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Breathing against a fan/pressure I'm confused any help would be great

  1. Aug 21, 2012 #1
    Breathing against a fan/pressure .... I'm confused... any help would be great

    Hi all,

    This post is in relation to breathing on a CPAP device which is commonly used to treat sleep apnea. What I am confused about is the effects of changing the pressure and the perceived affect on comfort of breathing.

    Essentially a CPAP device is a fan contained in a housing, with one side open to atmosphere and the fan used to generate a positive pressure that is transferred to the nose/mouth via a tube and mask which in turn provides positive pressure to the respiratory system. The fan is used to generate the positive pressure. A very simple system will have a fan controlled to operate at a constant speed. A typical pressure generated is between 4 - 20 cmH2O.

    If the fan is set to a speed to generate 10cmH2O, my understanding is that a fan generates pressure by accelerating air which creates dynamic pressure that is then "converted" in static pressure via Bernoulli’s law. Making the assumption that the pressure is set to approximately 10cmH2O on the high pressure side of the fan, when a flow is introduced to the system (i.e. someone breathing), the pressure drops across the tube due to losses due to friction. Also, if the volume of the system was to be suddenly, the pressure would drop, until the fan has had a chance to increase the pressure.

    Therefore, if someone was to inhale (increase airflow through the system) on a simple device as described above, the pressure at the mask (i.e. at the opening of the nose) drops due to a sudden increase in volume as well as the drop in pressure across the system due to losses due to friction. This should oppose breathing and make inhaling more difficult than breathing at atmospheric pressure.

    Similarly, if someone was to exhale on the system (i.e. a reverse airflow against the fan is created) a higher pressure is created at the nose to account for loses due to friction and the fact that the volume has increased.

    For a system generating 10cmH2O you may get 9.5 cmH2O at peak inhalation and 10.5 cmH2O on peak inhalation.

    Sorry if this is taking a while to get to the point ... If my understanding is correct (good chance that is it not) breathing against a fan as described above should make breathing more difficult. Similar to breathing through a restriction or large straw. The bit I am confused about is does the fan have any significant effect I am not considering. The reason I ask is because on CPAP at pressure above 10 cmH2O it usually feels easy to inhale and hard to exhale. The higher the pressure the bigger this effect appears to be.

    When you breathe against a fan do you have to do work against the fan? Where does the energy go when a volume of air gets pushed from one-side of a fan to the other (i.e. change in volume with a change of pressure). Should the only effect on breathing effort by the change in pressure at the nose or are there other factors that need to be considered?

    If you put your head in a "box" that has a pressure of 10cmH2O would this feel the same as breathing against a fan based system that maintains the pressure at 10cmH2O at the entrance to your nose?

    Any input help with my general confusion would be much appreciated.

    Thanks
     
  2. jcsd
  3. Aug 21, 2012 #2
    Re: Breathing against a fan/pressure .... I'm confused... any help would be great

    Hello david and welcome to Physics Forums.

    Not quite sure what your question is however do you understand how breathing works?

    When you expand the lung, the volume increases and the lung pressure drops to below atmospheric so air is pushed into your lungs by the atmosphere, which is at higher pressure.

    When you contract the lung the lung pressure increases to above atmospheric and so pushes the air out of the lung back into the atmosphere.

    Consequently the higher the atmospheric pressure the easier it is to breathe in but the harder to breathe out.

    By atmospheric I mean the pressure around the mouth including any mechanically derived increase.
     
  4. Aug 21, 2012 #3
    Re: Breathing against a fan/pressure .... I'm confused... any help would be great

    Thanks Studiot. I understand respiratory mechanics and am not sure if I agree with your post. The lungs are attached to the chest wall. The chest wall keeps the lungs inflated. If you remove the connection to the chest wall the lungs will deflate and the chest wall will expand to its equilibrium position. At the beginning of inhalation the pressure in our lungs is the same as the atmospheric pressure. When you inhale you create a negative pressure relative to atmosphere pressure that drives air into the lungs. The pressure differential is relative to the atmospheric pressure and changes in atmospheric pressure shouldn't have much if any mechanical effect on the work of breathing.

    But, at higher altitude, air density is lower so there is less oxygen per litre than at sea level. This can cause issues with breathing comfort.

    My question was around the effects of breathing against a fan that generates positive pressure. If your lungs are at the pressure supplied by the fan at the end of inhalation should it be difficult to exhale against the pressure created by the fan?
     
  5. Aug 21, 2012 #4
    Re: Breathing against a fan/pressure .... I'm confused... any help would be great

    Well your description of breathing pretty much matches mine (wipes brow :approve:), the point I was trying to convey is that there is no such thing as 'suction'.

    Compressing the air space inside the chest will produce roughly the same pressure differential about a higher base pressure equal to normal atmospheric plus the fan pressure.
    Edit

    It is the pressure differential that counts.

    So I was wrong before, you should not experience additional difficulty breathing out because the base pressure is higher, in terms of the volume compression.

    But because the base pressure is slightly higher the body will have to work slightly harder to achieve this compression. This may be felt as additional load by the unfit. This effect must be very slight as the compressibility of air at 300K and 1 bar is 0.9999 and at 5 bar it is 0.9987. The compressibility is a measure of the work required to compress the air

    As a diver I do not experience any extra difficulty breathing air increased pressure - rather more than your fan.
     
    Last edited: Aug 21, 2012
  6. Aug 21, 2012 #5

    NascentOxygen

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    Re: Breathing against a fan/pressure .... I'm confused... any help would be great

    From what you wrote, can I conclude that the pressure differential is user-settable? If this is the case, I'd expect there would be something in the accompanying instruction manual that would provide the information you seek. This raises the question: how does the user go about determining what setting s/he should use? I'd expect this to be of major importance.
     
  7. Aug 21, 2012 #6
    Re: Breathing against a fan/pressure .... I'm confused... any help would be great

    Another way I have been thinking about it is as a spring attached to a movable piston. Diagram attached.

    As the pressure is positive the piston will move so upwards until forces are balanced via Hookes law. If I then applied a force about this new equilbrium position to move the piston upwards and then measured how far the piston moved then removed the force and appled the same force in the opposite direction I would assume the piston would move the same distance in either direction (F = -kx). Is this correct? Does having to move a volume of air from one side of the fan to the other have any significance?
     

    Attached Files:

  8. Aug 21, 2012 #7
    Re: Breathing against a fan/pressure .... I'm confused... any help would be great

    Setting is determined via a sleep study but I have tried machine that are set to various pressures.
     
  9. Aug 21, 2012 #8
    Re: Breathing against a fan/pressure .... I'm confused... any help would be great

    Seems to me that the difference is one of direction. With pressure one commonly assumes an averaged force over area but with a fan I'd assume some anisotropy unless the equipment is designed in a way that creates such a turbulent flow that the directional characteristics are inconsequential in which case it would be the same.

    The purpose of a CPAP is to cause greater expansion of the lungs isn't it?
    Lets say work is PdV but that pressure is actually the pressure differential between the internal and external portion of the lungs.

    I don't know much about physiology so I'm thinking about a balloon...

    Lets assume the external portion of the lungs stays at constant pressure and that the internal is controlled by the CPAP. Then at higher internal pressure it would be easier for expansion to occur and more difficult for contraction.

    That's what makes sense to me anyway.
     
  10. Aug 22, 2012 #9
    Re: Breathing against a fan/pressure .... I'm confused... any help would be great

    The purpose of CPAP isn't to increase the pressure in the lungs. CPAP is used to treat sleep apnea which is basically severe snoring which causes sleepiness. If you are overweight, tired and your partner says you snort/choke in your sleep odds are you have sleep apnea and will benefit from CPAP.

    CPAP works by providing pressure to the respiratory system and in particular the upper airway. It basically holds the floppy airway open via a pneumatic splint. As a consequence the pressure in the lungs is increased but you breath on top of this elevated pressure, the CPAP doesn't breath for you. Its basically like sticking you nose in a pressurized space/room and breathing on top of this elevated pressure.
     
  11. Aug 22, 2012 #10
    Re: Breathing against a fan/pressure .... I'm confused... any help would be great

    The balloon analogy isn't bad. If you put a balloon connected to a tube which has a fan in it the balloon will expand until the pressure in the balloon equals the pressure provided by the fan.
    I guess my question is if you can imagine an external force now applied to the balloon to expand it (i.e. using muscles to inhale) and then you remove the force, presumably the balloon would deflate back to the point prior to applying the external force? Does the work expanding the balloon the same as the work deflating the balloon?
     
  12. Aug 22, 2012 #11
    Re: Breathing against a fan/pressure .... I'm confused... any help would be great

    The work may be the same but I suspect they are by different sources. Inhaling is not due to a pressure change, it is due to a volume change by muscular activity then pressure equalizes. This work is being performed by the person's muscles causing the change in volume for a given pressure (or pressure differential if you prefer).

    Like I said before, I don't know enough about physiology or anatomy but I suspect that a person doesn't perform as much work when exhaling (mostly due to this being a relaxed state and comparing it to lifting an object requiring continuous work though I'm not sure I've thought enough about it to have an explanation).

    Maybe a good way of thinking about it is that expanding one's chest wall is increasing their potential energy (at the expense of some kinetic energy) and then when they exhale this is that potential energy being utilized like some form of a stretched spring returning to a relaxed state.
     
  13. Aug 22, 2012 #12
    Re: Breathing against a fan/pressure .... I'm confused... any help would be great

    I reread your post and I'm not sure I gave a clear answer. (Time for bed...) The work for the case of the balloon would be the same but given that the mechanism for expansion is more complex than that analogy I suspect that inhaling is actually more work due to that being the state requiring muscular tension.
    It seems like the answer you're looking for is to the question "if/why it is less work to stop flexing than it is to flex?"
     
  14. Aug 22, 2012 #13
    Re: Breathing against a fan/pressure .... I'm confused... any help would be great

    To aid the purpose of discussion here are some rather hasty sketches of the breathing process and a typical output from a spirometer.

    As you can see breathing is an asymmetric process.

    Since muscles can onlly pull not push, body muscles work in antagonistic pairs, each pulling the opposite way.

    For breathing the antagonistic pairs are the internal and external intercostal muscles.

    To inhale the external ic muscles contract and the internal ic muscles relax, forcing the ribcage up and out. At the same time the diaphragm moves down.
    This expands the thorax volume and since the lungs are fixed to the diaphragm and ribcage they are expanded with it, thus lowering their internal pressure. Outside air is then at a higher pressure and flows into the expanded space.

    To exhale the external ic muscles relax and the diaphragm relaxes.
    Note the internal ic muscles are still relaxed at this stage.
    This is enough to return the thorax and lungs to its previous volume, compressing the air in the lungs (raising its pressure) to above extenal so air flow out.

    this process produces the tidal volume ripple in the spirometer graph below.

    Further forced exhalation is possible by then contracting the internal ic muscles to take up the expiratory reserve volume.

    The lungs are never fully emptied, always maintaining a residual volume.
     

    Attached Files:

  15. Aug 22, 2012 #14
    Re: Breathing against a fan/pressure .... I'm confused... any help would be great

    Adding to Studiot great explanation, as the lungs naturally want to collapse but are being held open by the chest (ribs, etc) we breath on top of a baseline volume (functional residual capacity).

    CPAP increases the pressure in the lungs with increases the baseline volume we breath upon which changes the work of breathing a little bit. What I don't understand is why it is hard to pressure out and easy to breath at higher CPAP pressure.

    Similar question. If you where to lie-down and place a weight on your chest. When you relax, your chest acts like a spring so it would compress until the force due to the weight the chest force (spring type system) and lungs retraction force balance. Within reason you should be operating about this new equilibrium and the added weight shouldn't do to much except change your equilibrium point. But I know if I put a weight on my chest it is going to be hard to inhale and easy to exhale. I feel a little bit like I am missing something fundamental.
     
  16. Aug 22, 2012 #15
    Re: Breathing against a fan/pressure .... I'm confused... any help would be great

    If you put a weight on your chest you are doing extra work against gravity on the weight as you breath.

    You don't have any extra chest muscles to effect this so it must be seen by the intercostals as extra load.

    There are biology and medical sections here. Perhaps you should ask a moderator to move this there as many with better knowledge than mine frequent those sections.
     
  17. Aug 22, 2012 #16
    Re: Breathing against a fan/pressure .... I'm confused... any help would be great

    If I have a weigth attached to a spring to extend the spring downwards it will extend to x based on Hooke's law (F = -kx) until the force from the spring balances the force due to gravity.

    e.g. 1 kg mass so a force of approx 10N applied to the spring due to gravity. Let k = 100 N/m. Therefore x = 0.1 m. If I now apply another external force "down" of 1N, I get x = 11/100 = 0.11 m. Alternatively if I apply an external force of 1N "up", I get x = 0.09 m. Either way the addition of 1 N with or against gravity moves the weight 0.01 m in the appropiate direction.

    More work will be done moving the weight down as the force is greater (W = Fxd).

    What I have done above "feels" wrong yet I don't know why? If above is correct then working with or against gravity has little effect?
     
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