(08-04-2015 11:27 AM)PaytonA Wrote: ... I would like to know what velocity pressure is...
Quote:I am still having a problem wrapping my head around this. To me, pressure is simple and is caused by compressing the gas. I can understand that the pressure you feel when a strong wind blows on you is velocity pressure but it is very localized to near the surface of your skin.
I think where the confusion may lie, if there is confusion, is in clarity regarding definitions of pressure vs. flow. Flow is movement of air, and is not pressure, but is caused by differences in static pressure from one location to another, and ONLY happens when the open nature of the atmosphere (IOW, having no barrier between area A and area B) converts that into velocity pressure, and creates flow. If there were a barrier, there would be no velocity pressure, and no flow, and just two separate areas with different static pressure.
Wind is the primary example. Wind is flow. When there is a pressure difference between the atmosphere over North Dakota and South Dakota (ND being higher in pressure) since the pressure is stronger in ND, it pushes the air south. Residents of Sioux Falls will experience a northerly wind, or flow, which is associated with the V pressure of the moving air. They feel flow, not pressure.
You can also think of it this way, in that velocity pressure somewhat implies that there is flow, while static pressure implies no flow, which is also why you can feel the effects of flow associated with V pressure, but not feel S pressure, for the most part. If the static pressure between two enclosed areas is different, and then is vented between them, then the flow that is created is actually from the static pressure becoming velocity pressure for the moment when there is flow.
We evolved to be able to sense flow, because being able to do that is integral to survival, and since pressure is not something we need to sense constantly, we did not evolve to be able to really sense that. We can feel flow, because of the Venturi turbulence that is created as it moves past us. But we can't really feel pressure, other than when landing at JFK after being at 30,000 feet, and then only because of the differential pressure on either side of our eardrums.
But otherwise, we do not feel pressure, even velocity pressure. What we might feel is the effects of flow; cooling of the skin, air hair moving, increased evaporation of sweat, slight movement of the skin, etc. But we are not feeling pressure, we are feeling the effects of flow, and those feelings are always
felt at the surface of the skin rather than anywhere else. I have to lift up the mask to create flow
(something that I can feel on my skin and hear the turbulence of) when I am not sure the blower is on or not, because pressure is not something that can be felt, only flow.
A CPAP creates velocity pressure, and if the CPAP were ported into a completely closed system, that would quickly become static pressure, increasing the differential pressure between inside us and outside us. But since the system is vented and air also goes in and out of the airway, it mostly is categorized as velocity pressure.
Since we feel flow, and not pressure, and since flow is associated with V pressure but not S pressure, it is pretty easy to mistakenly
associate flow with V pressure as if they were the same thing. Although associated, they are actually different things.
Quote:Sorry, this was just me considering a fan just another type of impeller.
No worries, just a tiny terminology bump in our common understanding.
Quote:...[ me, from earlier] 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.
Quote:This I believe to be true to a certain extent. If EPAP is set too low it can allow the airway to begin to or completely collapse at the end of exhale and/or at the normal pause between exhale and inhale. This is why I consider that EPAP is what controls obstructive apneas.
I believe it to a certain extent also, but the extent is 99%. And here is where we may part ways in our understanding. To me, it is quite obvious that INHALATION
support is 99% of OSA therapy via xPAP, because the laws of physics of fluid dynamics supports this view completely, if for no other reasons, which may be many.
Exhalation on its own stents the airway and prevents it from collapsing, which is why it does not collapse on exhale. The example I would choose is letting the air out of a balloon. If you consider that analogous to "exhale", then notice that the balloon does not collapse completely until after
the "exhalation" has completed. The surface of the balloon still has a smooth, stented shape as it gets smaller and smaller. But once the "exhale" is complete, and there is no airflow because the pressures inside and outside have equalized, the balloon collapses, and goes limp. And if you suck the remaining air out of the balloon (analogous to inhale), it collapses completely, once again due to the differences in static pressure.
That said, there are certainly issues where EPR or pressure relief can cause issues for a small number of OSA patients. And that is the other one percent. And that one percent of classical OSA patients probably blurs into the category of non-OSA patients, who need ASV, Bi-PAP, VPAP, etc. But non-OSA is a separate discussion, and there are extenuating issues beyond simple OSA CPAP stenting therapy for those folks, maybe even including you.