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david316
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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
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