Need a primer on flow limits

[Ctyankee]

Based on some of the recent comments I've read, I'm thinking that flow limits during the night are important.  Up until now, I've largely paid attention to obstructive and central apneas, and largely ignored both hyponeas and flow limit readings.


The attached two printouts show my results for the last two nights.   After the first night, I felt terrible.  As a result, I changed my inhalation and exhalation pressures and increased the PS value.  The second night shows a better AHI, and what I believe is a substantial reduction in flow limits.  For the second night, I bought a cervical collar, to reduce any chin tucking that might be increasing flow resistance.


Can one or more of the numerous experts in reading these results give me some tips on how to interpret and respond to flow limit values?


Based on these two printouts, does my observation of feeling lousy after the first night and good after the second make sense.


If flow limits are a frequent issue for me, any tips on how to reduce flow limits?


I sleep with a nasal mask and my jaw tightly bound closed using a white cotton chin strap.  Leakage is often a problem for me.  Sometimes it's though my mouth and
sometimes just around the mask perimeter.


I've been using a CPAP for 15 years and recently converted to a BiPap. I recently purchased the Knightsbridge chin strap but haven't had any luck with it.


Thanks for your help.  What a blessing to have this community to turn to for help.

[Sleeprider]

In addition to flow limitation, there is a significant difference in leak rate between the two nights, not to mention a change in settings. Inspiratory flow limitation is a restriction in the airway that limits maximum flow, requiring more inspiratory effort to compensate. Alternatively, if we can use pressure support to provide that compensation, rather than inspiratory effort, sleep will be more comfortable and less disrupted. It also helps to think of obstructive sleep disordered breathing as a continuum from normal inspiratory effort and flow, to increasing flow limitation, to hypopnea, to complete obstruction or apnea. Increasing restriction or obstruction requires more effort to overcome, and is often interrupted by an arousal (RERA) which may cause the sleeper to change position, airway clearing, or stimulate greater breathing effort.

Pressure support (difference in pressure between IPAP and EPAP) is an effective tool to reduce flow limitation. As the inspiratory flow rate increases, greater flow resistance is mitigated by a supporting increase in positive pressure. This can make flow limitation much less or even nearly disappear. In your case, you increased EPAP pressure and increased PS, and this had a positive effect, at least based on the comparison of these two nights. Unfortunately, you also have some central apnea resulting from higher rates of ventilation, and if you increase PS high enough to fully mitigate flow limitation, it may also increase those CA events. Your therapy looks much better at EPAP min 12, IPAP max 12, PS 2 than at EPAP min 10, IPAP max 16, PS 1. With CA events, the only way to get significantly more pressure support to maintain flow volume, and treat the inevitable CA events, an ASV machine is the best option. Hard to get, but for those that are able to persuade their doctor and insurance of their need, or buy one out of pocket, therapy becomes effortless with near zero AHI.

The presence of leaks on the first night probably disrupts the reliability of a direct comparison of these two charts, but it's pretty clear which one represents better sleep.

[70sSanO]

I’m just starting to get into the ins-and-outs of flow limitations. I’ve been doing CPAP, APAP, ASV for 19 years.

I sleep well and don’t have issues with flow limitations. It will always be about how I feel, but I’m trying to understand what is going on.

The quick and dirty answer has been Pressure Support to address flow limitations. But that only works if your machine reacts to the flow limitation.

I’ve also been looking into Dr. Barry Krakow’s work on Expiratory Pressure Intolerance. The basic premise is that fighting to exhale against pressure can cause arousals that impacts sleep.

This would lead me to think that EPAP has to go down but pressure support needs to go up, otherwise someone could just raise pressure to a high constant and go on with life.

I can’t answer specifics as far as setting a range and a trigger on you VAuto, but there will be those more familiar with your machine that can do that.

John

[ApneaQuestions]

I second everything sleeprider said.
We've both been driving ourselves crazy recently on plmnb's thread struggling with an outlier case of flow limitations.
It's a long read but there is a lot to be learned from that thread if you've been following along.

[70sSanO]

I sleep well and don’t have issues with flow limitations.  It will always be about how I feel, but I’m trying to understand what is going on.

I should probably clarify that my FL’s are running around .5.

I did bring my EPAP up and didn’t sleep as well, so I decided to set it up in ASVAuto with a range of 5.0 to 7.4.  PS is 2.0-15.0.  It is comfortable and my AHI is under 1.0 with 90% hypopneas.

Winter without humidification is always a bit tougher, but I don’t want to chase down a problem that really isn’t a problem for me.

John

[SleeplessInPHX]

In addition to flow limitation, there is a significant difference in leak rate between the two nights, not to mention a change in settings.  Inspiratory flow limitation is a restriction in the airway that limits maximum flow, requiring more inspiratory effort to compensate. Alternatively, if we can use pressure support to provide that compensation, rather than inspiratory effort, sleep will be more comfortable and less disrupted.  It also helps to think of obstructive sleep disordered breathing as a continuum from normal inspiratory effort and flow, to increasing flow limitation, to hypopnea, to complete obstruction or apnea. Increasing restriction or obstruction requires more effort to overcome, and is often interrupted by an arousal (RERA) which may cause the sleeper to change position, airway clearing, or stimulate greater breathing effort.

Pressure support (difference in pressure between IPAP and EPAP) is an effective tool to reduce flow limitation. As the inspiratory flow rate increases, greater flow resistance is mitigated by a supporting increase in positive pressure. This can make flow limitation much less or even nearly disappear. In your case, you increased EPAP pressure and increased PS, and this had a positive effect, at least based on the comparison of these two nights.  Unfortunately, you also have some central apnea resulting from higher rates of ventilation, and if you increase PS high enough to fully mitigate flow limitation, it may also increase those CA events.  Your therapy looks much better at EPAP min 12, IPAP max 12, PS 2 than at EPAP min 10, IPAP max 16, PS 1.  With CA events, the only way to get significantly more pressure support to maintain flow volume, and treat the inevitable CA events, an ASV machine is the best option. Hard to get, but for those that are able to persuade their doctor and insurance of their need, or buy one out of pocket, therapy becomes effortless with near zero AHI.

The presence of leaks on the first night probably disrupts the reliability of a direct comparison of these two charts, but it's pretty clear which one represents better sleep.

I think I need to read this 20 more times. THANK YOU Sleeprider, excellent explanation!

[Ctyankee]

Thanks for your help.  I took a nap today and tried several things to work on reducing flow limits including elevating my head, wearing a cervical collar and uping PS value to 3.  Interesting result was flow limit reduced to basically 0 and an AHI of 3.5 all from central apneas ( no surprise).  Sometimes I wonder if CAs are the lesser evil if they are of short duration.

[Gideon]

You know us, we like to see charts.

[Sleeprider]

I generally find the CA events are not so disruptive. They are simply the normal response of your body to hypocapnea (lower CO2 from ventilation), and the pauses in breathing simply work to restore the balance in your body. These events occur without respiratory effort and may not cause arousal as would be found with obstructive events. The good news is that these events seem to diminish in time as you adapt to a new normal ventilation rate. The presence of CA events can be managed by rebreathing more CO2, and this is how EERS (enhanced expiratory rebreathing space) is used as a therapy addition by some physicians. http://www.apneaboard.com/wiki/index.php...ace_(EERS)