ASV: HOW DOES IT WORK?

[máirtín]

How does it work?
I understand how a regular CPAP machine works: it blows pressurized air past the obstruction and breathing resumes.
I don't understand the mechanism of the ASV. If my brain doesn't send a signal to breath how does the machine start my lungs pumping again?

[adipasqu]

How does it work?
I understand how a regular CPAP machine works: it blows pressurized air past the obstruction and breathing resumes.
I don't understand the mechanism of the ASV. If my brain doesn't send a signal to breath how does the machine start my lungs pumping again?

Timed breaths. The machine calculates your average breathing rate over a period of time (4 minutes, I believe for the PRS1 ASV). If you fail to breathe according to that rate, the machine will initiate a breath for you.

-A

[máirtín]

the machine will initiate a breath for you.

[/quote]

How?

[adipasqu]

How?

It jacks up the pressure well above the current IPAP/PS, basically turning the machine into a ventilator (forcing air down into your lungs) until your spontaneous breathing resumes.

-A

[ameriken]

The RT at the DME told me that the Respironics ASV is designed to give you 10 BPM. I have low patient triggered breathing so there are nights when I am breathing at 14 - 18 BPM and then my PTB's will fall through the floor and thats when my chart shows the machine has kept me going at the 10 BPM.

[Bons]

You can program the Respironics machine to a set breath per minute minimum between 4 and 24, or leave it on auto.

[Bons]

Should've added - if you want to see how it works, exhale and then don't inhale; you will feel the pressure increase as it tries to make you breathe

[dsm]

máirtín

Depends on brand & they work quite differently. I'll assume you are 1stly interested in the Respironics ASV. This is a simplified explanation as it gets too messy trying to explain all the algorithms at play in the latest version of the machine. Also these algorithms are a changing thing & so whilst I am pretty sure the below is reasonably accurate, don't bet on it

Basics.
The basic premise is to think of it as similar to a bilevel that can become a trilevel when it needs to.
When all is going ok it will operate between the epap (exhale) pressure and ipap (inhale) pressure (Bilevel).
epap, ipap. When centrals start to occur it can decide to got to a third level ipap + ps.

Epap & OSA.
The Resp machine does have the ability to adjust the epap pressure in response to obstructive events.
It does that using a conventional Auto cpap algorithm and if it raises epap, it also raises ipap by the
same amount thus maintaining the same epap to ipap gap.

Epap, Ipap & PS (Respironics).
Where the ASV differs from a bilevel is that it has an ASV algorithm that constantly monitors the Average Peak Flow
in a 4 min window (this may be shorter in the new version of the Resp ASV - the older 'tank' models used 4 mins. Also
the older Resmed ASVs use 3 min window & I think they still do).

In this 'window' the machine gathers info on Av peak flow & calculates what each new breath should reach to meet a
target of flow. If the new breath flow is going to drop under 95% of the passing window's Av Peak Flow target, the machine
will *instantly* boost ipap by up to approx 3 CMs for the breath that is falling below target. It will perform the boost
(adding approx 3 CMs each boost, for up to 3 breaths).

Say normal ipap pressure is at 12 CMs & breath number x is going to miss its 95% of target, the machine will add approx 3 CMs.
Ipap is thus boosted to 15 CMs, if breath x+1 is also going to fall short of its 95% of target, add approx 3 CMs more to ipap (now
at 18 CMs), if breath x+2 is not going to meet its 95% of target then add one last boost of approx 3 CMs (now ipap is at 21 CMs).
But no more PS gets applied. As the peak av flow returns to normal the ipap boost is dropped back at a similar rate. (NOTE
Resmed ASV machines target flow + rate and apply PS if the breath won't meet 90% of a calculated combination of these).

So during an ASV central event epap hasn't changed, but ipap has now become ipap + an incremental PS. My recollection is
that the machine will not try to raise ipap more than approx 3 CMs in any single breath nor will it raise ipap more than
10 CMs total while trying to normalize respiratory flow. IIRC, the machine does aim to normalize respiration within 3 breaths.

Hope this is not too complex to follow. Also if anyone has any additional detail (supported by links) please add them
so I can learn about any new changes / differences.

DSM

Here is a very simple description
- ASV seeks to zap central apneas dynamically in real-time using pressure support but within certain constraints
- Auto cpap mode preempts OSA by slowly raising the base pressure (epap) in advance of obstructive apneas based on detecting
OSA pre-cursor activity (snoring, flattening, flow-lims (& in the case of Respironics auto algorithm) hypopneas and apneas).

# added refinements to wording re the 90% target
# refined target further
# corrected remaining ref to 90% (changed to 95% for Respironics whereas Resmed its 90%) also modified the last para ref to OSA pre-cursor ...

[Bons]

dsm,
Thanks for the explanation. Unfortunately, you can do it all over again because the original poster has a Resmed Vpap, not the Respironics version

[dsm]

dsm,
Thanks for the explanation. Unfortunately, you can do it all over again because the original poster has a Resmed Vpap, not the Respironics version

Actually, one of you with a Resmed ASV can help me here (if you have the new model - I already use the old Resmed unit).

Can anyone look at the nights data (down to breath by breath) & see what the highest pressure boost was from any one breath to the next where the machine responded to an ASV event.

Pls post any images if you have them. Also, does the new Resmed ASV mention varying epap in response to OSA events ?

Thanks

DSM

PS Resmed used to track breathing rate and volume in a 3-min window & raise ipap in response to not meeting 90% target for flow & rate as tracked in the 3 min moving window. This may have changed a bit with the new machine.

[dsm]

This link offers some more technical insights but is still referring to the older 'tank' & 'egg' models from Resp & Resmed.

http://www.lakesidepress.com/CPAP/ASV.htm

Note also they say the Bipap AutoSV targets 95% of Av Peak Flow & I'll defer to that figure. My recall of 90% may relate to the Resmed target volume & rate (or it got changed).

This link hints at new models (now out and available) and also points out how this area of xPAP is evolving fast. Also, I agree with their assessment that no one brand is particularly better than the other at applying ASV but I will say I think the Resmed easy-breathe is a better flow waveform & in my own experience with both todate, the Resmeds are quieter but not so much that it would deter me from using either.

DSM

#2
This link confirms that Resmed was using 90% target.
http://www.resmed.com/us/documents/1010 ... -sheet.pdf

The difficulty with most docs on ASV, is knowing which model a document really refers to, sometimes the documents don't seem to know. The algorithms are changing in these machines & we are not always told when or why. Sometimes it is possible to 2nd guess what may have changed but the secrecy surrounding the exact algorithmic details doesn't look like going away any time soon.

D

[dsm]

This Resmed diag confirms what I have long believed (& obtained from running pressure sensing tests). It is from the Resmed tech spec PDF linked to in an earlier post.

It shows 3 pressure increases from the moment the machine decides to respond to an event. It also shows roughly 3 pressure drops back to where it was before after the breathing is considered normalized. The total PS increase indicated in the below chart shows it going from 7CMs to 15CMs which is a PS increase of 8 CMs. I don't believe either Resmed's or Respironics' machines exceed around 3 CMs in any one breath boost & also I don't believe either machine exceeds 10 CMs in any one pattern of 3 boosts.

Note EEP (End Expiratory Pressure (also sometimes called epap)) is shown as 4 CMs. PS is shown as 3 CMs (that is the lowest & also the default PS the Resmed machine can be set to). so epap=4 ipap=7, in the diagram below the machine boosts ipap to 15 CMs ( 7 + 8 ). As said before I think the max the machine might boost by is actually around 10 CMs of boost in 3 breaths.

The diag talks of PS varying from 3 to 11. 4=epap 4+3=7 (ipap) 4+11=15 (or max pressure achieved in normalizing that imaginary patient).

DSM

[Mr Bill]

Very interesting reply DSM. I have my min BPM set to 8 so I can fall asleep. However, I always seem to end up around 18BPM once I am asleep.

[JohnBFisher]

... How does it [ASV] work? ...

Rather than answer the technical side of it, that DSM did a good job with, let me explain the WHY behind an ASV.

Basically, an ASV attempts to break the undershoot / overshoot cycle that creates Central Apneas (CA) and Cheyne-Stokes Respiration (CSR). In both cases, these start as depressed respiration. This less than ideal respiration (the waning side of CSR) causes a build up of CO2 in the blood. This is the undershoot side of the cycle. The body does not breathe as much as needed to blow off the CO2.

As the CO2 builds up, the body tends to over react. The respiration rate increases, the breathing deepens. This is the waxing side of CSR. During this over compensation, the body blows off too much CO2. This is the overshoot side of the cycle.

By blowing off too much CO2, the respiratory drive is depressed. And we start the cycle over again.

To interrupt this cycle, the ASV switches from a BiPAP (or even CPAP) to become a ventilator. In effect, if I don't breathe in a certain amount of time, the pressure increases, to inflate my lungs. Respiration occurs, even if my body is not involved in the process. Thus, the normal overcompensation that follows the undershoot side of the cycle does not occur. The cycle is broken.

So, by being an adaptive servo-ventilator, the ASV combines BiPAP and ventilator to meet my specific needs. And it works! My AHI is almost always less than 2. And it is normally 0 or 1. I sleep MUCH better with my ASV. Without it, I was starved for sleep and had uncontrollable high blood pressure.

Hope that helps explain WHY an ASV does what it does.

[dsm]

Thanks John

Always helps a ton to have the other side explained - am sure there is a bigger audience for the basic why
Cheers

DSM