Bilevels, Timed Mode, & Ventilation - Comments

In a few posts I have seen references to timed mode Bilevels as implying that they always get a person with a central breathing again.

Just wanted to explore this issue a little & hope to clarify just what a Bilevel can & can't really do. ..

Timed Ventilation:
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1) Timed mode means the bilevel can be set to switch between epap & ipap after a pre-configured period of time. For this discussion I'll call that cycling ...
> a. Fixed timed mode will cause the bilevel to cycle between epap & ipap at a set timed rate - the values that determine this are a set BPM number (Breaths per Minute) rate & the length of inspiration (max 3 or 4 secs dep on brand) & in some machines the ratio of inspiration to exhalation
> b. Spontaneous timed mode allows a back-up rate to be set (again using BPM or Breaths per Minute) and if the sleeper's breathing rate drops below that BPM then the machine starts cycling at the rate detrmined by the BPM setting. As with the other mode there can be settings for time, insp duration & insp to exhale ratio. So timed spontaneous mode adds a 'backup rate'

2) Does the 'cycling' of itself actually make the sleeper who is having a central breath again ?
> not always - especially if the machine was not set with an adequate epap to ipap gap & adequate risetime, which typically can only be done in a sleep lab bilevel titration

Lets look at the mechanisms.
The cycling of a conventional bilevel is usually based on time & relies on the ipap to epap gap being wide enough that when the machine cycles from breath-out (epap) to the breath-in pressure (ipap), that change in pressure is enough to start to inflate the sleepers lungs & trigger their natural breathing again but. But if that gap is very small or the rise time has been fiddled with and the change in pressure is done too slowly such as at a risetime of 6 (600 millisecs) vs say 2 (200 millisecs) the sleeper may well not react at all. We have had one case here where the person's machine would push ipap to 30 CMs & he still wouldn't start breathing, whilst that is an extreme example it shows that just cycling doesn't always achieve the desired result & thus any dilution of the gap effect or risetime makes it harder for the machine to achieve its goal.

What inflates a sleepers lungs ?
> air under pressure

Who needs their lungs to be inflated under pressure ?
> usually patients in a hospital

What gets someone with a central breathing again ?
> pressure from a ventialtor can do it (inflate the patients lungs) if the pressure is high enough
> the rhythm of epap-ipap-epap-ipap cycling can of itself trigger the sleeper to start breathing again
> doing the above and boosting pressure each ipap cycle is the technique used on some SV machines
> the CO2 level in their blood rises to a point that the brain finally responds & triggers breathing again

Points here ...
a) The gap really needs to be tuned in the lab to match the needs of the person suffering centrals. The epap is of course the 1st thing to set and that deals with obstructive apneas. The gap & speed of air delivery is what is going to best get the sleeper with a central breathing again.
b) The only certain way to get a person breathing again is to pump a known (and safe) volume of air into them (volume ventilation)
c) Home timed only bilevels rely on the simplest of techniques to try 'nudge' a central afflicted sleeper into breathing again - timed cycling
d) Too big a gap between epap & ipap can create other problems and can harm parts of some people's respiratory tract - titration is a balancing act


Volume Ventilation:
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Another type of bilevel is the Volume ventilator - typically found in hospitals but also as specialized home ventilators. It uses time but also will have settings that deliver volume and can also control flow as well. If the tidal flow for a patient is such that they need to breathe 550ml per breath, a volume ventilator can be set to make sure that patient gets approx that volume of air per breath (volume) and the rate at which it is delivered can also be set (time & flow). The pressure needed to archive that tidal volume & flow is be applied as needed by the ventilator.

One machine that can do volume *assist* is the Respironics AVAPS Synchrony machine. It is a specialist machine really needing careful selection for the user.
http://www.talkaboutsleep.com/sleep-dis ... irator.htm

There are a number of other brands home 'volume ventilators' including ...

Resmed Models ...
http://www.resmed.com/en-uk/products/fl ... u=products
http://www.resmed.com/en-uk/products/fl ... u=products

Also another more robust Respironics home 'Volume Ventilator' for people with more serious complications ...
http://www.medexsupply.com/respiratory- ... id-86.html

#2 I was just thinking - I would also probably put the Vpap Adapt SV in the category of a volume ventilator but not the Bipap Auto SV. The Adapt SV sure tries to inflate even if one resists it - the Bipap SV is far too gentle to do the same level of forced breathing I have experienced with the Vpap Adapt SV


The point here
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If someone implies or says that a Timed mode bilevel will resolve a central, that can be more wishful thinking than fact - a timed bilevel 'can' resolve a central but 'only' if set up properly. So, if a timed bilevel has been tuned to the person's centrals the wishful thinking is closer to fact. If the set up is done by anyone who didn't tune the machine to the particular persons real needs (epap, ipap, the gap & the risetime (or flow control)) then it becomes more wishful thinking rather than fact. Anyone who has centrals & plays with the mentioned settings really needs to know what they are doing esp the risetime setting. For central sufferers it is *not* a comfort setting. It regulates the speed of flow when cycling and thus can change the volume delivered and if risetime is too long it can overly soften the trigger effect some people need to start spontaneously breathing again.

The only bilevel I can think of that can be safely claimed that it 'will' resolve a central is a 'volume ventilator'. Timed only bilevels 'hope' they are resolving centrals & if set up well should do so.

All debate welcome

DSM

Following on posts in another thread re the Bipap SV, I did some tests last night to see what my machine would do if I stopped breathing altogether. This is in effect simulating 1) a non-responsive central, or 2) a non-responsive obstructive apnea (the machine can't tell the difference & relies on epap being set high enough that #2 above will rarely happen).

The specs for the machine (as I understand them) say that if there is zero flow, it will start to cycle. Cycling means switching from epap to ipap in a regular pattern based on the current 1) calculated BPM (BPM=AUTO), or hard set BPM ( where BPM is set to a number like 10).

Each time the machine went to ipap it boosted the pressure. I believe it will add pressure three times (not 100% certain of this though) & will only stop adding pressure when it reaches IpapMAX or the user starts normal breathing again.

Whilst this is not volume ventilation, it is going someway towards it. Real volume ventilation is where the machine is trying to restore a particular volume. This is one of the major difference between the Bipap SV and the Adapt SV, The Vpap SV monitors and drives peak volume + BPM wheras the Bipap SV monitors a target peak flow + BPM. Using both machines one gets to feel what that actually means.

DSM

(#2 UPDATE) - have determined that the below patent actually applies to an Auto & not a Bipap so the Bipap SV does handle irregular breathing differently from an std Auto machine - The SV uses cycling & the Auto merely adjusts pressure over time ...)

(#3 UPDATE)
Because this is Auto & not bilevel related I moved it to the other thread where the question was 1st raised.
viewtopic/t35402/What-are-Flow-Limitati ... thing.html

DSM