How does Flow Limitation affect pressure

[Roy289]

New to using APAP ResMed Autoset for her and trying to understand how pressure is increased. I know that OA's cause the pressure to increase but CA's have no affect.

Can someone explain how/if Flow Limits cause increase in pressure. I believe on Resmed units the shape of the inhalation over a sample time period determines Flow Limitation. Does this sampling cause both increase and decrease in pressure?

[PoolQ]

The way I understand it is that an OA is a 100% flow limit, so when the CPAP starts seeing flow limits it starts expecting to see OA's. To prevent the start of OA's the CPAP starts raising pressure at the first sign of flow limits

[Sleeprider]

You can find Resmed's definition and reaction to flow limitation here: http://www.resmed.com/int/clinicians/com...nc=dealers

Read the linked page, and if you have further questions, post back.

1. Flow Limitation

What is flow limitation?

A physiological change in the status of the upper airway
Causes a spectrum of closure, from subtle narrowing to partial collapse

What is the significance of flow limitation?

It usually precedes snoring and apnea. It is therefore, usually, the earliest sign of impending airway collapse3
Airway changes limit flow despite respiratory effort and in fact, cause increased respiratory effort. This may lead to arousal

How do I observe flow limitation?

The inspiratory flow time curve represents inspiratory flow limitation
The shape of the curve indicates the status of the upper airway. A normal upper airway is observed as a bell-shaped curve. The curve flattens with increasing flow limitation and airway narrowing/ closure

How does a ResMed AutoSet device measure flow limitation?
It calculates flow limitation on a breath-by-breath basis, detecting and responding to subtle changes

How does a ResMed AutoSet device respond to flow limitation?

If flow limitation is detected, AutoSet Spirit responds by gradually increasing pressure to bring the airway back to normal. Typically, this helps prevent snoring and apneas4
If no further events occur, AutoSet Spirit gently decreases the pressure, towards the minimum set pressure

[Roy289]

The Autoset line probably has the same algorithm for managing pressure thru the Flow Limitation. My Flow rate has inhalation flat topping and the pressure shows little upticks (the inspir. time is flat). Assume the flow rate is being sampled but most of my inhalations are more flat than rounded so hard to see relationship to pressure adjustment. My pressure range is currently set to 7-11 (originally at 7-14) by DME). Would resetting the pressure back to 7-14 be recommended?

[Sleeprider]

The Autoset line probably has the same algorithm for managing pressure thru the Flow Limitation. My Flow rate has inhalation flat topping and the pressure shows little upticks (the inspir. time is flat). Assume the flow rate is being sampled but most of my inhalations are more flat than rounded so hard to see relationship to pressure adjustment. My pressure range is currently set to 7-11 (originally at 7-14) by DME). Would resetting the pressure back to 7-14 be recommended?

What is your average and 95% pressure currently? If the machine is reaching its maximum pressure regularly, then you probably need a higher maximum pressure. Your question is pretty hard to answer without seeing the data to understand why a higher pressure may, or may not be needed. Generally, I don't recommend chasing flow limitation events unless they are disrupting your sleep or are precursors of OA or H events.

[Roy289]

Statistics
Channel Min Med 95% Max

W-Avg: 9.51 5.00 9.90 11.00 11.00

Should I increase by increments of 1? Brought it down originally as was chasing mask leakage.

[Crimson Nape]

My Flow Limitation graph provides a false positive to the Resmed and will cause the unit to go to .2cm below my programmed upper limit, regardless of how high the pressure setting. I've found that a straight pressure of 11 produces the lowest AHI and best sleep quality. Any pressure higher than 11 isn't beneficial and doesn't reduce the flow limitation graph. . . it just stays the same. If I had the money I'd like to try a P-R to see if "likes" me in this area.

[palerider]

apneas are not flow limitations, and vice versa, just to clarify.

an apnea is a > 90% reduction in flow from the baseline for ten or more seconds.

a hypopnea is a > 30% reduction in flow from baseline for ten or more seconds.

a flow limitation does not necessarily correspond with either of the above, a flow limitation is a restriction in the airflow, making it harder to breath, which causes increased breathing effort, and can lead to arousals, however there does not have to be any reduction in total flow due to flow limitations, (thinking gulping a drink vs sipping it, you get the same amount of drink, just takes longer one way than the other)

[0rangebear]

Palerider
Where does you definition of an Apnea come from?

Is the formula use in SleepyHead or Rescan.

I ask because I have been reading medical Journals and I find the definition varies by source

apneas are not flow limitations, and vice versa, just to clarify.

an apnea is a > 90% reduction in flow from the baseline for ten or more seconds.

a hypopnea is a > 30% reduction in flow from baseline for ten or more seconds.

a flow limitation does not necessarily correspond with either of the above, a flow limitation is a restriction in the airflow, making it harder to breath, which causes increased breathing effort, and can lead to arousals, however there does not have to be any reduction in total flow due to flow limitations, (thinking gulping a drink vs sipping it, you get the same amount of drink, just takes longer one way than the other)

[robysue]

Palerider
Where does you definition of an Apnea come from?

Is the formula use in SleepyHead or Rescan.

SleepyHead does NOT have a "formula" for defining apneas or hypopneas. It reads the data that is scored by the machine and displays that data.

Each manufacturer has their own algorithms for determining when to score an apnea or a hypopnea. They all are based on "reduction in airflow" from a moving baseline, although how to compute that moving baseline and long the "moving baseline window" is may vary from manufacturer to manufacturer. There's a fascinating white paper from the NHI at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4629962/ that contains a table that has some pretty extensive technical information about the criteria that Resmed, PR, and DeVilbass machines use to detect events. The table can be found at http://www.ncbi.nlm.nih.gov/pmc/articles...der-8-425/

The table indicates that Resmed machines score an apnea when the flow is reduced to less than 25% of baseline for at least 10 seconds whereas the PR machines score an apnea when the flow is reduced to less than 20% of baseline for at least 10 seconds. DeVilbass APAPs score apneas when the flow is reduced to less 10% of baseline for at least 10 seconds.

Hypopnea definitions also vary from machine to machine:

• Resmed scores a hypopnea when the flow rate is between 25%--50% of baseline for at least 10 seconds. In other words, the flow must be reduced by at least 50% for a Resmed machine to score a H. There's also some interesting information in the table that may explain why a lot of people saw their HI decrease (sometimes dramatically) when they switched from the S8 to the S9: In order to score a H, the S9 and A10 machines require "at least one obstructed breath", whereas the S8's did not have that requirement.
  
• PR scores a hypopnea when the flow rate is between 20%--60% of baseline for at least 10 seconds. In other words, the flow rate must be reduced by at least 40% for a PR machine to score an H. Interestingly, PR requires a "recovery breath" at the end of the event unless the flow rate is reduced for at least 60 seconds.
  
• DeVilbass scores an H when the flow rate is between 10%--50% of baseline for at least 10 seconds. In other wrods, the flow rate must be reduced by at least 50% for a DeVilbass machine to score an H.
  

The detection of flow limitations also vary by manufacturer: Resmed looks at each and every breath, PR does a 4-breath average, and DeVilbass uses an average of all breaths over a 12-second range.

Another fascinating thing about the table is that it details how each of the three manufacturers have programmed their auto algorithms to respond to apneas, hypopneas, flow limitations, and snoring.

Snoring is discussed in the paper itself, and I haven't dug into that part. (Tables are much easier to process quickly.) There's a lot of really interesting info in this paper for tech-geeks and data-nerds.

Finally, it's also worth noting that there are two different ways that a lab can choose to score a hyponea on an in-lab sleep test according to the titration guidelines set up by the AASM, although the two ways are now based mainly on Medicare and some other insurance companies refusing to recognize the newer standards.

Both ways to score hypopneas now seem to require a 30% drop in airflow for at least 10 second. The one that the AASM wants to use requires either a 3% drop in O2 OR an arousal. The one that Medicare insists on requires a 4% drop in O2. I wonder if PR has updated their algorithm for scoring Hs on the DreamStations.

See http://www.aasmnet.org/articles.aspx?id=4132
and http://www.aasmnet.org/articles.aspx?id=3782 for details about the AASM rules for scoring hyponeas.