School Me - EPR (EPAP) & Flow Limitations

[70sSanO]

I recall EPR came about to add more comfort when exhaling, especially at higher pressures than was possible with straight CPAP or APAP.

Years ago, the use of EPR was typically discouraged long term, as it reduced the pressure at end-expiration and during that pause there could be an upper airway narrowing which could lead to an obstructive event.  The corrective measure was to increase the inspiration pressure (or range) by a corresponding amount.  For someone starting out, I'm all for comfort as any reasonable xPAP therapy is better than none.

I have read posts suggesting that increasing EPR can help to eliminate flow limitations.  From what I have read, (also Resmed video), flow limitations are a less restricted airway as opposed to apneas or hypopneas.  This seems counter to increased pressure that keeps the airway open and prevents events (apneas, hypopneas, and flow limitations), unless the flow limitation is on the expiratory side.

I do know that BiPAP is like EPR on steroids and that it is used not only for additional comfort, but if other medical conditions exist.  Apart from the S/T and now ASV treatment, it does seem to pose some of the same questions.

Just trying to get my head around how the therapy, beyond comfort, works.

John

[Sleeprider]

John, good questions. The evidence of EPR helping with flow limitations is more than just anecdotal, we have seen charts of the same individual with EPR on and off, and the changes can be significant.  EPR is limited to 3-cm but it works identically to bilevel pressure in the VPAP series.  Resmed does not discuss flow limitation in its VPAP titration protocols, but as you note, a FL is very similar to a hypopnea that didn't result in enough air flow reduction to be flagged, but the titration protocol for hypopnea is to increase pressure support.  It's not surprising then the same thing works for flow limitation. 

Also as you note the use of EPR reduces exhale pressure or EPAP, and it may fall below the intended therapeutic level, especially in fixed CPAP pressure machines.  A higher minimum pressure or CPAP pressure may be advisable when EPR is used.  The irony in that is, when we add EPR to someone using auto-pressure with significant flow limitation, the resulting median, 95% and maximum IPAP pressures are nearly always lower with EPR than without, and of course the exhale pressure is much lower.  The Resmed machines target flow limitation in the Autoset and Vauto algorithms to increase pressure more than any other factor other than an OA event. This is why they succeed in preventing pressure so effectively, and why the machine is so much more responsive ahead of events than Philips which uses primarily snores. 

It is surprising to me that Resmed in particular has not emphasized in its literature the importance of flow limitation, because it is a key to their auto-pressure algorithm, and it works!  Most of the medical community are walking in a trance repeating the AHI dogma of the insurance companies, but flow limitation needs to be emphasized much more, and only a few physicians like Dr. Barry Krakow have caught on to this.  Out of flow limitation, arises sleep disruption from RERA, hypopnea and even obstructive apnea.  Flow limitation can exist for the entire duration of sleep making it a miserable, unsatisfying experience, and leading patients to the ultimate conclusion that CPAP doesn't work or make them feel better.  People with high flow limitation are tired, cranky and don't have any ideal why, and their doctors don't have a clue either.  Flow limitation needs to become a much more discussed and emphasized issue with physicians and clinicians who currently dismiss it as unimportant, and the use of EPR or more appropriately, pressure support needs to become the way flow limitation is mitigated and treated.

[70sSanO]

Sleeprider, thanks for the info.

I’ve always been in the if-it-works camp.

The response rate to increase pressure makes a lot of sense. Being able to catch the limitation before it becomes an apnea event (after the expiratory end) seems to be key to why EPR works with Resmed.

John

[sleepyp]

In my own situation my sleep study showed no apeas, only hypopneas.
I'm suspecting that the hypopneas I'm seeing are flow limits that develop. EPR clearly reduces flow limits (I've got charts to prove it, plus I've seen heaps others on this forum) and so I think EPR is stopping the hypopneas by prevention rather than reaction (jumping to high pressures after the event).

I suspect if you have apneas or obstructive events that are more sudden (ie not a result of flow limits developing) then the reduced EPAP with EPR would reduce treatment efficacy. You'd need the higher constant pressure to prevent sudden airway collapse.
That's why I'm surprised that sleep docs don't analyse the root cause of the vents in the first place before deciding on machines and settings.
I'm a process control systems engineer and the way DMEs behave would be considered highly irresponsible in my industry. A CPAP machine is a closed loop pressure/flow control system so I do understand a lot of that aspect, I'm just learning about OSA itself.

[Sleeprider]

Sleepyp, it's a good thing you have the Resmed because the Philips AFlex and CFlex does not appear to have any effect on flow limitation or hypopnea. Resmed is selling limited bilevel machines as CPAP, and they may be sensitive to that capability being discussed. which explains the silence on the matter. We can treat someone experiencing hypopnea at generally lower pressure using EPR, than CPAP pressure without EPR or using CFlex.

[sleepyp]

Sleepyp, it's a good thing you have the Resmed because the Philips AFlex and CFlex does not appear to have any effect on flow limitation or hypopnea.  Resmed is selling limited bilevel machines as CPAP, and they may be sensitive to that capability being discussed. which explains the silence on the matter.  We can treat someone experiencing hypopnea at generally lower pressure using EPR, than CPAP pressure without EPR or using CFlex.

Funny you say that, I've only moved to the Airsense Auto since last week. I was using a Dreamstation Go Auto full time for three months previously.

I can say, with no vested interest, that the Airsense Auto is a far superior machine to the Dreamstation Auto in treating my particular case of OSA.
Lower AHI, lower pressures, and feeling way better. I feel an equal improvement moving from Dreamstation to Airsense as I did going from non treated to the Dreamstation.

I'm sure the Dreamstation works better than the Resmed in other cases, and I'm not suggesting one is universally better than the other.
The Airsense algorithm detects and responds with pressure very differently to the Dreamstation, and the pressure relief as you said is worlds apart.

I think it would be far more helpful for Dreamstation and Airsense to be considered different treatment tools, as opposed two different brands of the same treatment tool.

[70sSanO]

If I might ask another question?

How does someone decipher the Flow Limitaton index numbers... for Median, 95%, and Max?

In other words is there a quantitative value to the number of events for, say, .10, .20, .50 indices or is more an evaluation of the graph... that looks good or bad?

John

[Dormeo]

If I might ask another question?

How does someone decipher the Flow Limitaton index numbers... for Median, 95%, and Max?

In other words is there a quantitative value to the number of events for, say, .10, .20, .50 indices or is more an evaluation of the graph... that looks good or bad?

John

My big push has been to reduce my FLs. I find by far the best way to tell how it’s going is 1) to see how much of the FL graph is densely flagged, and 2) to sample areas of the flow rate graph with a close zoom so I can see the tops if the inhalation traces. 

See whether those two methods correlate with subjective feelings of more restful sleep. I think they do for me, though I have pain as a confounding variable, unfortunately.

[Gideon]

ResMed uses a flattening index with 1 being the worst.  

Respironics I don't know anything about their flagging algorithm.

[Geer1]

In all mechanical systems pressure differential is what initiates flow. The higher the pressure differential, the higher the flow rate. Restrictions cause pressure loss and more restrictive passageways require higher pressure differentials to provide the same flow rate.

Now speaking in a CPAP machine sense, greater pressure support helps fight flow limitations because it increases the pressure differential which increases flow rate and it does this for both the inspiratory and exhilatory processes. Without pressure support the only pressure differential is that which is created by your own body (muscles/diaphragm creating a vacuum lowering the pressure in your lungs which sucks air in and then the muscles relaxing and weight muscle tension pushing air out). This is why people notice that especially if EPR/PS is turned off that it is harder to exhale against larger pressures, if you weren't wearing a CPAP machine your body doesn't have to exhale against any pressure but if it is being supplied a constant pressure it must now overcome that pressure.

Another real life example most people can probably relate to is blowing up a balloon. In order to blow up a balloon you must provide enough pressure to overcome the current pressure inside the balloon. The harder you blow, the larger the pressure difference you create and the faster the balloon inflates.

Edit: Regarding EPAP, EPAP is your minimum pressure and that is what holds airway in open position. That is why titration protocols say to increase EPAP until apneas do not occur then you increase PS to deal with hypopneas and flow limitations. Raising EPAP can help a bit with hypopneas and flow limitations by keeping the airway more open to begin the breath with but EPR/PS will have greater affect on flowing more air volume.