Quote:Would you please educate me. I would like to know what velocity pressure is and how it is that we have 2 kinds of pressure in our PAP air system. I also do not know of any static pressure in the system.
I have a lot of trouble just educating myself (which may be evident). The pressure from a xPAP is velocity pressure. Total pressure is velocity pressure plus static pressure. Static pressure is somewhat like atmospheric pressure. A blown up balloon has static pressure inside it, for instance. Wikipedia can probably explain this better than I ever could, but neither velocity pressure nor static pressure exist in a vacuum (no pun) in an xPAP-aided breathing system; respiration is always a function of total
And the reason I got so deep as to make the distinction is because of the system we have, which is partially closed. It is a closed system if you ignore the CO2 venting (and of course the fact that we have an airway to the outside), so for all intents and purposes you can then consider a simplified version of human breathing with xPAP a closed system, at least for this explanation.
In normal breathing, the diaphragm changes the volume of the cavity where the lungs exist, essentially changing the volume of the lungs themselves. When you breath in, the static pressure inside your lungs decreases differentially as opposed to the atmospheric pressure outside your lungs, and the greater static pressure outside is what pushes the air into your lungs. Actually, I said that wrong; the change in differential pressure is what causes you
to breathe in. When you exhale, the diaphragm attempts to reduce the size of the volume of the cavity, and the static pressure inside your lungs is then greater than the atmospheric pressure, and the internal, higher pressure is what pushes the air back out of your lungs. Static pressure always attempts to equalize via flow when there is no barrier, and those changes in pressure caused by the diaphragm are what causes airflow in and out.
So that static pressure oscillates mildly, with or without the presence of the CPAP velocity pressure introduced into the system, and only due to the diaphragm modulating the internal static pressure in comparison to the external static pressure. Since total pressure is V pressure plus S pressure, total pressure also changes slightly depending upon whether you are inhaling or exhaling. But the V pressure from the CPAP does not change, and does not need to change (once set properly, of course).
Quote:The pump must have an impeller in order to maintain even pressure. A reciprocating pump can not do that.
The only thing unique about an impeller is that it is the "fan blade" part of a centrifugal pump. But any fan can blow air, and will work just the same as an impeller-driven centrifugal pump, and could theoretically power an xPAP. Modern xPAPs might indeed all use impellers, but that is not critical to blowing air. Either a centrifugal pump or a simple fan can blow air, and all either needs to modulate pressure is a servo feedback system to accelerate or retard the speed of the fan blade or the impeller. Certainly a reciprocating pump would never be appropriate, exactly for the reason you state. But whether an xPAP uses an impeller or not is not relevant to being able to modulate pressure or maintain even pressure, because a simple fan can do this as well.
Quote:EPAP pressure *IS* is what controls obstructive apnea otherwise the airway might start to close during exhale and be harder to maintain open for the inhale.
"OSA is a problem with airway collapse on inhale. The airway does not generally collapse on exhale. So whether the total pressure in the system goes up during exhale is not relevant to the therapy, and compensating for that change in total pressure during exhalation is not needed."
I restated that here because I believe it to be true and to be the very central to how xPAP therapy works. And how physics works. Blowing air into (increasing the differential pressure) into any cavity results in a stenting effect which is due to the difference between the total pressure outside the cavity as compared to inside the cavity. When that pressure is greater inside the cavity, this helps prevent the walls of the cavity from collapsing. When the pressure outside the cavity is greater than inside the cavity, this instead contributes to the tendency for the walls of the cavity to collapse.
Since the differential pressure is greater on the outside of your lungs when you inhale, this is when the airway is in danger of collapse. When you exhale and the pressure is greater inside your airway, there is no danger of the airway collapsing, barring any highly-unusual exceptional cases, due to the stenting effect. In fact, the higher pressure "balloons" out your airway during exhale. That's Fluid Dynamics 101.
And what xPAP does is eliminate the negative differential pressure by adding positive velocity pressure, which stents your airway even during inhale. Theoretically, you could turn the blower off during exhale altogether (other than the danger of CO2 rebreathing) and the airway would never collapse, although that is not practical because it would be hella annoying. But bottom line, exhaling is never the problem with OSA, only inhaling. The CPAP blows air as a therapeutic aid to inhalation, not exhalation.
Back in the day I sold Electrolux solid tank vacuums door to door to work through college. One of the tricks we used was to turn on the customer's old bag vacuum, then turn on the Electrolux vacuum, and then connect the two hoses together. It was like a suction (pressure) tug-of-war. Since the Electrolux solid tank vacuum had more structure and suction than the customers' old garden-variety bag vacuum, all you had to do was wait about 5 seconds, and the bag on their old vacuum would shrivel into a pitiful little noodle. The look on house-mom's face alone was worth it. I sold a lot of vacuums that way, and if ever there was an illustration of how differential pressure worked, I think this might have been it.