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CPAP Pressure vs. Volume
#1
I am new to this forum so this may be a repeat....I searched but couldn't find this topic.

What is the difference between CPAP pressure and volume of air flow?

I just replaced my control unit (Respironics System One) two weeks ago when the first one malfunctioned after a year of use. The new unit appears to be exactly the same model (DS 150). However, the difference is like night and day. The new unit is much more quite but seems to have less air flow than the old one. I took it back to the medical provider to have the pressure checked and was told it was the same (9.0). I asked to have it set to 10.0 but after two weeks I set it back to 9.0 due to chest pain. So, now I am getting a pressure of 9.0 but it sure feels and sounds like the volume of air is less than before.

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#2
Talking out loud as I go....

The CPAP pressure (yours is 7) is how much the force of air is (I think it means it can push water up 7cm).

If I press solidly down on a scale so it registers 7lbs, if I press with more force, the weight changes. If I press less, the weight changes. There's no "pressing gentler" to reach the 7lbs cause I'd still be pressing 7lbs worth. It might take me more effort to reach that 7lbs than it might my younger and stronger nephew but it's still 7lbs.

So if an older machine is exerting more effort to push out 7cm of air pressure, is it using more air to do so than, say, a newer machine that can push out the same 7cm?

If the hose is the same, I'd say CPAP pressure and air volume would be the same.

Using my twisted logic, I'd say that no, the volume of air and the CPAP pressure are the same.

But, like all things, I could be wrong.

Now you got me curious!
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#3
An old saying goes..."a pound is a pound the world around"
A measured pressure of 9 cmH2O in one machine is going to be
a measured pressure of 9 cmH2O in another machine. There are many posts of people remarking that because they changed masks from a full face or nasal mask to a pillows style, that the pressure increased, or that if the pillows was changed to a mask , led them to believe that the pressure decreased. Neither is true. The pressure to the patient remains the same.
Paula has the principle.. CPAP pressure can be measured with a device (an ez homemade version can be made) called a "manometer". A simple manometer can be a plastic tube attached to a plywood board, in a "U" shape with some water in it. Without any added pressure to either end of the tubing, the water will be at equal level. If one puts some centimeter measured marks along one of the upright sides of the tube and a CPAP hose; w/ CPAP set at 9 cmH2O is attached to the opposite side of the hose, the result would be the water would rise in the marked side to the 9cm mark.
Yesterday is history; Tomorrow is a mystery; Today is a gift; Thats why its called "The Present".
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#4
Wow, I was right about something? First time today! (it's been a rough day)
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#5
(04-10-2012, 09:00 AM)Jaygee Wrote: What is the difference between CPAP pressure and volume of air flow?

Actually, pressure and volume are inversely proportional. As I recall from college years ago, and from SCUBA certifications at a later age, it is based on Boyles Law. I can't, however, recall the equation, only that for the two to act as stated requires constant temperature.

Breathing keeps you alive. And PAP helps keep you breathing!
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#6
(04-11-2012, 07:37 PM)JumpStart Wrote:
(04-10-2012, 09:00 AM)Jaygee Wrote: What is the difference between CPAP pressure and volume of air flow?

Actually, pressure and volume are inversely proportional. As I recall from college years ago, and from SCUBA certifications at a later age, it is based on Boyles Law. I can't, however, recall the equation, only that for the two to act as stated requires constant temperature.

From Wikipedia here, The mathematical equation for Boyle's law is:

pV = k

where:

p denotes the pressure of the system.
V denotes the volume of the gas.
k is a constant value representative of the pressure and volume of the system.

So long as temperature remains constant the same amount of energy given to the system persists throughout its operation and therefore, theoretically, the value of k will remain constant. However, due to the derivation of pressure as perpendicular applied force and the probabilistic likelihood of collisions with other particles through collision theory, the application of force to a surface may not be infinitely constant for such values of k, but will have a limit when differentiating such values over a given time.

Forcing the volume V of the fixed quantity of gas to increase, keeping the gas at the initially measured temperature, the pressure p must decrease proportionally. Conversely, reducing the volume of the gas increases the pressure.

Sorry, that's as far as I go with the math. I love physics, but I hate the math... Coffee

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#7
If by volumn you mean Tidal Volume - from Wikipedia- "Tidal volume is the lung volume representing the normal volume of air displaced between normal inspiration and expiration when extra effort is not applied. Typical values are around 500ml or 7ml/kg bodyweight." The CPAP doesn't influence your tidal volume except by keeping the airway open.
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#8
(04-12-2012, 07:55 AM)zimlich Wrote: If by volumn you mean Tidal Volume - from Wikipedia- "Tidal volume is the lung volume representing the normal volume of air displaced between normal inspiration and expiration when extra effort is not applied. Typical values are around 500ml or 7ml/kg bodyweight." The CPAP doesn't influence your tidal volume except by keeping the airway open.

Wouldn't tidal volume - the "normal" volume of air displaced, be different when there is an applied pressure to the lungs such as in cpap? For example, from my limited knowledge, you inspire/exhale a greater volume of air when under external pressure (eg, 100' under water) than you do at sea level. Here, wouldn't the reverse, greater internal pressure, have a similar effect? I am the reverse of SS - math is OK, but physics, uh-uh.
Breathing keeps you alive. And PAP helps keep you breathing!
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#9
CPAP doesn't actually really apply pressure to the lungs like a ventilator. It uses air pressure to splint the airway open. In another thread someone used their CPAP at a pressure of 20 to see if it would blow up a balloon. It wouldn't.
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#10
(04-13-2012, 03:07 PM)zimlich Wrote: CPAP doesn't actually really apply pressure to the lungs like a ventilator. It uses air pressure to splint the airway open. In another thread someone used their CPAP at a pressure of 20 to see if it would blow up a balloon. It wouldn't.

Whether it does or doesn't blow up a balloon (or a dirigible, for that matter) is irrlevant to the issue, IMO. Cpap provides a higher pressure to the airway than is "normal", ie, normal being the normal atmospheric pressure at the location of the person breathing without cpap. Whatever that amount of increased pressure provided by cpap may be, and whatever its purpose may be, unless there is a corresponding decrease in pressure between airway entrance and lungs, there must absolutely be increased pressure in the lungs. Whether it is minimal or not does not change the standards as defined by Boyles Law.
Breathing keeps you alive. And PAP helps keep you breathing!
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