where there is only a few centimeters of difference. At that point, you may want to try straight pressure to see if that may give you even better sleep quality. Some people are sensitive to pressure changes which can bump a person out of their needed sleep stages, which leaves the person feeling unrested.

Hopefully you're using software to see your data and not just relying on the LCD info.


Den

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[gary1001]

I tried 5.6 and 6.6 last night. Both made a huge difference (without much between them). Primarily, I felt much better in the morning and data shined a spot light on a couple of areas I assumed were fine or didn't make a difference. Firstly, I had almost no flow limitation events. Normal there would be minor events happening all night, this time only a handful. Secondly, my tidal volume increased from a medium of 360 to 440. Lastly my AHI dropped from a low 2.0 to and even lower 0.2. Additionally, the max pressure dropped from around 12 to 7.6 which I guess means the machine is no longer chasing events in "auto" mode.

The takeaway message for me is that while my AHI numbers were low to start with, flow limitation events were happening and impacting my ability to breath freely (which is reflected in the tide volume).

I'm going to try again with 6.6 for the whole night and monitor SpO2. Looking at the graph, only one flow limitation occurred on 6.6 oppose to 5.6 and the tide volume looked slightly higher. Who knows, maybe straight CPAP is a better fit for me or a least a tight pressure range?

(But the way, I was told when the machine was setup that is "auto" adjusting and to leave the range wide open. I've always had low AHI numbers so have thought nothing of it)

[djhall]

(But the way, I was told when the machine was setup that is "auto" adjusting and to leave the range wide open. I've always had low AHI numbers so have throught nothing of it)

That is a common belief. As you've already discovered, experience shows the auto machines deliver the best results when the minimum is set at a pressure that is effective for the majority of the night and the auto functionality is only relied upon to assist with shorter periods of time where our pressure needs temporarily increase above normal.

[Kennerly]

djhall wrote:

"That is a common belief. As you've already discovered, experience shows the auto machines deliver the best results when the minimum is set at a pressure that is effective for the majority of the night and the auto functionality is only relied upon to assist with shorter periods of time where our pressure needs temporarily increase above normal."

Good explanation, and also Den's above.

Does the approach change much if it's complex apnea? I understand that the general approach to minimize centrals is to bring the IPAP down from the usual setting - accepting some obstructive events - in search of the optimal compromise between obstructive and central events. I really haven't seen much info about how EPAP plays into the mix.

If bringing EPAP up can create an opportunity to bring IPAP down a little without an increase in obstructions, this would be an ideal approach for complex apnea. But it would follow then that plain cpap would be the recommended approach for complex apnea, instead of bipap.

What am I missing?

[djhall]

Does the approach change much if it's complex apnea? I understand that the general approach to minimize centrals is to bring the IPAP down from the usual setting - accepting some obstructive events - in search of the optimal compromise between obstructive and central events. I really haven't seen much info about how EPAP plays into the mix.

If bringing EPAP up can create an opportunity to bring IPAP down a little without an increase in obstructions, this would be an ideal approach for complex apnea. But it would follow then that plain cpap would be the recommended approach for complex apnea, instead of bipap.

What am I missing?

Well, for one thing you are certainly missing an easy question! I'm not sure I can address some of the complex apnea specific parts of your questions, but I can help a little. EPAP is the low pressure a bi-level switches to during your exhale phase. If the airway is allowed to fully collapse (OA) at the end of the exhale then the machine cannot sense the inhalation attempt that prompts a bi-level machine to switch to the IPAP pressure. Therefore, you have to treat OAs with EPAP. H, RERA, and FL are all partial collapse of the airway of varying severity. Because the airway is only partially collapsed during these events the machine is still able to detect the inhalation attempt and switches to the IPAP pressure. If the IPAP pressure is high enough it can force the airway the rest of the way open for the inhale, so H, RERA, and FL can be treated with EPAP to prevent them from starting or IPAP to blow them open during inhalation.

The formula with bi-level machines is that EPAP + Pressure Support = IPAP. EPAP has to be sufficient to treat OA. IPAP should be sufficient to treat the other events, but you can get IPAP there by using more EPAP or by using more PS (PS = 0 being straight CPAP). Bringing EPAP up doesn't give you the opportunity to bring IPAP down, it gives you the opportunity to bring pressure support down while still keeping IPAP up. The balance with CA events is figuring out what combination bothers you less. I personally can deal with lots of EPAP if PS is between 2 and 4, but once PS hits 5 or more my CA events start climbing rapidly regardless of how low EPAP is. Other people find that high EPAP triggers increased CA events, but they tolerate a large PS differential just fine, so they keep EPAP as low as possible and crank up the PS. If you can't find an EPAP & PS combination that is effective at controlling the other events without triggering too many CAs, then you end up having to go to an ASV.

[gary1001]

Something I was wondering, if you increase your lowest pressure past the minimum effective point (say, you are trying to find the minimum point like me however overshoot it), do you increase your chances of central apneas occurring?

[Pugsy]

Centrals popping up as a response to cpap therapy pressure can happen at any pressure..even as little as 5 cm.
It happens in a small % of people who start cpap therapy.
It doesn't have to be high pressures that do it.
If you are using a machine that auto adjusts say up to 14...with the minimum of say 8...and you see 14 fairly often without centrals happening..then it is unlikely that increasing the minimum from that 8 is going to trigger any centrals because a person would already be seeing centrals because of the time spent at 14 cm.

Higher pressures don't always cause centrals...
When I was using the APAP I often saw pressures pushing 20 cm and not one central and my minimum was originally 8 cm and I moved it to 10 cm to better stent the airway open.
People talk about it like it is common but it really isn't.
There are people using bilevel machines with pressures exceeding 20 cm all night long and they don't have an abnormal number of centrals pop up. An occasional "central"...real or not isn't a big deal.

[djhall]

Something I was wondering, if you increase your lowest pressure past the minimum effective point (say, you are trying to find the minimum point like me however overshoot it), do you increase your chances of central apneas occurring?

As Pugsy said, "yes, no, and maybe". It all depends on what your system finds disruptive. I am fine at bi-level 22 exhale and 24 inhale, but increase the spread to 14 exhale and 20 inhale and my CAs go way up, even though the pressure is lower. Other people are the reverse. The result usually isn't worse than a bad night sleep if you are monitoring the effect of your changes.... I go from 1.5 AHI to 8 or 9 in the scenario above, and it goes back down when I back off the change. It is a fairly small number of people who actually experience serious complex apnea symptoms.

[gary1001]

Yes, no and maybe it is! Makes sense really, everyone responses differently. The message I take away is to monitor closely and give any changes a couple of days to settle. The reason I ask is I noticed a cluster of CA events with a higher lower limit. Not many but something different. It could have been other factors so I'll give it a few days.