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o2 in cpap
#1


I'm just curious about the o2 going into the cpap. i have the hose adapter for my wife and know how to hook it up but am curious about how the pressures work. the cpap at 6 or 11 cm/h2o seems to have alot more pressure than the o2 set at the 2 liter rate. how is the o2 able to get into the stronger pressure cpap air without the cpap pressure blowing back in the o2, stopping it from entering. i know the cpap number is pressure and the o2 number is volume. what is the pressure of the o2 and how does this work? thanks.
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#2
PRS1 manual
• When using oxygen with this system, the oxygen supply must comply with local regulations for medical oxygen.
• Oxygen supports combustion. Oxygen should not be used while smoking or in the presence of an open flame.
• When using oxygen with this system, turn the device on before turning on the oxygen. Turn the oxygen off before turning the device off. This will prevent oxygen accumulation in the device. Explanation of the Warning: When the device is not in operation and the oxygen flow is left on, oxygen delivered into the tubing may accumulate within the device’s enclosure. Oxygen accumulated in the device enclosure will create a risk of fire.
• When using oxygen with this system, a Respironics Pressure Valve must be placed in-line with the patient circuit between the device and the oxygen source. The pressure valve helps prevent the backflow of oxygen from the patient circuit into the device when the unit is off. Failure to use the pressure valve could result in a fire hazard.
• Do not connect the device to an unregulated or high pressure oxygen source.
• Do not use the device in the presence of a flammable anaesthetic mixture in combination with oxygen or air, or in the presence of nitrous oxide.
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#3
(04-20-2012, 11:24 PM)greatunclebill Wrote: I'm just curious about the o2 going into the cpap. i have the hose adapter for my wife and know how to hook it up but am curious about how the pressures work. the cpap at 6 or 11 cm/h2o seems to have alot more pressure than the o2 set at the 2 liter rate. how is the o2 able to get into the stronger pressure cpap air without the cpap pressure blowing back in the o2, stopping it from entering. i know the cpap number is pressure and the o2 number is volume. what is the pressure of the o2 and how does this work? thanks.

Darn good question. I think you got everybody stumped, at least so far. Fortunately, I have never had to consider how to do this although, like everyone else, I knew where the O2 was supposed to enter the mask. Just assumed it would mix, but no, you had to ask how!HuhsignCrazy Now I woke up last night thinking about this!!!Oh-jeez

Breathing keeps you alive. And PAP helps keep you breathing!
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#4
(04-21-2012, 07:42 AM)JumpStart Wrote:
(04-20-2012, 11:24 PM)greatunclebill Wrote: I'm just curious about the o2 going into the cpap. i have the hose adapter for my wife and know how to hook it up but am curious about how the pressures work. the cpap at 6 or 11 cm/h2o seems to have alot more pressure than the o2 set at the 2 liter rate. how is the o2 able to get into the stronger pressure cpap air without the cpap pressure blowing back in the o2, stopping it from entering. i know the cpap number is pressure and the o2 number is volume. what is the pressure of the o2 and how does this work? thanks.

Darn good question. I think you got everybody stumped, at least so far. Fortunately, I have never had to consider how to do this although, like everyone else, I knew where the O2 was supposed to enter the mask. Just assumed it would mix, but no, you had to ask how!HuhsignCrazy Now I woke up last night thinking about this!!!Oh-jeez
for starters for those that don't know, the o2 tubing doesn't go directly in the mask ports. there is a apprx 4" piece of solid tubing (adapter) that goes between the cpap machine and the hose that goes to the mask. there is a port on that tubing that the o2 tubing fits on. the adapter can actually go on either end of the hose, close to the machine or close to the mask. close to the machine makes more sense logistically or otherwise you have an o2 line coming out the side of the adapter next to the mask making one more thing to get goofed up and another tube in your face while sleeping. when it is attached near the machine it is as if isn't there. it is not in the way of anything. i guess it is physically possible to stick the o2 line directly into the mask but i was given an o2 adapter and told to do it that way. o2 line into the mask would cause the same problems as the adapter being near the mask. the other problem going directly to the mask is that there's a chance the good o2 could flush directly out the exhaust vent.

the o2 adapter looks like the below picture.

this o2 situation came out of the blue so to speak. my wife had her sleep study 2 years ago and nothing was said about o2. when i bought my pulse oximeter i tested us with and without cpap. hers was low with cpap so i asked the doc to have a professional test done. the result was an o2 generator, so i guess my little rig works. this is another good reason to buy a pulse oximeter. without it we would have had no idea. in a sense it probably saved her life.

[attachment=116]
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#5
I think the adapter you describe is simply a back-flow preventer. I suppose it could also be a pressure regulator, but sure doesn't look like one to me. If it isn't, how do you equalize the pressures? Particularly on a multi-pressure system like apap? That is the 64 dollar question, in my mind. And if you don't have to do that, why not?
Breathing keeps you alive. And PAP helps keep you breathing!
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#6
(04-21-2012, 09:07 AM)JumpStart Wrote: I think the adapter you describe is simply a back-flow preventer. I suppose it could also be a pressure regulator, but sure doesn't look like one to me. If it isn't, how do you equalize the pressures? Particularly on a multi-pressure system like apap? That is the 64 dollar question, in my mind. And if you don't have to do that, why not?
it is a three way connector, 2 ends to put it in line with the hose and an o2 port for the o2. there is nothing preventer or regulator about it. if i take off the o2 line, hold my hand over the mask end of the hose and turn the machine on, cpap air comes out of the o2 port. it has to be the right thing because everybody sells them for the purpose and there is too much $$$ liability involved if it isn't. for that matter there is nothing preventer or regulator in mask ports.

i have a feeling we're all gonna learn something before this thread ends.
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#7
Pressure vs. flow. As you said, completely different critters.

11cm/h20 is, of course, a pressure. 2 liters per minute o2 is a flow rate. A full tank of o2 is pressured at ~2000 psi. The tanks regulator knocks that down to something in the neighborhood of 50 psi where a CPAP at 20cm/h2o is, as we found recently, only around 1/4 psi.

Even though the o2 is being delivered at a much lower flow rate than the pap, the pressure is significantly higher, so no problem.

At least, that's how I see it...
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#8
The O2 line is pressurized and injects the O2 into the CPAP airstream? I don't know cause I don't have one.

Regardless, O2 will be pulled into the CPAP airstream due to the Bernoulli effect. The air in the CPAP airstream is of course moving, and when a fluid moves its pressure is reduced. Thus the O2 is pulled into this lower-pressure air stream.

Pressure washers use this same principle to introduce soap into the water stream.

In our automobile engines we used to have carburetors. There's a stream of air that's pulled into the carburetor. This same effect is used to pull the gasoline into that airstream.
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#9
(04-21-2012, 08:57 PM)mckevin32 Wrote: .... A full tank of o2 is pressured at ~2000 psi. The tanks regulator knocks that down to something in the neighborhood of 50 psi where a CPAP at 20cm/h2o is, as we found recently, only around 1/4 psi.

If the O2 is at a higher pressure, it will have no difficulty entering the cpap airstream to the mask. However, something will have to prevent it from flowing the other way, IMO. Pure O2, much less O2 @ a much higher pressure, will need to be prevented from entering the device. I believe some form of backflow preventer will be necessary. And I would think that some equalization of pressures between cpap and O2 would be appropriate. Just my thought.
Breathing keeps you alive. And PAP helps keep you breathing!
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#10
(04-21-2012, 09:40 PM)JumpStart Wrote: If the O2 is at a higher pressure, it will have no difficulty entering the cpap airstream to the mask. However, something will have to prevent it from flowing the other way, IMO. Pure O2, much less O2 @ a much higher pressure, will need to be prevented from entering the device. I believe some form of backflow preventer will be necessary. And I would think that some equalization of pressures between cpap and O2 would be appropriate. Just my thought.

Though I doubt there would be any harm in o2 entering the device, I suppose there could be an increased fire risk given that o2 promotes combustion. In any case the point should be moot since the flow rate of the o2 is so very much lower than that from the pap. Keep in mind that the flow rate for the o2 is only 2 or 3 liters per minute, I have no idea what the typical flow rate is for a cpap, but my Sleepyhead reports commonly show leaks over 25 lpm, so actual output has to be a number greater than that.

The o2 pressure is several hundred times greater than that of the air from the device, so it has no trouble at all being introduced into the stream, but the flow rate is proportionally miniscule compared to the flow from the pap, ensuring the delivery to the patient rather than backflowing to the machine.
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