How does CPAP work? Doesn't make sense

MRRPM wrote: Anyway, feels like I can't breathe while using the mask, done some calculating, seems the tidal lung volume of healthy adults is around .5 liter. My hose & mask volume of air is around .3 liter. If I'm simply pushing air back & fourth through a hose, that has over half the volume as I'm breathing in, I'm getting less than half the air I would normally get, without the mask/hose assembly, attached to my face. Seems this is almost like breathing into a bag? What am I missing here??

and

As small as these holes are, there is no way that all the air that I'm exhaling will go through them, in the time it takes me to exhale, so some, (who knows how much) is going back up the tube to be rebreathed

MRRPM,

You have the basic arithmetic wrong. Your arithmetic assumes that no new air is coming into the system through the blower unit.

Here's the correct arithmetic as I see it: Yes, tidal volume is about .5 liter for a healthy adult---during WAKE breathing. Respiratory rate is 12-18 breaths per minute. Let's over estimate the respiratory rate as 20 breaths per minute. That means that in one minute you're taking in (and breathing out) approximately:

(0.5 L/breath) * (20 breaths/min) = 10 L/min

So you need at least 10 Liters of fresh room air available each minute in order to breathe with a normal tidal volume. With the mask on, your upper airway, the mask, and the hose form a "semi-closed" pressurized system. The system is "semi-closed" because enough air is being blown INTO the system to keep the pressure constant in the presence of the intentional leak (venting) of air through those small holes in the mask. (A closed pressurized system would need no new air being blown in to the system to maintain pressure; in a semi-closed system you have to push as much air INTO the system as you are losing OUT of the system in order to maintain pressure; in order to INCREASE the pressure, you have to push MORE air INTO the system than you are losing OUT of the system.)

The typical intentional vent rate for masks ranges anywhere from 20 L/min to 60 L/min depending on mask type and the pressure used. (The higher the pressure, the larger the volume of air that gets blown through the system.) In other words, the blower unit is pumping 20-60 liters of air into the semi-closed system every single minute. All that air that's being pumped into the system is oxygen-rich room air pulled into the blower unit and through the hose. And that 20-60 L/min of air is available for your lungs to take in as much of it as they want when you are inhaling. And every single minute 20-60 L/min of air is EXITING out of those little holes in the mask. The air you exhale mixes with the air being forced into the system and it is immediately blown out of those little holes in the mask. The force of the exhaled air pales in comparison with the constant stream of 20-60 L/min of air coming INTO the mask through the hose; hence it gets blown out of the holes instead of going down the hose.

In other words, the purpose of those exhaust vents is three-fold: It provides a exist for the "bad" air you are exhaling AND the intentional leak caused by those holes forces the exhaled air out of the vents as soon as you exhale AND the intentional leak caused by those holes forces the machine to draw in far more room air than you actually need to maintain your tidal volume in order to maintain the desired pressure in the semi-closed system.

robysue wrote:

MRRPM wrote: MRRPM,

You have the basic arithmetic wrong. Your arithmetic assumes that no new air is coming into the system through the blower unit.

Here's the correct arithmetic as I see it: Yes, tidal volume is about .5 liter for a healthy adult---during WAKE breathing. Respiratory rate is 12-18 breaths per minute. Let's over estimate the respiratory rate as 20 breaths per minute. That means that in one minute you're taking in (and breathing out) approximately:

(0.5 L/breath) * (20 breaths/min) = 10 L/min

So you need at least 10 Liters of fresh room air available each minute in order to breathe with a normal tidal volume. With the mask on, your upper airway, the mask, and the hose form a "semi-closed" pressurized system. The system is "semi-closed" because enough air is being blown INTO the system to keep the pressure constant in the presence of the intentional leak (venting) of air through those small holes in the mask. (A closed pressurized system would need no new air being blown in to the system to maintain pressure; in a semi-closed system you have to push as much air INTO the system as you are losing OUT of the system in order to maintain pressure; in order to INCREASE the pressure, you have to push MORE air INTO the system than you are losing OUT of the system.)

The typical intentional vent rate for masks ranges anywhere from 20 L/min to 60 L/min depending on mask type and the pressure used. (The higher the pressure, the larger the volume of air that gets blown through the system.) In other words, the blower unit is pumping 20-60 liters of air into the semi-closed system every single minute. All that air that's being pumped into the system is oxygen-rich room air pulled into the blower unit and through the hose. And that 20-60 L/min of air is available for your lungs to take in as much of it as they want when you are inhaling. And every single minute 20-60 L/min of air is EXITING out of those little holes in the mask. The air you exhale mixes with the air being forced into the system and it is immediately blown out of those little holes in the mask. The force of the exhaled air pales in comparison with the constant stream of 20-60 L/min of air coming INTO the mask through the hose; hence it gets blown out of the holes instead of going down the hose.

In other words, the purpose of those exhaust vents is three-fold: It provides a exist for the "bad" air you are exhaling AND the intentional leak caused by those holes forces the exhaled air out of the vents as soon as you exhale AND the intentional leak caused by those holes forces the machine to draw in far more room air than you actually need to maintain your tidal volume in order to maintain the desired pressure in the semi-closed system.

I understand what you're saying, and it makes sense, but let's break it down to one breath at a time.
On inhale, I will breathe 100% room air, whatever it's makeup is. Upon exhale, some air goes out of the intentional leak holes, back into the room, and some gets pushed back up into the hose, since there's no check valve keeping air out of the hose. Upon the next inhale, I am rebreathing some of the air that was previously exhaled, right?

How is air getting into the hose? Your machine only exhales does not inhale.

The 2-4 feet between your mask's vents and the intake of your machine will further reduce the amount of "used" air you can inhale.
The cpap makes it nearly impossible to smell some of the usual scents that occur in bed (especially the foul ones)

MRRPM wrote: I understand what you're saying, and it makes sense, but let's break it down to one breath at a time.
On inhale, I will breathe 100% room air, whatever it's makeup is. Upon exhale, some air goes out of the intentional leak holes, back into the room, and some gets pushed back up into the hose, since there's no check valve keeping air out of the hose. Upon the next inhale, I am rebreathing some of the air that was previously exhaled, right?

No, you are still not understanding.

Under normal operating conditions, on each exhalation 100% of the exhaled air is immediately FORCED out of the mask through those vent holes by the CONSTANT 20-60 L/min of air constantly being blown INTO the hose by the blower unit. None of the exhaled air is "pushed back up into the hose" because the force of the incoming air is too great to allow the exhaled air to "back up" into the hose.

Think of it this way: The air being pumped into the system is a rather strong wind compared to your exhalation. That wind is bringing air UP the tube, INTO the mask, and OUT the vent holes, and it sets up a very STRONG one-way airflow. In other words, the airflow through the semi-closed system is like a strong stream of water where the blower unit is at the upstream end and the vent holes are at the downstream end. When you exhale the air you exhale is being exhaled with much less force than the one-way air current coming into the mask from the hose. The exhaled air is like a stick thrown into a stream of water: The exhaled air (the stick) is carried out the vent holes (downstream) by the force of the wind blowing through the system (the current of the stream).

The exhaled air cannot get pushed "back up into the hose" just like the stick cannot move "upstream" from where it lands in the water.

In other words, you don't need a check valve because force of the incoming air (the wind speed) is great enough to prevent air from backing up into the hose.

Bill44133 wrote:How is air getting into the hose? Your machine only exhales does not inhale.

The blower unit BRINGS IN FRESH AIR through the INTAKE vent on the back of the machine. The INTAKE vent is usually covered by an air filter so that you are NOT breathing concentrated amounts of dust, pet dander, and pollen.

Here's a rough schematic:

AIR is pulled INTO --> the intake vent of the blower unit by the fan in the blower unit --> and the AIR is then slightly compressed by the blower unit --> and the AIR is then blown INTO the hose --> and the AIR is then blown INTO the mask (where you breath it in if you are inhaling) --> and the AIR is then blown OUT of the mask through the exhaust vents.

The air you exhale enters the air stream at the mask (the italicized part of the schematic), but the AIR current in the system is too strong for the exhaled air to flow BACKWARDS in the system. Like a stick carried downstream by the water's current, the exhaled air is carried by the air blowing THROUGH the system OUT the exhaust vents and so you don't rebreathe your exhaled air.

Don't forget that the air you exhale is not depleted of oxygen. Atmospheric air is ~20% oxygen. Breathing uses 4-6% of that so there is oxygen left if the air is inhaled again.

Also, your mask doesn't just leak when you are forcefully exhaling. Air is constantly mixing and leaking all throughout the breathing process, when you're exhaling obviously, but also when you're inhaling and the pauses in between.

robysue wrote:

Bill44133 wrote:How is air getting into the hose? Your machine only exhales does not inhale.

The blower unit BRINGS IN FRESH AIR through the INTAKE vent on the back of the machine. The INTAKE vent is usually covered by an air filter so that you are NOT breathing concentrated amounts of dust, pet dander, and pollen.

Here's a rough schematic:

AIR is pulled INTO --> the intake vent of the blower unit by the fan in the blower unit --> and the AIR is then slightly compressed by the blower unit --> and the AIR is then blown INTO the hose --> and the AIR is then blown INTO the mask (where you breath it in if you are inhaling) --> and the AIR is then blown OUT of the mask through the exhaust vents.

The air you exhale enters the air stream at the mask (the italicized part of the schematic), but the AIR current in the system is too strong for the exhaled air to flow BACKWARDS in the system. Like a stick carried downstream by the water's current, the exhaled air is carried by the air blowing THROUGH the system OUT the exhaust vents and so you don't rebreathe your exhaled air.

I thought he(the op) is making the same mistake I made on the BiLevel Pap. I initially thought that the machine was breathing for me(inhale and exhale). I later discovered that the lower pressure when exhale just makes it easier for exhale. I have high settings of IPap 23 and EPap19.

There is much to learn on this CPap Therapy. I could not have gotten through all this without this website.

Yes, the air only goes one way--from the machine, to the mask, and into the room.
A playful spouse would have to walk around the bed, etc. to "share the sauerkraut".

MRRPM wrote: Anyway, feels like I can't breathe while using the mask, done some calculating, seems the tidal lung volume of healthy adults is around .5 liter. My hose & mask volume of air is around .3 liter. If I'm simply pushing air back & fourth through a hose, that has over half the volume as I'm breathing in, I'm getting less than half the air I would normally get, without the mask/hose assembly, attached to my face. Seems this is almost like breathing into a bag? What am I missing here??

Either you know just enough science to be dangerous or you are using the machine without turning it on.

I don't know what calculations you did, but I suspect they dealt with a static volume of air at room pressure. Remember 1)the air in that hose and mask is under a greater pressure than room air and 2)"fresh" air is constantly flowing into the system even when you are exhaling.

Now don't try to tell me that you exhale hard enough that the little fan in the machine reverses direction.

ChicagoGramps

ChicagoGranny wrote:

MRRPM wrote: Anyway, feels like I can't breathe while using the mask, done some calculating, seems the tidal lung volume of healthy adults is around .5 liter. My hose & mask volume of air is around .3 liter. If I'm simply pushing air back & fourth through a hose, that has over half the volume as I'm breathing in, I'm getting less than half the air I would normally get, without the mask/hose assembly, attached to my face. Seems this is almost like breathing into a bag? What am I missing here??

Either you know just enough science to be dangerous or you are using the machine without turning it on.

I don't know what calculations you did, but I suspect they dealt with a static volume of air at room pressure. Remember 1)the air in that hose and mask is under a greater pressure than room air and 2)"fresh" air is constantly flowing into the system even when you are exhaling.

Now don't try to tell me that you exhale hard enough that the little fan in the machine reverses direction.

ChicagoGramps

That 0.3 L calculation is tacitly assuming that the CPAP itself is a closed system though. However, it is not, actually. The "effective" tidal volume of the system is the absolute flow rate (in units of volume per time) times the change in time (delta t, in units of seconds). One recognizes this calculation as an integral from calculus.

MRRPM wrote:New to CPAP actually BiPAP in my case, can't sleep at nite because of the mask setup waking me up. How can sleep be better, if I'm waking up more?
Anyway, feels like I can't breathe while using the mask, done some calculating, seems the tidal lung volume of healthy adults is around .5 liter. My hose & mask volume of air is around .3 liter. If I'm simply pushing air back & fourth through a hose, that has over half the volume as I'm breathing in, I'm getting less than half the air I would normally get, without the mask/hose assembly, attached to my face. Seems this is almost like breathing into a bag? What am I missing here??

Your particular mask, the Mirage FX Nasal mask, has a built-in vent rate that allows air from you and from the CPAP machine to be vented out from the mask and away from you. Looking at page 7 of the User Guide ( see link below ) we see that at a CPAP pressure of 12 cm H2O, the vent rate is 34 liters per minute. For purposes of discussion, I'm using the 12 cm pressure number because the math for bi-level pressures gets pretty bad. With an average tidal volume of 500 mL and a breath rate of 16 BPM, you will be inhaling 8 liters of air each minute or roughly one fourth of the air that the mask vents away from you. Since we sometimes see leak rates of 100 liters per minute for pretty bad mask leaks, this means that the CPAP machine can easily provide 134 liters of air per minute. You are not even going to use 10% of that volume. Assuming no leaks at all with a really, really good mask fit then the machine will be delivering 42 liters of air per minute up the CPAP hose and to the mask. 80% of that total air is vented, 20% you breathe.

The absolute worst case scenario has you re-breathing 100 mL of air each breath and 400 mL of clean, new air along with it. But that is not how it really works. Your mask has a dead volume of 93mL and none of that goes down the CPAP hose to be re-breathed. The pressure in the hose keeps the air you exhale from going back down the hose and forces it out the mask vents. While that is happening, the incoming air from the CPAP machine is mixing with your exhaled air in your mask dead space at a ratio of 5 to 1 diluting the residual 93 mL to be only 20 mL of exhaled air and 80 mL of new air ( rounding for convenience). Then you breathe in and you get that original 20 mL of exhaled air and 480 mL of new air. For practical purposes, the inhaled air is almost immeasurably different from room air with and extraordinarily low increase in CO2 levels from the room air percentage of 0.03%.

LinK:

http://www.resmed.com/us/assets/documen ... er_eng.pdf

I don't think very many of you have actually played around with your machine.
I can put my mask up to my nose, breathe out, and air will come out of the filter in the back of the machine.

So...that means air is leaving my mouth, going up the hose, going thru the fan, and out the filter, at the same time it's going out the little holes in the mask. I'm not, but if I were to breathe out the same amount of air that is in the hose, and what goes out thru the holes, I would not get fresh air, unless I could create a greater depression than the fan can make a water column, basically sucking air past the fan, or in thru the holes.

Someone said that air does not go backup the hose, but goes back out the holes. This is not true, if it were, therewould be no need for the lower pressure in a BiPAP machine. What is happening in a conventional machine is it's making a high pressure area around your nose/mouth, at all times. When you breathe out, it's harder to push the air back up the hose, hence the need to lower pressure, on a BiPAP, to allow the air to go up the hose easier. BiPAP wouldn't be needed if the air could freely go out the holes, unobstructed.

The other uncomfortable thing I find about this setup, is the mass of the air & fan having to slow down when the air reverses & travels up the hose. Pressure is whatever it's programmed at, then when the air stops, pressure at the mouth/nose is higher, from the inertia of the mass of air & rotating fan blades forcing air down the tube.

MRRPM wrote: Anyway, feels like I can't breathe while using the mask

Many people get this feeling, myself included, when any one of the pressure settings is below 7 or 6. This could be the starting ramp pressure or the minimum pressure.

What are your pressure settings?

MRRPM - your idea is a little off.

Yes if you take a deep breath and blow it all out at once you can fill the hose and machine and make air come out of the back of the machine.

However you normally don't breathe like that when you are sleeping.

If you were able to fill the hose with every breath and re-breathe you exhaled air over and over again your blood oxygen levels would drop. You can pick up an oximeter and check to see if that happens.

Since you pause at the end of each breath the pause allows the machine to clear the air out of the vents. This gives you a good supply of fresh air with every breath.

Now if you cough or sneeze with every breath all bets are off. The same goes if you pant like a dog.